chapter
stringlengths 1.97k
1.53M
| path
stringlengths 47
241
|
|---|---|
Insect (Drosophila) and frog (Xenopus) development passes through three rather different (although often overlapping) phases: (1) establishing the main axes (dorsal-ventral; anterior-posterior; left-right). This is done by gradients of mRNAs and proteins encoded by the mother's genes and placed in the egg by her. (2) Establishing the main body parts such as the notochord and central nervous system in vertebrates and the segments in Drosophila These are run by genes of the zygote itself.
Now let us look for clues as to how the final working out of the embryo is done. We shall examine four examples:
1. the formation of wings (in Drosophila)
2. the formation of legs (also in Drosophila)
3. the formation of the bones (radius and ulna) of the front limb in mammals (mice)
4. the formation of eyes (probably in all animals)
Wings
The insect body plan consists of head, thorax, and abdomen. The thorax is built from three segments, T1, T2, and T3. Each carries a pair of legs; hence insects are six-legged creatures. In most of the insect orders, T2 and T3 each carry a pair of wings (the honeybee is an example). However, flies belong to the insect order diptera; they have only a single pair of wings (on T2). The third thoracic segment, T3, carries instead a pair of balancing organs called halteres.
In Drosophila, a gene called Ultrabithorax (Ubx) acts within the cells of T3 to suppress the formation of wings. By creating a double mutation in the Ultrabithorax gene (in its introns, as it turned out), Professor E. B. Lewis of Caltech was able to produce flies in which the halteres had been replaced by a second pair of wings. Ultrabithorax (Ubx) is an example of a "selector gene". Selector genes are genes that regulate (turning on or off) the expression of other genes. Thus selector genes act as "master switches" in development.
Wings and all their associated structures are complicated pieces of machinery. Nonetheless, mutations in a single gene, were able to cause the reprogramming of the building of T3 (and deprived the flies of their ability to fly). Selector genes encode transcription factors. Ultrabithorax encodes a transcription factor that is normally expressed at high levels in T3 (as well as in the first abdominal segment) of Drosophila.
These photographs were taken by, and kindly supplied by, Professor Lewis. He has spent his entire career studying selector genes in Drosophila. His life's work was honored when he shared the 1995 Nobel Prize for physiology or medicine.
Legs
Another selector gene, called Antennapedia (Antp), is normally turned "on" (expressed) in the thorax and turned "off" (repressed) in the cells of the head. However, mutations in Antp can cause it to turn on in the head and form a pair of legs where the antennae would normally be.
When you consider the many genes that must be involved in building a complex structure like an insect leg (or wing), it is remarkable that a single gene can switch them all on. It is also clear that once a selector gene turns "on" in certain cells of the embryo, it remains "on" in all the cells derived from those cells. Those cells become irrevocably committed to carrying out the genetic program leading to the formation of a leg or wing.
Homeobox Genes
Most selector genes, including Antp and Ubx, are homeobox genes
Antp, Ubx, and a number of other selector genes have been cloned and sequenced. They all contain within their coding regions a sequence of some 180 nucleotides called a homeobox. The approximately 60 amino acids encoded by the homeobox are called a homeodomain. It mediates DNA binding by these proteins. Many proteins containing homeodomains have been shown to be transcription factors; probably they all are.
The table shows the sequence of 60 amino acids in the homeodomain of the protein encoded by the Drosophila homeobox gene Antennapedia (Antp) compared with the homeodomain encoded by the mouse gene HoxB7; by bicoid (bcd), another homeobox gene in Drosophila; by goosecoid, a homeobox gene in Xenopus; and by mab-5, a homeobox gene in the roundworm Caenorhabditis elegans. A dash indicates that the amino acid at that position is identical to the one in the Antennapedia homeobox domain. Note that the mouse homeobox in HoxB7 differs from the Antp homeobox by only two amino acids (even though some 700 millions years have passed since these animals shared a common ancestor). HoxB6, used in the experiment described in the next section, differs from Antp in only 4 amino acids.
The Hox Cluster
Antp and Ubx are two of 8 homeobox genes that are linked in a cluster on one Drosophila chromosome. All of them encode transcription factors, each with a DNA-binding homeodomain and act in sequential zones of the embryo in the same order that they occur on the chromosome! The entire cluster is designated HOM-C with lab, Pb, Dfd, Scr, and Antp belonging to the ANT-C complex and Ubx, Abd-A, and Abd-B designated the BX-C complex, All animals that have been examined have at least one Hox cluster. Their genes show strong homology to the genes in Drosophila. Mice and humans have 4 Hox clusters (a total of 39 genes in humans) located on four different chromosomes.
• In mice: HoxA, HoxB (shown here), HoxC, HoxD
• In humans: HOXA, HOXB, HOXC, HOXD
As in Drosophila, they act along the developing embryo in the same sequence that they occupy on the chromosome. All the genes in the mammalian Hox clusters show some sequence homology to each other (especially in their homeobox) but very strong sequence homology to the equivalent genes in Drosophila. HoxB7 differs from Antp at only two amino acids, HoxB6 at four. In fact, when the mouse HoxB6 gene is inserted in Drosophila, it can substitute for Antennapedia and produce legs in place of antennae just as mutant Antp genes do. This fascinating result indicates clearly that these selector genes have retained, through millions of years of evolution, their function of assigning particular positions in the embryo, but the structures actually built depend on a different set of genes specific for a particular species.
The Mammalian Skeleton
The foreleg of the mouse and the arm of humans contain a single upper bone, the humerus, and two lower bones, the radius and ulna. The building of the entire arm, including carpals and the phalanges of the fingers, is controlled by Hox cluster genes.
When mice were bred with homozygous mutations for both HoxA11 and HoxD11, they were born with neither radius nor ulna in the forelimbs. Here, then, is another example of the power of selector genes to initiate a whole program, perhaps involving hundreds of other genes, to form a structure as complex as a forelimb. Mice that are homozygous for mutant HoxA10, C10, and D10 genes fail to form a lumbar and sacrum region in their vertebral column ("backbone"). Instead these vertebrae develop ribs like the thoracic vertebrae above them. However, if any one of these 6 Hox alleles is normal, the mice are much less severely affected. This shows the high degree of redundancy of these Hox genes.
Eyes
The compound eye of Drosophila is a marvel of precisely-organized structural elements. No one knows how many genes it takes to make the eye, but it must be a large number. Nevertheless, a single selector gene, eyeless (ey) (named, as is so often the case, for its mutant phenotype) can serve as a master switch turning on the entire cascade of genes needed to build the eye. Through genetic manipulation, it is possible to get the eyeless gene to be expressed in tissues where it is ordinarily not expressed. When eyeless is turned on in cells destined to form
• the insect's antennae, eyes form on the antennae
• wings, extra eyes form on the wings
• legs, eyes form on the legs.
Mice have a gene, small eyes (Sey; also known as Pax6) that is similar in sequence to the Drosophila eyeless gene. As its name suggests, it, too, is involved in eye formation (even though the structure of the mouse eye is entirely different from the compound eye of Drosophila).
However, the sequences of the mouse small eyes gene and the Drosophila eyeless genes are so similar that the mouse gene can substitute for eyeless when introduced into Drosophila. So, like the genes of the Hox clusters, Drosophila eyeless and mouse small eyes have retained, through millions of years of independent evolution, their function of assigning particular positions in the embryo where certain structures should be built, but the structures actually built depend on a different set of genes specific for a particular species.
Humans also have a gene that is homologous to small eyes and eyeless: it is called aniridia. Those rare humans who inherit a single mutant version of aniridia lack irises in their eyes. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/14%3A_Embryonic_Development_and_its_Regulation/14.06%3A_Homeobox_Genes.txt |
Stem cells are cells that divide by mitosis to form either two stem cells, thus increasing the size of the stem cell "pool", or one daughter that goes on to differentiate, and one daughter that retains its stem-cell properties. How the choice is made is still unknown. However, several genes have been found whose activity prevents a daughter cell from differentiating.
Types of Stem Cells
Several adjectives are used to describe the developmental potential of stem cells; that is, the number of different kinds of differentiated cell that they can become.
1. Totipotent cells. In mammals, totipotent cells have the potential to become any type in the adult body and any cell of the extraembryonic membranes (e.g., placenta). The only totipotent cells are the fertilized egg and the first 4 or so cells produced by its cleavage (as shown by the ability of mammals to produce identical twins, triplets, etc.). In mammals, the expression totipotent stem cells is a misnomer — totipotent cells cannot make more of themselves.
2. Pluripotent stem cells. These are true stem cells, with the potential to make any differentiated cell in the body (but probably not those of the placenta which is derived from the trophoblast).
Three types of pluripotent stem cells occur naturally:
• Embryonic Stem (ES) Cells. These can be isolated from the inner cell mass (ICM) of the blastocyst — the stage of embryonic development when implantation occurs. For humans, excess embryos produced during in vitro fertilization (IVF) procedures are used. Harvesting ES cells from human blastocysts is controversial because it destroys the embryo, which could have been implanted to produce another baby (but often was simply going to be discarded).
• Embryonic Germ (EG) Cells. These can be isolated from the precursor to the gonads in aborted fetuses.
• Embryonic Carcinoma (EC) Cells. These can be isolated from teratocarcinomas, a tumor that occasionally occurs in a gonad of a fetus. Unlike the other two, they are usually aneuploid.
All three of these types of pluripotent stem cells can only be isolated from embryonic or fetal tissue. They can be grown in culture, but only with special methods to prevent them from differentiating.
In mice and rats, embryonic stem cells can also:
• contribute to the formation of a healthy chimeric adult when injected into a blastocyst which is then implanted in a surrogate mother;
• enter the germline of these animals; that is, contribute to their pool of gametes;
• develop into teratomas when injected into immunodeficient (SCID) mice. These tumors produce a wide variety of cell types representing all three germ layers (ectoderm, mesoderm, and endoderm).
Using genetic manipulation in the laboratory, pluripotent stem cells can now be generated from differentiated cells. These induced pluripotent stem cells (iPSCs) are described below.
1. Multipotent stem cells. These are true stem cells but can only differentiate into a limited number of types. For example, the bone marrow contains multipotent stem cells that give rise to all the cells of the blood but not to other types of cells. Multipotent stem cells are found in adult animals; perhaps most organs in the body (e.g., brain, liver, lungs) contain them where they can replace dead or damaged cells. These adult stem cells may also be the cells that — when one accumulates sufficient mutations — produce a clone of cancer cells.
Stem Cells for Human Therapy
The Dream
Many medical problems arise from damage to differentiated cells. Examples:
• Type 1 diabetes mellitus where the beta cells of the pancreas have been destroyed by an autoimmune attack
• Parkinson's disease; where dopamine-secreting cells of the brain have been destroyed
• Spinal cord injuries leading to paralysis of the skeletal muscles
• Ischemic stroke where a blood clot in the brain has caused neurons to die from oxygen starvation
• Multiple sclerosis with its loss of myelin sheaths around axons
• Blindness caused by damage to the cornea
The great developmental potential of stem cells has created intense research into enlisting them to aid in replacing the lost cells of such disorders. While progress has been slow, some procedures already show promise. Using multipotent "adult" stem cells.
• culturing human epithelial stem cells and using their differentiated progeny to replace a damaged cornea. This works best when the stem cells are from the patient (e.g. from the other eye). Corneal cells from another person (an allograft) are always at risk of rejection by the recipient's immune system.
• the successful repair of a damaged left bronchus using a section of a donated trachea that was first cleansed of all donor cells and then seeded with the recipient's epithelial cells and cartilage-forming cells grown from stem cells in her bone marrow. So far the patient is doing well and needs no drugs to suppress her immune system.
Using differentiated cells derived from embryonic stem (ES) cells. Phase I clinical trials are underway to assess the safety of
• injecting retinal cells derived from ES cells
• into the eyes of young people with an inherited form of juvenile blindness;
• into the eyes of adults with age-related macular degeneration.
• injecting glial cells derived from ES cells into patients paralyzed by spinal cord injuries.
The Immunological Problems
One major problem that must be solved before human stem cell therapy becomes a reality is the threat of rejection of the transplanted cells by the host's immune system (if the stem cells are allografts; that is, come from a genetically-different individual).
A Possible Solution
One way to avoid the problem of rejection is to use stem cells that are genetically identical to the host. This is already possible in the rare situations when the patient has healthy stem cells in an undamaged part of the body (like the stem cells being used to replace damaged corneas). But even where no "autologous" stems cells are available, there may be a solution: using somatic-cell nuclear transfer .
In this technique,
1. An egg has its own nucleus removed and replaced by
2. a nucleus taken from a somatic (e.g., skin) cell of the donor.
3. The now-diploid egg is allowed to develop in culture to the blastocyst stage when
4. embryonic stem cells can be harvested and grown up in culture.
5. When they have acquired the desired properties, they can be implanted in the donor with no fear of rejection.
Using this procedure it possible to not only grow blastocysts but even have these go on to develop into adult animals — cloning — with a nuclear genome identical to that of the donor of the nucleus. The first successful cloning by SCNT was with amphibians. Later, mammals such as sheep (Dolly), cows, mice and others were successfully cloned. And in the 11 November 2007 issue of Science, researchers in Oregon reported success with steps 1–4 in rhesus monkeys (primates like us).
Their procedure:
• Remove the spindle and thus all nuclear material from secondary oocytes at metaphase of meiosis II.
• Fuse each enucleated egg with a skin cell taken from a male monkey.
• Culture until the blastocyst stage is reached.
• Extract embryonic stem cells from the inner cell mass.
• Establish that they have the nuclear genome of the male (but mostly the mitochondrial genome of the female).
• Culture with factors to encourage differentiation: they grew cardiac muscle cells (which contracted), and even neuron-like cells.
• Inject into SCID mice and examine the tumors that formed. These contained cells of all three germ layers: ectoderm, mesoderm, and endoderm.
• However, even after more than 100 attempts, they have not been able to implant their monkey blastocysts in the uterus of a surrogate mother to produce a cloned monkey.
This should reassure people who view with alarm the report in May 2013 by the same workers that they have finally succeeded in producing embryonic stem cells (ESCs) using SCNT from differentiated human tissue. The workers assure us that they will not attempt to implant these blastocysts in a surrogate mother to produce a cloned human. And their failure with monkeys suggests that they would fail even if they did try.
While cloning humans still seems impossible, patient-specific ESCs
• could be used in cell-replacement therapy or, failing that,
• provide the material for laboratory study of the basis of — and perhaps treatment of — genetic diseases.
Whether they will be more efficient and more useful than induced pluripotent stem cells remains to be seen.
Questions that Remain to be Answered
• Imprinted Genes. Sperm and eggs each contain certain genes that carry an "imprint" identifying them later in the fertilized egg as being derived from the father or mother respectively. Creating an egg with a nucleus taken from an adult cell may not allow a proper pattern of imprinting to be established. When the diploid adult nucleus is inserted into the enucleated egg (at least those of sheep and mice), the new nucleus becomes "reprogrammed". What reprogramming actually means still must be learned, but perhaps it involves the proper methylation and demethylation of imprinted genes. For example, the inactive X chromosome in adult female cells must be reactivated in the egg, and this actually seems to happen.
• Aneuploidy. In primates (in contrast to sheep, cattle, and mice), the process of removing the resident nucleus causes molecules associated with the centrosome to be lost as well. Although injecting a donor nucleus allows mitosis to begin, spindle formation may be disrupted, and the resulting cells fail to get the correct complement of chromosomes (aneuploidy).
• Somatic Mutations. This procedure also raises the spectre of amplifying the effect(s) of somatic mutations. In other words, mutations that might be well-tolerated in a single somatic cell of the adult (used to provide the nucleus) might well turn out to be quite harmful when they become replicated in a clone of cells injected later into the patient.
• Political Controversy. The goal of this procedure (which is often called therapeutic cloning even though no new individual is produced) is to culture a blastocyst that can serve as a source of ES cells. But that same blastocyst could theoretically be implanted in a human uterus and develop into a baby that was genetically identical to the donor of the nucleus. In this way, a human would be cloned. And in fact, Dolly and other animals are now routinely cloned this way. The spectre of this is so abhorrent to many that they would like to see the procedure banned despite its promise for helping humans. In fact, many are so strongly opposed to using human blastocysts — even when produced by nuclear transfer — that they would like to limit stem cell research to adult stem cells (even though these are only multipotent).
Possible Solutions to the Ethical Controversy
Induced pluripotent stem cells (iPSCs)
A promising alternative to the use of embryonic stem cells in human therapy are recently-developed methods of genetically reprogramming the nuclei of differentiated adult cells so that they regain the pluripotency of embryonic stem (ES) cells.
In June 2007, three laboratories reported that introducing extra copies of only 4 genes into adult mouse skin cells (fibroblasts) enables them to regain the properties of ES cells. When these cells, named induced pluripotent stem cells (iPSCs for short), were placed in mouse blastocysts, they participated in building all the tissues of the chimeric mice that resulted. (When placed in tetraploid (4n) blastocysts — unable by themselves to develop normally — embryos were formed that thus were clones of the skin cell donor.) The four genes: c-Myc, Sox2, Oct3/4, Klf4.
By 2009, several labs had succeeded in producing fertile adult mice from iPSCs derived from mouse embryonic fibroblasts. This shows that iPSCs are just a capable of driving complete development (pluripotency) as embryonic stem cells.
Reprogramming works in humans, too! Using the same four genes, the Yamanaka lab in Japan reported on 20 November 2007, that they now had reprogrammed human skin cells to become induced pluripotent stem cells (iPSCs). And the Thomson lab in Wisconsin accomplished the same thing using SOX2, OCT4, NANOG, and LIN28.
Further evidence of the remarkable role played by these few genes is the finding that during normal embryonic development of the zebrafish, the same or similar genes (SoxB1, Oct4, Nanog) are responsible for turning on the genes of the zygote. Earlier in development of the blastula, all the genes being expressed (including these) are the mother's — mRNAs and proteins that the mother deposited in the unfertilized egg. It makes sense that the same proteins that can reprogram a differentiated cell into a pluripotent state (iPSCs) are those that produce the pluripotent cells of the early embryo.
These achievements open the possibility of
• creating cells for laboratory study of the basis of genetic diseases.
Examples: researchers have succeeded in deriving iPSCs from
• patients with amyotrophic lateral sclerosis (ALS, "Lou Gehrig's disease"), and then causing them to differentiate into motor neurons (the cells affected in the disease) for study of their properties;
• the skin cells of a patient with an inherited heart disease (long QT syndrome) and causing these to differentiate into beating heart cells for study in the laboratory.
• The Jaenisch lab reported in the 6 March 2009 issue of Cell that they have succeeded in making iPSCs (they call them hiPSCs) from fibroblasts taken from patients with Parkinson's disease. The cells were then differentiated into dopamine-releasing cells — the cells lacking in this disease. What is particularly exciting is that they accomplished this after using the Cre-lox system to remove all the genes (e.g., SOX2, OCT4, KLF4) needed for reprogramming the fibroblasts to an embryonic-stem-cell-like condition.
• Since that report, other laboratories — using other methods — have also created iPSCs from which all foreign DNA (vector and transgenes) has been removed. Not only should such cells be safer to use in therapy, but these results show that the stimulus to reprogram a differentiated cell into a pluripotent state need only be transitory.
• creating patient-specific cell transplants — avoiding the threat of immunological rejection — that could be used for human therapy.
Therapy with iPSCs has already been demonstrated in mice. Three examples:
1. The Jaenisch lab in Cambridge, MA reported (in Science, 21 December 2007) that they had successfully treated knock-in mice that make sickle-cell hemoglobin with the human βS genes (and show many of the signs of sickle-cell disease in humans) by
• harvesting some fully-differentiated fibroblasts from a sickle-cell mouse;
• reprogramming these to become iPSCs by infecting them with Oct4, Sox2, Klf4, and c-Myc;
• then removing (using the Cre-lox system) the c-Myc to avoid the danger of this oncogene later causing cancer in the recipient mice;
• replacing the βS genes in the iPSCs with normal human βA genes;
• coaxing, with a cocktail of cytokines, these iPSCs to differentiate in vitro into hematopoietic (blood cell) precursors;
• injecting these into sickle-cell mice that had been irradiated to destroy their own bone marrow (as is done with human bone marrow transplants). (Although the recipient mice were different animals from the fibroblast donor, they were of the same inbred strain and thus genetically the same — like identical human twins. So the procedure fully qualifies as "patient-specific", i.e., with no danger of the injected cells being rejected by the recipient's immune system.)
The result: all the signs of sickle-cell disease (e.g., anemia) in the treated animals showed marked improvement.
2. In the 25 July 2013 issue of Nature, a team of Japanese scientists report that they were able to manufacture three-dimensional buds of human liver cells. Their process:
• create human iPSCs from human fibroblasts using the techniques described above;
• treat these with the substances needed for them to differentiate in liver cell precursors;
• culture these with a mixture of human endothelial cells and mesenchymal stem cells (to mimic the conditions that occur in normal embryonic development of the liver);
• implant the resulting solid masses (buds) of liver-like cells into immunodeficient mice.
The result: the implanted buds developed a blood supply and the mice began to secrete human albumin, human alpha-1-antitrypsin, and to to detoxify injected chemicals just as human livers do.
3. Workers in the Melton lab at Harvard University reported in the 9 October 2014 issue of Cell that they had succeeded in differentiating large numbers of human beta cells from human iPSCs (as well as from human ES cells). When transplanted into diabetic mice, these cells brought their elevated blood sugar levels back down.
Let us hope that what works in mice can someday be developed into a safe therapy that will work in humans. (In the case of Type 1 diabetes mellitus, however, even patient-derived beta cells will still be at risk of the same autoimmune rejection that caused the disease in the first place.)
Despite these successes, iPSCs may not be able to completely replace the need for embryonic stem cells and may even be dangerous to use in human therapy. Several groups have found that human iPSCs contain mutations as well as epigenetic patterns (e.g., methylation of their DNA) that are not found in embryonic stem cells. Some of the mutations are also commonly found in cancer cells.
Other approaches being explored
• ES cells can be derived from a single cell removed from an 8-cell morula. The success of preimplantation genetic diagnosis (PGD) in humans shows that removing a single cell from the morula does not destroy it — the remaining cells can develop into a blastocyst, implant, and develop into a healthy baby. Furthermore, the single cell removed for PGD can first be allowed to divide with one daughter used for PGD and the other a potential source of an ES cell line.
• In altered nuclear transfer (ANT) — a modified version of SCNT (somatic-cell nuclear transfer) — a gene necessary for later implantation (Cdx2 — encoding a homeobox transcription factor) is turned off (by RNA interference) in the donor nucleus before the nucleus is inserted into the egg. The blastocyst that develops
• has a defective trophoblast that cannot implant in a uterus
• but the cells of the inner cell mass are still capable of developing into cultures of ES cells. (The gene encoding the interfering RNA can then be removed using the Cre/loxP technique.)
• Jose Cibelli and his team at Advanced Cell Technology reported in the 1 February 2002 issue of Science that they had succeeded inIf this form of cloning by parthenogenesis works in humans [It does! — success with unfertilized human eggs was reported in June 2007.], it would have
• stimulating monkey oocytes to begin dividing without completing meiosis II (therefore still 2n)
• growing these until the blastocyst stage, from which they were able to harvest
• ES cells.
• the advantage that no babies could be produced if the blastocyst should be implanted (two identical genomes cannot produce a viable mammal — probably because of incorrect imprinting);
• the disadvantage that it will only help females (because only they can provide an oocyte!) (But men may have a procedure that works for them next.)
• On 24 March 2006, Nature published an online report that a group of German scientists had been able to derive pluripotent stem cells from the stem cells that make spermatogonia in the mouse. Both in vitro and when injected into mouse blastocysts, these cells differentiated in a variety of ways including representatives of all three germ layers. If this could work in humans, it would
• provide a source of stem cells whose descendants would be "patient-specific"; that is, could be transplanted back into the donor (men only!) without fear of immune rejection.
• avoid the controversy surrounding the need to destroy human blastocysts to provide embryonic stem cells.
• The 7 January 2007 issue of Nature Biotechnology reports the successful production of amniotic fluid-derived stem cells ("AFS"). These are present in the amniotic fluid removed during amniocentesis. With the proper culture conditions, they have been shown to be able to differentiate into a variety of cell types includingSo these cells are pluripotent. Although perhaps not as versatile as embryonic stem cells, they are more versatile than adult stem cells.
• ectoderm (neural tissue)
• mesoderm (e.g., bone, muscle)
• endoderm (e.g., liver)
Applied to humans, none of the above procedures would involve the destruction of a potential human life. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/14%3A_Embryonic_Development_and_its_Regulation/14.07%3A_Stem_Cells.txt |
The other pages describe:
• the properties and potential therapeutic applications of embryonic (and other types of) stem cells
• how mouse embryonic stem cells can be used to make transgenic mice
• how the fusion of a differentiated cell from an adult sheep with an enucleated sheep egg can produce a clone of the cell donor ("Dolly")
The techniques used in the early steps of each process have been achieved with human cells.
Thirteen years ago a research team led by James Thomson of the University of Wisconsin reported (in the 6 November 1998 issue of Science) that they were able to grow human embryonic stem (ES) cells in culture.
At the time of implantation, the mammalian embryo is a blastocyst. It consists of the
• trophoblast — a hollow sphere of cells that will go on to implant in the uterus and develop into the placenta and umbilical cord.
• inner cell mass (ICM) that will develop into the baby as well as the extraembryonic amnion and yolk sac.
The cells of the inner cell mass are considered pluripotent; that is, each is capable of producing descendants representing all of the hundreds of differentiated cell types in the newborn baby, including
• ectodermal cells like neurons and skin (epithelial cells)
• mesodermal cells like striated muscle, smooth muscle, cartilage, and bone
• endodermal cells like the liver and the lining of the intestine
The Process
• Remove the trophoblast cells from a human blastocyst (these were extras not needed for assisted reproductive technology).
• Separate the cells of the inner cell mass and culture them on a plate of "feeder" cells (mouse fibroblasts were used).
• Isolate single cells and grow them as clones.
• Test the clones.
The Results
• Each successful clone maintained a normal human karyotype (unlike most cultured human cells — HeLa cells, for example).
• These cells had high levels of the enzyme telomerase, which maintains normal chromosome length and is characteristic of cells with unlimited potential to divide ("immortal").
• When injected into SCID mice, these cells formed teratomas; tumors containing a mix of differentiated human cell types, including cells characteristic of
• ectoderm
• mesoderm
• endoderm
Note
SCID = severe combined immunodeficiency.
SCID mice lack a functioning immune system (have neither T cells nor B cells) and so cannot reject foreign tissue. Some rare inherited diseases of humans are also called SCID. They produce a similar phenotype but involve different molecular defects.
Human embryonic stem cells have the potential to
• teach us about the process of human embryonic development, its genetic control, etc.
• provide a source of replacement cells to repair damaged human tissue. As the proper signals are discovered, it will be possible to cause these cells to differentiate along a particular pathway, e.g., to form insulin-secreting beta cells of the islets of Langerhans. Such cells might be able to replace lost or non-functioning cells in a human patient (e.g., with Type 1 diabetes mellitus).
However, there are problems that remain to be solved before this hope can be realized.
• Production of human ES cells requires the destruction of the blastocyst, and this is morally-repugnant to many people.
• Cell replacement therapy had better be "patient-specific"; that is, the donated cells should be genetically identical to the recipient. Otherwise, the replaced cells are at risk of being rejected by the host's immune system. [Link to a discussion of "therapeutic cloning" — a method to avoid this.
• ES cells are pluripotent and might differentiate in unwanted ways when introduced into the patient. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/14%3A_Embryonic_Development_and_its_Regulation/14.08%3A_Embryonic_Stem_Cells.txt |
Could a mutation in one of your liver cells ever be passed on to your children?
No!
Why not?
• The fusion of one sperm cell and one egg cell represents the only genetic link between the bodies of parents and the body of their child and
• the cells destined to produce sperm and eggs are set aside very early in embryonic life.
Example 1:
By the 15th week of gestation, the human female fetus has already set aside each and every cell that may someday develop into a mature egg. (In fact, each of these cells has already entered its final meiotic division!)
Example 2:
Caenorhabditis elegans is a microscopic (~ 1 mm) nematode (roundworm) that normally lives in soil. Like all animals, it starts life as a fertilized egg (zygote) which then undergoes the mitotic divisions needed to produce the
• 556 cells of the newly-hatched worm and, later,
• the 959 somatic cells, and a variable number of germ cells, of the adult worm.
In Weismann's view, the somaplasm simply provides the housing for the germplasm, seeing to it that the germplasm is protected, nourished, and conveyed to the germplasm of the opposite sex to create the next generation. The old riddle about which came first, the chicken or the egg, would have been no puzzle to Weismann. In his view, the chicken is simply one egg's device for laying another egg.
Weismann also understood the implications of his theory for aging. Once the opportunity to pass germplasm on has passed, there is no need to maintain the integrity of the somaplasm ("disposable soma"); hence the decline in body function with aging .
Today we know that only the germplasm — the gametes and the cells that form them — continue to express high levels of the enzyme telomerase. These cells are able to maintain the length of their chromosomes forever and are immortal. The cells of the somaplasm, in contrast, stop producing telomerase, lose a portion of their chromosome tips at each mitosis, and eventually die.
Deciding between germline and soma
What determines
• which of the early cells of the embryo will be destined to go on to form sperm or eggs, that is to become germline and
• which are destined to develop into the body tissues (soma) of the animal?
At the 4th mitotic division in the gall midge (an insect) egg, 2 of the 16 nuclei become pinched off in a small amount of cytoplasm at one end of the egg. At the 5th mitosis, these two nuclei divide normally, producing daughter cells with the full complement of chromosomes (2n = 40) of the species. But not so for the other nuclei. When each of these reaches anaphase, only 8 of their 40 chromosomes (dyads) separate and move to opposite ends of the spindle. The remaining 32 chromosomes stay at the metaphase plate and eventually disintegrate.
• The descendants of the two "normal" nuclei ultimately differentiate to form sperm or eggs, i.e., the germline.
• The descendants of the rest of the nuclei, those with the sharply-reduced chromosome number, go on to form all the other tissues of the insect body, i.e., the soma.
The top row of this figure shows normal development in the gall midge. The gametes are descended from the two nuclei, each with a full diploid set of 40 chromosomes, that were partitioned off in a mass of special cytoplasm (here called "germplasm"). The remaining nuclei lose 32 chromosomes before going on to form the rest of the insect body (the somaplasm).
The bottom row shows that destruction of the germplasm causes the nuclei that move there to undergo chromosome elimination also. The animal that develops is sterile but otherwise normal.
Mammals
In mammals (and birds), the decision to become germ cells is not intrinsic but is the result of cell-to-cell signaling during early embryonic development.
In mice, the setting aside of the future germ cells begins at the start of gastrulation (6 days after fertilization in the mouse) as the mesoderm is forming. Ectoderm cells of the future extraembryonic membranes secrete cytokines (including bone morphogenic proteins) which signal cells in the developing mesoderm to differentiate into cells which — under the influence of other inter-cellular chemical signals — will go on to form either
• mesoderm of the extraembryonic membranes (amnion and allantois)
• primordial germ cells (PGCs)
The PGCs migrate into the part of the developing embryo that will go on to form the gonads (ovaries or testes).
Exceptions to Weismann's Theory
The distinction between germline and soma exists only in animals.
In plants, cells destined to become gametes do arise from somatic tissues. In the flowering plants (angiosperms), for example, certain signals cause meristems that had been making stem tissue to become converted into flower buds which go on to make the gametes.
In microorganisms, all life's functions are embodied in a single cell. (However, some unicellular organisms, like the ciliated protozoan Tetrahymena thermophila, have a complete genome in their micronuclei, which are passed on to the next generation, as well as genes in a macronucleus, which is not. Thus, even here, there is the equivalent of a distinction between germline and soma.)
Somatic vs. Germline Mutations
The significance of mutations is profoundly influenced by the distinction between germline and soma. Mutations that occur in a somatic cell, in the bone marrow or liver for example, may
• damage the cell
• make the cell cancerous
• kill the cell
Whatever the effect, the ultimate fate of that somatic mutation is to disappear when the cell in which it occurred, or its owner, dies.
Germline mutations, in contrast, will be found in every cell descended from the zygote to which that mutant gamete contributed. If an adult is successfully produced, every one of its cells will contain the mutation. Included among these will be the next generation of gametes, so if the owner is able to become a parent, that mutation will pass down to yet another generation.
Example 1:
More than 8000 people living in South Africa today carry a gene for the metabolic disease called porphyria. Every one of them has acquired their gene through a chain of ancestors leading back to a single couple: Ariaantje Jacobs and Gerrit Jansz. This woman and man emigrated from Holland to South Africa late in the seventeenth century and one or the other of them passed the gene — through the germline — on to their descendants. Fortunately, the ailment is usually mild (unless the person is given a barbiturate sedative, which triggers a violent reaction).
Example 2:
Retinoblastoma, a tumor that occurs in humans in
• a sporadic form, which is caused by a somatic mutation in each of the two RB genes in a cell
• a familial (inherited) form, which is caused by a somatic mutation to one RB gene in a cell that already has one mutant RB gene inherited through the germline from a parent. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/14%3A_Embryonic_Development_and_its_Regulation/14.09%3A_Germline_vs._Soma.txt |
When some species of flatworms (left) are decapitated, they can regenerate a new head. Double-amputees can regenerate both a new head at the anterior surface and a new tail at the posterior surface (right). They do this by the proliferation and differentiation of the pluripotent stem cells (called neoblasts) that it retains in its body throughout its life.
How do the cells know whether to develop into a head or a tail? Thanks to the ease with which individual genes can be knocked out by RNA interference (RNAi), it has been shown that Wnt/β-catenin signaling dictates where the head and tail form.
• Blocking Wnt/β-catenin signaling by RNAi causes a head to form where a tail should (producing a two-headed animal) while
• blocking part of the β-catenin degradation complex (thus enhancing the pathway) causes a tail to develop where a head should (producing a two-tailed animal).
These amphibians can regenerate a missing tail, legs, even eyes. This remarkable ability is particularly pronounced in the larval stage. For this reason, larval salamanders are favorites for doing research on regeneration. For example, cutting the tail off a larval salamander initiates the following sequence of events:
• A layer of epidermal cells grows over and covers the stump.
• A mass of undifferentiated cells — called the blastema — develops just beneath.
• Muscle and cartilage form in the regrowing tail.
• The notochord and spinal cord grow out into the regrowing tail.
• After a few weeks, a new, fully-functional and anatomically-correct tail is complete.
The Mechanism
For years, it has been unclear as to whether this regeneration depends on
• a population of pluripotent stem cells that have resided in the animal body prepared for such an event (as occurs in the hydra) or
• the dedifferentiation of specialized cells, e.g. muscle and cartilage cells, in the stump.
The answer appears to be both.
• Stem cells in the spinal cord migrate into the regrowing tail and differentiate into several cell types, including muscle and cartilage. Although the stem cells are ectoderm, they are able to develop into mesoderm.
• Muscle cells in the stump migrate into the blastema while
• reentering the cell cycle to produce thousands of descendants;
• dedifferentiate as they do so; that is, they lose the characteristic proteins, etc. of muscle cells.
• Even though there is as yet no sign of a tail, its final pattern is established during this process for if the blastema is removed and transplanted elsewhere, it will continue the process of regenerating a tail.
• Finally the cells of the blastema differentiate into all the cell types — nerve, muscle, cartilage, skin — used to build the regenerated tail.
Mammals
Don't we wish that we had the same powers of regeneration that salamanders do: able to regenerate a severed spinal cord or grow a new heart! But unfortunately, we cannot. We can regenerate some skin, a large amount of liver, and the very tips of fingers and toes. But that's about it. Just why we are so limited is not known (but is the subject of intense research). Much of the excitement surrounding research on stem cells is because of the hope that they may provide a means of regrowing damaged or lost tissues or even organs.
In contrast to the situation that appears to hold for salamanders, dedifferention of specialized cells does not appear to play a role in the formation of a blastema in mice. Instead, the various tissues — epidermis, hair follicles, sweat glands, neurons (all ectoderm) and muscle, bone, tendon, blood vessels (mesoderm) — that participate in regenerating the tip of an amputated mouse digit (finger or toe) develop from a diverse population of "adult" stem cells in the stump that retain their restricted developmental potential. You can read about the evidence for this in Rinkevich, Y., et al., Nature, 476, 409-413 (25 August 2011).
Genetic Control of Regeneration
A number of genes have been found to be implicated in regeneration. One of the most potent of these is Wnt.
• Injection of agents (e.g. antisense RNA molecules) that interfere with the Wnt/β-catenin pathway
• blocks limb regeneration in salamanders and, as we saw above,
• promotes head formation in regenerating planarians, while
• injection of agents that enhance the Wnt/β-catenin pathway
• enable chicks (that, like mammals, are normally incapable of regenerating limbs) to regenerate a wing;
• as well as enabling a regenerating planarian to form a tail where a head should go. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/14%3A_Embryonic_Development_and_its_Regulation/14.10%3A_Regeneration.txt |
Strategy and Topology
Humans (and most animals) digest all their food extracellularly; that is, outside of cells. Digestive enzymes are secreted from cells lining the inner surfaces of various exocrine glands. The enzymes hydrolyze the macromolecules in food into small, soluble molecules that can be absorbed into cells.
Figure 15.1.1.1 shows the major topological relationships in the body. The linings of all
• exocrine glands, including digestive glands
• nasal passages, trachea, and lungs
• kidney tubules, collecting ducts, and bladder
• reproductive structures like the vagina, uterus, and fallopian tubes
are all continuous with the surface of the body. Anything placed within their lumen is, strictly speaking, outside the body. This includes the secretions of all exocrine glands (in contrast to the secretions of endocrine glands, which are deposited in the blood) and any indigestible material placed in the mouth which will appear, in due course, at the other end.
Ingestion
Food placed in the mouth is ground into finer particles by the teeth. It is then moistened and lubricated by saliva (secreted by three pairs of salivary glands). Small amounts of starch are digested by the amylase present in saliva. The resulting bolus of food is swallowed into the esophagus and carried by peristalsis to the stomach.
The Stomach
The wall of the stomach is lined with millions of gastric glands, which together secrete 400–800 ml of gastric juice at each meal. Several kinds of cells are found in the gastric glands inlcuding parietal cells, chief cells, mucus-secreting cells, and hormone-secreting (endocrine) cells.
Parietal cells
Parietal cells secrete hydrochloric acid and intrinsic factor.
Hydrochloric acid (HCl)
Parietal cells contain a H+/K+ ATPase. This transmembrane protein secretes H+ ions (protons) by active transport, using the energy of ATP. The concentration of H+ in the gastric juice can be as high as 0.15 M, giving gastric juice a pH somewhat less than 1. With a concentration of H+ within these cells of only about 4 x 10-8 M, this example of active transport produces more than a million-fold increase in concentration. No wonder that these cells are stuffed with mitochondria and are extravagant consumers of energy.
Intrinsic factor is a protein that binds ingested vitamin B12 and enables it to be absorbed by the intestine. A deficiency of intrinsic factor — as a result of an autoimmune attack against parietal cells — causes pernicious anemia.
Chief cells
The chief cells synthesize and secrete pepsinogen, the precursor to the proteolytic enzyme pepsin. Pepsin cleaves peptide bonds, favoring those on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues. Its action breaks long polypeptide chains into shorter lengths. Secretion by the gastric glands is stimulated by the hormone gastrin. Gastrin is released by endocrine cells in the stomach in response to the arrival of food.
Absorption in the stomach
Very little occurs. However, some water, certain ions, and such drugs as aspirin and ethanol are absorbed from the stomach into the blood (accounting for the quick relief of a headache after swallowing aspirin and the rapid appearance of ethanol in the blood after drinking alcohol). As the contents of the stomach become thoroughly liquefied, they pass into the duodenum, the first segment (about 10 inches [25 cm] long) of the small intestine. Most of our ingested vitamins and minerals are absorbed here. Two ducts enter the duodenum:
1. one draining the gall bladder and hence the liver and
2. the other draining the exocrine portion of the pancreas.
The Liver
The liver secretes bile. Between meals it accumulates in the gall bladder. When food, especially when it contains fat, enters the duodenum, the release of the hormone cholecystokinin (CCK) stimulates the gall bladder to contract and discharge its bile into the duodenum. Bile contains:
• bile acids. These amphiphilic steroids emulsify ingested fat. The hydrophobic portion of the steroid dissolves in the fat while the negatively-charged side chain interacts with water molecules. The mutual repulsion of these negatively-charged droplets keeps them from coalescing. Thus large globules of fat (liquid at body temperature) are emulsified into tiny droplets (about 1 µm in diameter) that can be more easily digested and absorbed.
• bile pigments. These are the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish color of the bile pigments imparts the characteristic brown color of the feces.
The Hepatic Portal System
The capillary beds of most tissues drain into veins that lead directly back to the heart. But blood draining the intestines is an exception. The veins draining the intestine lead to a second set of capillary beds in the liver.
Here the liver removes many of the materials that were absorbed by the intestine:
• Glucose is removed and converted into glycogen.
• Other monosaccharides are removed and converted into glucose.
• Excess amino acids are removed and deaminated.
• The amino group is converted into urea.
• The residue can then enter the pathways of cellular respiration and be oxidized for energy.
• Many nonnutritive molecules, such as ingested drugs, are removed by the liver and, often, detoxified.
The liver serves as a gatekeeper between the intestines and the general circulation. It screens blood reaching it in the hepatic portal system so that its composition when it leaves will be close to normal for the body. Furthermore, this homeostatic mechanism works both ways. When, for example, the concentration of glucose in the blood drops between meals, the liver releases more to the blood by converting its glycogen stores to glucose (glycogenolysis) and converting certain amino acids into glucose (gluconeogenesis)
The Pancreas
The pancreas consists of clusters of endocrine cells (the islets of Langerhans) and exocrine cells whose secretions drain into the duodenum. Pancreatic fluid contains:
• sodium bicarbonate (NaHCO3). This neutralizes the acidity of the fluid arriving from the stomach raising its pH to about 8.
• pancreatic amylase. This enzyme hydrolyzes starch into a mixture of maltose and glucose.
• pancreatic lipase. The enzyme hydrolyzes ingested fats into a mixture of fatty acids and monoglycerides. Its action is enhanced by the detergent effect of bile.
• 4 "zymogens" — proteins that are precursors to active proteases. These are immediately converted into the active proteolytic enzymes:
• trypsin. Trypsin cleaves peptide bonds on the C-terminal side of arginines and lysines.
• chymotrypsin. Chymotrypsin cuts on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues (the same bonds as pepsin, whose action ceases when the NaHCO3 raises the pH of the intestinal contents).
• elastase. Elastase cuts peptide bonds next to small, uncharged side chains such as those of alanine and serine.
• carboxypeptidase. This enzyme removes, one by one, the amino acids at the C-terminal of peptides.
• nucleases. These hydrolyze ingested nucleic acids (RNA and DNA) into their component nucleotides.
The secretion of pancreatic fluid is controlled by two hormones: secretin, which mainly affects the release of sodium bicarbonate and cholecystokinin (CCK), which stimulates the release of the digestive enzymes
The Small Intestine
Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi, which line the inner surface of the small intestine. This scanning electron micrograph shows the villi carpeting the inner surface of the small intestine.
The crypts at the base of the villi contain stem cells that continuously divide by mitosis producing
• More stem cells
• Paneth cells, which secrete antimicrobial peptides that suppress the concentration of bacteria in the small intestine
• Cells that migrate up the surface of the villus while differentiating into
• columnar epithelial cells (the majority),which are responsible for digestion and absorption
• goblet cells, which secrete mucus
• endocrine cells, which secrete a variety of hormones
The continuous production of new epithelial cells replace older cells that after about 5 days die by apoptosis. The villi increase the surface area of the small intestine to many times what it would be if it were simply a tube with smooth walls. In addition, the apical (exposed) surface of the epithelial cells of each villus is covered with microvilli (also known as a "brush border"). Thanks largely to these, the total surface area of the intestine is almost 200 square meters, about the size of the singles area of a tennis court and some 100 times the surface area of the exterior of the body.
The electron micrograph (courtesy of Dr. Sam L. Clark) shows the microvilli of a mouse intestinal cell. Incorporated in the plasma membrane of the microvilli are a number of enzymes that complete digestion:
• aminopeptidases attack the amino terminal (N-terminal) of peptides producing amino acids
• disaccharidases convert disaccharides into their monosaccharide subunits
• maltase hydrolyzes maltose into glucose
• sucrase hydrolyzes sucrose (common table sugar) into glucose and fructose
• lactase hydrolyzes lactose (milk sugar) into glucose and galactose
Fructose simply diffuses into the villi, but both glucose and galactose are absorbed by active transport
• fatty acids and monoglycerides. These become resynthesized into fats as they enter the cells of the villus. The resulting small droplets of fat are then discharged by exocytosis into the lymph vessels, called lacteals, draining the villi.
Humans with a rare genetic inability to form microvilli die of starvation.
The Large Intestine (colon)
The large intestine receives the liquid residue after digestion and absorption are complete. This residue consists mostly of water as well as any materials that were not digested. The colon contains an enormous (~413) population of microorganisms. Our bodies consist of about the same number (~313) of cells. Most of the species live there perfectly harmlessly; that is, they are commensals. Some are actually beneficial as they synthesize vitamins and digest polysaccharides for which we have no enzymes (providing an estimated 10% of the calories we acquire from our food).
Most of the bacteria belong to the Firmicutes and Bacteroidetes (although used as an indicator of water pollution by feces, E. coli is actually a minor component). In both obese mice (ob/ob) and humans, the relative proportion of Bacteroidetes declines and, in mice at least, the efficiency with which residual food is absorbed increases. Putting humans on a diet causes them to regain the normal proportion of Bacteroidetes. Why this relationships exists remains to be discovered. Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces may consist of bacterial cells.
Reabsorption of water is the chief function of the large intestine. The large amounts of water secreted into the stomach and small intestine by the various digestive glands must be reclaimed to avoid dehydration. If the large intestine becomes irritated, it may discharge its contents before water reabsorption is complete causing diarrhea. On the other hand, if the colon retains its contents too long, the fecal matter becomes dried out and compressed into hard masses causing constipation.
15.1B: Metabolism
All living things must have an unceasing supply of energy and matter. The transformation of this energy and matter within the body is called metabolism. Catabolism is destructive metabolism. Typically, in catabolism, larger organic molecules are broken down into smaller constituents. This usually occurs with the release of energy (usually as ATP). Anabolism is constructive metabolism. Typically, in anabolism, small precursor molecules are assembled into larger organic molecules. This always requires the input of energy (often as ATP).
Autotrophic and Heterotrophic Nutrition
Green plants, algae, and some bacteria are autotrophs ("self-feeders"). Most of them use the energy of sunlight to assemble inorganic precursors, chiefly carbon dioxide and water, into the array of organic macromolecules of which they are made. The process is photosynthesis. Photosynthesis makes the ATP needed for the anabolic reactions in the cell.
All other organisms, including ourselves, are heterotrophs. We secure all our energy from organic molecules taken in from our surroundings ("food"). Although heterotrophs may feed partially (as most of us do) or exclusively on other heterotrophs, all the food molecules come ultimately from autotrophs. We may eat beef but the steer ate grass. Heterotrophs degrade some of the organic molecules they take in (catabolism) to make the ATP that they need to synthesize the others into the macromolecules of which they are made (anabolism).
How humans (and other animals) do it
Humans are heterotrophs. We are totally dependent on ingested preformed organic molecules to meet all our energy needs. We are also dependent on preformed organic molecules as the building blocks to meet our anabolic needs.
• Ingestion: taking food within the body (although as the figure shows, it is still topologically in the external world, not the internal).
• Digestion.
The enzyme-catalyzed hydrolysis of
• polysaccharides (e.g., starch) to sugars
• proteins to amino acids
• fats to fatty acids and glycerol
• nucleic acids to nucleotides
• Absorption into the body and transport to the cells.
• Absorption into cells
Within cells, these molecules are further degraded into still simpler molecules containing two to four carbon atoms. These fragments (acetyl-CoA for example) face one of two alternatives:
• They may proceed up various metabolic pathways and serve as the building blocks of, for example, sugars and fatty acids. From these will be assembled the macromolecules of the cell:
• polysaccharides
• fats
• proteins
• nucleic acids
• Or the molecules in this pool of two- to four-carbon fragments may be still further degraded — ultimately to simple inorganic molecules such as carbon dioxide (CO2), H2O, and ammonia (NH3). This phase of catabolism releases large amounts of energy (in the form of ATP). One use to which this energy is put is to run the anabolic activities of the cell. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.01%3A_Nutrition/15.1A%3A_The_Human_Gastrointestinal_Tract.txt |
The human diet must provide the following:
• calories - enough to meet our daily energy needs
• amino acids - there are nine, or so, "essential" amino acids that we need for protein synthesis and that we cannot synthesize from other precursors
• fatty acids - there are three "essential" fatty acids that we cannot synthesize from other precursors
• minerals - inorganic ions generally 18 different ones, calcium in relatively large amounts, zinc in "trace" amounts
• vitamins - a dozen or so, small organic molecules that we cannot synthesize from other precursors in our diet
Determining what substances must be incorporated in the human diet and how much of each is incorporated even after years of research still under active study. Why the uncertainty? Inadequate intake of some vitamins produces easily-recognized deficiency diseases like However, it is so difficult to exclude some other possible vitamins from the diet that deficiency diseases are hard to demonstrate.
• scurvy: lack of ascorbic acid (vitamin C)
• beriberi: lack of thiamine (vitamin B1)
• pellagra: lack of niacin
Similarly, some minerals are needed is such vanishingly small amounts that it is practically impossible to prepare a diet that does not include them. However, totally synthetic diets are now available for intravenous feeding of people who cannot eat. This so-called total parenteral nutrition has revealed, unexpectedly, some additional trace element needs: chromium and molybdenum.
Despite some uncertainties, the Food and Nutrition Board of the U. S. National Academy of Sciences publishes guidelines. One of the most useful of these is called recommended daily allowances or RDAs. These provide the basis for the nutrition labels on food.
Carbohydrates
Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets and starches provide the bulk of that. Age, sex, size, health, and the intensity of physical activity strongly affect the daily need for calories. Moderately active females (19–30 years old) need 1500–2500 kcal/day, while males of the same age need 2500–3300 kcal/day. In some poor countries, too many children do not receive enough calories to grow properly. In order to maintain blood sugar levels, they attack their own protein. This condition of semi-starvation is known as marasmus.
Protein
Humans must include adequate amounts of 9 amino acids in their diet. These "essential" amino acids cannot be synthesized from other precursors. However, cysteine can partially meet the need for methionine (they both contain sulfur), and tyrosine can partially substitute for phenylalanine.
The Essential Amino Acids for Humans
Histidine
Isoleucine
Leucine
Lysine
Methionine (and/or cysteine)
Phenylalanine (and/or tyrosine)
Threonine
Tryptophan
Valine
Two of the essential amino acids, lysine and tryptophan, are poorly represented in most plant proteins. Thus strict vegetarians should take special pains to ensure that their diet contains sufficient amounts of these two amino acids. Birds, mammals, and some other animals are able to discriminate food that contains a nutrient, e.g., an essential amino acid, that they need from food that doesn't. If offered a food lacking that nutrient, they quickly stop eating it. How is this done? In rats, at least, it turns out that certain neurons in the brain detect the lack of an essential amino acid and signal the appetite centers of the brain to stop feeding on deficient food. The neurons detect the lack by the failure of their transfer RNAs (tRNAs) for that amino acid to acquire it. Rats whose tRNAs for threonine have been blocked from loading threonine cease feeding even if their food contains adequate concentrations of it.
Fats
Ingested fats provide the precursors from which we synthesize our own fat as well as cholesterol and various phospholipids. Fat provides our most concentrated form of energy. Its energy content (9 kcal/gram) is over twice as great as carbohydrates and proteins (4 kcal/gram). Humans can synthesize fat from carbohydrates (as most of us know all too well). However, three essential fatty acids cannot be synthesized this way and must be incorporated in the diet. These are
• linoleic acid
• linolenic acid
• arachidonic acid
All are unsaturated; that is, have double bonds.
Types of fats
• Saturated. No double bonds between the carbon atoms in the fatty acid chains. Most animal fats (e.g., butter) are highly saturated.
• Monounsaturated. Have a single double bond in the fatty acid chains. Examples are olive, peanut, and rapeseed (canola) oil.
• Polyunsaturated. Have two or more double bonds in their fatty acid chains. Examples: corn, soy bean, cottonseed, sunflower, and safflower oils.
• Trans Fats. Have been partially hydrogenated producing fewer double bonds and of those that remain, converting them from a cis to a trans configuration.
• Omega-3 fats. Have at least one double bond three carbon atoms in from the end of the fatty acid molecule. Linolenic acid is an example. Fish oils are a rich source of omega-3 fatty acids.
Many studies have examined the relationship between fat in the diet and cardiovascular disease. There is still no consensus, but the evidence seems to indicate that Mono and polyunsaturated fats are less harmful than saturated ones, except that trans unsaturated fats may be worse than saturated fats. Ingestion of omega-3 unsaturated fats may be protective. For this reason, 1.1 grams/day for women (1.6 for men) is recommended.
Read the label
At present, food labels in the U.S. list the total amount of fat in a serving of the product (5 g in the example shown here) with a breakdown of the amounts of saturated (1 g), polyunsaturated (0.5 g), and monounsaturated fat (1.5 g).
What about trans fats? There is a proposal to have them included, but at present they are not. However, if you add the amounts of saturated, polyunsaturated, and monounsaturated fat, and the total does not equal "Total Fat" , the discrepancy (2 g in this example) represents the amount of trans fat. Baked goods (like the one whose label is shown here) tend to have quite a bit of trans fat.
Minerals
Calcium
Calcium is essential to almost every function in the body. Blood clotting, intracellular signaling and muscle contraction need only trace amounts. However, large amounts of calcium are needed to make bone (which is 18% calcium), So substantial amounts are needed in the diet, especially during infancy, childhood, and pregnancy. Three hormones parathyroid hormone (PTH), calcitonin and calciferol (vitamin D) work together to regulate how much calcium
• is absorbed from your food
• is taken from, or added to, bone
• is excreted in the urine.
A temporary deficit in the amount of calcium in the diet can be compensated for by its removal from the huge reserves in bone.
Iron (Fe)
Iron is incorporated in a number of body constituents, notably cytochromes, myoglobin and hemoglobin. Not surprisingly, an iron deficiency shows up first as anemia.
In developed countries like the U.S., iron deficiency is the most common mineral deficiency. It is particularly common among women because of the loss of blood during menstruation and the need for extra iron during pregnancy and breast feeding.
Marginal iron intake is so widespread that some nutritionists want to have iron added to common foods like bread and cereals, just as some vitamins now are. However, excess iron in the body also leads to problems, and this has made the proposal controversial. Even iron supplement tablets pose risks: thousands of children in the U.S. are accidentally poisoned each year by swallowing too many iron tablets. In fact, iron is the most frequent cause of poisoning deaths among children in the U.S.
Iodine
• Incorporated in the hormones thyroxine (T4) and triiodothyronine (T3).
• In regions with iodine-deficient soils, food may not contain enough iodine to meet body needs. The result is goiter: a swelling of the thyroid gland.
• The use of iodized salt (table salt to which a small amount of sodium iodide, KI, is added) has reduced the incidence of goiter in most developed countries.
Because iodine deficiency during pregnancy can lead to mental retardation of the infant, it is recommended that pregnant women receive 150–250 µg of iodine daily during both pregnancy and lactation. Hundreds of supplements — both prescription and nonprescription — are sold for this purpose. However, a study of 60 of them reported in the 2/26/09 issue of The New England Journal of Medicine found that only 9 of the 60 contained an amount of iodine within 5% of the amount claimed on the label. Others ranged from only 11% of the amount claimed to almost 3 times as much. Examples: one (prescription) preparation claiming a daily dose of 150 µg actually provided only 26 µg while another (nonprescription) preparation claiming 226 µg of iodine actually contained 610 µg!
Fluoride
The value of fluoride (in ionized form, F) was first recognized as a preventive for dental caries (cavities). This makes sense because fluoride ions are incorporated along with calcium and phosphate ions in the crystalline structure of which both bones and teeth are constructed. But it may have other functions.
In order to grow properly, a rat must consume 0.5 parts per million (ppm) of fluoride ions in its diet. The rat in the bottom photo received the same diet as that in the top except that tin, vanadium, and fluorides were carefully excluded for 20 days. When tin and vanadium were then given to the deprived rat, it still did not grow normally. But adding 0.5 ppm of potassium fluoride (KF) to its diet restored normal growth and health. (Photos courtesy of Klaus Schwarz, VA Hospital, Long Beach, CA.)
Humans get most of their fluoride in drinking water. In regions where the natural amount is less than 1 ppm, many communities add enough fluoride to bring the concentration up to 1 ppm. Perhaps because the range between optimum and excess is more narrow for fluoride than for most minerals in the diet, water fluoridation has been controversial. Leaving aside the philosophical and political questions raised by proponents and opponents of fluoridation, the safety and efficacy of this public health measure has been thoroughly established.
Zinc
Zinc is incorporated in many enzymes and transcription factors. Zinc supplements are popular for their supposed antioxidant properties and to hasten the recovery from colds. Excessive intake of zinc causes a brief illness. Its most frequent cause is from ingested acidic food or drink that has been stored in galvanized (zinc-coated) containers.
Vitamins
Vitamin A (Retinol)
• Functions: Multiple, including serving as the precursor to retinal, the prosthetic group of all four of the light-absorbing pigments in the eye and regulating gene expression essential for the health of epithelia.
• Sources: cream, butter, fish liver oils, eggs. Carrots and some other vegetables provide beta-carotene, which the liver can convert into vitamin A.
• Deficiency: night-blindness.
• Excess: stored in the liver, but can be toxic in large doses, especially in children. Even in adults the range between too little and too much is narrow: ingesting vitamin A in amounts not much greater than the recommended dietary allowance (RDA) leads to an increase in bone fractures later in life. High doses taken early in pregnancy have been linked to a greater risk of birth defects. (Its chemical relative isotretinoin — the acne treatment Accutane® — is such a notorious teratogen that it should not be used when there is any chance of a pregnancy occurring).
Thiamine ( Vitamin B1)
• Function: coenzyme in cellular respiration.
• Sources: meat, yeast, unpolished cereal grains, enriched bread and breakfast cereals.
• Deficiency: beriberi. Rarely found in developed countries except among alcoholics.
• Excess: water soluble and any excess easily excreted.
Riboflavin ( Vitamin B2)
• Function: prosthetic group of flavoprotein enzymes, e.g., flavin adenine dinucleotide (FAD) used in cellular respiration.
• Sources: liver, eggs, cheese, milk, enriched bread and breakfast cereals.
• Deficiency: damage to eyes, mouth, and genitals.
• Excess: water soluble and any excess easily excreted.
Niacin (Nicotinic acid or Vitamin B3)
• Function: this member of the B vitamins is a precursor of NAD and NADP.
• Sources: meat, yeast, milk, enriched bread and breakfast cereals.
• Deficiency: pellagra (producing skin lesions); a risk where corn (maize) is the staple carbohydrate.
• Excess: accidental ingestion of very high doses produces a brief illness, but niacin is water-soluble and any excess is quickly excreted.
Biotin (Vitamin B7)
• Function: this member of the B vitamins is a cofactor in many essential metabolic enzymes.
• Sources: liver, egg yolks, corn (maize), intestinal bacteria.
• Deficiency: rare except perhaps during pregnancy.
• Excess: none identified.
Vitamin B12
• Function: needed for DNA synthesis.
• Sources: liver, eggs, milk; needs intrinsic factor to be absorbed.
• Deficiency: pernicious anemia; caused by lack of intrinsic factor or a vegan diet.
• Excess: none identified.
Folic acid ( Folacin)
• Function: synthesis of purines and pyrimidines.
• Sources: green leafy vegetables, but destroyed by cooking.
• Deficiency: anemia, birth defects. Women who expect to become pregnant should be extra careful that they receive adequate amounts (400 µg/day). Starting 1 January 1998, any bread or breakfast cereal described as "enriched" must have enough folic acid added to it so that a single serving will provide 10% of this requirement.
• Excess: water soluble and any excess easily excreted.
Vitamin C (Ascorbic acid)
• Functions: coenzyme in the synthesis of collagen.
• Sources: citrus fruits, green peppers, tomatoes; destroyed by cooking.
• Deficiency: scurvy.
• Excess: Many people take huge amounts of vitamin C, hoping to ward off colds, cancer, etc. They seem to suffer no harm except, perhaps, to their wallets.
Vitamin D
• Functions: absorption of calcium from the intestine and bone formation.
• Sources:
• synthesized when ultraviolet light (mostly UV-B) strikes the skin (forms vitamin D3).
• present in some fish (e.g., salmon), cod liver oil, eggs, and steroid-containing foods irradiated with ultraviolet light.
• Deficiency:
• rickets — inadequate conversion of cartilage to bone — in children;
• osteomalacia — softening of the bones — in adults.
Until recently, rickets has been very rare in North America. But the combination of two growing trends
• breast feeding and
• protecting children from exposure to the sun
has caused cases to reappear especially in northern latitudes with their short winter days.
Breast milk provides less than 20% of the recommended daily dose for infants. Until the infant is old enough to eat foods fortified with vitamin D, many pediatricians recommend vitamin supplements for breast-fed babies.
• Excess: However, this fat-soluble vitamin is dangerous in very high doses, especially in infants, causing excessive calcium deposits and mental retardation. So some pediatricians view the use of vitamin D supplements for infants with caution (especially since certain preparations have been found to contain amounts far higher than that listed on the label).
Vitamin E (Tocopherol)
• Function: acts as an antioxidant agent in cells.
• Sources: vegetable oils, nuts, spinach.
• Deficiency: anemia, damage to the retinas.
• Excess: high doses may be toxic.
Vitamin K
• Function: needed for the synthesis of blood clotting factors.
• Sources: spinach and other green leafy vegetables; synthesized by intestinal bacteria.
• Deficiency: slow clotting of blood. Because
• little or no vitamin K crosses the placenta,
• the colon of newborn babies has not yet been colonized by vitamin K-synthesizing bacteria,
• breast milk is a poor source of the vitamin,
babies are routinely given vitamin K at birth to eliminate the risk of uncontrolled bleeding.
• Excess: No risk from natural forms of the vitamin (K1 and K2).
"Natural" versus "Synthetic" Vitamins
There is no scientific distinction between them. The thiamine molecule (or any other molecule) is the same entity whether synthesized by a plant or by an organic chemist or whether it is still in plant or animal material or has been extracted and incorporated in a pill.
Control of Food Intake
A complex web of signals controls appetite and the intake of food. These include both nerve signals and hormones - both of which signal centers in the brain - chiefly in the hypothalamus. This table lists some of the hormonal signals that have been identified, their effect on appetite and weight gain. Such complexity probably reflects the need for redundant circuits in such a vital activity as acquiring food. But, it also frustrates the search for treatments to attack the increasing incidence of obesity.
Appetite Stimulants Appetite Suppressants
Ghrelin Leptin
Agouti-related protein (AgRP) α-MSH and β-MSH
Neuropeptide Y (NPY) β-endorphin
Melanin-concentrating hormone (MCH) Cholecystokinin (CCK)
Anandamide Incretins
Orexins (also called hypocretins) Insulin
Amylin
Pancreatic polypeptide
PYY3-36
Brain-derived neurotrophic factor (BDNF)
This diagram presents a model of how some of the chief players interact.
• After a period of fasting, secretion of ghrelin activates neurons ("X") in the hypothalamus. They release the excitatory neurotransmitter glutamate where they synapse with AgRP/NPY-releasing neurons. These set in motion the signals that induce feeding.
• A positive feedback loop strengthens the response: AgRP and NPY inhibit the activity of proopiomelanocortin (POMC) neurons whose function is to inhibit "X" neurons (a double-negative is a positive).
• When satiety is finally reached, leptin activates the POMC neurons which release α-MSH and β-endorphin where they synapse with the "X" neurons and the stimulus to continue feeding is stopped. (The precise identity of the "X" neurons remains to be determined.)
15.1D: Recommended Dietary Allowances
For many years the Food and Nutrition Board of the United States National Academy of Sciences has taken responsibility for establishing guidelines on what quantities of the various nutrients should be eaten by human males and females at various ages. These were called RDAs (for Recommended Dietary Allowances, and often referred to as Recommended Daily Allowances). They provide the data on which food labels are based. In 1997, the Institute of Medicine of the National Academy published a report that added three new categories, including:
• adequate intake ("AI"), where no RDA has been established
• tolerable upper intake levels ("UL"), to caution against excess intake of nutrients — like vitamin D — that can be harmful in large amounts.
As their findings trickle in, here is a table of RDAs (or AIs) for young adult women and men.
Females Males Females Males
Protein 46 g 56 g Folacin 400 µg same
Vitamin A (retinol) 700 µg* 900 µg* Biotin 30 µg (AI) same
Thiamine (Vitamin B1) 1.1 mg 1.2 mg Calcium 1000 mg (AI) same
Riboflavin (Vitamin B2) 1.1 mg 1.3 mg Phosphorus 700 mg same
Niacin (Vitamin B3) 14 mg 16 mg Selenium 55 µg same
Pantothenic acid (Vitamin B5) 5 mg (AI) same Iron 18 mg 8 mg
Vitamin B6 1.3 mg same Zinc 8 mg 11 mg
Vitamin B12 2.4 µg same Magnesium 310 mg 400 mg
Vitamin C 75 mg* 90 mg* Iodine 150 µg same
Vitamin D 15 µg ** same Fluoride 3 mg (AI) 4 mg (AI)
Vitamin E 15 mg** same Linoleic acid 12 g (AI) 17 g (AI)
Vitamin K 90 µg (AI) 120 µg α-Linolenic acid 1.1 g (AI) 1.6 g (AI)
*This value has been questioned following the publication of data indicating that a high intake of vitamin A in older people leads to an increased risk of hip fractures. Vitamin A stimulates osteoclasts, the cells that degrade bone, and inhibits osteoblasts, the cells that build bone. To the extent that the vitamin A requirement is met by ingested beta-carotene, these amounts should be multiplied by 12. And that is probably the best way to get your vitamin A as the body only converts enough beta-carotene into vitamin A to meet its needs. There is also evidence that beta-carotene has important functions besides being the precursor of vitamin A and therefore should be ingested in amounts even greater than needed to meet the vitamin A requirement. In short, one should consume vitamin tablets containing beta-carotene and not vitamin A.
*Smokers should add 35 mg to these values, and some nutritionists believe that 200 mg of vitamin C per day is probably optimal for everyone. This is more than twice the current RDA, but far lower than the 2,000 mg/day that is the upper limit (UL) and that some people exceed in the hope of warding off colds, cancer, etc.
**15 µg = 600 IU ("International Units").
**The upper limit (UL) is 1,000 mg/day. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.01%3A_Nutrition/15.1C%3A_Nutrition.txt |
Breathing
In mammals, the diaphragm divides the body cavity into the abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the thoracic cavity, which contains the heart and lungs. The inner surface of the thoracic cavity and the outer surface of the lungs are lined with pleural membranes which adhere to each other. If air is introduced between them, the adhesion is broken and the natural elasticity of the lung causes it to collapse. This can occur from trauma. And it is sometimes induced deliberately to allow the lung to rest. In either case, reinflation occurs as the air is gradually absorbed by the tissues. Because of this adhesion, any action that increases the volume of the thoracic cavity causes the lungs to expand, drawing air into them.
• During inspiration (inhaling), the external intercostal muscles contract, lifting the ribs up and out. This is accomplished by the contraction of the diaphragm muscle, which draws it down. During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breath 15–18 times a minute exchanging about 500 ml of air.
• In more vigorous expiration, the internal intercostal muscles draw the ribs down and inward and the the wall of the abdomen contracts pushing the stomach and liver upward. Under these conditions, an average adult male can flush his lungs with about 4 liters of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain.
The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the hemoglobin therein. At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled.
Component Atmospheric Air (%) Expired Air (%)
Table 15.2.1.1: Composition of atmospheric air and expired air in a typical subject. Note that only a fraction of the oxygen inhaled is taken up by the lungs.
N2 (plus inert gases) 78.62 74.9
O2 20.85 15.3
CO2 0.03 3.6
H2O 0.5 6.2
100.0% 100.0%
Central Control of Breathing
The rate of cellular respiration (and hence oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20–25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing. It is a rising concentration of carbon dioxide — not a declining concentration of oxygen — that plays the major role in regulating the ventilation of the lungs. Certain cells in the medulla oblongata are very sensitive to a drop in pH. As the CO2 content of the blood rises above normal levels, the pH drops
. Those rare people who inherit two defective genes for alpha-1 antitrypsin are particularly susceptible to developing emphysema.
15.2B: Control of Breathing
• The subject begins by breathing pure air (21% oxygen, 0.03% carbon dioxide, and about 79% inert gases by volume), first from the room and then from the tank. This control reveals what, if any, changes in response can be expected just by breathing from the tank (because of an unpleasant taste or increased air resistance, for example). The two graphs show that no appreciable change does occur when breathing air.
• When 100% oxygen is used instead, no marked change in rate ("breaths/minute") or depth ("vital capacity") of breathing occurs either, although there is a tendency for depth of breathing to decrease slightly.
• When the subject inhales a gas mixture consisting of 92% oxygen and 8% carbon dioxide, however, a most dramatic increase in the rate and depth of inspiration takes place. Note that there is no question of tissues lacking oxygen. The gas mixture contains four times as much oxygen as air.
• Note that after a period of breath-holding, the rate and depth of inspiration are markedly greater than before the breath-holding began (1). This can be explained by the build-up of CO2 during the breath-holding period.
• Vigorous, forced hyperventilation reduces the CO2 content of the alveolar air and blood to below its normal value, leading to a period of shallow breathing before its concentration builds back up to normal (2).
• The length of time that one can hold his or her breath to the breaking point can be substantially increased by hyperventilating just prior to the period of breath-holding (3).
It may seem curious that the rate at which one breathes and thus supplies oxygen to the body is controlled by carbon dioxide rather than oxygen. But cellular respiration produces CO2 as fast as oxygen is consumed, so the CO2 given off by active muscles triggers increased ventilation of the lungs and thus automatically supplies additional oxygen. While CO2 is the major stimulus for controlling breathing, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate, but there has been no increase in the production of CO2. People who live at high altitudes, e.g., in the Andes, have enlarged carotid bodies.
15.2C: Vertebrate Lungs
Terrestrial vertebrates (amphibians, reptiles, birds, and mammals) use a pair of lungs to exchange oxygen and carbon dioxide between their tissues and the air.
Frog Lungs
The frog's lungs are a pair of thin-walled sacs connected to the mouth through an opening, the glottis. The surface area of the lungs is increased by inner partitions which are richly supplied with blood vessels. The frog inflates its lungs by
• filling its mouth with air
• then closing its mouth
• closing the internal openings to its nostrils
• opening its glottis
• raising the floor of its mouth thus forcing air into the lungs.
The frog's skin serves as a supplementary organ of gas exchange. However, it must remain moist to do this, which is one reason that frogs, like other amphibians, live in moist places. The frog's circulatory system, which brings oxygen-depleted blood to its lungs (and skin) and takes oxygen-enriched blood away is described in a separate page.
Reptile Lungs
The skin of reptiles is dry and scaly, so they can live in arid locations (although many do not). However, they cannot use their skin as an organ of gas exchange. Reptiles depend entirely on their lungs for this. Their lungs are considerably more efficient than those of amphibians.
• They have a much greater surface area for the exchange of gases.
• They are inflated and deflated by the bellowslike expansion and contraction of the rib cage.
While fresh air flows in and stale air out of the lizard's lungs, another reptile, the American alligator, uses a more efficient mechanism similar to that described below in birds.)
The lizard's circulatory system, which brings oxygen-depleted blood to its lungs and takes oxygen-enriched blood away is described in a separate page.
Bird Lungs
Unlike reptiles, birds are homeothermic ("warm blooded"), maintaining a constant body temperature (usually around 40°C) despite wide fluctuations in the temperature of their surroundings. They maintain their body temperature with the heat produced by muscular activity. This depends, in turn, on a high rate of cellular respiration. So the demands on the gas-exchange efficiency of the lungs of a small, active bird are great.
Although the ventilation of bird lungs is similar to that of reptiles, their effectiveness is increased by the presence of air sacs. Although no gas exchange occurs in the air sacs, their arrangement increases the efficiency of lung ventilation by enabling fresh air to pass in one direction through the lungs during both inhalation and exhalation. The air sacs also aid in reducing the density of the body by substituting air for tissue or fluid in many places. Even some of the bird's bones are penetrated by air sacs.
Mammalian Lungs
Ventilation of mammalian lungs is assisted by the diaphragm - a muscular partition that divides the thoracic cavity from the abdominal cavity.
15.2D: Tracheal Breathing
Tracheae open to the outside through small holes called spiracles. In the grasshopper, the first and third segments of the thorax have a spiracle on each side. Another 8 pairs of spiracles are arranged in a line on either side of the abdomen. The spiracles are guarded by
• valves controlled by muscles that enables the grasshopper to open and close them
• hairs that filter out dust as the air enters the spiracles
The experiment illustrated (first performed by the insect physiologist Gottfried Fraenkel) shows that there is a one-way flow of air through the grasshopper. The liquid seals in the tubing move to the right as air enters the spiracles in the thorax and is discharged through the spiracles in the abdomen. The rubber diaphragm seals the thorax from the abdomen. The one-way flow of air increases the efficiency of gas exchange as CO2-enriched air can be expelled without mingling with the incoming flow of fresh air.
Gas Exchange in Aquatic Insects
Even aquatic insects use a tracheal system for gas exchange.
• Some, like mosquito larvae ("wigglers"), get their air by poking a breathing tube — connected to their tracheal system — through the water surface.
• Some insects that can submerge for long periods carry a bubble of air with them from which they breathe.
• Still others have spiracles mounted on the tips of spines. With these they pierce the leaves of underwater plants and obtain oxygen from the bubbles formed (by photosynthesis) within the leaves.
• Even in aquatic insects that have gills, after oxygen diffuses from the water into the gills, it then diffuses through a gas-filled tracheal system for transport through the body. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.02%3A_Gas_Exchange/15.2A%3A_Human_Respiratory_System.txt |
The circulatory system is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis.
Simplified diagram of the human Circulatory system in anterior view. (Public Domain; LadyofHats)
Main Features of the Human Circulatory System
• A liquid, blood, to transport nutrients, wastes, oxygen and carbon dioxide, and hormones.
• Two pumps (in a single heart): one to pump deoxygenated blood to the lungs and the other to pump oxygenated blood to all the other organs and tissues of the body
• A system of blood vessels to distribute blood throughout the body
• Specialized organs for exchange of materials between the blood and the external environment; for example organs like the lungs and intestine that add materials to the blood and organs like the lungs and kidneys that remove materials from the blood and deposit them back in the external environment
The Heart and Pulmonary System
The heart is located roughly in the center of the chest cavity. It is covered by a protective membrane, the pericardium.
• Deoxygenated blood from the body enters the right atrium.
• It flows through the tricuspid valve into the right ventricle. The term tricuspid refers to the three flaps of tissue that make up the valve.
• Contraction of the ventricle then closes the tricuspid valve and forces open the pulmonary valve.
• Blood flows into the pulmonary artery.
• This branches immediately, carrying blood to the right and left lungs.
• Here the blood gives up carbon dioxide and takes on a fresh supply of oxygen.
• The capillary beds of the lungs are drained by venules that are the tributaries of the pulmonary veins.
• Four pulmonary veins, two draining each lung, carry oxygenated blood to the left atrium of the heart.
The above figure shows the human heart, with a schematic view of the pathway of blood through the lungs and internal organs. Oxygenated blood is shown in red; deoxygenated blood in blue. Note that the blood draining the stomach, spleen, and intestines passes through the liver before it is returned to the heart. Here surplus or harmful materials picked up from those organs can be removed before the blood returns to the general circulation.
The Coronary System
From the left atrium,
• Blood flows through the mitral valve (also known as the bicuspid valve) into the left ventricle.
• Contraction of the ventricle closes the mitral valve and opens the aortic valve at the entrance to the aorta.
• The first branches from the aorta occur just beyond the aortic valve still within the heart.
• Two openings lead to the right and left coronary arteries, which supply blood to the heart itself. Although the coronary arteries arise within the heart, they pass directly out to the surface of the heart and extend down across it. They supply blood to the network of capillaries that penetrate every portion of the heart.
• The capillaries drain into two coronary veins that empty into the right atrium.
Diseases of the Coronary system: Arteriosclerosis and Atherosclerosis
The coronary arteries arise at the point of maximum blood pressure in the circulatory system. Over the course of time, the arterial walls are apt to lose elasticity, which limits the amount of blood that can surge through them and hence limits the supply of oxygen to the heart. This condition is known as arteriosclerosis.
Alternatively, fatty deposits, called plaque, may accumulate on the interior surface of the coronary arteries; this condition is known as atherosclerosis. This is particularly common in people who have high levels of cholesterol in their blood. Plaque deposits reduce the bore of the coronary arteries and thus the amount of blood they can carry. Atherosclerosis (usually along with arteriosclerosis) may limit the blood supply to the heart that during times of stress the heart muscle is so deprived of oxygen that the pain of angina is created. It triggers the formation of a clot causing a coronary thrombosis. This stops the flow of blood through the vessel and the capillary network it supplies causing a heart attack. The portion of the heart muscle deprived of oxygen dies quickly of oxygen starvation. If the area is not too large, the undamaged part of the heart can, in time, compensate for the damage.
Coronary bypass surgery uses segments of leg veins to bypass the clogged portions of the coronary arteries.
The Systemic Circulation
The remainder of the system is known as the systemic circulation. The graphic shows the major arteries (in bright red) and veins (dark red) of the system. Blood from the aorta passes into a branching system of arteries that lead to all parts of the body. It then flows into a system of capillaries where its exchange functions take place.
Blood from the capillaries flows into venules which are drained by veins.
• Veins draining the upper portion of the body lead to the superior vena cava.
• Veins draining the lower part of the body lead to the inferior vena cava.
• Both empty into the right atrium. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3A%3A_Anatomy_of_Human_Circulatory_System.txt |
All the functions of the circulatory system occur in the capillary beds. The rest of the system consists of two pumps (in the heart) and associated plumbing:
• arteries and their terminal branches, the arterioles
• veins and their tributaries, the venules.
Blood Pressure
Blood moves through the arteries, arterioles, and capillaries because of the force created by the contraction of the ventricles.
Blood pressure in the arteries.
The surge of blood that occurs at each contraction is transmitted through the elastic walls of the entire arterial system where it can be detected as the pulse. Even during the brief interval when the heart is relaxed — called diastole — there is still pressure in the arteries. When the heart contracts — called systole — the pressure increases.
Blood pressure is expressed as two numbers, e.g., 120/80. The first is the pressure during systole. The unit of measure is the torr, in this example, the pressure equivalent to that produced by a column of mercury 120 mm high. The second number is the pressure at diastole.
Although blood pressure can vary greatly in an individual, continual high pressure — especially diastolic pressure — may be the symptom or cause of a variety of ailments. The medical term for high blood pressure is hypertension.
Blood pressure in the capillaries
The pressure of arterial blood is largely dissipated when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7.5 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.
Blood pressure in the veins
When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins.
Exchanges Between Blood and Cells
With rare exceptions, our blood does not come into direct contact with the cells it nourishes. As blood enters the capillaries surrounding a tissue space, a large fraction of it is filtered into the tissue space. It is this interstitial or extracellular fluid (ECF) that brings to cells all of their requirements and takes away their products. The number and distribution of capillaries is such that probably no cell is ever farther away than 50 µm from a capillary.
When blood enters the arteriole end of a capillary, it is still under pressure (about 35 torr) produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space. Thus fluid, called interstitial fluid, is simply blood plasma minus most of the proteins. It has the same composition and is formed in the same way as the nephric filtrate in kidneys. Interstitial fluid bathes the cells in the tissue space and substances in it can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.
Near the venous end of a capillary, the blood pressure is greatly reduced (to about 15 torr). Here another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and thus a somewhat greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small (~ 25 torr), it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid reenters the capillary here.
The first of the four graphs (a) shows this balanced relationship in the capillary bed; the others show what happens when the system is altered.
Pressure relations in the capillaries.
PA = blood pressure at the arteriole end of the capillary.
PV = blood pressure at the venule end of the capillary.
The horizontal line represents the osmotic pressure of the blood.
When the blood pressure is greater than the osmotic pressure, filtration of interstitial fluid occurs (downward-pointing arrows).
When the blood pressure is less than the osmotic pressure, reabsorption of interstitial fluid occurs (up arrows).
(a) The normal situation. Filtration and absorption are balance.
(b) Result of dilating the arterioles. PA increases and the tissue space becomes engorged with interstitial fluid.
(c) Result of constricting the arterioles. PA decreases and interstitial fluid is withdrawn from the tissue space.
(d) Result of a lowered concentration of protein in the blood (such as occurs during prolonged malnutrition). Because of the reduced osmotic pressure (lower horizontal line), fluid accumulates in the tissue spaces resulting in edema.
Control of the Capillary Beds
An adult human has been estimated to have some 60,000 miles (96,560 km) of capillaries with a total surface area of some 800–1000 m2 (an area greater than three tennis courts). The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. So a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. During vigorous exercise, for example, capillary beds in the skeletal muscles open at the expense of those in the viscera. The reverse occurs after a heavy meal.
The table shows the distribution of blood in the human body at rest and during vigorous exercise. Note the increase in blood supply to the working organs (skeletal muscles and heart). The increased blood supply to the skin aids in the dissipation of the heat produced by the muscles. Note also that the blood supply to the brain remains constant. The total blood flow during exercise increases because of a more rapid heartbeat and also a greater volume of blood pumped at each beat.
Blood Flow ml/min
At Rest During Strenuous
Exercise
Heart 250 750
Kidneys 1,200 600
Skeletal Muscles 1,000 12,5000
Skin 400 1,900
Viscera 1,400 600
Brain 750 750
Other 600 400
Total 5,600 17,500
The walls of arterioles are encased in smooth muscle. Constriction of arterioles decreases blood flow into the capillary beds they supply while dilation has the opposite effect. In time of danger or other stress, for example, the arterioles supplying the skeletal muscles will be dilated while the bore of those supplying the digestive organs will decrease. These actions are carried out by
• the autonomic nervous system
• local controls in the capillary beds
Local Control in the Capillary Beds
• Nitric oxide (NO) is a potent dilator of arteries and arterioles.
• When the endothelial cells that line these vessels are stimulated, they synthesize nitric oxide. It quickly diffuses into the muscular walls of the vessels causing them to relax.
• In addition, as the hemoglobin in red blood cells releases its O2 in actively-respiring tissues, the lowered pH causes it to also release NO which helps dilate the vessels to meet the increased need of the tissue.
Nitroglycerine, which is often prescribed to reduce the pain of angina, does so by generating nitric oxide, which relaxes the walls of the arteries and arterioles. The prescription drug sildenafil citrate ("Viagra") does the same for vessels supplying blood to the penis. The effects of these two drugs are additive and using them together could precipitate a dangerous drop in blood pressure.
• Cells where infection or other damage is occurring release substances like histamine that dilate the arterioles and thus increase blood flow in the area.
• In most of the body, the flow of blood through a capillary is controlled by the arteriole supplying it. In the brain, however, another mechanism participates. The degree of contraction of pericytes, cells that surround the capillary, also adjusts the flow of blood through the capillary. The changes in brain activity seen by such imaging procedures as fMRI and PET scans are probably influenced by pericyte activity.
Shock
Under some circumstances, capillary beds may open without others closing in compensation. Although the volume of blood remains unchanged, blood pressure declines abruptly as blood pools in the capillary beds. If untreated shock is usually fatal.
Shock can also result from severe bleeding. The heart can only pump as much blood as it receives. If insufficient blood gets back to the heart, its output — and hence blood pressure — drops. The tissues fail to receive enough oxygen. This is especially critical for the brain and the heart itself. To cope with the problem, arterioles constrict and shut down the capillary beds — except those in the brain and heart. This reduces the volume of the system and helps maintain normal blood pressure.
Air-breathing vertebrates that spend long periods under water (e.g., seals, penguins, turtles, and alligators) employ a similar mechanism to ensure that the oxygen supply of the heart and brain is not seriously diminished. When the animal dives, the blood supply to the rest of the body is sharply reduced so that what oxygen remains will be available for those organs needing it most: the brain and heart.
Regulation of Blood Pressure by Hormones
One of the functions of the kidney is to monitor blood pressure and take corrective action if it should drop. The kidney does this by secreting the protease renin.
• Renin acts on angiotensinogen, a plasma peptide, splitting off a fragment containing 10 amino acids called angiotensin I.
• angiotensin I is cleaved by a peptidase secreted by blood vessels called angiotensin converting enzyme (ACE) producing angiotensin II, which contains 8 amino acids.
• angiotensin II
• constricts the walls of arterioles closing down capillary beds
• stimulates the proximal tubules in the kidney to reabsorb sodium ions
• stimulates the adrenal cortex to release aldosterone. Aldosterone causes the kidneys to reclaim still more sodium and thus water
• increases the strength of the heartbeat
• stimulates the pituitary to release the vasopressin
All of these actions, which are mediated by its binding to G-protein-coupled receptors on the target cells, lead to an increase in blood pressure.
The Heart
A rise in blood pressure stretches the atria of the heart. This triggers the release of atrial natriuretic peptide (ANP). ANP is a peptide of 28 amino acids. ANP lowers blood pressure by:
• relaxing arterioles
• inhibiting the secretion of renin and aldosterone
• inhibiting the reabsorption of sodium ions in the collecting ducts of the kidneys
The effects on the kidney reduce the reabsorption of water by them thus increasing the flow of urine and the amount of sodium excreted in it (These actions give ANP its name: natrium = sodium; uresis = urinate). The net effect of these actions is to reduce blood pressure by reducing the volume of blood volume in the system. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3B%3A_How_the_Human_Circulatory_System_Works.txt |
During rest, the heart beats about 70 times a minute in the adult male, while pumping about 5 liters of blood.
The stimulus that maintains this rhythm is self-contained. Embedded in the wall of the right atrium is a mass of specialized heart tissue called the sino-atrial (S-A) node. The S-A node is also called the pacemaker because it establishes the basic frequency at which the heart beats.
The interior of the fibers of heart muscle, like all cells, is negatively charged with respect to the exterior. In the cells of the pacemaker, this charge breaks down spontaneously about 70 times each minute. This, in turn, initiates a similar discharge of the nearby muscle fibers of the atrium. A tiny wave of current sweeps over the atria, causing them to contract.
When this current reaches the region of insulating connective tissue between the atria and the ventricles, it is picked up by the A-V node (atrio-ventricular node). This leads to a system of branching fibers that carries the current to all parts of the ventricles. The contraction of the heart in response to this electrical activity creates systole. A period of recovery follows called diastole.
• The heart muscle and S-A node become recharged.
• The heart muscle relaxes.
• The atria refill.
The Electrocardiogram
The electrical activity of the heart can be detected by electrodes placed at the surface of the body. Analysis of an electrocardiogram (ECG or EKG) aids in determining, for example, the extent of damage following a heart attack. This is because death of a portion of the heart muscle blocks electrical transmission through that area and alters the appearance of the ECG.
Ventricular Fibrillation
The ventricles can maintain a beat even without a functioning A-V node, although the beat is slower. There is, however, a danger that impulses arising in the ventricles may become disorganized and random. If this happens, they begin to twitch spasmodically, a condition called ventricular fibrillation. Blood flow ceases and unless the heart rhythm is restarted, death follows swiftly. In fact, ventricular fibrillation is the immediate cause of as much as 25% of all deaths.
Hospital emergency rooms, ambulances, commercial air craft and many other public places are now equipped with defibrillators which, by giving the heart a jolt of direct current, may restore its natural rhythm and save the victim's life.
Artificial Pacemakers
These are devices that generate rhythmic impulses that are transmitted to the heart by fine wires. Thanks to miniaturization and long-lived batteries, pacemakers can be implanted just under the skin and reached through a small incision when maintenance is needed.
Auxiliary Control of the Heart
Although the A-V node sets the basic rhythm of the heart, the rate and strength of its beating can be modified by two auxiliary control centers located in the medulla oblongata of the brain.
• One sends nerve impulses down accelerans nerves.
• The other sends nerve impulses down a pair of vagus nerves
The Accelerans Nerve
The accelerans nerve is part of the sympathetic branch of the autonomic nervous system, and like all post-ganglionic sympathetic neurons releases noradrenaline at its endings on the heart. It increases the rate and strength of the heartbeat and thus increase the flow of blood. Its activation usually arises from some stress such as fear or violent exertion. The heartbeat may increase to 180 beats per minute. The strength of contraction increases as well so the amount of blood pumped may increase to as much as 25–30 liters/minute.
Note
The 24 Feb 2000 issue of the New England Journal of Medicine reports on a family some of whose members have inherited a mutant gene for the transporter that is responsible for reuptake of noradrenaline back into the neuron that released it. Those with the mutation are prone to bouts of rapid heartbeat and fainting when they suddenly stand up.
Vigorous exercise accelerates heartbeat in two ways:
• As cellular respiration increases, so does the carbon dioxide level in the blood. This stimulates receptors in the carotid arteries and aorta, and these transmit impulses to the medulla for relay by the accelerans nerve to the heart.
• As muscular activity increases, the muscle pump drives more blood back to the right atrium. The atrium becomes distended with blood, thus stimulating stretch receptors in its wall. These, too, send impulses to the medulla for relay to the heart.
Distention of the wall of the right atrium also triggers the release of atrial natriuretic peptide (ANP) which initiates a set of responses leading to a lowering of blood pressure.
The Vagus Nerves
The vagus nerves are part of the parasympathetic branch of the autonomic nervous system. They, too, run from the medulla oblongata to the heart. Their activity slows the heartbeat. Pressure receptors in the aorta and carotid arteries send impulses to the medulla which relays these by way of the vagus nerves to the heart. Heartbeat and blood pressure diminish.
15.3D: The Lymphatic System
Most (~90%) of this interstitial fluid returns at the venule end of the capillary. The 10% that does not is picked up by tiny vessels called lymph capillaries. The cells forming the walls of the lymph capillaries are loosely fitted together, thus making the wall very porous. Any serum proteins that filtered through the capillary wall pass easily from the tissue space into the interior of the lymph capillary. The lymph capillaries of the intestinal villi, called lacteals, also pick up fat droplets. White blood cells (leukocytes) migrate from the tissue space into the lymph capillary squeezing between the cells that make up its wall.
The lymph capillaries drain into still larger collecting vessels. The flow through the collecting vessels is quite slow. Like blood in the veins, contraction of skeletal muscles compresses the collecting vessels and squeezes the fluid — now called lymph — along. Again, like the return of blood in the veins, the lymph can flow only in one direction because of valves in the vessels. All the lymph collected from the entire left side of the body, the digestive tract and the right side of the lower part of the body flows into a single major vessel, the thoracic duct.
The thoracic duct empties into the left subclavian vein. The lymph in the right side of the head, neck, and chest is collected by the right lymph duct and empties into the right subclavian vein. Together they empty 1–2 liters of lymph into the blood each day.
Lymph Nodes
The collecting vessels of the lymphatic system are interrupted by lymph nodes. These are especially abundant in the groin, armpits, abdomen and neck. These contain cavities called sinuses into which the lymph flows bringing various leukocytes (e.g., lymphocytes and dendritic cells) and out of which pass antibodies and lymphocytes which then enter the blood at the subclavian veins.
The walls of the sinuses are lined with phagocytic cells, which engulf any foreign particles, e.g., bacteria, that might be present in the lymph. Tests have demonstrated that over 99% of the bacteria carried into a node are screened out before the lymph leaves the node on its return to the blood. This filtering mechanism is one of the most important body defenses against infectious disease. When combating a heavy infection, the lymph nodes enlarge producing "swollen glands".
Edema
The production of lymph is increased by
• increased blood pressure in the capillaries
• increased capillary permeability such as occurs if the adherens junctions between the cells lining the capillaries are damaged
• a decreased concentration of plasma proteins (such as occurs in prolonged malnutrition).
The lymphatic system may be unable to handle the increased volume of lymph, and it may accumulate in the tissues and distend them. This condition is known as edema. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3C%3A_The_Heartbeat.txt |
Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.
1. red blood cells (RBCs) or erythrocytes
2. platelets or thrombocytes
3. five kinds of white blood cells (WBCs) or leukocytes
• Three kinds of granulocytes
• neutrophils
• eosinophils
• basophils
• Two kinds of leukocytes without granules in their cytoplasm
• lymphocytes
• monocytes
If one takes a sample of blood, treats it with an agent to prevent clotting, and spins it in a centrifuge, the red cells settle to the bottom and the white cells settle on top of them forming the "buffy coat". The fraction occupied by the red cells is called the hematocrit. Normally it is approximately 45%. Values much lower than this are a sign of anemia
Functions of the blood
Blood performs two major functions:
• transport through the body of
• oxygen and carbon dioxide
• food molecules (glucose, lipids, amino acids)
• ions (e.g., Na+, Ca2+, HCO3)
• wastes (e.g., urea)
• hormones
• heat
• defense of the body against infections and other foreign materials. All the WBCs participate in these defenses.
The formation of blood cells
All the various types of blood cells are produced in the bone marrow (some 1011 of them each day in an adult human). They arise from a single type of cell called a hematopoietic stem cell — an "adult" multipotent stem cell. These stem cells are very rare (only about one in 10,000 bone marrow cells) and are attached (probably by adherens junctions) to osteoblasts lining the inner surface of bone cavities. They also express a cell-surface protein designated CD34.
Hematopoietic stem cell produce, by mitosis, two kinds of progeny: (1) more stem cells (A mouse that has had all its blood stem cells killed by a lethal dose of radiation can be saved by the injection of a single living stem cell) and (2) cells that begin to differentiate along the paths leading to the various kinds of blood cells. Which path is taken is regulated by the need for more of that type of blood cell which is, in turn, controlled by appropriate cytokines and/or hormones. For example, Interleukin-7 (IL-7) is the major cytokine in stimulating bone marrow stem cells to start down the "lymphoid" path leading to the various lymphocytes (mostly B cells and T cells).
Some of the cytokines that drive the differentiation of the "myeloid" leukocytes are
• Erythropoietin (EPO), produced by the kidneys, enhances the production of red blood cells (RBCs).
• Thrombopoietin (TPO), assisted by Interleukin-11 (IL-11), stimulates the production of megakaryocytes. Their fragmentation produces platelets.
• Granulocyte-macrophage colony-stimulating factor (GM-CSF), as its name suggests, sends cells down the path leading to both those cell types. In due course, one path or the other is taken.
• Under the influence of granulocyte colony-stimulating factor (G-CSF), they differentiate into neutrophils.
• Further stimulated by interleukin-5 (IL-5) they develop into eosinophils.
• Interleukin-3 (IL-3) participates in the differentiation of most of the white blood cells but plays a particularly prominent role in the formation of basophils (responsible for some allergies).
• Stimulated by macrophage colony-stimulating factor (M-CSF) the granulocyte/macrophage progenitor cells differentiate into monocytes, macrophages, and dendritic cells (DCs).
Red Blood Cells (erythrocytes)
The most numerous type in the blood. They average 7 µm in diameter. Women average about 4.8 million of these cells per cubic millimeter (mm3; which is the same as a microliter [µl]) of blood. Men average about 5.4 x 106 per µl. These values can vary over quite a range depending on such factors as health and altitude. (Peruvians living at 18,000 feet may have as many as 8.3 x 106 RBCs per µl.)
RBC precursors mature in the bone marrow closely attached to a macrophage. They manufacture hemoglobin until it accounts for some 90% of the dry weight of the cell. In mammals, the nucleus is squeezed out of the cell and is ingested by the macrophage. All the mitochondria as well as the endoplasmic reticulum and Golgi apparatus are destroyed. No-longer-needed proteins are expelled from the cell in vesicles called exosomes. Figure \(2\) shows a scanning electron micrograph that shows the characteristic biconcave shape of red blood cells.
Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded into bile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.
Red blood cells are responsible for the transport of oxygen and carbon dioxide.
Oxygen Transport
In adult humans the hemoglobin (Hb) molecule consists of four polypeptides:
• two alpha (α) chains of 141 amino acids
• two beta (β) chains of 146 amino acids
To each of these is attached the prosthetic group heme. There is one atom of iron at the center of each heme. One molecule of oxygen can bind to each heme. The reaction is reversible. Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood. Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.
The pressure of oxygen in the lungs is 90–95 torr; in the interior tissues it is about 40 torr. Therefore, only a portion of the oxygen carried by the red blood cells is normally unloaded in the tissues. However, vigorous activity can lower the oxygen pressure in skeletal muscles below 40 torr, which causes a large increase in the amount of oxygen released. This effect is enhanced by the high concentration of carbon dioxide in the muscles and the resulting lower pH (7.2). The lower carbon dioxide concentration (and hence higher pH) at the lungs promotes the binding of oxygen to hemoglobin and hence the uptake of oxygen.
Temperature changes also influence the binding of oxygen to hemoglobin. In the relative warmth of the interior organs, the curve is shifted to the right (like the curve for pH 7.2), helping to unload oxygen. In the relative coolness of the lungs, the curve is shifted to the left, aiding the uptake of oxygen.
Although the oxygen transported by RBCs make possible cellular respiration throughout the body, RBCs lack mitochondria and so cannot perform cellular respiration themselves and must subsist on glycolysis.
Carbon Dioxide Transport
Carbon dioxide (CO2) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H+) and a bicarbonate ion:
\[\ce{CO2 + H2O <=> H2CO3 <=> H+ + HCO3^{−}}\]
95% of the CO2 generated in the tissues is carried in the red blood cells. It enters (and leaves) the cell by diffusion through the plasma membrane. Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen). The rest is converted following the equation above by the enzyme carbonic anhydrase into bicarbonate ions that diffuse back out into the plasma and hydrogen ions (H+) that bind to the protein portion of the hemoglobin (thus having no effect on pH). The bicarbonate ions pass out of the red cell by facilitated diffusion through transmembrane channels in the plasma membrane. Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5) When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli.
Anemia
Anemia is a shortage of RBCs and/or the amount of hemoglobin in them. Anemia has many causes. One of the most common is an inadequate intake of iron in the diet.
Blood Groups
Red blood cells have surface antigens that differ between people and that create the so-called blood groups such as the ABO system and the Rh system.
White Blood Cells (leukocytes)
White blood cells are much less numerous than red (the ratio between the two is around 1:700) and have nuclei. They consist of lymphocytes and monocytes with relatively clear cytoplasm, and three types of granulocytes, whose cytoplasm is filled with granules. They participate in protecting the body from infection.
Lymphocytes
There are several kinds of lymphocytes (although they all look alike under the microscope), each with different functions to perform . The most common types of lymphocytes are:
• B lymphocytes ("B cells"). These are responsible for making antibodies.
• T lymphocytes ("T cells"). There are several subsets of these:
• inflammatory T cells that recruit macrophages and neutrophils to the site of infection or other tissue damage
• cytotoxic T lymphocytes (CTLs) that kill virus-infected and, perhaps, tumor cells
• helper T cells that enhance the production of antibodies by B cells
Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they encounter antigens, continue to divide by mitosis, mature into fully functional cells.
Monocytes
Monocytes leave the blood and become macrophages and one type of dendritic cell. See Figure 15.3.5.6
Neutrophils
The most abundant of the WBCs. This photomicrograph shows a single neutrophil surrounded by red blood cells. Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis. This is a never-ending task, even in healthy people: Our throat, nasal passages, and colon harbor vast numbers of bacteria. Most of these are commensals, and do us no harm. But that is because neutrophils keep them in check.
However heavy doses of radiation, chemotherapy and many other forms of stress can reduce the numbers of neutrophils so that formerly harmless bacteria begin to proliferate. The resulting opportunistic infection can be life-threatening.
Eosinophils
The number of eosinophils in the blood is normally quite low (0–450/µl). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.
Basophils
Ordinarily representing less than 1% of the WBCs, their numbers also increase during infection. Basophils leave the blood and accumulate at the site of infection or other inflammation. There they discharge the contents of their granules, releasing a variety of mediators such as histamine, serotonin, prostaglandins and leukotrienes which increase the blood flow to the area and in other ways add to the inflammatory process. The mediators released by basophils also play an important part in some allergic responses such as hay fever and an anaphylactic response to insect stings.
Platelets
Platelets are cell fragments produced from megakaryocytes. These polyploid (128n) cells in the bone marrow send pseudopodia-like projections into the lumen of adjacent blood vessels. Blood flowing through the vessel shears off the platelets. Blood normally contains 150,000–400,000 per microliter (µl) or cubic millimeter (mm3). This number is normally maintained by a homeostatic (negative-feedback) mechanism. If this value should drop much below 20,000/µl, there is a danger of uncontrolled bleeding. Some causes include certain drugs and herbal remedies as well as autoimmunity.
When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting. A blood clot consists of a plug of platelets enmeshed in a network of insoluble fibrin molecules. Platelets also promote inflammation.
Plasma
Plasma is the straw-colored liquid in which the blood cells are suspended.
Composition of blood plasma
Component Percent
Water ~92
Proteins 6–8
Salts 0.8
Lipids 0.6
Glucose (blood sugar) 0.1
Plasma transports materials needed by cells and materials that must be removed from cells:
• various ions (Na+, Ca2+, HCO3, etc.)
• glucose and traces of other sugars
• amino acids
• other organic acids
• cholesterol and other lipids
• hormones
• urea and other wastes
Most of these materials are in transit from a place where they are added to the blood (a "source")
• exchange organs like the intestine
• depots of materials like the liver
to places ("sinks") where they will be removed from the blood.
• every cell
• exchange organs like the kidney, and skin.
Serum Proteins
Proteins make up 6–8% of the blood. They are about equally divided between serum albumin and a great variety of serum globulins. After blood is withdrawn from a vein and allowed to clot, the clot slowly shrinks. As it does so, a clear fluid called serum is squeezed out. Thus Serum is blood plasma without fibrinogen and other clotting factors. The serum proteins can be separated by electrophoresis.
• A drop of serum is applied in a band to a thin sheet of supporting material, like paper, that has been soaked in a slightly-alkaline salt solution.
• At pH 8.6, which is commonly used, all the proteins are negatively charged, but some more strongly than others.
• A direct current can flow through the paper because of the conductivity of the buffer with which it is moistened.
• As the current flows, the serum proteins move toward the positive electrode.
• The stronger the negative charge on a protein, the faster it migrates.
• After a time (typically 20 min), the current is turned off and the proteins stained to make them visible (most are otherwise colorless).
• The separated proteins appear as distinct bands.
• The most prominent of these and the one that moves closest to the positive electrode is serum albumin.
• Serum albumin
• is made in the liver
• binds many small molecules for transport through the blood
• helps maintain the osmotic pressure of the blood
• The other proteins are the various serum globulins.
• They migrate in the order
• alpha globulins (e.g., the proteins that transport thyroxine and retinol [vitamin A])
• beta globulins (e.g., the iron-transporting protein transferrin)
• gamma globulins.
• Gamma globulins are the least negatively-charged serum proteins. (They are so weakly charged, in fact, that some are swept in the flow of buffer back toward the negative electrode.)
• Most antibodies are gamma globulins.
• Therefore gamma globulins become more abundant following infections or immunizations.
• Gamma globulins can be harvested from donated blood (usually pooled from several thousand donors) and injected into persons exposed to certain diseases such as chicken pox and hepatitis. Because such preparations of immune globulin contain antibodies against most common infectious diseases, the patient gains temporary protection against the disease.
If a precursor of an antibody-secreting cell becomes cancerous, it divides uncontrollably to generate a clone of plasma cells secreting a single kind of antibody molecule.
The above image shows — from left to right — the electrophoretic separation of:
• normal human serum with its diffuse band of gamma globulins
• serum from a patient with multiple myeloma producing an IgG myeloma protein
• serum from a patient with Waldenström's macroglobulinemia where the cancerous clone secretes an IgM antibody
• serum with an IgA myeloma protein
Serum Lipids
Because of their relationship to cardiovascular disease, the analysis of serum lipids has become an important health measure. The table shows the range of typical values as well as the values above (or below) which the subject may be at increased risk of developing atherosclerosis.
LIPID Typical values (mg/dl) Desirable (mg/dl)
Cholesterol (total) 170–210 <200
LDL cholesterol 60–140 <100
HDL cholesterol 35–85 >40
Triglycerides 40–160 <160
• Total cholesterol is the sum of HDL cholesterol, LDL cholesterol and 20% of the triglyceride value
• Note that high LDL values are bad, but high HDL values are good.
• Using the various values, one can calculate a
cardiac risk ratio = total cholesterol divided by HDL cholesterol
• A cardiac risk ratio greater than 7 is considered a warning.
Blood Transfusions
In the United States, in 2001, some 15 million "units" (~475 ml) of blood were collected from blood donors.
• Some of these units ("whole blood") were transfused directly into patients (e.g., to replace blood lost by trauma or during surgery).
• Most were further fractionated into components, including:
• RBCs - When refrigerated these can be used for up to 42 days.
• Platelets - These must be stored at room temperature and thus can be saved for only 5 days.
• Plasma - This can be frozen and stored for up to a year.
Ensuring the safety of donated blood
A variety of infectious agents can be present in blood. Infections such as
• viruses (e.g., HIV-1, hepatitis B and C, West Nile virus)
• bacteria like the spirochete of syphilis
• protozoans like the agents of malaria and babesiosis
• prions (e.g., the agent of variant Crueutzfeldt-Jakob disease)
These could be transmitted to recipients. To minimize these risks donors are questioned about their possible exposure to these agents. Each unit of blood is tested for a variety of infectious agents. Most of these tests are performed with enzyme immunoassays (EIA) and detect antibodies against the agents. However, it takes a period of time for the immune system to produce antibodies following infection, and during this period ("window"), infectious virus is present in the blood. For this reason, blood is now also checked for the presence of the RNA of these RNA viruses HIV-1, hepatitis C and West Nile virus by the nucleic acid-amplification test (NAT).
Thanks to all these precautions, the risk of acquiring an infection from any of these agents is vanishingly small. Despite this, some people — in anticipation of need — donate their own blood ("autologous blood donation") prior to surgery.
Blood Typing
Donated blood must also be tested for certain cell-surface antigens that might cause a dangerous transfusion reaction in an improperly-matched recipient.
Blood Substitutes
Years of research have gone into trying to avoid the problems of blood perishability and safety by developing blood substitutes. Most of these have focused on materials that will transport adequate amounts of oxygen to the tissues.
• Some are totally synthetic substances.
• Others are derivatives of hemoglobin.
Although some have reached clinical testing, none has as yet proved acceptable for routine use. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3E%3A_Blood.txt |
Blood groups are created by molecules present on the surface of red blood cells (and often on other cells as well).
The ABO Blood Groups
The ABO blood groups were the first to be discovered (in 1900) and are the most important in assuring safe blood transfusions. The table shows the four ABO phenotypes ("blood groups") present in the human population and the genotypes that give rise to them.
Blood Group Antigens on RBCs Antibodies in Serum Genotypes
A A Anti-B AA or AO
B B Anti-A BB or BO
AB A and B Neither AB
O Neither Anti-A and Anti-B OO
When red blood cells carrying one or both antigens are exposed to the corresponding antibodies, they agglutinate; that is, clump together. People usually have antibodies against those red cell antigens that they lack. The antigens in the ABO system are O-linked glycoproteins with their sugar residues exposed at the cell surface. The terminal sugar determines whether the antigen is A or B.
The critical principle to be followed is that transfused blood must not contain red cells that the recipient's antibodies can clump. Although theoretically it is possible to transfuse group O blood into any recipient, the antibodies in the donated plasma can damage the recipient's red cells. Thus, when possible, transfusions should be done with exactly-matched blood.
In 2007, Danish and French investigators reported the properties of two bacterial glycosidases that specifically remove the sugars responsible for the A and B antigens. This discovery raises the possibility of being able to treat A, B, or AB blood with these enzymes and thus convert the blood to Group O, the "universal donor".
Why do we have antibodies against red cell antigens that we lack? Bacteria living in our intestine, and probably some foods, express epitopes similar to those on A and B. We synthesize antibodies against these if we do not have the corresponding epitopes; that is, if our immune system sees them as "foreign" rather than "self".
The Rh System
Rh antigens are transmembrane proteins with loops exposed at the surface of red blood cells. They appear to be used for the transport of carbon dioxide and/or ammonia across the plasma membrane. They are named for the rhesus monkey in which they were first discovered.
There are a number of Rh antigens. Red cells that are "Rh-positive" express the one designated D. About 15% of the population have no RhD antigen and thus are "Rh-negative". The major importance of the Rh system for human health is to avoid the danger of RhD incompatibility between mother and fetus.
During birth, there is often a leakage of the baby's red blood cells into the mother's circulation. If the baby is Rh-positive (having inherited the trait from its father) and the mother Rh-negative, these red cells will cause her to develop antibodies against the RhD antigen. The antibodies, usually of the IgG class, do not cause any problems for that child, but can cross the placenta and attack the red cells of a subsequent Rh+ fetus. This destroys the red cells producing anemia and jaundice. The disease, called erythroblastosis fetalis or hemolytic disease of the newborn, may be so severe as to kill the fetus or even the newborn infant. It is an example of an antibody-mediated cytotoxicity disorder.
Although certain other red cell antigens (in addition to Rh) sometimes cause problems for a fetus, an ABO incompatibility does not. Why is an Rh incompatibility so dangerous when ABO incompatibility is not?
It turns out that most anti-A or anti-B antibodies are of the IgM class and these do not cross the placenta. In fact, an Rh/type O mother carrying an Rh+/type A, B, or AB fetus is resistant to sensitization to the Rh antigen. Presumably her anti-A and anti-B antibodies destroy any fetal cells that enter her blood before they can elicit anti-Rh antibodies in her. This phenomenon has led to an extremely effective preventive measure to avoid Rh sensitization. Shortly after each birth of an Rh+ baby, the mother is given an injection of anti-Rh antibodies. The preparation is called Rh immune globulin (RhIG) or Rhogam. These passively acquired antibodies destroy any fetal cells that got into her circulation before they can elicit an active immune response in her.
Rh immune globulin came into common use in the United States in 1968, and within a decade the incidence of Rh hemolytic disease became very low.
Other blood groups
Several other blood group antigens have been identified in humans. Some examples: MN, Duffy, Lewis, Kell.
These groups also sometimes cause transfusion reactions and even hemolytic disease of the newborn in cases where there is no ABO or Rh incompatibility. The Duffy red cell antigen also serves as the receptor for entry by the malaria parasite Plasmodium vivax. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3F%3A_Blood_Groups.txt |
In addition to its role in the transport of materials, the circulatory system is responsible for the distribution of heat throughout the body. This is true both of
• endotherms, animals — birds and mammals — that generate internally the heat needed to maintain their body temperature. Birds and mammals are "warm-blooded' or homeothermic, maintaining their body temperature within narrow limits, no matter what the ambient temperature.
• mesotherms, animals that generate heat internally but do not maintain a fixed body temperature. Tuna, some sharks, and the echidna are mesotherms.
• ectotherms, animals — the other vertebrates and the invertebrates — that secure their heat from their surroundings (e.g., by basking in the sun). Ectotherms are "cold-blooded" or poikilothermic.
The major source of heat for endotherms is the metabolism of their internal organs. Over two-thirds of the heat generated in a resting human is created by the organs of the thoracic and abdominal cavities and the brain (which contributes 16% of the total — about the same as all our skeletal muscles when they are at rest). There are several measures that an endothermic animal can take if it begins to lose heat to its surroundings faster than it can generate heat (i.e., it begins to grow cold). It can increase the metabolic rate of its tissues. Many small mammals and human infants do this as their surroundings get colder, but it is still uncertain whether adult humans can. The increase in metabolism, with the accompanying release of heat, occurs in brown adipose tissue. It can also increase its physical activity. At rest, muscles make only a small contribution (about 16%) to body heat. During vigorous exercise, this can increase greatly. In the absence of voluntary muscle action, the same effect is achieved by shivering. The greater the surface-to-volume ratio of a part of the body, the faster is can transfer heat to its surroundings. This is why you first notice cold in your hands and feet. The loss of heat from the extremities can be sharply reduced by diminishing their blood supply. In extreme cold, for example, the blood supply to the fingers can drop to 1% or so of its normal value.
Countercurrent heat exchanger
Many animals (including humans) have another way to conserve heat. The arteries of our arms and legs run parallel to a set of deep veins. As warm blood passes down the arteries, the blood gives up some of its heat to the colder blood returning from the extremities in these veins. Such a mechanism is called a countercurrent heat exchanger. When heat loss is no problem, most of the venous blood from the extremities returns through veins located near the surface.
Countercurrent heat exchangers can operate with remarkable efficiency. A sea gull can maintain a normal temperature in its torso while standing with its unprotected feet in freezing water. When you consider that the blood of fishes passes over the gills which are bathed in the surrounding water, it is easy to see why fishes are "cold-blooded". Nonetheless, some marine fishes (e.g., the tuna) are mesotherms — able to keep their most active swimming muscles warmer than the sea by using a countercurrent heat exchanger.
The above photograph on the right shows a cross section through a skipjack tuna. The dark muscle on either side of the vertebral column is maintained at a higher temperature than the rest of the fish thanks to its countercurrent heat exchanger. The cold, oxygen-rich arterial blood passes into a series of fine arteries that take the blood into the active muscles. These fine arteries lie side by side with veins draining those muscles. So as the cold blood passes into the muscles, it picks up the heat that had been generated by these muscles and keeps it from being lost to the surroundings. Thanks to this countercurrent heat exchanger, a tuna swimming in the winter can maintain its active swimming muscles 14°C warmer than the surrounding water. The photomicrograph on the left is of a cross section through the heat exchanger. Note the close, parallel packing of the arteries (thick walls) and veins (thin walls).
Countercurrent exchangers also operate in the kidney and are built into the design of artificial kidneys.
The circulatory system is also responsible for cooling an animal. If the animal's "core" body temperature gets too high, the blood supply to the surface and extremities is increased enabling heat to be released to the surroundings. If this is insufficient, the animal can evaporate water from the blood — in the form of sweat for those animals with sweat glands. The evaporation of 1 gram of water absorbs some 540 calories of heat.
Most endotherms cannot tolerate a rise in body temperature of more than 5°C or so. The brain is the organ most susceptible to damage by a high temperature. Some mammals, dogs for example, have a countercurrent heat exchanger located between the carotid arteries and the vessels that distribute blood to the brain. This heat exchanger transfers some of the heat of the arterial blood to the relatively cool venous blood returning from the nose and mouth. This cools their arterial blood before it reaches the brain.
The shifting of blood flow as needed to maintain homeothermy is controlled by temperature receptors in the hypothalamus of the brain. One set of receptors here responds to small (0.01°C) increases in the temperature of the blood. When triggered, all the activities such as shunting blood vessels to the skin and extremities and sweating by which the body cools itself are brought into play. It is this center that enables us to maintain a constant body temperature (homeothermy) during periods of extreme exertion or in hot surroundings.
A second region of the hypothalamus triggers warming responses such as shunting blood away from the skin and extremities and shivering when the body becomes chilled. It is the hypothalamus that executes the fever response. In effect, the hypothalamus is the body's thermostat. The release of prostaglandins during inflammation increases the setting; that is, turns the thermostat "up". If the body temperature is not yet there, the body begins shivering violently — causing "chills" — to generate the heat needed. The result is fever when the new set point is reached. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3G%3A_The_Transport_of_Heat.txt |
When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting. A blood clot consists of a plug of platelets enmeshed in a network of insoluble fibrin molecules. Platelet aggregation and fibrin formation both require the proteolytic enzyme thrombin. Clotting also requires calcium ions (Ca2+) (which is why blood banks use a chelating agent to bind the calcium in donated blood so the blood will not clot in the bag) and about a dozen other protein clotting factors. Most of these circulate in the blood as inactive precursors. They are activated by proteolytic cleavage becoming, in turn, active proteases for other factors in the system. By tradition, these factors are designated by Roman numerals.
There are two processes that can initiate clotting: A very rapid process the so-called extrinsic pathway or a slower but larger intrinsic pathway.
The Extrinsic Pathway
Damaged cells display a surface protein called tissue factor (TF). Tissue factor then binds to activated Factor 7. The TF-7 heterodimer is a protease with two substrates: Factor 9 and Factor 10.
The Intrinsic Pathway
Factor 12 (also called the Hageman factor) circulates in the blood. If blood escapes into tissue spaces (e.g., as a result of an injury), contact with collagens in the tissue space activates Factor 12. Activated Factor 12 is a serine protease that activates Factor 11 which in turn activates Factor 9 which in turn activates Factor 10.
The Two Pathways Converge at Factor 10
Factor 10 - produced by either or, more likely, both pathways - binds and activates Factor 5. This heterodimer is called prothrombinase because it is a protease that converts prothrombin (also known as Factor II) to thrombin. Thrombin has several different activities. Two of them are:
• proteolytic cleavage of fibrinogen (aka "Factor I") to form soluble molecules of fibrin and a collection of small and fibrinopeptides.
• activation of Factor 13 which forms covalent bonds between the soluble fibrin molecules converting them into an insoluble meshwork — the clot.
Thrombin and activated Factors 10 ("Xa") and 11 ("XIa") are also serine proteases.
Amplifying the Clotting Process
The clotting process also has several positive feedback loops which quickly magnify a tiny initial event into what may well be a lifesaving plug to stop bleeding.
• The TF-7 complex (which started the process) also activates Factor 9.
• Factor 9 binds to Factor 8, a protein that circulates in the blood stabilized by another protein, von Willebrand Factor (vWF).
• This complex activates more Factor 10.
• As thrombin is generated, it activates more
• Factor 5
• Factor 8
• Factor 11 (all shown above with green arrows).
• Factor 11 amplifies the production of activated Factor 9.
Thus what may have begun as a tiny, localized event rapidly expands into a cascade of activity.
Platelets
Platelets are cell fragments produced from megakaryocytes. Blood normally contains 150,000 to 400,000 per microliter (µl). If this value should drop much below 20,000/µl, there is a danger of uncontrolled bleeding. This is because of the essential role of platelets in maintaining the integrity of the adherens junctions that provide a tight seal between the endothelial cells that line the blood vessels and in forming a clot where blood vessels have been broken.
When blood vessels are damaged, fibrils of collagen in the extracellular matrix (ECM) are exposed. Platelets then begin to adhere to the collagen through the action of specific receptors for collagen present on their plasma membrane and von Willebrand factor which links the platelets to the collagen. These actions cause a plug of platelets to form at the site.
The bound platelets release ADP and thromboxane A2, which recruit and activate still more platelets circulating in the blood. (This role of thromboxane accounts for the beneficial effect of low doses of aspirin — a cyclooxygenase inhibitor — in avoiding heart attacks.), tissue factor, and serotonin, which enhances their clumping and promotes constriction of the blood vessel.
ReoPro
ReoPro s a monoclonal antibody directed against platelet receptors. It inhibits platelet aggregation and appears to reduce the risk that "reamed out" coronary arteries (after coronary angioplasty) will plug up again.
Bleeding Disorders
A deficiency of a clotting factor can lead to uncontrolled bleeding. The deficiency may arise because not enough of the factor is produced or a mutant version of the factor fails to perform properly.
Examples:
• von Willebrand disease (the most common)
• hemophilia A for factor 8 deficiency
• hemophilia B for factor 9 deficiency.
• hemophilia C for factor 11 deficiency
In some cases of von Willebrand disease, either a deficient level or a mutant version of the factor eliminates its protective effect on factor 8. The resulting low level of factor 8 mimics hemophilia A.
Why do all the human bleeding disorders involve factors in amplification pathways? Probably because they are the only deficiencies that can be tolerated. Loss of the genes for tissue factor or factor 7 in knockout mice is lethal.
Hemophilia A and B
The genes encoding factors 8 and 9 are on the X chromosome. Thus their inheritance is X-linked. Like other X-linked disorders, hemophilia A and B are found almost exclusively in males because they inherit just a single X chromosome, and if the gene for factor 8 (or 9) on it is defective, they will suffer from the disease.
Queen Victoria of the UK was a carrier of a mutant factor 9 gene and passed it on to several of her descendants.
There are many different mutant versions of the genes for factors 8 and 9. Although some produce only a minor effect on the function of their protein, others fail to produce any functioning clotting factor.
Treating Hemophilia A and B
What can be done?
Factor 8 and 9 can be extracted from donated blood, usually pooled from several thousand donors, and purified. Injections of this material can halt episodes of bleeding in hemophiliacs and have allowed countless young men to live relatively normal lives. However, in the early 1980s, blood contaminated with the human immunodeficiency virus (HIV) was unknowingly used to manufacture preparations of factors 8 and 9. In some areas, 90% or more of the hemophiliacs became infected by these contaminated preparations. Many have since died of AIDS. The future now looks brighter because:
• all donated blood is now tested to see if the donor has been infected with HIV (as well as hepatitis B and C)
• plasma-derived preparations of factors 8 and 9 are now treated with heat and/or solvents to destroy any viruses that might be present
• recombinant factor 8 and recombinant factor 9 made by genetic engineering are now available
These recombinant factors are made by inserting the DNA encoding the human protein into mammalian cells grown in culture. E. coli cannot be used because these factors are glycoproteins, and E. coli lacks the machinery to attach carbohydrate properly.
And the team that brought us Dolly reported in the 19 December 1997 issue of Science that they have succeeded in cloning female sheep transgenic for the human factor 9 gene. The human gene is coupled to the promoter for the ovine (sheep) milk protein beta-lactoglobulin. When the lambs mature, it is hoped that they will secrete large amounts of human factor 9 in their milk, which can then be purified for human therapy.
Attempts to cure hemophilia by gene therapy also look promising. It is difficult to see how even the most worried critics of genetic engineering can fail to approve its potential to save the lives of thousands of hemophiliacs in the years to come.
Liver Transplants
People with liver failure can be cured with a liver transplant. On the rare occasions when the patient has happened to be a hemophiliac (A, B or C), the transplant cured not only the patient's liver disease but cured his hemophilia as well!
Controlling Clotting
While the ability to clot is essential to life, the process must be carefully regulated. Inappropriate clot formation, especially in the brain or lungs, can be life-threatening.
Antithrombin III
As its name suggests, this plasma protein (a serpin) inhibits the formation of thrombin. It does so by binding to and thus inactivating prothrombin, factor 9 and factor 10. Heparin is a mixture of polysaccharides that bind to antithrombin III, inducing an allosteric change that greatly enhances its inhibition of thrombin synthesis. Some surgical patients, especially those receiving hip or heart valve replacements, and people at risk of ischemic stroke (clots in the brain), are given heparin.
Protein C
With its many clot-promoting activities, it is probably no accident that thrombin sits at the center of the control mechanism.
• Excess thrombin binds to cell-surface receptors called thrombomodulin.
• The resulting complex activates a plasma protein called Protein C and its cofactor Protein S.
• Together these inhibit further thrombin formation
• directly — by inactivating Factor 5
• indirectly — by inactivating Factor 8.
Some inherited disorders that predispose to spontaneous clots, especially in the leg veins:
• inherited deficiency of Protein C or Protein S
• inherited mutation in the Factor 5 gene producing a protein that no longer responds to the inhibitory effect of Protein C
Recombinant Protein C is now available to treat people threatened with inappropriate clotting, e.g., as a result of widespread infection (sepsis).
Vitamin K
Vitamin K is a cofactor needed for the synthesis (in the liver) of factors 2 (prothrombin), 7, 9, and 10 and proteins C and S. A deficiency of Vitamin K predisposes to bleeding. Conversely, blocking the action of vitamin K helps to prevent inappropriate clotting.
Warfarin (Coumadin®) is sometimes prescribed as a "blood thinner" because it is an effective vitamin K antagonist. (Warfarin is also used as a rat poison because it can cause lethal internal bleeding.) Warfarin treatment is tricky because the therapeutic window (neither too much nor too little) is very narrow, and there is substantial variability between people in their response. So treatment requires regular monitoring of clotting time until the proper dosage is established. However, a number of new anti-clotting agents — that work by inhibiting activated factor 10 (Factor Xa) — are being tested and may turn out to be safer and effective alternatives to warfarin.
Dissolving clots
Plasma contains plasminogen, which binds to the fibrin molecules in a clot. Nearby healthy cells release tissue plasminogen activator (TPA), which also binds to fibrin and, as its name suggests, activates plasminogen forming plasmin. Plasmin (another serine protease) proceeds to digest fibrin, thus dissolving the clot.
Recombinant human TPA is now produced by recombinant DNA technology. Injected within the first hours after a heart attack, it has saved many lives by dissolving the clot blocking the coronary artery and restoring blood flow before the heart muscle becomes irreversibly damaged. It is also used for people who suffer an ischemic stroke; that is, a clot in the brain. (It must not, of course, be used for hemorrhagic strokes, that is, a burst blood vessel)
Angiogenesis
Thrombin (as well as factors 7 and 10) promotes healing by stimulating the growth of new blood vessels at the site of damage. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3H%3A_Blood_Clotting.txt |
The most common hemoglobin of adult humans is known as hemoglobin A (HbA). However, other variant forms of hemoglobin are found in some humans. One of these is called hemoglobin S (HbS). Individuals who manufacture HbS exclusively suffer from sickle-cell disease. Sickle-cell disease is quite common in malaria-ridden parts of Africa and Asia and also occurs in African-Americans and others descended from inhabitants of those regions. The disease gets its name from the fact that the patient's red cells become crescent or sickle-shaped while passing through the capillaries, especially in tissues that are metabolizing actively. The distorted cells are very fragile and are apt to rupture long before their normal life span (about 120 days) is over. This causes a severe anemia (giving rise to an alternate name for the disease: sickle-cell anemia).
Causes of sickle-cell anemia
Individuals with sickle-cell disease have inherited from each parent a gene — βS — encoding the beta chain of hemoglobin. Individuals who inherit only one βS gene along with the βA allele have both HbA and HbS in their red cells. In the malaria-free United States, these heterozygotes are well. In regions where malaria is common, having one of each beta chain gene (βA and βS) confers resistance to one of the most dangerous types of malaria (falciparum). This would explain why the HbS gene is so prevalent in those regions.
The amino acid sequences of the beta chains of HbA and HbS have been determined. The beta chains are identical except for the amino acid at position 6 (counting, as always, from the amino terminal). This position is occupied by glutamic acid in HbA chains, but in HbS beta chains, valine is found there instead.
Why does this single amino acid change in a chain of 146 amino acids so drastically alter the properties of deoxygenated hemoglobin? In switching from glutamic acid to valine, a strongly hydrophilic molecule has been replaced by a strongly hydrophobic one. Position 6 is located at the surface of the beta chain, where it would normally be exposed to water. This switch from a hydrophilic to a hydrophobic region on the surface reduces the solubility of the molecule and promotes the formation of large insoluble aggregates.
The mutation that produces HbS is a single-base substitution in which the substitution of a T for an A at the 17th nucleotide of the sense strand of the first exon of the beta chain gene converts a codon for glutamic acid (GAG) to a codon for valine (GTG). Although this change might at first appear trivial, the resulting substitution of valine for glutamic acid so alters the physical properties of hemoglobin that a serious disease is produced in people carrying both genes for the trait.
15.3J: Serine Proteases
The serine proteases are a family of enzymes that cut certain peptide bonds in other proteins. This activity depends on a set of amino acid residues in the active site of the enzyme — one of which is always a serine (thus accounting for their name). In mammals, serine proteases perform many important functions, especially in digestion, blood clotting, and the complement system.
Digestive Enzymes
Three protein-digesting enzymes secreted by the pancreas are serine proteases: chymotrypsin, trypsin and elastase. These three share closely-similar structures (tertiary as well as primary). In fact, their active serine residue is at the same position (Ser-195) in all three. Despite their similarities, they have different substrate specificities; that is, they cleave different peptide bonds during protein digestion.
Clotting Factors
Several activated clotting factors are serine proteases, including Factors 10 (X), 11 (XI), and 12 (XII), Thrombin. and Plasmin.
Complement Factors
Several proteins involved in the complement cascade are serine proteases, including
• C1r and C1s
• the C3 convertases
• C4b,2a
• C3b,Bb
Serpins
Serpins are Serine Protease Inhibitors. Here is a list of a few important serine proteases and the serpins that control them.
Serine Protease Serpin
Chymotrypsin alpha-1-antichymotrypsin
Complement factors C1r and C1s C1 Inhibitor (C1INH)
Elastase (secreted by neutrophils) alpha-1-antitrypsin
Clotting factor 10 (X) antithrombin III
Thrombin antithrombin III
Plasmin alpha-2-antiplasmin
Trypsin pancreatic trypsin inhibitor
How Serpins Work
The serpins inhibit the action of their respective serine protease by mimicking the three-dimensional structure of the normal substrate of the protease. The serine protease binds the serpin instead of its normal substrate. This alone would block any further activity by the protease. But the serpin has another trick to play. The protease makes a cut in the serpin leading to the formation of a covalent bond linking the two molecules, a massive allosteric change in the tertiary structure of the serpin, which moves the attached protease to a site where it can be destroyed.
Importance of Serpins
Almost 20% of the proteins found in blood plasma are serpins. Their abundance reflects their importance: putting a stop to proteolytic activity when the need for it is over. This is especially important for the clotting and complement systems where a tiny initial activating event leads to a rapidly amplifying cascade of activity.
Serpin Deficiencies
A number of inherited human diseases are caused by a deficiency of a particular serpin. The deficiency usually results from a mutation in the gene encoding the serpin.
Alpha-1-antitrypsin deficiency
Alpha-1-antitrypsin inactivates the elastase secreted by neutrophils. When the lungs become inflamed, neutrophils secrete elastase as a defensive measure. However, it is important to inactivate this elastase as soon as its job is done. That is the function of alpha-1-antitrypsin. Its name, alpha-1-antitrypsin, suggests that it attacks the digestive enzyme, trypsin. In vitro, it does, but in the body, alpha-1-antitrypsin is found in the blood, not the intestine. Inactivation of trypsin in the intestine is the function of another serpin, pancreatic trypsin inhibitor. People with an inherited deficiency of alpha-1-antitrypsin (they are homozygous for a point mutation in its gene) are prone to emphysema. An effective treatment is on the horizon now that genetic engineering has produced goats that secrete human alpha-1-antitrypsin in their milk.
Alpha-1-antitrypsin deficiency can also lead to liver damage. Alpha-1-antitrypsin is synthesized in the liver. However, some mutant versions of the molecule form insoluble aggregates within the liver cells. This mechanism is similar to that of the prion diseases where protein aggregates destroy neurons in the brain. A drug that enhances autophagy protects mice from the liver damage caused by aggregates of mutant alpha-1-antitrypsin.
C1INH deficiency
A deficiency of C1INH produces hereditary angioedema (HAE). In addition to C1r and C1s, C1INH also inhibits several other serine proteases including kallikrein, the enzyme responsible for forming the potent vasodilator bradykinin. Hence, a deficiency of C1INH can trigger a dangerous swelling (edema) of the airways, as well as of the skin and intestine.
Antiplasmin deficiency
A deficiency in antiplasmin puts the person at risk of uncontrollable bleeding.
Antithrombin deficiency
A deficiency in antithrombin puts the person at risk of spontaneous blood clots, which can lead to a heart attack or stroke. In January 2009, an advisory committee of the U.S. FDA decided that a recombinant human antithrombin (ATryn®) secreted into the milk of transgenic goats was safe for use in therapy.
The Evolution of the Serine Proteases
The close sequence similarity of the various mammalian serine proteases suggests that each is the product of a gene descended by repeated gene duplication from a single ancestral gene.
Other Serine Proteases
Serine proteases and molecules similar to them are found elsewhere in nature.
Subtilisin
Subtilisin is a serine protease secreted by the bacterium Bacillus subtilis. Although it has the same mechanism of action as the serine proteases of mammals, its primary structure and tertiary structure are entirely different. An example at the molecular level of convergent evolution: two molecules acquiring the same function (analogous) but having evolved from different genes.
Acetylcholinesterase
This enzyme is built like and acts like the other serine proteases, but its substrate is the neurotransmitter acetylcholine, not a protein. It is found at several types of synapses as well as at the neuromuscular junction — the specialized synapse that triggers the contraction of skeletal muscle. The organophosphate compounds used as insecticides (e.g., parathion) and nerve gases (e.g. Sarin) bind to the serine at the active site of acetylcholinesterase blocking its action.
Serpinlike Molecules
Angiotensinogen
This peptide is the precursor of angiotensin II — a major factor in maintaining blood pressure.
Chicken Ovalbumin
This is the major protein in the "white" of the egg (and a favorite antigen in immunological research). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3I%3A_Sickle-Cell_Disease.txt |
An efficient circulatory system has:
• a fluid, e.g., blood, to carry the materials to be transported
• a system of vessels to distribute the blood
• a pump to push the blood through the system
• exchange organs to carry out exchanges between the blood and external environment, e.g.
• lungs and intestine to add materials to the blood;
• lungs and kidneys to remove materials from the blood.
• The most crucial demand on the circulatory system is the transport of oxygen and carbon dioxide to and from a gas exchange organ such as lungs or gills and the tissues.
• All exchanges between blood and cells occur in the capillaries.
• The force of the pump that pushes blood through the arteries is dissipated as the blood flows through capillaries. Although capillaries are tiny, the total cross-sectional area of all the capillaries supplied by a single artery is much greater than that of the artery itself. Like a rapid, narrowly-confined stream spreading out over a flat plain, the force and velocity of flow diminish quickly.
This creates a problem:
• If the pump is used to deliver blood with force to the gas exchange organ, little force remains to distribute the oxygenated blood to the tissues.
• If the pump is used to deliver blood with force to the tissues, little force remains to send the deoxygenated blood to the gas exchange organ.
The Fish Heart
Most fishes have never solved this problem, which is probably why most of them are "cold-blooded".
• Blood collected from throughout the fish's body enters a thin-walled receiving chamber, the atrium.
• As the heart relaxes, the blood passes through a valve into the thick-walled, muscular ventricle.
• Contraction of the ventricle forces the blood into the capillary networks of the gills where gas exchange occurs.
• The blood then passes on to the capillary networks that supply the rest of the body where exchanges with the tissues occur.
• Then the blood returns to the atrium.
While obviously adequate to the fish's needs, this is not a very efficient system. The pressure generated by contraction of the ventricle is almost entirely dissipated when the blood enters the gills.
The Squid Hearts
This group of marine invertebrates has solved the problem by having separate pumps:
• two gill hearts to force blood under pressure to the gills
• a systemic heart to force blood under pressure to the rest of the body
The Frog and Lizard heart
The Frog Heart
The frog heart has 3 chambers: two atria and a single ventricle.
• The atrium receives deoxygenated blood from the blood vessels (veins) that drain the various organs of the body.
• The left atrium receives oxygenated blood from the lungs and skin (which also serves as a gas exchange organ in most amphibians).
• Both atria empty into the single ventricle.
• While this might appear to waste the opportunity to keep oxygenated and deoxygenated bloods separate, the ventricle is divided into narrow chambers that reduce the mixing of the two blood.
• So when the ventricle contracts
• oxygenated blood from the left atrium is sent, relatively pure, into the carotid arteries taking blood to the head (and brain)
• deoxygenated blood from the right atrium is sent, relatively pure, to the pulmocutaneous arteries taking blood to the skin and lungs where fresh oxygen can be picked up
• Only the blood passing into the aortic arches has been thoroughly mixed, but even so it contains enough oxygen to supply the needs of the rest of the body
• Note that in contrast to the fish, both the gas exchange organs and the interior tissues of the body get their blood under full pressure.
The Lizard Heart
• Lizards have a muscular septum which partially divides the ventricle.
• When the ventricle contracts, the opening in the septum closes and the ventricle is momentarily divided into two separate chambers.
• This prevents mixing of the two bloods.
• The left half of the ventricle pumps oxygenated blood (received from the left atrium) to the body.
• The right half pumps deoxygenated blood (received from the right atrium) to the lungs.
Birds, Crocodiles, and Mammals
The septum is complete in the hearts of birds, crocodiles, and mammals providing two separate circulatory systems:
• pulmonary for gas exchange with the environment
• systemic for gas exchange (and all other exchange needs) of the rest of the body
The efficiency that results makes possible the high rate of metabolism on which the endothermy ("warm-bloodedness") of birds and mammals depends. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.03%3A_Circulatory_Systems/15.3K%3A_Animal_Circulatory_Systems.txt |
Immunologists, as well as the general public, use the term allergy in several different ways. An allergy is a harmful immune response elicited by an antigen that is not itself intrinsically harmful. Examples:
• The windblown pollen released by orchard grass has no effect on me but produces a violent attack of hay fever (known to physicians as allergic rhinitis) in my wife.
• She, on the other hand, can safely handle the leaves of poison ivy while if I do so, I break out in a massive skin rash a day or two later.
Antigens that trigger allergies are often called allergens.
Four different immune mechanisms can result in allergic responses.
• Immediate Hypersensitivities: These occur quickly after exposure to the allergen. They are usually mediated by antibodies of the IgE class. Examples include hay fever, hives and asthma.
• Antibody-Mediated Cytotoxicity: Cell damage caused by antibodies directed against cell surface antigens. Hence a form of autoimmunity. Examples include Hemolytic disease of the newborn (Rh disease) and Myasthenia gravis (MG)
• Immune Complex Disorders: Damage caused by the deposit in the tissues of complexes of antigen and their antibodies. Examples include Serum sickness and Systemic lupus erythematosus (SLE)
• Cell-Mediated Hypersensitivities: These reactions are mediated by CD4+ T cells. Examples:
The rash produced following exposure to poison ivy. Because it takes a day or two for the T cells to mobilize following exposure to the antigen, these responses are called delayed-type hypersensitivities (DTH). Those, like poison ivy, that are caused by skin contact with the antigen are also known as contact sensitivities or contact dermatitis.
• certain autoimmune diseases, including
• Type 1 diabetes mellitus
• Multiple sclerosis (MS)
• Rheumatoid arthritis (RA)
Immediate Hypersensitivities
Local Anaphylaxis
The constant region of IgE antibodies (shown in blue) has a binding site for a receptor present on the surface of basophils and their tissue-equivalent the mast cell. These cell-bound antibodies have no effect until and unless they encounter allergens (shown in red) with epitopes that can bind to their antigen-binding sites. When this occurs, the mast cells to which they are attached
• explosively discharge their granules by exocytosis. The granules contain a variety of active agents including histamine;
• synthesize and secrete other mediators including leukotrienes and prostaglandins.
Release of these substances into the surrounding tissue causes local anaphylaxis: swelling, redness, and itching. In effect, each IgE-sensitized mast cell is a tiny bomb that can be exploded by a particular antigen. The most common types of local anaphylaxis are:
• allergic rhinitis (hay fever) in which airborne allergens react with IgE-sensitized mast cells in the nasal mucosa and the tissues around the eyes;
• bronchial asthma in which the allergen reaches the lungs either by inhalation or in the blood
• hives (physicians call it urticaria) where the allergen usually enters the body in food.
Leukotrienes are far more potent than histamine in mediating these reactions.
Leukotrienes and prostaglandins are derivatives of arachidonic acid (AA) an unsaturated fatty acid produced from membrane phospholipids. The principal pathways of arachidonic acid metabolism are
• the 5-lipoxygenase pathway, which produces a collection of leukotrienes (LT) and
• the cyclooxygenase pathway, which yields a number of prostaglandins (PG) and thromboxanes (Tx).
All three are synthesized by monocytes and macrophages. Mast cells and basophils generate a mixture of leukotrienes. The products of both pathways act in concert to cause inflammation with prostaglandins producing fever and pain. Aspirin, ibuprofen, and certain other nonsteroidal anti-inflammatory drugs (NSAIDs) achieve their effects (fever and pain reduction) by blocking the activity of cyclooxygenase.
Some people respond to environmental antigens (e.g., pollen grains, mold spores) with an unusually vigorous production of IgE antibodies. Why this is so is unclear; heredity certainly plays a role. In any case, the immune system of these people is tilted toward the production of T helper cells of the Th2 subtype. These release interleukin 4 (IL-4) and interleukin 13 (IL-13) on the B cells that they "help". These lymphokines promote class switching in the B cell causing it to synthesize IgE antibodies.
An inherited predisposition to making IgE antibodies is called atopy. Atopic people are apt to have higher levels of circulating IgE (up to 12 µg/ml) than is found usually (about 0.3 µg/ml). Whereas only 20–50% of the receptors on mast cells are normally occupied by IgE, all the receptors may be occupied in atopic individuals.
Skin Testing
When the problem allergen is not obvious, it can often be identified by skin testing. A panel of suspected allergens is injected into separate sites in the skin and each site is observed for the development of a "wheal and flare" reaction. The wheal is a sharply delineated soft swelling surrounded by the flare - a reddened area. Both are caused by the release of leukotrienes at the site, which increase the flow of blood to the site making it swollen and red.
A positive skin test occurs within minutes or even seconds (in contrast to patch testing for DTH responses).
Systemic Anaphylaxis
Some allergens can precipitate such a massive IgE-mediated response that a life-threatening collapse of the circulatory and respiratory systems may occur.
Frequent causes:
• insect (e.g., bee) stings
• many drugs (e.g., penicillin)
• a wide variety of foods. Egg white, cow's milk, and nuts are common offenders in children; in fact, some school systems in the US now ban peanuts and peanut-butter sandwiches when they have a student at risk of systemic anaphylaxis from exposure to peanuts. Fish and shellfish are frequent causes of anaphylaxis in adults.
Treatment of systemic anaphylaxis centers on the quick administration of adrenaline, antihistamines, and if shock has occurred then intravenous fluid replacement.
An example of systemic anaphylaxis
The three graphs show the physiological responses of a physician (Dr. Vick) stung by a single bee while on a picnic with coworkers (fortunately some with medical training!). Dr. Vick required cardiac massage and intravenous injections of adrenaline at the times shown. He and his colleagues worked in a laboratory studying bee venom, but prior to this episode he had no idea that he had developed such extreme susceptibility. [Courtesy of Dr. J. Vick from L. M. Lichtenstein, "Allergic Responses to Airborne Allergens and Insect Venoms", Fed. Proc. 36:1727, 1977.]
Desensitization
So far, the most effective preventive for IgE-mediated allergies is to inject the patient with gradually-increasing doses of the allergen itself. The goal is to shift the response of the immune system away from Th2 cells in favor of Th1 cells. Unfortunately, this therapy takes a long time and the results are too often disappointing. Clinical trials are now underway to test the safety and efficacy of a complex of ragweed pollen allergen with chemically-modified DNA. This complex binds to the immune receptor TLR-9 causing a shift of the immune response from Th2 to Th1 much more rapidly than desensitization by the allergen alone.
Anti-IgE Antibodies
IgE molecules bind to mast cells and basophils through their constant region. If you could block this region, you could interfere with binding — hence sensitization of — these cells. Humanized monoclonal antibodies specific for the constant region of IgE are in clinical trials. They have shown some promise against asthma and peanut allergy, but such treatment will probably have to be continued indefinitely (and will be very expensive).
IgE-Independent Allergic Reactions
Mast cells have surface receptors in addition to IgE molecules. Binding of ligands to these other receptors can also trigger degranulation and immediate anaphylactic responses. Some culprits are:
• pathogen-associated molecular patterns (PAMPs)
• substance P
• some components of wasp venoms
• some antibiotics
Antibody-Mediated Cytotoxicity
In these disorders, the person produces antibodies directed against antigens present on the surface of his or her own cells. Thus these qualify as autoimmune disorders. Some examples:
• hemolytic disease of the newborn (Rh disease)
• immune hemolytic anemia
• immune thrombocytopenic purpura
• myasthenia gravis (MG)
• thyrotoxicosis (Graves' disease)
• pemphigus and pemphigoid, in which the antibodies are directed against the proteins in desmosomes (pemphigus) or hemidesmosomes (pemphigoid).
• Goodpasture's Syndrome
Binding of antibodies to the surface of the cell can result in:
• phagocytosis of the cell
• lysis of the cell
• damage to molecules on the cell surface (e.g., myasthenia gravis)
• activation of cell-surface receptors (e.g., thyrotoxicosis)
Hemolytic Disease of the Newborn (Rh Disease)
Rh antigens are expressed at the surface of red blood cells. During pregnancy, there is often a tiny leakage of the baby's red blood cells into the mother's circulation. If the baby is Rh-positive (having inherited the trait from its father) and the mother Rh-negative, these red cells will cause her to develop antibodies against the Rh antigen. The antibodies, usually of the IgG class, may not develop fast enough to cause problems for that child, but can cross the placenta and attack the red cells of a subsequent Rh+ fetus. This destroys the red cells producing anemia and jaundice. The disease may be so severe as to kill the fetus or even the newborn infant.
Although certain other red cell antigens (in addition to Rh) sometimes cause problems for a fetus, an ABO incompatibility does not. Why is an Rh incompatibility so dangerous when ABO incompatibility is not? It turns out that most anti-A or anti-B antibodies are of the IgM class and these do not cross the placenta. In fact, an Rh-/type O mother carrying an Rh+/type A, B, or AB fetus is resistant to sensitization to the Rh antigen. Presumably her anti-A and anti-B antibodies destroy any fetal cells that enter her blood before they can elicit anti-Rh antibodies in her.
This phenomenon has led to an extremely effective preventive measure to avoid Rh sensitization. Shortly after each birth of an Rh+ baby, the mother is given an injection of anti-Rh antibodies. The preparation is called Rh immune globulin (RhIG) or Rhogam. These passively acquired antibodies destroy any fetal cells that got into her circulation before they can elicit an active immune response in her.
Rh immune globulin came into common use in the United States in 1968, and within a decade the incidence of Rh hemolytic disease became very low.
Immune Hemolytic Anemia
Some people synthesize antibodies against their own red blood cells, and these may lyze the cells producing anemia. Infections, cancer, or an autoimmune disease like systemic lupus erythematosus (SLE) are often involved. Many drugs (e.g. penicillin, quinidine) can also trigger the disorder. In these cases, stopping the drug usually brings about a quick cure.
Immune Thrombocytopenic Purpura
This is an autoimmune disorder in which the patient develops antibodies against his or her own platelets (thrombocytes). The life span of the platelets may be reduced from the normal of 8 days to as little as 1 hour, and platelet counts may drop from a normal of 140,000–440,000/µl to 20,000/µl or less. This greatly interferes with normal clotting, causing
• external bleeding (e.g., from the nose)
• internal bleeding into the skin causing purple patches (called purpura)
The fact that antibodies are the culprit was dramatically demonstrated by using a patient's serum to passively — but only temporarily —transfer the disorder to a normal recipient. The graph shows the decline and recovery in the platelet count of a normal human subject receiving two transfusions of serum from a patient with thrombocytopenic purpura. [From W. J. Harrington et al., J. Lab. Clin. Med. 38:1, 1951.]
Often no cause of the disorder can be found (the physicians call it "idiopathic"). Some cases are triggered by drugs like quinine, aspirin, digitoxin, and sulfa drugs. These cases can be cured by stopping the drug. The idiopathic cases can sometimes be helped by giving corticosteroids and/or removing the patient's spleen. Rituximab, a monoclonal antibody directed against B cells is also used. If these treatments are inadequate, attempts can be made to increase the platelet count by giving synthetic agonists (e.g., Romiplastin [Nplate®]) that stimulate the production of thrombopoietin.
Myasthenia Gravis (MG)
The hallmark of this autoimmune disorder is weakness of the skeletal muscles, especially those in the upper part of the body. It is caused by antibodies that attack the acetylcholine (ACh) receptors at the subsynaptic membrane of neuromuscular junctions. As the number of receptors declines, the ACh released with the arrival of a volley of nerve impulses is inadequate to generate end-plate potentials (EPPs) of the normal size. After repeated stimulation, the EPPs fail to reach the threshold needed to generate an action potential and the muscle stops responding.
The signs and symptoms of myasthenia gravis can be quickly but only temporarily relieved by injecting a drug that inhibits the action of cholinesterase. This prolongs the action of ACh at the neuromuscular junction. The immunosuppressant action of corticosteroids, like prednisone, can provide long-term improvement for patients. The exclusive role of antibodies (of the IgG class) in this disorder is demonstrated by the presence of the disease in the newborn babies of mothers with the disorder. As these antibodies, which the fetus had received from the mother's circulation, disappear (in 1–2 weeks), so do all signs of the disease.
Thyrotoxicosis (Graves' disease))
In this disorder, the patient has antibodies that bind to the TSH receptors on the thyroxine-secreting cells of the thyroid. These antibodies mimic the action of TSH itself (thus they behave as a TSH agonist) and trigger secretion of thyroxine (T4) and T3 by the thyroid gland. The role of antibodies (of the IgG class) in this disorder is demonstrated by the presence of the disease in the newborn babies of mothers with the disorder. As these antibodies, which the fetus had received from the mother's circulation, disappear (in 1–2 weeks), so do all signs of the disease.
Immune Complex Disorders
While binding of antibody to antigen is often a helpful — even life-saving — response, in some circumstances it causes pathological changes.
Serum Sickness
In passive immunization, an antiserum containing needed antibodies is injected into the patient. At one time, these antisera were prepared by immunizing horses or sheep. While they did their intended work (usually to provide immediate protection to a person exposed to diphtheria or tetanus), they also often later lead to a syndrome called serum sickness. The patient developed fever, hives, arthritis and protein in the urine. After a week or two, the symptoms would disappear spontaneously.
Serum sickness is caused by the many extraneous proteins present in the antiserum. Being foreign to the recipient, an active immunity develops against these proteins. The resulting antibodies bind to them forming immune complexes. These are carried by the blood and deposited in the walls of blood vessels as well as in the glomeruli of the kidneys (see figure).
Antigen-antibody complexes
• bind to Hageman factor — one (XII) of the blood clotting factors. This activates inflammatory kinins.
• bind to a system of serum proteins collectively known as complement. This generates
• complement factor C3a, an anaphylatoxin which activates basophils and mast cells and causes them to release their histamine and leukotrienes producing inflammation.
• complement factor C5a, another anaphylatoxin which also attracts neutrophils to the site.
Thanks to nearly universal active immunization against both tetanus and diphtheria, serum sickness is now quite rare. However, kidney damage (called glomerulonephritis) produced by deposits of immune complexes is found in some persistent infections.
Examples:
• the protozoans that cause malaria
• the flatworms that cause schistosomiasis and the filarial worms that cause elephantiasis and other diseases in humans.
• the virus that causes hepatitis B.
In these cases, the continued presence of the pathogen provides a renewable source of antigen to combine with antibodies synthesized by the host resulting in deposits of immune complexes.
Systemic Lupus Erythematosus (SLE)
Humans with SLE develop (for unknown reasons) antibodies against a wide variety of self components:
• their own double-stranded DNA
• nucleosomes
• red blood cells
• platelets
• NMDA receptors in the brain
• even their own IgG molecules. (These "anti-antibodies" are called rheumatoid factors. They are also found in people with rheumatoid arthritis (hence the name) and, for a time, in people with mononucleosis.)
In all these cases of autoimmunity, immune complexes form and are deposited in the skin, joints, and kidneys where they initiate inflammation.
Farmer's Lung
Repeated exposure to airborne organic particles, like mold spores, can elicit formation of antibodies. When these interact with inhaled antigen, inflammation of the alveoli occurs. The sufferer develops a cough, fever, and difficulty in breathing. Once removed from the source of antigen, the attack subsides within a few days.
Farmers exposed to moldy hay often develop this problem (technically known as extrinsic allergic alveolitis). Sugarcane workers, cheese makers, mushroom growers, pigeon fanciers, and a number of other occupational or hobby groups are apt to develop allergic alveolitis from exposure to the spores and dusts associated with their activities.
Cell-Mediated Hypersensitivities
Cell-mediated hypersensitivities can occur with extrinsic antigens or with internal ("self") antigens.
Extrinsic Antigens
The most common example of cell-mediated hypersensitivity to external antigens is the contact dermatitis caused in some people when their skin is exposed to a chemical to which they are allergic. Some examples:
• the catechols found in poison ivy, poison oak, and poison sumac
• nickel (often used in jewelry)
• some dyes
• certain organic chemicals used in industry
In every case, these simple chemicals probably form covalent bonds with proteins in the skin, forming the antigen that initiates the immune response. Dendritic cells in the skin take up the complex, process it, and "present" it to CD4+ T cells in nearby lymph nodes. Because it takes a day or two for the CD4+ T cells to mobilize to the affected area of skin, these cases are examples of delayed-type hypersensitivity (DTH).
When a patient is unsure of what chemical is causing the dermatitis, the physician can try a patch test. Pieces of gauze impregnated with suspected allergens are placed on the skin. After 48 hours, they are removed and each site is examined for a positive response (a reddened, itching, swollen area).
Intrinsic ("self") Antigens
Cell-mediated hypersensitivities to "self" cause autoimmune diseases. Examples:
• Type 1 diabetes mellitus
• Multiple sclerosis (MS)
• Rheumatoid arthritis (RA)
Type 1 diabetes mellitus
In this disease, T cells initiate the destruction of the insulin-producing beta cells of the islets of Langerhans in the pancreas. The chief culprits are CD8+ cytotoxic T lymphocytes (CTL) aided and abetted by CD4+ helper T cells of the Th1 subset. Although antibodies against beta cell antigens are also found, these appear to be a secondary effect. Evidence: a diabetic boy with X-linked agammaglobulinemia so unable to make any antibodies at all.
Multiple Sclerosis (MS)
In this case, T cells — again mostly Th1 cells — initiate an attack that destroys the myelin sheath of neurons. As the disease progresses, other cells (e.g. macrophages) as well as antibodies participate in causing the damage.
Rheumatoid Arthritis (RA)
In this disorder, antibodies and T cells — again probably Th1 cells — react to antigens in the joints and release tumor necrosis factor-alpha (TNF-α) with resulting inflammation and damage to the joints.
A genetically engineered fusion protein consisting of the TNF receptor fused to the constant region of human IgG1 has shown promise as a treatment for RA. Given by injection, the fusion protein binds TNF-α and interferes with its action.
Autoimmune disorders are more common in females than in males
Graves' disease, systemic lupus erythematosus (SLE), multiple sclerosis, and rheumatoid arthritis are all more common in women than in men. The sex bias ranges from 9:1 for SLE to >2:1 for multiple sclerosis and rheumatoid arthritis. Why?
The answer is unclear, but hormones are probably involved.
A few clues:
• In mice susceptible to Type 1 diabetes, testosterone seems to play a key role.
• Castration causes male mice to become as susceptible as females.
• Giving androgens like testosterone to females protects them.
• High levels of estrogen and progesterone suppress Th1 responses (cell-mediated immunity).
• Pregnant women — with extra-high levels of these hormones — produce large numbers of immunosuppressive regulatory T cells. Together these two responses may account for the improvement that often occurs in multiple sclerosis and rheumatoid arthritis during pregnancy (an improvement that ends after birth).
• High levels of these hormones promote Th2 responses (antibody-mediated immunity). SLE results from antigen-antibody complexes and so it is not surprising that pregnancy does not help — and in some women actually exacerbates — this autoimmune disorder.
The Hygiene Hypothesis
Allergies, like asthma and hay fever, and some autoimmune diseases are more common in regions with good sanitation.
For example,
• Crohn's disease, an inflammation of the small intestine and
• ulcerative colitis, an inflammation of the large intestine
are rare in developing countries with poor sanitation but have become more common in developed regions with good sanitation.
Why?
No one knows for certain, but one intriguing possibility, called the hygiene hypothesis, is that infection by parasitic worms (helminths) shifts the balance of the immune response:
• promoting regulatory T cells (Treg) with their anti-inflammatory cytokines IL-10 and TGF-β and
• inhibiting pro-inflammatory effector T cells, e.g., Th17 cells.
Small clinical trials of feeding whipworm (a nematode) eggs to patients with Crohn's disease or ulcerative colitis have begun. After the eggs develop into mature worms in their intestine, most patients showed marked improvement of their symptoms. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4.01%3A_15.4T_Allergies.txt |
The ability of the immune system to respond to an antigen exists before it ever encounters that antigen. The immune system relies on the prior formation of an incredibly diverse population of:
• B cells (B lymphocytes) each with its surface covered with thousands of identical copies of a receptor for antigen (the B-cell receptor for antigen = BCR)
• T cells (T lymphocytes) each with its surface covered with thousands of identical copies of a T-cell receptor for antigen (TCR)
The above figure illustrates the activation of the one B cell in a pool of B cells whose BCR is specific for an epitope (small dark spheres) on the antigen. This phenomenon is called clonal selection because it is antigen that selects particular lymphocytes for clonal expansion. Clonal selection leads to the eventual production of:
• A pool of antibody-secreting plasma cells. Plasma cells are B-cells that have tooled up (e.g., forming a large endoplasmic reticulum) for massive synthesis and secretion of an antibody. The antibody is the secreted version of the BCR. (For clarity, each BCR is shown with a single binding site for the epitope; actually, the BCRs are IgM and each has 10 identical binding sites.
• A pool of "memory" cells. These are B lymphocytes with receptors of the same specificity as those on the original activated B cell.
How B cells and T cells meet antigens
Fig.15.4.1.2 Postcapillary venule
What is the probability that those few lymphocytes able to bind to a particular epitope will actually encounter the antigen carrying that epitope?
Surprisingly, it is quite high because both B cells and T cells migrate in and out of lymph nodes and the spleen. Lymph nodes serve as lymph filters, trapping foreign matter that gains access to the tissues. It has been shown that as much as 99% of the bacteria entering a node are removed by it. Similarly, the spleen traps antigens that gain access to the blood. Even if an invader fails to enter either lymphatic or blood vessels, its antigens can still reach lymph nodes and spleen carried there by dendritic cells that
• engulf the antigen in the tissues
• migrate in the lymphatic vessels to nearby lymph nodes or spleen
• process the antigen and "present" it to T cells and also B cells
Graft rejection is a form of cell-mediated immunity. If a piece of skin from a mouse of one strain (B) is grafted onto the flank of a mouse of a second strain (A), the graft does well at first. Blood vessels from the host grow into it, and it functions normally. After some 10–14 days, however, the blood supply to the graft breaks down and the graft degenerates. Finally it is sloughed off like an old scab. This is called a first-set rejection. That the graft rejection is an immune response is demonstrated by now grafting the mouse with two pieces of skin a repeat of skin from strain B and skin from another strain (C)
The results are quite different.
• The B skin may not even survive long enough to acquire a blood supply. It is rejected in a much shorter period (less than a week). This "second-set" phenomenon is the T-cell equivalent of the secondary "memory" response of B cells. Its specificity and memory is shown by the fact that
• The graft of C skin is rejected in the normal "first-set" period. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4A%3A_Clonal_Selection_and_Immunological_Memory.txt |
Antibodies are proteins synthesized and secreted by B cells that bind to antigens. Most antigens are macromolecules: proteins, polysaccharides, even DNA and RNA. The interaction occurs by noncovalent forces (like that between enzymes and their substrate) between the antigen-combining site on the antibody and a portion of the antigen called the antigenic determinant or epitope.
These photos show one type of interaction — precipitation — between antibodies and antigen.
1. The tube contains antibodies to the Type III pneumococcal polysaccharide isolated from the capsule surrounding the bacteria.
2. A solution of the polysaccharide is added.
3. The formation of insoluble antigen-antibody complexes is revealed by the almost instantaneous appearance of turbidity.
4. After an hour, the complexes settle out as a precipitate. If the proportion of antigen to antibody in the mixture is selected properly, the fluid above the precipitate will be devoid of both.
In the human body, this binding can literally be life-saving. The capsule that surrounds pneumococci protects them from phagocytosis. Pneumococci that fail to make a capsule — "R" forms — do not cause disease. If the appropriate antibodies are present in the body, they combine with the capsule. Coated with protein instead of polysaccharide, the pneumococci are now easy to ingest.
These photomicrographs show phagocytosis of antibody-coated pneumococci.
• Left: A neutrophil extends a pseudopod toward two pneumococci.
• Center: these bacteria have been engulfed (arrows), and the neutrophil is beginning to engulf four more pneumococci at the upper right.
• Right: Two pneumococci have escaped.
In the days before antibiotics, the start of antibody production by the immune system of the patient marked the turning point in the progression of the disease.
The Antigen-Combining Site
The fig. 15.4.2.3 shows the primary structure of an IgG antibody. Different IgG antibodies differ most markedly at the so-called hypervariable regions (shown in red):
• three in the heavy chain
• three in the light chain
In the three-dimensional (tertiary) structure of the molecule, the 6 hypervariable regions are brought close together and make up the antigen-combining site. For this reason, the hypervariable regions are also called complementarity determining regions (CDRs).
The fig. 15.4.2.4 shows the tertiary structure of one arm of an antibody specific for the lysozyme found in hen's egg white (upper right) bound to lysozyme (lower left).
The following are indicated on the diagram:
• constant region of the light chain (CL) and the first constant region of the heavy chain (CH)
• variable regions of the heavy (VH) and light chains (VL)
• the third hypervariable region of the heavy (Hv3H) and light (Hv3L) chains. The importance of the pair of third hypervariable or complementarity determining regions in binding the epitope is typical of both antibodies and of T cell receptors for antigen. This makes good sense as the opportunities for genetic diversity at those sites is far greater than for the first and second CDRs.
• Gln121 is a glutamine that is a dominant feature of the epitope on lysozyme
The fig. 15.4.2.5 is a space-filling model of the same antibody with the light chain in yellow, the heavy chain in blue. The view is looking down on the epitope-binding surface (left) and the epitope on lysozyme (right). In the antigen-antibody complex, the two surfaces fit snugly together. The 17 amino acid residues that contact lysozyme in the antigen-antibody complex are numbered (1–7 on the L chain, 8–17 on the H chain. The 16 amino acid residues of lysozyme that contact the antibody; that is, that make up its epitope are also numbered. Number 14 is Gln-121. The complementarity of the antigen-binding site and the epitope, their respective shapes and the opportunities for multiple noncovalent interactions determine how strongly the two bind together. The strength of the binding of an antibody to its antigen is called its affinity. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4B%3A_Antibody-Antigen_Binding.txt |
Lymphocytes are one of the five kinds of white blood cells (leukocytes) that circulate in the blood. Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. The most abundant lymphocytes are B lymphocytes (B cells) and T lymphocytes (T cells). B cells are produced in the bone marrow. The precursors of T cells are also produced in the bone marrow but leave the bone marrow and mature in the thymus (which accounts for their designation). Each B cell and T cell is specific for a particular antigen. What this means is that each is able to bind to a particular molecular structure. The specificity of binding resides in a receptor for antigen: the B cell receptor (BCR) for antigen and the T cell receptor (TCR) respectively.
Both BCRs and TCRs share these properties:
• They are integral membrane proteins.
• They are present in thousands of identical copies exposed at the cell surface.
• They are made before the cell ever encounters an antigen.
• They are encoded by genes assembled by the recombination of segments of DNA.
• They have a unique binding site.
• This site binds to a portion of the antigen called an antigenic determinant or epitope.
• The binding, like that between an enzyme and its substrate depends on complementarity of the surface of the receptor and the surface of the epitope.
• The binding occurs by non-covalent forces (again, like an enzyme binding to its substrate).
• Successful binding of the antigen receptor to the epitope, if accompanied by additional signals, results in:
• stimulation of the cell to leave G0 and enter the cell cycle.
• Repeated mitosis leads to the development of a clone of cells bearing the same antigen receptor; that is, a clone of cells of the identical specificity.
BCRs and TCRs differ in their structure, the genes that encode them and the type of epitope to which they bind.
B Cells
Fig.15.4.3.1 B cell and lymphokines
• BCRs bind intact antigens (like diphtheria toxoid, the protein introduced into your body in the DTP vaccine). These may be
• soluble molecules present in the extracellular fluid
• intact molecules that the B cell plucks from the surface of antigen-presenting cells like macrophages and dendritic cells
• The bound antigen molecules are engulfed into the B cell by receptor-mediated endocytosis.
• The antigen is digested into fragments which are then displayed at the cell surface nestled inside a class II histocompatibility molecule.
• Helper T cells specific for this structure (i.e., with complementary TCRs) bind the B cell and secrete lymphokines that:
• stimulate the B cell to enter the cell cycle and develop, by repeated mitosis, into a clone of cells with identical BCRs
• switch from synthesizing their BCRs as integral membrane proteins to a soluble version
• differentiate into plasma cells that secrete these soluble BCRs, which we now call antibodies
T Cells
The surface of each T cell also displays thousands of identical T cell receptors (TCRs). There are two types of T cells that differ in their TCR:
• alpha/beta (αβ) T cells. Their TCR is a heterodimer of an alpha chain with a beta chain. Each chain has a variable (V) region and a constant (C) region. The V regions each contain 3 hypervariable regions that make up the antigen-binding site.
• gamma/delta (γδ) T cells. Their TCR is also a heterodimer of a gamma chain paired with a delta chain.
The discussion that follows now concerns alpha/beta T cells. Gamma/delta T cells, which are less well understood, are discussed at the end.
The TCR (of alpha/beta T cells) binds a bimolecular complex displayed at the surface of some other cell called an antigen-presenting cell (APC). This complex consists of a fragment of an antigen lying within the groove of a histocompatibility molecule. The complex has been compared to a "hot dog in a bun".
Most of the T cells in the body belong to one of two subsets. These are distinguished by the presence on their surface of one or the other of two glycoproteins designated:
• CD4
• CD8
Which of these molecules is present determines what types of cells the T cell can bind to.
• CD8+ T cells bind epitopes that are part of class I histocompatibility molecules. Almost all the cells of the body express class I molecules.
• CD4+ T cells bind epitopes that are part of class II histocompatibility molecules. Only specialized antigen-presenting cells express class II molecules. These include dendritic cells, phagocytic cells like macrophages and B cells.
CD8+ T cells
The best understood CD8+ T cells are cytotoxic T lymphocytes (CTLs). They secrete molecules that destroy the cell to which they have bound.
This is a very useful function if the target cell is infected with a virus because the cell is usually destroyed before it can release a fresh crop of viruses able to infect other cells. An example will show the beauty and biological efficiency of this mechanism.
Example
Every time you get a virus infection, say influenza (flu), the virus invades certain cells of your body (in this case cells of the respiratory passages). Once inside, the virus subverts the metabolism of the cell to make more virus. This involves synthesizing a number of different macromolecules encoded by the viral genome. In due course, these are assembled into a fresh crop of virus particles that leave the cell (often killing it in the process) and spread to new target cells. Except while in transit from their old homes to their new, the viruses work inside of your cells safe from any antibodies that might be present in blood, lymph, and secretions. But early in the process, infected cells display fragments of the viral proteins in their surface class I molecules. CTLs specific for that antigen will be able to bind to the infected cell and often will be able to destroy it before it can release a fresh crop of viruses.
In general, the role of the CD8+ T cells is to monitor all the cells of the body, ready to destroy any that express foreign antigen fragments in their class I molecules.
CD4+ T cells
CD4+ T cells bind an epitope consisting of an antigen fragment lying in the groove of a class II histocompatibility molecule. CD4+ T cells are essential for both the cell-mediated and antibody-mediated branches of the immune system:
• cell-mediated immunity: These CD4+ cells bind to antigen presented by antigen-presenting cells (APCs) like phagocytic macrophages and dendritic cells. The T cells then release lymphokines that attract other cells to the area. The result is inflammation: the accumulation of cells and molecules that attempt to wall off and destroy the antigenic material (an abscess is one example, the rash following exposure to poison ivy is another).
• antibody-mediated immunity: These CD4+ cells, called helper T cells, bind to antigen presented by B cells. The result is the development of clones of plasma cells secreting antibodies against the antigenic material.
T cells
AIDS provides a vivid illustration of the importance of CD4+ T cells in immunity. The human immunodeficiency virus (HIV) binds to CD4 molecules and thus is able to invade and infect CD4+ T cells. As the disease progresses, the number of CD4+ T cells declines below its normal level of about 1000 per microliter (µl). A partial explanation for this may be the unceasing efforts of the patient's CD8+ T cells to destroy the infected CD4+ cells. However, it turns out that only a small fraction of the patients CD4+ T cells are infected at any given time. How uninfected CD4+ cells may be induced to commit suicide is discussed in the page on apoptosis.
When the number of CD4+ T cells drops below 400 per microliter, the ability of the patient to mount an immune response declines dangerously. Not only does the patient become hypersusceptible to pathogens that give all of us grief but also to microorganisms, especially viruses and fungi, that normally inhabit our tissues without harming us. These opportunistic infections can be fatal.
Building the T-cell Repertoire
T cells have receptors (TCRs) that bind to antigen fragments nestled in MHC molecules. But all cells express class I MHC molecules containing fragments derived from self proteins. Many cells express class II MHC molecules that also contain self peptides. This presents a risk to the animal of the T cells recognizing these self-peptide/self-MHC complexes and mounting an autoimmune attack against them. Fortunately, this is usually avoided by a process of selection that goes on in the thymus (where all T cells develop).
The process works like this:
• The precursors of T cells like all blood cells are formed in the bone marrow.
• These cells then migrate to the cortex of the thymus. At this time they have neither a complete TCR nor either CD4 or CD8 (thus are called "double-negative" or DN cells).
• In the cortex of the thymus they begin to form a TCR and synthesize both CD4 and CD8 (so now they are "double-positive" or DP cells). The cortical cells of the thymus express a wide variety of small molecules, usually a peptide of 6–8 amino acids derived from body proteins; that is, "self" proteins such as proteins within the cytosol and serum proteins - proteins circulating in the blood and lymph nestled in a histocompatibility molecule (encoded by the MHC).
• Most of the cells (~97%) will produce a TCR that does not bind to any of the peptide-MHC molecules present on the surface of the cortical cells. Unless they can try again with a new TCR, these cells die by "neglect" (by apoptosis, actually).
• Those remaining cells whose TCR has bound a peptide antigen presented in class II MHC molecule stop expressing CD8 and become CD4+ T cells. It is these cells that will go on to become
• Th1 cells in cell-mediated immune responses
• Th1 helper cells for cytotoxic T lymphocytes (CTLs)
• Th2 helper cells for B cells
• Those remaining cells whose TCR has bound a peptide antigen presented in class I MHC molecule stop expressing CD4 and become CD8+ T cells.
• Both sets of cells are said to have undergone positive selection.
• After positive selection, these cells migrate to the medulla of the thymus.
• There those cells whose TCR binds very strongly to complexes of self-peptide and self-MHC are destroyed (again by apoptosis).
• This process of negative selection is important as it eliminates cells that might otherwise mount an autoimmune attack. It is one of the ways in which tolerance to self antigens is achieved. [Link to discussion of T-cell tolerance.]
• The cells whose TCRs bind antigen at an affinity below the threshold that triggers apoptosis are free to leave the thymus and migrate throughout the immune system (lymph nodes, spleen, etc.)
• It is this population that we depend on to mount immune responses against foreign antigens. A TCR that binds self-peptide/self-MHC with low affinity may well bind a foreign-peptide in self MHC with high affinity.
Gamma/Delta (γδ) T Cells
Gamma/delta T cells differ from their alpha/beta cousins in several ways:
• Their TCR is encoded by different gene segments.
• Their TCR binds to
• antigens that can be intact proteins (just as antibodies do) as well as a variety of other types of organic molecules (often containing phosphorus atoms).
• antigens that are not "presented" within class I or class II histocompatibility molecules;
• antigens that are not presented by "professional" antigen-presenting cells (APCs) like dendritic cells.
• Most of these T cells have neither CD8 nor CD4 on their surface. This makes sense because they have no need to recognize class I and class II histocompatibility molecules.
• Gamma/Delta T cells, like alpha/beta T cells, develop in the thymus. However, they migrate from there into body tissues, especially epithelia (e.g., intestine, skin, lining of the vagina), and don't recirculate between blood and lymph nodes (they represent no more than 5% of the T cells in the blood and are even rarer in lymph nodes). They encounter antigens on the surface of the epithelial cells that surround them rather than relying on the APCs found in lymph nodes.
What is the Function of γδ T cells?
That is still something of a mystery. Situated as they are at the interfaces between the external and internal worlds, they may represent a first line of defense against invading pathogens. Their response does seem to be quicker than that of αβ T cells. Curiously, many of the antigens to which γδ T cells respond are found not only on certain types of invaders (e.g., Mycobacterium tuberculosis, the agent of tuberculosis) but also on host cells that are under attack by pathogens.
Knockout mice that cannot make γδ T cells are slower to heal injuries to their skin. They are also much more susceptible to skin cancers than normal mice. Perhaps immune surveillance is one of the functions of γδ T cells. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4C%3A_B_Cells_and_T_Cells.txt |
Both B cells and T cells have surface receptors for antigen. Each cell has thousands of receptors of a single specificity; that is, with a binding site for a particular epitope. T-cell receptors (TCRs) enable the cell to bind to and, if additional signals are present, to be activated by and respond to an epitope presented by another cell called the antigen-presenting cell or APC. B-cell receptors (BCRs) enable the cell to bind to and, if additional signals are present, to be activated by and respond to an epitope on molecules of a soluble antigen. The response ends with descendants of the B cell secreting vast numbers of a soluble form of its receptors. These are antibodies.
Antibodies
Antibodies are glycoproteins. They are built of subunits containing two identical light chains (L chains), each containing about 200 amino acids and two identical heavy chains (H chains), which are at least twice as long as light chains. The first 100 or so amino acids at the N-terminal of both H and L chains vary greatly from antibody to antibody. T These are the variable (V) regions. Unless members of the same clone (and often not even then), no two B cells are likely to secrete antibodies with the same variable region. The amino acid sequence variability in the V regions is especially pronounced in 3 hypervariable regions. The tertiary structure of antibodies brings the 3 hypervariable regions of both the L and the H chains together. Together they construct the antigen binding site against which the epitope fits. For this reason, the hypervariable regions are also called complementarity determining regions (CDRs). Only a few different amino acid sequences are found in the C-terminals of H and L chains. These are the constant (C) regions.
Humans make two different kinds of C regions for their L chains producing kappa (κ) L chains and lambda (λ) L chains. They also make five different kinds of C regions for their H chains producing:
• mu (µ) chains (the H chain of IgM antibodies)
• gamma (γ) chains (IgG)
• alpha (α) chains (IgA)
• delta (δ) chains (IgD)
• epsilon (ε) chains (IgE)
The images above() represent the folded (tertiary) structure of an entire L chain (right side with thin connecting lines) and the V region plus the first third of the C region of a heavy chain (left side; darker lines). Each circle represents the location in 3D space of an alpha carbon. The filled circles at the top are amino acids in the hypervariable or complementary determining regions (CDRs); they form the site that binds the antigen.
Antibody molecules have two functions to perform:
• recognize and bind to an epitope on an antigen
• trigger a useful response to the antigen
The division of labor is:
• V regions are responsible for epitope recognition.
• C regions are responsible for triggering a useful response
So, V regions finger the culprit; the C regions take action.
If an antibody's H chains (see IgG above), are cut at its hinge region on the N-terminal side of the disulfide bonds holding the H chains together, 3 fragments are produced:
• 2 Fab fragments ("fragment antigen-binding") and
• 1 Fc fragment ("fragment crystalline" — because the uniformity of this region allows crystals to form while the great diversity of V regions prevents them from forming).
Why 5 kinds of heavy chains? To provide for different effector functions.
The 5 classes of antibodies
Class H chain L chain Subunits mg/ml Notes
IgG gamma kappa or lambda H2L2 6–13 transferred across placenta; four subclasses: IgG1-4 in humans
IgM mu kappa or lambda (H2L2)5 0.5–3 first antibodies to appear after immunization
IgA alpha kappa or lambda (H2L2)2 0.6–3 much higher concentrations in secretions; two subclasses
IgD delta kappa or lambda H2L2 <0.14 function uncertain
IgE epsilon kappa or lambda H2L2 <0.0004 binds to basophils and mast cells sensitizing them for certain allergic reactions
"mg/ml" gives the concentration normally found in human serum. The subclasses of IgG and IgA are encoded by different C-region gene segments.
If an antibody-secreting cell becomes cancerous, it will grow into a clone secreting its single class of molecule. The disease is called multiple myeloma. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4D%3A_Antigen_Receptors.txt |
Histocompatibility molecules are glycoproteins expressed at the surface of almost all vertebrate cells. They get their name because they are responsible for the compatibility — or rather the lack of it — of the tissues of genetically different individuals. Monozygotic ("identical") human twins have the same histocompatibility molecules on their cells, and they can accept transplants of tissue from each other. The rest of us have a set of histocompatibility molecules that is probably unique to us. A graft of our tissue into another human will provoke an immune response which, if left unchecked, will end in the rejection of the transplant. So the histocompatibility molecules of one individual act as antigens when introduced into a different individual. In fact, the histocompatibility molecules are often called histocompatibility antigens or transplantation antigens.
The most rapid and severe rejection of foreign tissue occurs when there is a failure to properly match the donor and recipient for the major histocompatibility molecules. There are two categories: class I and class II.
Class I Histocompatibility Molecules
Class I molecules consist of two polypeptide chains, a long one (on the left) of 346 amino acids — it is called the heavy chain — and a short one (on the right) of 99 amino acids. The heavy chain consists of 5 main regions or domains:
• three extracellular domains, designated here as N (includes the N terminal), C1, and C2
• a transmembrane domain where the polypeptide chain passes through the plasma membrane of the cell
• a cytoplasmic domain (with the C terminal) within the cytoplasm of the cell
he above figure shows a protein molecule called beta-2 microglobulin ("β2M"). It is not attached to the heavy chain by any covalent bonds, but rather by a number of noncovalent interactions. The dark bars represent disulfide (S-S) bridges linking portions of each domain (except the N domain). However, the bonds in S-S bridges are no longer than any other covalent bond, so if this molecule could be viewed in its actual tertiary (3D) configuration, we would find that the portions of the polypeptide chains containing the linked Cys are actually close together. The outermost domains ("N" and "C1") contain two segments of alpha helix that form two ridges with a groove between them. A small molecule (e.g., a short peptide) is attached noncovalently in the groove between the two alpha helices, rather like a hot dog in a bun(See fig. 15.4.5.2)
The two objects on the left of the image that look like candelabra represent the short, branched chains of sugars in this glycoprotein. The regions marked "Papain" represent the places on the heavy chain that are attacked by the proteinase papain (and made it possible to release the extracellular domains from the plasma membrane for easier analysis). The image represents the structure of a class I histocompatibility molecule, called H-2K. Almost all the cells of an animal's body (in this case, a mouse) have thousands of these molecules present in their plasma membrane. These molecules provide tissue identity and serve as major targets in the rejection of transplanted tissue and organs. But tissue rejection is not their natural function. Class I molecules serve to display antigens on the surface of the cell so that they can be "recognized" by T cells.
Humans synthesize three different types of class I molecules designated HLA-A, HLA-B, and HLA-C. (HLA stands for human leukocyte antigen; because the molecules were first studied on leukocytes). These differ only in their heavy chain, all sharing the same type of beta-2 microglobulin. The genes encoding the different heavy chains are clustered on chromosome 6 in the major histocompatibility complex (MHC). We inherit a gene for each of the three types of heavy chain from each parent so it is possible, in fact common, to express two allelic versions of each type. Thus a person heterozygous for HLA-A, HLA-B, and HLA-C expresses six different class I proteins. These are synthesized and displayed by most of the cells of the body (except those of the central nervous system).
Histocompatibility molecules present antigens to T cells
Although histocompatibility molecules were discovered because of the crucial role they play in graft rejection, clearly evolution did not give vertebrates these molecules for that function. So what is their normal function? The answer: to display antigens so that they can be "seen" by T lymphocytes. The antigen receptor on T lymphocytes (or T cells, as they are commonly called) "sees" an epitope that is a mosaic of the small molecule in the groove and portions of the alpha helices flanking it.
The small molecules ("hot dogs") are enormously diverse. They probably represent fragments derived from all the proteins present within the cell. These would include fragments of normal cell constituents, fragments of proteins encoded by intracellular parasites (like viruses), and fragments of proteins encoded by mutated genes in cancer cells.
Class II Histocompatibility Molecules
Human class II molecules are designated HLA-D, and the genes encoding them are also located in the major histocompatibility complex (MHC). Class II molecules consist of two transmembrane polypeptides. These interact to form a groove at their outer end which, like class I molecules, always contains a fragment of antigen. But the fragments bound to class II molecules are derived from antigens that the cell has taken in from its surroundings. Extracellular molecules are engulfed by endocytosis. The endosomes fuse with lysosomes and their contents are partially digested. The resulting fragments are placed in class II molecules and returned to the cell surface.
Class II molecules, in contrast to class I, are normally expressed on only certain types of cells. These are cells like macrophages and B lymphocytes that specialize in processing and presenting extracellular antigens to T lymphocytes. Thus antigen presentation by class II molecules differs from that by class I in two important ways:
• All cells can present antigens with class I molecules, whereas only certain cells can do so with class II.
• The antigen fragments (hot dogs) displayed in class I molecules are generated from macromolecules synthesized within the cell, whereas those displayed in class II molecules have been acquired from outside the cell.
Class I and Class II molecules present antigen fragments to different subsets of T cells
Most of the T cells of the body belong to one of two distinct subsets: CD4+ or CD8+. CD4 and CD8 are surface glycoproteins. Both CD4+ and CD8+ T cells have an antigen receptor (TCR) that "sees" a complex hot-dog-in-bun epitope. The CD8 molecules on CD8+ T cells bind to a site found only on class I histocompatibility molecules (shown here as a gray hemisphere). The CD4 molecules on CD4+ T cells bind to a site found only on class II histocompatibility molecules (shown below as a yellow triangle). However, neither type can be activated by simply binding its complementary epitope. Additional molecular interactions must take place.
The CD8+ T Cell/Class I Interaction
Because of the need for CD8 to bind to a receptor site found only on class I histocompatibility molecules, CD8+ T cells are only able to respond to antigens presented by class I molecules. Most CD8+ T cells are cytotoxic T cells (CTLs). They contain the machinery for destroying cells whose class I epitope they recognize.
Example
Every time you get a viral infection, say influenza (flu), the virus invades certain cells of your body. Once inside, the virus subverts the metabolism of the cell to make more virus. This involves synthesizing molecules encoded by the viral genome. In due course, these are assembled into a fresh crop of virus particles that leave the cell (often killing it in the process) and spread to new cells. Except while in transit from their old home to their new, the virus works inside cells safe from any antibodies. But early in their intracellular life, infected cells display fragments of the viral proteins being synthesized in the cytoplasm in their surface class I molecules. Any cytotoxic T cells specific for that antigen will bind to the infected cell and often will be able to destroy it before it can release a fresh crop of virus.
The bottom line: the function of the body's CD8+ T cells is to monitor all the cells of the body ready to destroy any that express foreign antigen fragments in their class I molecules.
The CD4+ T Cell/Class II Interaction
The CD4 molecules expressed on the surface of CD4+ T cells enable them to bind to cells presenting antigen fragments in class II molecules but not in class I. Only certain types of cells, those specialized for taking up antigen from extracellular fluids, express class II molecules. Among the most important of these are dendritic cells, macrophages (phagocytic cells that develop from monocytes that have migrated from the blood to the tissues), and B lymphocytes ("B cells") that take up exogenous antigen by receptor-mediated endocytosis.
So CD4+ T cells see antigen derived from extracellular fluids and processed by specialized antigen-presenting cells. To respond to an antigen, a CD4+ T cell must have a T cell receptor (TCR) able to recognize (bind to) a complex epitope comprising an antigenic fragment displayed by a class II molecule, bind a site on the class II molecule (shown above as a yellow triangle) with its CD4, and bind to costimulatory molecules on the antigen-presenting cell. If these conditions are met, the T cell becomes activated. Activated T cells enter the cell cycle leading to the growth of a clone of identical T cells and begin to secrete lymphokines.
Lymphokines activate and recruit other cells (e.g., mast cells) to the region producing inflammation (e.g., to cope with a bacterial infection). They also activate B cells enabling them to develop into a clone of antibody-secreting cells. The CD4+ T cells that activate B cells are called Helper T cells.
Contributors and Attributions
John W. Kimball. This content is distributed under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4E%3A_Histocompatibility_Molecules.txt |
The human genome is presently estimated to contain 20–25 thousand genes. The number of T-cell receptors for antigen (TCRs) that we make is estimated at 2.5 x 107; the number of different kinds of antibody molecules (BCRs) is probably about the same.
How could 2.5 x 104 genes encode 2.5 x 107 different TCRs and the same number of different BCRs?
The answer: each receptor chain
• heavy (H) plus kappa (κ) or lambda (λ) chains for BCRs;
• alpha (α) and beta (β) or gamma (γ) and delta (δ) chains for TCRs)
is encoded by several different gene segments. The genome contains a pool of gene segments for each type of chain. Random assortment of these segments makes the largest contribution to receptor diversity.
B cells
Gene segment usage for BCRs
For the heavy (H) chains of BCRs (antibodies), the gene segments are:
• 51 VH segments. Each of these encodes most of the N-terminal of the antibody, including the first two (but not the third) hypervariable or complementarity determining region (CDR).
• 27 DH (="diversity") gene segments. These encode part of the third CDR ("CDR3").
• 6 JH (="joining") gene segments. These encode the remainder of the V region of the BCR (including the remainder of CDR3).
• 9 CH gene segments. These encode the C region of the BCR (and the antibody derived from it). The C gene segments are
• 1 mu (µ); encodes the C region of IgM
• 1 delta (δ) for IgD
• 4 gamma (γ) gene segments for the four types of IgG
• 1 epsilon (ε) for IgE
• 2 alpha (α) gene segments for the two types of IgA
All of these gene segments are clustered in a complex locus on chromosome 14. During the differentiation of the B cell (and long before any encounter with an antigen), the DNA in this locus is cut and recombined to make an intact gene for the heavy chain. This gene can then be transcribed into pre-mRNA, which is then processed to form the mRNA that will be translated into the heavy (H) polypeptide chain.
V(D)J Joining
• Each gene segment (V, D, and J) has an adjacent Recombination Signal Sequence (RSS)
• at the 3' end of each V segment
• at both ends of each D segment
• at the 3' end of each J segment
• These are recognized by two proteins encoded by two Recombination Activating Genes
• RAG-1
• RAG-2
• The RAG-1 and RAG-2 proteins cut through both strands of DNA at the RSS forming double-stranded breaks (DSBs).
• Then the regular machinery for repairing DSBs (by nonhomologous end-joining) swings into action.
• The cut ends are stitched together (ligated) to form:
• a coding joint (D-J or V-DJ for heavy chains; V-J for light chains)
• a signal joint (usually a loop of DNA deleting all the intervening DNA initially present between the 2 gene segments chosen).
• D-J joining occurs first; then the combined DJ segment (still attached to the cluster of constant region gene segments) is joined to a V segment (as shown in the figure).
• The V gene segment chosen may be thousands of base pairs away from the D-J segment so the chromosome must be drawn into a loop to bring the two together. The loop is stabilized by
• a protein designated CTCF ("CCCTC binding factor"; named for the nucleotide sequence to which it binds). The CTCF at the D-J site on the DNA forms a dimer with the CTCF at the V site on the DNA binding the two regions together.
• cohesin — the same protein complex that holds sister chromatids together during mitosis and meiosis.
• In the process, the cluster of gene segments moves from the periphery of the nucleus (a region of inactive genes) to the center of the nucleus (a region of active gene transcription).
SCID
Some cases of severe combined immunodeficiency in humans (SCID) are caused by defects in V(D)J joining.
• One version is caused by mutations in both copies of either RAG1 or RAG2.
• Another is caused by mutations in a gene needed for nonhomologous end-joining. (No coding joint is formed even though a signal joint forms normally.)
If the 51 VH, 27 DH, and 6 JH gene segments were assembled randomly (they probably are not), that would provide a minimum of 8.3 x 103 different possible combinations. But the possibilities of antibody V region diversity turn out to be greater than that. The recombination process is not precise.
• The exact points of splicing between VH and DH and between DH and JHcan vary over several nucleotides
• Extra nucleotides, called N regions, can also be inserted at these joints.
• All of these add greatly to the diversity of CDR3.
Light chains
Once the H chain gene is assembled, transcribed, and translated, the resulting H chain can pair with an L chain that is itself the product of a similar recombination process occurring
• on chromosome 2 for kappa gene segments
• on chromosome 22 for lambda gene segments
Antibodies (BCRs) Gene Segments Combinations
40
5 200 κ chains
31
4 124 λ chains
VH 51
DH 25
JH 6 7,650 H chains
Any H chain with any L chain (324) 2.5 x 106
As the table shows, this lays the foundation for a potential B-cell repertoire of 2.5 x 106 different antibody V regions. But the true number is probably virtually limitless because of variation in the exact splice point and the introduction of N nucleotides both of which increase the diversity of CDR3.
Diversity comes at a price
The combining of V, D, and J gene segments coupled with the random incorporation of extra nucleotides (N regions) at the joints, creates enormous coding variability. It also creates a high risk (two times out of three) of introducing a frameshift so that the codons for the rest of the V region encode nonsense. Although many B cells are wasted, the odds are not quite as bad as they seem. If the B cell fails to make a functional product from the cluster of gene segments on one of its chromosomes, it can turn to the gene segments on its homolog and try again. If it fails both times to make a functional kappa L chains, it still has two tries at making a functional lambda L chain.
Somatic Hypermutation (SHM) and Antibody Diversity
The diversifying mechanisms described above take place before the B cell encounters antigen. After a B cell encounters antigen, it may begin mitosis, growing into a clone of cells synthesizing the same BCR (and, eventually, secreting antibodies with the same binding site). Point mutations can occur while this is going on. Some of these may generate a binding site with increased affinity for its epitope. These are favorable mutations, and the "subclone" in which they occur tends to be favored and may replace the ancestral clone. The result is affinity maturation — the production of antibodies of ever-increasing affinity for the antigen.
Class Switch Recombination (CSR)
As B cells grow into a clone in response to antigen, they may rearrange their DNA once again. For example, a B cell that has assembled a complete gene for the H chain of IgM (µ), may cut the gene on the 3´ side of the assembled V-region segments and move the assembly to the 5´ side of another of its CH gene segments. Now the cell begins to make a different class of antibody, such as IgG or IgA. But the antigen specificity of the antibody remains the same because the N-terminal of the H chain remains unchanged (as does the entire L chain).
Class switch recombination enables the body to produce antibodies with different effector functions; that is, different means of dealing with the same antigen. The ability of a B cell to switch CH gene segments depends on its receiving help from helper T cells.
Alpha/beta (αβ) T cells
The most abundant T cells in the blood express a receptor for antigen (TCR) that is a heterodimer of two chains designated alpha (α) and beta (β). Each of these is encoded by a gene assembled from V, D, J, and C gene segments. Like BCRs, there are multiple variants of these gene segments arranged in clusters:
• alpha chain gene segments on chromosome 14
• beta chain gene segments on chromosome 7
T cell receptors (TCRs) Gene segments Combinations
50
50 2.5 x 103 alpha chains
20
13
2 520 beta chains
Any alpha with any beta chain 1.3 x 106
And like B cells, the greatest diversity in the receptors of αβ T cells occurs in the third complementarity determining region (CDR3) of the alpha and beta chains because of the junctional diversity between the V, J, and D segments and the addition of N region nucleotides. However, T cells do not seem to use somatic mutation to increase receptor diversity. Actual measurements of the repertoire in humans reveals a figure of about 2.5 x 107.
Gamma/delta (γδ) T cells
The TCR repertoire of γδ T cells seems much smaller than that of their αβ cousins. The gamma chain gene segments are clustered on chromosome 7. The delta chain gene segments are clustered on chromosome 14 (within the alpha chain cluster). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4F%3A_Antigen_Receptor_Diversity.txt |
T cells are T lymphocytes that belong to the CD4+ subset. These cells have a number of direct functions, but they get their name from the help they provide to other types of effector cells, such as B cells and cytotoxic T lymphocytes (CTLs). The help consists of secreted cytokines that stimulate the helped cells.
Types of Helper T Cells
Four kinds have been identified:
• Th1
• These participate in both cell-mediated immunity and antibody-mediated immunity. They are essential for controlling such intracellular pathogens as viruses and certain bacteria, e.g., Listeria and Mycobacterium tuberculosis (the bacillus that causes TB). They provide cytokine-mediated "help" to cytotoxic T lymphocytes — perhaps the body's most potent weapon against intracellular pathogens.
• Th2
• These provide help for B cells and are essential for the production of IgE antibodies and assist in the production of some subclasses of IgG as well. Antibodies are needed to control extracellular pathogens (which unlike intracellular parasites are exposed to antibodies in blood and other body fluids).
• Tfh
• These also provide help to B cells enabling them to develop into antibody-secreting plasma cells. This occurs in nests of lymphoid cells called follicles — in the lymph nodes. The most abundant helper T cells there are B-cell helpers called follicular helper T (Tfh) cells.
• Th17
• These protect surfaces (e.g., skin, lining of the intestine) against extracellular bacteria.
In addition, there is another related subset that dampens rather than promotes immune responses. These cells, designated Treg, are discussed on another page. Link to it.
Origin of Helper T Cells
Like all T cells, Th cells arise in the thymus.
• When they acquire CD4, they are called pre-Th cells.
• When they are presented with both an antigen and appropriate cytokines,they begin to proliferate and become activated.
• It is the nature of the stimulation — the type of antigen-presenting cell and cytokine(s) — that determines which path they enter.
Th1 Cells
Th1 cells are produced when dendritic cells and pre-Th cells form an immunological synapse in which the dendritic cell presents antigen to the T cell's receptor for antigen (TCR), and secretes interleukin 12 (IL-12) as well as IFN-γ. The paracrine stimulation by these cytokines causes the Th1 cells to secrete their own lymphokines:
• interferon-gamma (IFN-γ)
• tumor-necrosis factor-beta (TNF-β) (also known as lymphotoxin)
These stimulate macrophages to kill the bacteria they have engulfed, recruit other leukocytes to the site producing inflammation, act on B cells to promote antibody class switching, and help cytotoxic T cells (CTL) do their work and probably help convert some of them to memory cells.
Th2 Cells
Th2 cells are produced when antigen-presenting cells (APCs) present antigen (e.g., on parasitic helminth worms or certain allergens) to the T cell's receptor for antigen (TCR) along with
• the costimulatory molecule B7 (CD80 & 86)
• the paracrine stimulants interleukin 4 (IL-4) and interleukin 2 (IL-2).
The identity of the APCs for Th2 responses is still uncertain. Some research indicates that basophils are the APCs, but other research questions this role.
The major lymphokines secreted by Th2 cells are
• interleukin 4 (IL-4). This
• stimulates class-switching in B cells and promotes their synthesis of IgE antibodies.
• acts as a positive-feedback device promoting more pre-Th cells to enter the Th2 pathway.
• blocks the IFN-γ receptors from entering the immunological synapse on pre-Th cells thus inhibiting them from entering the Th1 path (shown in red).
• Interleukin 13 (IL-13). This also promotes the synthesis of IgE antibodies as well as recruiting and activating basophils.
• Interleukin 5 (IL-5). Attracts and activates eosinophils.
Two transcription factors have been found that play a critical role in the choice between becoming a Th1 or a Th2 cell.
• T-bet for Th1 cells
• GATA3 for Th2 cells
T-bet produces Th1 cells by
• turning on the genes needed for Th1 function (e.g., for IFN-γ)
• blocking the activity of GATA3.
Mice whose genes for T-bet have been "knocked-out" lack Th1 cells and have elevated numbers of Th2 cells (making them susceptible to such Th2-mediated disorders as asthma).
GATA3 produces Th2 cells by
• turning on the genes needed for Th2 function (e.g., for IL-5)
• blocking the activity of T-bet
Reciprocal inhibition of Th1 and Th2 cells.
The antigenic stimulus that sends pre-Th cells down one path or the other also sets the stage for reinforcing the response.
A Th1 response inhibits the Th2 path in two ways:
• IFN-γ (shown above in red) and IL-12 inhibit the formation of Th2 cells
• IFN-γ also inhibits class-switching in B cells
A Th2 response inhibits the Th1 path:
• IL-4 suppresses Th1 formation (shown above in red)
• significance for public health: infection by helminths — common in the tropics — increases one's risk of viral and bacterial diseases, and in laboratory mice has been shown to enhance viral infections
Negative feedback of Th1 and Th2 cell formation
There is also evidence that late in the immune response, negative feedback mechanisms come into play to dampen the response.
• IL-4 kills (by apoptosis) the precursors of the dendritic cells that induce the Th2 path and thus further production of IL-4.
• IFN-γ may eventually turn off the Th1 response that produced it.
Th1 and Th2 cells have different chemokine receptors.
Chemokines are cytokines that are chemotactic for (attract) leukocytes. The members of one group, who share a pair of adjacent cysteine (C) residues near their N-terminal, are designated CC chemokines. Chemokines bind to receptors on the responding leukocyte. The receptors are transmembrane proteins with the chemokine binding site exposed at the surface of the plasma membrane. CC chemokine receptors are designated CCR.
With their different functions, we might expect that Th1 cells and Th2 cells would respond differently to chemokines. And so they do.
Th1 cells express the chemokine receptor CCR5 (but not CCR3).
Th2 cells express the chemokine receptor CCR3 (but not CCR5).
CCR3
One chemokine that binds to CCR3 is called eotaxin. It is secreted by epithelial cells and phagocytic cells in regions where allergic reactions are occurring.
CCR3 is found on all cells implicated in allergic responses (e.g., asthma). They are:
• Th2 cells
• eosinophils
• basophils
CCR5
CCR5 is found on
• Th1 cells, especially those in the lymphoid tissue of the intestine
• macrophages
CCR5 also acts — along with the CD4 molecule — as a coreceptor for HIV-1, the retrovirus that causes AIDS. This fact may explain
• why destruction of the lymphoid tissue of the intestine occurs soon after HIV infection;
• why certain HIV-infected men
• with inherited mutations preventing the expression of CCR5 or
• who produce high levels of the natural ligand for CCR5 (a chemokine designated CCL3L1, which presumably competes with HIV for access to CCR5)
can tolerate their infection for long periods without progressing to a full-blown case of AIDS;
• the collapse of cell-mediated immunity in the late stages of AIDS
Example
One striking illustration: an AIDS patient with leukemia was given a bone marrow transplant from a donor whose cells expressed a nonfunctional version of CCR5. Two years later, the patient was not only cured of his leukemia but of AIDS as well.
Another: Gene therapy in which samples of a patient's CD4+ T cells were treated in vitro so that their CCR5 gene became nonfunctiona. Expanded in culture and then returned to the donor, five (of six) patients had their CD4+ T cell counts rebound.
Tfh Cells
Follicular helper T cells (Tfh) are CD4+ helper T cells found in nests of B cells called follicles — in the lymph nodes. When exposed (in the paracortical area of the lymph node) to cells presenting antigen to them, e.g., dendritic cells and a cocktail of cytokines they migrate into the follicles. The combined stimuli of antigen binding to their TCR and exposure to cytokines activate a transcription factor called Bcl-6 (first identified in a B-cell lymphoma). Bcl-6 turns on a collection of genes which, among other things, cause the Tfh cells to form an immunological synapse with those B cells expressing the antigen fragments in class II histocompatibility molecules that match their TCR.
Several other pairs of ligands and their receptors stabilize the synapse, including the interaction between CD28 on the Tfh cell and its ligand, B7, on the B cell. These binding interactions stimulate the B cell to
• undergo class switching with the synthesis of other antibody classes (except IgE)
• undergo affinity maturation
• form antibody-secreting plasma cells and memory B cells
This intense activity within the follicle forms a germinal center. It is not yet certain whether Tfh cells represent a distinct class of Th cells or are simply a further stage in the maturation of Th1, or Th2, or Th17 cells.
Th17 Cells
Th17 cells are a recently-identified subset of CD4+ T helper cells. They are found at the interfaces between the external environment and the internal environment, e.g., skin and lining of the GI tract. They probably start out like other "naive" Th cells, but when exposed to
• cells presenting antigen to them, e.g., dendritic cells
• several cytokines notably transforming growth factor-beta TGF-β and IL-6
they enter a pathway distinct from that of Th1, Th2, and Tfh cells. The combined stimuli of antigen binding to the TCR and exposure to the cytokinesactivate a nuclear retinoid receptor designated RORγt. This is a transcription factor that turns on a collection of genes which, among other things, leads to
• the synthesis and secretion of IL-17 (giving the cells their name)
• increased synthesis of the plasma membrane receptor for the interleukin IL-23. Interaction of IL-23 (perhaps secreted from nearby dendritic cells) with the receptor drives the rapid proliferation of the Th17 cells
Situated in the skin and the lining of the GI tract, Th17 cells are positioned to attack fungi and bacteria at those locations. They do this by secreting defensins and recruiting scavenging cells, especially neutrophils, to the site. The result: clearing away of the invaders with accompanying inflammation.
But inflammation is a double-edged sword. So it is not surprising that Th17 cells have been implicated as potent effectors of such damaging inflammatory disorders as
• Crohn's disease (an inflammation of the small intestine)
• Ulcerative colitis (inflammation of the large intestine)
• Psoriasis (inflammation of the skin)
• An animal model (in mice) of multiple sclerosis
• Rheumatoid arthritis
Summary Table
Type Cytokine Stimulus Master
Transcription Factor
Effector Cytokine(s) Main Target Cells Effector Targets/Functions Pathological Effects
Th1 T-bet IFN-γ & TNF-β Macrophages, dendritic cells Intracellular pathogens Autoimmunity;
cell-mediated allergies
Th2 IL-4 GATA3 Eosinophils, basophils, B cells Various helminths Asthma and IgE-mediated allergies
Tfh Bcl-6 B cells Class Switch Recombination and Affinity Maturation of antibodies Autoimmune diseases?
Th17 TGF-β plus IL-6
Inhibited by retinoic acid
RORγt Neutrophils Extracellular bacteria and fungi
mediates inflammation
Autoimmune diseases
pTreg TGF-β minus IL-6
Stimulated by retinoic acid and IL-2
Foxp3 all the other types of T cells Immunosuppression; anti-inflammatory None? | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4H%3A_T_Helper_cells.txt |
Cytotoxic T lymphocytes are lymphocytes that kill other ("target") cells.
Targets may include:
• virus-infected cells (e.g., HIV-infected CD4+ T cells)
• cells infected with intracellular bacterial or protozoal parasites
• allografts such as transplanted kidney, heart, lungs, etc.
• cancer cells. (The tumor-infiltrating lymphocytes, TIL, that have shown some promise in cancer therapy contain CTLs.)
There is also evidence that CTL are active in some autoimmune disorders, e.g. help destroy the beta cells of the islets of Langerhans, leading to Type I diabetes mellitus.
Properties of CTLs
Most of them
• belong to the CD8+ subset of T cells;
• use the αβ T-cell receptor for antigen (TCR)
• thus recognize antigens nestled in the groove of class I histocompatibility (MHC) molecules.
• If they encounter (on a dendritic cell) the antigen/MHC for which their TCR is specific, they
• enter the cell cycle and go through several rounds of mitosis ("clonal expansion") followed by
• differentiation into effector ("killer") cells. Their differentiation includes forming a large number of modified lysosomes stuffed with proteins: perforin and several types of granzyme. They are helped in these activities by helper T cells that secrete stimulatory cytokines like IL-21.
• Most of these CTLs will die (of apoptosis) when they have done their job, but some (especially those that have received "help" from helper T cells) will become memory cells — long-lived cells poised to respond to the antigen if it should reappear.
An example will show the beauty and biological efficiency of CTLs.
Every time you get a virus infection, say influenza (flu), the virus invades certain cells of your body (in this case cells of the respiratory passages). Once inside, the virus subverts the metabolism of the cell to make more virus. This involves synthesizing a number of different macromolecules encoded by the viral genome. In due course, these are assembled into a fresh crop of virus particles that leave the cell (often killing it in the process) and spread to new target cells. Except while in transit from their old homes to their new, the viruses work inside of your cells safe from any antibodies that might be present in blood, lymph, and secretions. But early in the process, infected cells display fragments of the viral proteins in their surface class I molecules. CTLs specific for that antigen will be able to bind to the infected cell and often will be able to destroy it before it can release a fresh crop of viruses.
In general, the role of the CD8+ T cells is to monitor all the cells of the body, ready to destroy any that express foreign antigen fragments in their class I molecules. Some CD4+ T cells can develop into CTLs, but they can attack only those cell types (e.g. B cells, macrophages, dendritic cells) that express class II MHC molecules. Virtually every cell in the body expresses class I MHC molecules, so CD8+ CTLs are not limited in the targets they can attack.
Mechanisms of Killing
There are two different types of mechanisms.
Perforin/Granzyme Killing
CTLs have cytoplasmic granules that contain the proteins perforin and granzymes. When the CTL binds to its target, the contents of the granules are discharged by exocytosis. A dozen or more perforin molecules insert themselves into the plasma membrane of target cells forming a pore that enables granzymes to enter the cell. Granzymes are serine proteases. The two most abundant ones are
• Granzyme A. Once inside the cell, it enters the mitochondria and cleaves a subunit of complex I (the NADH dehydrogenase) of the electron transport chain producing reactive oxygen species (ROS) that kill the cell.
• Granzyme B. Once inside the cell, it proceeds to cleave the precursors of caspases thus activating them to cause the cell to self-destruct by apoptosis.
Both in structure and function, the interaction of CTL and its target resembles the synapses of the nervous system. The two cells are attached tightly at a small patch of plasma membrane. Special adhesion molecules hold them together. The granules are discharged only at that small portion of the plasma membrane (like the neurotransmitters released at a synapse). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4I%3A_Cytotoxic_T_lymphocytes_%28CTL%29.txt |
The human body can respond to antigen in many different ways. These fall into two major categories:
• antibody-mediated immunity. Antibodies — dissolved in blood, lymph, and other body fluids — bind the antigen and trigger a response to it. (This form of immunity is also called humoral immunity.)
• cell-mediated immunity (CMI). T cells (lymphocytes) bind to the surface of other cells that display the antigen and trigger a response. The response may involve
• other lymphocytes
• any of the other white blood cells (leukocytes)
Examples of Cell-Mediated Immunity
Delayed-Type Hypersensitivity (DTH): the tuberculin test
Many states in the United States require that professors and teachers (among others) be checked periodically for tuberculosis. This chronic disease, caused by Mycobacterium tuberculosis, evokes an immune response that, unfortunately, does not cure the patient, but does provide an inexpensive test for the disease called the tuberculin test (or Mantoux test). A tiny amount of protein, extracted from the bacteria, is injected into the skin. If the subject is currently infected, or has ever been infected, with the bacteria, a positive test results. In 24 hours or so, a hard, red nodule develops at the site of the injection. This nodule is densely packed with lymphocytes and macrophages.
In Europe, most people produce a positive tuberculin reaction, not because they have had the infection, but because earlier they had been vaccinated against tuberculosis with a preparation of a related (but harmless) bacterium called BCG.
The response to tuberculin is called "delayed" because of the time it takes to occur (in contrast to the "immediate" responses characteristic of many antibody-mediated sensitivities like an allergic response to a bee sting).
DTH is a cell-mediated response (in fact, anti-tuberculin antibodies are rarely found in tuberculin-positive people). The T cells responsible for DTH are members of the CD4+ subset.
Contact Sensitivity
Many people develop rashes on their skin following contact with certain chemicals. Nickel, certain dyes, and the active ingredient of the poison ivy plant are common examples. The response takes some 24 hours to occur, and like DTH, is triggered by CD4+ T cells. The actual antigen is probably created by the binding of the chemical to proteins in the skin. After the antigen is engulfed by dendritic cells in the skin, they migrate to nearby lymph nodes where they present fragments of the antigen to CD4+ T cells. The activated T cells migrate from the lymph nodes to the skin to elicit the inflammatory response.
Killing intracellular parasites
Some human pathogens avoid exposure to antibodies by taking up residence within cells. These include all viruses (discussed in the next section), and some bacteria such as
• the bacterium that causes Legionnaires's disease
• Listeria monocytogenes, that humans sometimes acquire from contaminated food and even some protozoans.
These microorganisms are engulfed by phagocytic cells, like macrophages, but evade the normal intracellular mechanisms that should destroy them. However, the macrophages can present fragments of antigens derived from these parasites. These are displayed in the class II histocompatibility molecules of the macrophages. CD4+ T cells responding to these epitopes release lymphokines that stimulate the macrophages sufficiently that they can now begin to destroy the organisms. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4J%3A_Cell-Mediated_Immunity.txt |
New parts for old
Many organs and tissues are now routinely transplanted from one human to another. Except for the rare cases where the donor and recipient are monozygotic ("identical") twins, such grafts are called allografts.
• kidney
• Living donors can be used because they have two kidneys and can get along with only one.
• heart
• For patients with failing hearts (often because of inherited defects). Only cadavers can be used as donors.
• lungs
• Usually transplanted along with a heart. Some attempts have been made with portions of lungs from living donors.
• pancreas
• For people with Type 1 diabetes mellitus.
• liver
• For irreversible liver failure (e.g., from toxins, hepatitis B infection).
• skin
• For burns; usually taken from elsewhere on the patient's own body.
• cornea
• To restore sight; taken from cadavers.
• blood
• To temporarily restore blood volume.
• bone marrow
• As a source of blood ("hematopoietic") stem cells to repopulate the patient's own marrow that is
• congenitally deficient in its ability to make one or more kinds of blood cells — Example: severe combined immunodeficiency (SCID)
• has been destroyed by cancer therapy.
• cord blood
• Blood drained (through the umbilical cord) from the placenta of newborn infants. A convenient source of blood stem cells.
• ovary
• Has restored fertility and produced healthy babies but so far only when donor and recipient were monozygotic (identical) twins.
The Problems
• Graft rejection
• The patient's immune system "sees" an allograft as foreign (antigenic) and mounts an immune response against it.
• Graft-versus-host disease (GVHD)
• T cells in the graft "see" the tissues of the recipient as foreign antigens and mount an immune attack against them. This a particularly serious problem with grafts of bone marrow because of the many T cells in it.
• Infections
• Attempts to suppress the immune response to avoid graft rejection and GVHD weaken the ability of the body to combat infectious agents (bacteria, viruses, fungi).
• More rarely, the donated organ may be infected and transmit the agent to the recipient. Tuberculosis, rabies, syphilis, hepatitis B, HIV-1, and several other diseases have been transmitted in this way. Potential organ donors are now routinely tested for evidence of infection by HIV-1 and -2, HTLV-1 and -2, hepatitis B and C (HBV, HCV), human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) as well as by Treponema pallidum (syphilis).
• Cancer
• Suppressing the host's immune responses also increases the risk of cancer.
Coping with the Immunological Problems
• Use the patient's own tissue when possible (skin, bone marrow, blood vessels).
• Use tissue from an "identical" (monozygotic) twin in the very rare cases that one is available. Being genetically identical, the recipient sees the transplant as "self", not as foreign, and does not mount an attack against it. The first successful kidney transplants (done in the mid 1950s) were between identical twins, and both donors and recipients went on to lead normal lives.
• Use an "immunologically privileged" site. These are parts of the body where the immune system is prevented from mounting an attack. They include the eye, testes, and brain, but only the eye's privileged status has so far been exploited (for corneal grafts).
• Use a relative, preferably a sibling, as the donor. While never identical, they may have inherited some of the same histocompatibility antigens so the recipient's immune response may not be as strong as it otherwise would be.
• Tissue-typing. Determine the histocompatibility antigens of both recipient and potential donor and use the organ with the fewest mismatches.
• Immunosuppression. Use immunosuppressive agents to blunt the recipient's immune response. Invariably required for all allografts.
Tissue Typing
The strongest antigens expressed by tissues are the class I and class II histocompatibility molecules. These are encoded by an array of genes on chromosome 6 called the major histocompatibility complex (MHC).
Class I molecules consist of a transmembrane protein to which are attached (noncovalently), a molecule of beta-2 microglobulin, and a short peptide. The class I transmembrane proteins are encoded by three loci: HLA-A, HLA-B, and HLA-C. Class I molecules are expressed at the surface of almost all the cells of the body (except for red blood cells and the cells of the central nervous system).
Class II molecules consist of two transmembrane polypeptides: an alpha (α) chain and beta (β) chain between which is nestled (noncovalently) a short peptide. The alpha and beta chains are encoded by clusters of loci in the region of chromosome 6 designated HLA-D. Unlike class I molecules, class II molecules are expressed on only a few types of cells, chiefly antigen-presenting cells (APCs) such as dendritic cells and macrophages, as well as other cells where inflammation is occurring.
Why so many MHC alleles
The genes of the MHC are the most polymorphic known. The graphic above shows the latest counts of alleles found at each locus in the human population. Of course, any one human can inherit a maximum of two alleles at each locus. The diversity of alleles in the population makes possible thousands of different combinations. In a study of 1000 blood and organ donors in San Francisco that were typed for HLA-A and HLA-B,
• Over half the group had a combination that was unique.
• Another 111 donors had a set of these molecules that they shared with only one other person in the group.
• The most frequent phenotype (HLA-A1, HLA-A3, HLA-B7, and HLA-B8) was found in 11 donors.
The extreme polymorphism of the MHC did not evolve to frustrate transplant surgeons and their patients.
Why, then?
The function of class I and class II molecules is to "present" antigenic peptides to the T cells of the immune system. The peptides — usually about 9 amino acids long — are bound by noncovalent forces in a groove at the surface of the MHC molecule. These peptides can include fragments of protein antigens derived from intracellular pathogens (e.g. viruses). Different pathogens generate different antigenic fragments. Different MHC molecules differ in the efficiency with which they bind particular sequences of amino acids in these fragments. Therefore, we might expect that some MHC products would be better than others at presenting pathogen antigens to the immune system.
One piece of evidence: People infected with the human immunodeficiency virus (HIV-1) who have one particular HLA-B molecule are more resistant to developing a full-blown case of AIDS than those with other HLA-B molecules (even though some of these differ by only a single amino acid).
Another piece of evidence: Wegner and colleagues reported in the 5 September 2003 issue of Science the results of a direct test of this hypothesis. They exposed groups of sticklebacks — differing in the number of their class II alleles — to three types of parasite (all at once). They used two species of parasitic nematode and one species of trematode). Those fish with 5–6 different alleles resisted infection better than those with fewer (or more).
So it may well be that the great diversity of class I and class II alleles in the human population has helped ensure that no single pathogen can wipe out the entire population.
Techniques of tissue typing
Most tissue typing is done using serological methods: antibodies specific for those HLA antigens that have been identified in the human population. A reaction between cells of the subject and, for example, anti-HLA-A28 antibodies and HLA-A9 antibodies — but no other antibodies — establishes the phenotype. At the present time, routine typing is limited to establishing the phenotype at HLA-A, HLA-B, and HLA-DR.
Coming into wider use is DNA typing, especially for HLA-D antigens. It promises to improve the sensitivity and specificity of tissue typing. The totals of numbers of alleles at each HLA locus given in the graphic above are based on DNA typing.
How useful is tissue typing?
So what hope do these data hold for the dialysis patient awaiting a kidney transplant? If the patient has a large family of willing donors, the odds for a good match are not bad because of the tight linkage of the HLA loci.
Assuming that no crossing over occurs within the HLA region of either the mother's or the father's two number 6 chromosomes, there are four possible combinations in which they may transmit their alleles to their children. So even if the parents carry different alleles at each locus (which is often the case), there is a 1:4 probability that any one of their children will be an exact match with any other. (Only the HLA-A and HLA-B antigens are shown here, but the tight linkage of the entire HLA region makes it likely that all the loci on each chromosome will be passed on as a block.)
But most organs are transplanted from cadavers to complete strangers. In the United States, the program is monitored by the United Network for Organ Sharing (UNOS). Tissue typing is usually limited to looking for 6 HLA antigens: the two each at HLA-A, HLA-B, and HLA-DR. If only one antigen is found at a locus, it means that either the tissue is homozygous for that allele or no reagent exists yet to detect the second allele. This table shows the results of one study of several thousand kidney recipients.
Number of HLA
mismatches
% kidneys surviving
after 5 years
0 68
1 61
2 61
3 58
4 58
5 57
6 56
The results tell us that:
• Having no mismatches provides a clear, but modest, advantage over mismatched kidneys. (This advantage is cumulative: at 17 years, 50% of the kidneys with no mismatches are still functioning while 50% of those with one or more mismatches have been lost after 8 years.)
• However, the incremental disadvantage of additional mismatches is small. In fact, the procedures to prevent rejection are now sufficiently good that ~90% of all kidneys — even those with all loci mismatched — can be expected to be functioning at the end of the first year.
Minor histocompatibility antigens
Even if it were possible to match donor and recipient at every locus of the MHC, some tissue incompatibility would still remain (except between identical twins). Few of the antigens responsible have been identified, but they include:
• H-Y, an antigen encoded on the Y chromosome and thus present in male, but not female, tissue
• HA-2, an antigen derived from the contractile protein myosin.
The number and variety of histocompatibility antigens tell us that probably no two humans (again, except for identical twins) exist on earth with perfectly compatible tissues. Therefore successful transplantation of allografts requires some degree of immunosuppression to avoid graft rejection.
Graft-versus-host disease (GVHD)
Allografts that contain T cells of the donor can cause graft-versus-host disease (GVHD). The T cells in the transplant see the host as "foreign" and proceed to mount a widespread attack against the tissues of the host. GVHD is an especially serious problem with grafts of bone marrow (the source of all blood cells) and cord blood. Even when the donor and recipient have identical HLA alleles, grafts of bone marrow often cause GVHD because of differences in their minor histocompatibility antigens.
Some cancer patients are now deliberately treated so vigorously with radiation and chemotherapy that their bone marrow is destroyed along with their cancer cells. In order to survive, these patients must be given stem cells to repopulate their marrow after their therapy. In some cases, their own bone marrow is used. Some of it is removed prior to the onset of treatment of the patient and is itself treated to remove any cancer cells that may be lurking in it. If allografted bone marrow is required, there is a strong danger of GVHD. If the GVHD can be controlled, the stem cells should eventually establish themselves in the bone marrow of their new host and in due course generate some or even all of the patient's blood cells. Cord blood — another source of stem cells — presents less of a risk of serious GVHD even from a donor with HLA molecules not present in the recipient. This is because cord blood does not contain any mature T cells.
In mice, GVHD can be minimized by injecting large numbers of regulatory T cells, but for humans, control of GVHD — like control of graft rejection — still depends on the use of immunosuppression.
Immunosuppression
Immunosuppression is the treatment of the patient with agents that inhibit the immune response. The following is the list of immunosuppressants currently used.
Purine analogs
These are relatives of the purines used in DNA synthesis. Because they interfere with DNA synthesis, they interfere with the rapid cell proliferation needed for immune responses. Azathioprine (trade name = Imuran) is a widely-used purine analog.
Unfortunately, these drugs also interfere with the many other tissues that depend on rapid cell division (e.g., lining of the intestine, hair follicles) so they have many unpleasant side effects. Therefore, the search for agents that specifically target immune cells goes on.
Corticosteroids
These relatives of cortisol interfere with a transcription factor needed to turn on the genes for T cells to become activated. Prednisone and prednisolone are most commonly used.
Tacrolimus (Prograf®) and cyclosporine (Neoral®)
These are natural products isolated from microbial cultures. They inhibit the signaling pathway used by T cells to turn on their genes for activation, e.g., for IL-2 secretion.
Rapamycin
This is a macrolide antibiotic produced by an actinomycete found on Easter Island (which the inhabitants call Rapa Nui — hence the name). Rapamycin inhibits T cell proliferation, and shows great promise in reducing the problems of transplant rejection.
Rapamycin is also known as sirolimus and is sold under the trade name Rapamune.
Mycophenolate mofetil
This small molecule inhibits an enzyme needed by B and T cells for purine synthesis. Other types of cells are not dependent on the enzyme so side effects are mild. The trade name is CellCept.
Antithymocyte globulin (ATG)
This preparation contain antibodies — raised in horses or rabbits — directed against T cells.
Monoclonal antibodies
Several preparations are now used:
• Muromonab-CD3 (OKT3) and two humanized anti-CD3 monoclonals. They bind to the CD3 molecule on the surface of T cells.
• Daclizumab and basiliximab. Target the IL-2 receptor and thus inhibit only activated T cells.
• Alemtuzumab (Campath-1H®). Binds to CD52, a molecule found on lymphocytes and depletes both T cells and B cells.
Belatacept
This is a protein, produced by recombinant DNA technology, that combines
• the extracellular portion of CTLA-4 ("cytotoxic T-lymphocyte-associated antigen 4", one of the ligands for B7) with
• the Fc region (the C-terminal two-thirds of the constant region) of a human IgG1 antibody.
It blocks the "Signal Two" needed to activate T cells.
Side effects of immunosuppression
They are serious.
Infections
The immune system is vital to protect us against infectious agents (bacteria, viruses, fungi). So infection is a frequent side effect of immunosuppression in transplant recipients. Fortunately, the infections can usually be controlled by the appropriate antibiotic, antiviral drug, etc.
Cancer
5% or more of transplant recipients will develop cancer within a few years of receiving their allograft. This may not seem to represent a great risk, but it does represent a 100-fold increase in risk compared to the general population. Allograft recipients are particularly prone to developing skin cancers and lymphomas. Curiously, transplant recipients do not seem to be at any greater risk for developing the most common types of cancer in the rest of the population: cancers of the lung, breast, colon, and prostate. One exception: recipients of allografted bone marrow run a slightly, but significantly, higher (2–3 fold) risk of developing these types of tumors. In most cases, these cancers arise from a cell of the host. But in some cases of melanoma and Kaposi's sarcoma the cancer cells were present in the graft and proliferated in the immunosuppressed host.
Things that can be done to help in such cases include stopping the immunosuppression. The chief culprit seems to be the immunosuppression that these patients have been receiving. In most cases,this leads to regression of the cancer, but often rejection of the transplant as well. The choice is usually clear for patients with allografted kidneys; they can go back on dialysis and anticipate receiving another kidney at a future date. But what of the cancer patient with a heart transplant?
Future prospects for transplantation
Although organ transplants have helped thousands of people, much remains to be done. In particular, ways need to be found to
• increase the number of available organs (the need now far exceeds the supply)
• find more precise methods of immunosuppression in order to prevent rejection without the dangerous side effects of infection and cancer.
Both these problems may be helped by xenotransplantation.
Xenotransplantation
Xenotransplantation is the use of organs from other animals. A number of attempts have been made to use hearts, livers, and kidneys from such primates as chimpanzees and baboons — so far with limited success. One reason is that xenotransplants usually are attacked immediately by antibodies of the host resulting in hyperacute rejection. But perhaps the use of pigs as organ donors will be feasible.
• Their organs are about the right size for use in humans.
• They can be made transgenic for molecules that may circumvent
• hyperacute rejection (by knocking out the genes responsible for cell-surface antigens that humans have preformed antibodies against);
• the chronic, T-cell-mediated, rejection that plagues all allografts.
• They can be produced in the numbers needed.
However, pigs contain retroviruses (called PERV = porcine endogenous retrovirus), and there is fear that these might infect the human recipient.
Only a few transplants of pig tissue into humans have been done to date: skin grafts and grafts of pancreatic islets. A larger number of people have been temporarily hooked up to pig organs or "bioreactors" containing pig cells to provide support for their failing spleen, liver, or kidneys. Most of these recipients have been monitored for signs of infection by PERV and — even though PERV can infect human cells growing in culture — there is no evidence that any of these people exposed to pig tissue have become infected.
Is xenotransplantation safe?
Pigs are not the only animals that contain latent viruses in their cells. Could the viruses in the cells of other kinds of animal donors infect the transplant recipient? start an epidemic? The danger seems greater for xenotransplants from other primates. (HIV, the retrovirus that causes AIDS appears to have entered humans from a primate host)
For these reasons, many biologists are urging that transplant surgeons proceed cautiously with xenotransplants.
Immune Privilege
It has long been know that certain sites in the body, for example,
• the anterior chamber of the eye
• the testes
• the brain
are "privileged". They are protected from attack by the immune system.
This can cause problems. Several cases have emerged of survivors of Ebola who no longer have Ebola virus in their blood and are symptom-free but still retain live virus in such privileged sites as brain, testis, and aqueous humor of the eye where the virus has escaped attack by the immune system.
Many factors are involved in immune privilege, such as
• tight junctions between the cells of the tissue
• little expression of class I histocompatibility molecules
• expression of the Fas ligand, FasL.
The presence of FasL on their surface protects cells in privileged locations from immune attack because when threatened by a cytotoxic T cell, they force the T cell to commit suicide by apoptosis. Activated cytotoxic T cells express Fas on their surface. When they engage (with their T cell receptor) a privileged cell expressing FasL, instead of killing the target, the target kills them!
So if the organs of transgenic pigs can be made to expresses human FasL, perhaps they will be resistant to T-cell mediated rejection.
The human placenta also enjoys immune privilege. It is almost as foreign to the mother as a kidney transplant from her husband would be, but unlike the kidney, she will not reject it (at least not for 9 months). In lab rats, the embryos (and the mother's endometrium) secrete corticotropin-releasing hormone (CRH). This hormone induces the expression of Fas ligand (FasL) on the cells of the placenta. Activated T cells express Fas, so any threatening T cells would commit suicide by apoptosis when they encounter FasL on their target. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4K%3A_Organ_Transplants.txt |
Transplants of Hematopoietic Stem Cells
These can be:
• autologous — from hematopoietic stem cells that were
• removed from the patient before cancer therapy began,
• stored alive,
• and, if there were cancer cells in the bone marrow (the case with multiple myeloma and leukemias), treated to "purge" them. Most failures of autologous stem cell transplants occur because of failure to get all the cancer cells out of the harvested cells rather than failure to eliminate them from the patient.
• allogeneic — hematopoietic stem cells removed from someone else, often a close relative. Another source of hematopoietic stem cells is cord blood — blood drained (through the umbilical cord) from the placenta of newborn infants.
Allogeneic stem cells
• avoid the problem of lurking residual cancer cells but
• should be closely matched to the major histocompatibility loci (MHC) of the patient. If not, the donor cells will attack the recipient causing often-fatal graft-versus-host disease (GVHD). Even with an exact match at the MHC, some GVHD is likely.
In one remarkable case, an AIDS patient with leukemia was given a bone marrow transplant from a donor whose cells did not express a functional version of CCR5 — a coreceptor needed by HIV to infect T cells. Two years later, the patient was not only cured of his leukemia but of AIDS as well. Autologous hematopoietic stem cell transplants also show promise of being an effective treatment for the autoimmune disorder systemic lupus erythematosus (SLE). If the patient's own marrow was not completely destroyed, the donor lymphocytes and the patient's lymphocytes can exist together. Then a later infusion of the donor's T cells may be able to kill off all the patient's remaining malignant cells leaving the patient with a bone marrow that produces donor-type cells exclusively. So hematopoietic stem cell transplants (HSCT) can be life-saving but create their own problems. (Another example: an "immediate"-type allergy like hay fever or asthma of the donor can create the same allergy in the recipient.)
15.4M: Antigen Presentation
Antigens are macromolecules that elicit an immune response in the body. Antigens can be proteins, polysaccharides, conjugates of lipids with proteins (lipoproteins) and polysaccharides (glycolipids). Most of this page will describe how protein antigens are presented to the immune system. The presentation of lipid and polysaccharide antigens will be mentioned at the end. It will be helpful to distinguish between two limiting cases.
Antigens that enter the body from the environment; these would include inhaled macromolecules (e.g., proteins on cat hairs that can trigger an attack of asthma in susceptible people), ingested macromolecules (e.g., shellfish proteins that trigger an allergic response in susceptible people), and molecules that are introduced beneath the skin (e.g., on a splinter or in an injected vaccine). Alternatively, antigens can be generated within the cells of the body; these would include proteins encoded by the genes of viruses that have infected a cell and aberrant proteins that are encoded by mutant genes; such as mutated genes in cancer cells. In all cases, however, the initial immune response to any antigen absolutely requires that the antigen be recognized by a T lymphocyte ("T cell"). The truth of this rule is clearly demonstrated in AIDS: the infections (viral or fungal or bacterial) that so often claim the life of AIDS patients do so when the patient has lost virtually all of his or her CD4+ T cells. The two categories of antigens are processed and presented to T cells by quite different mechanisms.
Exogenous Antigens
Exogenous antigens (inhaled, ingested, or injected) are taken up by antigen-presenting cells (APCs). These include phagocytic cells like dendritic cells and macrophages and B lymphocytes ("B cells") which are responsible for producing antibodies against the antigen. Antigen-presenting cells
• engulf the antigen by endocytosis
• endosome fuses with a lysosome where the antigen is degraded into fragments (e.g. short peptides)
• these antigenic peptides are then displayed at the surface of the cell nestled within a class II histocompatibility molecule.
• they may be recognized by CD4+T cells
The Class I Pathway
Class I histocompatibility molecules are transmembrane proteins expressed at the cell surface. Like all transmembrane proteins, they are synthesized by ribosomes on the rough endoplasmic reticulum (RER) and assembled within its lumen. There are three subunits in each class I histocompatibility molecule:
• the transmembrane polypeptide (called the "heavy chain")
• the antigenic peptide
• beta-2 microglobulin
All of these must be present within the lumen of the endoplasmic reticulum if they are to assemble correctly and move through the Golgi apparatus to the cell surface. The Problem: proteins encoded by the genes of an infecting virus are synthesized in the cytosol. How to get them into the endoplasmic reticulum?
The Solution: TAP (= transporter associated with antigen processing).
• Viral proteins in the cytosol are degraded by proteasomes into viral peptides.
• The peptides are picked up by TAP proteins embedded in the membrane of the endoplasmic reticulum.
• Using the energy of ATP, the peptides are pumped into the lumen of the endoplasmic reticulum where they assemble with the transmembrane polypeptide and beta-2 microglobulin.
• This trimolecular complex then moves through the Golgi apparatus and is inserted in the plasma membrane.
• The complex can be bound by a T cell with a receptor (TCR) able to bind the peptide and flanking portions of the histocompatibility molecule (the hot dog in the bun) and CD8 molecules that bind the CD8 receptor (shown above as a gray hemisphere) on the histocompatibility molecule.
The Class II Pathway
Class II histocompatibility molecules consist of two transmembrane polypeptides and a third molecule nestled in the groove they form. All three components of this complex must be present in the endoplasmic reticulum for proper assembly. But antigenic peptides are not transported to the endoplasmic reticulum, so a protein called the invariant chain ("Ii") temporarily occupies the groove.
The steps:
• The two chains of the class II molecule are inserted into the membrane of the endoplasmic reticulum.
• They bind (in their groove) one molecule of invariant chain.
• This trimolecular complex is transported through the Golgi apparatus and into vesicles called lysosomes.
Meanwhile foreign antigenic material is engulfed by endocytosis forming endosomes. These also fuse with lysosomes. Then,
• The antigen is digested into fragments.
• The invariant (Ii) chain is digested.
• This frees the groove for occupancy by the antigenic fragment.
• The vesicles move to the plasma membrane and the complex is displayed at the cell surface.
• The complex can be bound by a T cell with
• a receptor (TCR) able to bind the peptide and flanking portions of the histocompatibility molecule (the hot dog in the bun) and
• CD4 molecules that bind the CD4 receptor (shown above as a yellow triangle) found on all class II histocompatibility molecules.
Interconnections Between the Class I and Class II Pathways
Cross-Presentation: Transferring Exogenous Antigens to the Class I Pathway
Cross-presentation is the transferring of extracellular antigens like bacteria, some tumor antigens, and antigens in cells infected by viruses into the class I pathway for stimulation of CD8+ cytotoxic T cells (CTL). Only certain "professional" antigen-presenting cells (APCs) like dendritic cells can do this - use the class I as well as the class II pathways of antigen presentation.
Cross-presentation following infection by viruses is important because:
• Most viruses infect cells other than APCs (e.g., liver cells, epithelial cells of the lung) (and, of course, are intracellular in these).
• While viral antigens displayed on the surface of any infected cell can serve as targets for cytotoxic T cells (CTLs),
• the lack of any costimulatory molecules on the cell surface makes them poor stimulants for the development of clones of CTLs in the first place.
However, when an infected cell dies, it can be engulfed by a professional APC, and the antigens within it can enter the class I pathway. One mechanism:
• The dead cell is engulfed by endocytosis.
• The endosome that forms fuses with a lysosome and degradation of the dead cell begins.
• Antigens pass into the cytosol and are degraded in proteasomes.
• The peptides formed are then are picked up by TAP and inserted into class I MHC molecules and displayed at the cell surface — along with the costimulatory molecules needed to start a vigorous clonal expansion of CD8+ cytotoxic T cells.
Diverting Antigens from the Class I to the Class II Pathway
Autophagy provides a mechanism by which cells can transfer endogenous (intracellular) antigens into the class II pathway, for example
• self-proteins so as to be able to delete CD4+ T cells with receptors capable of attacking them and thus potentially capable of causing autoimmunity
• proteins synthesized by an infecting virus. In this way viral infection can generate CD4+ T cells as well as cytotoxic T cells (CD8+)
B Lymphocytes: A Special Case
B lymphocytes are both antigen-receiving and antigen-presenting cells. They bind intact antigens (e.g., virus particles, proteins) with their B cell receptor (BCR). They can come in contact with these antigens by encountering them in the surrounding lymph or by being presented them by macrophages or dendritic cells. B lymphocytes process antigen by the class II pathway for presentation to T cells.
The process:
• B cells engulf antigen by receptor-mediated endocytosis
• The B cell receptors for antigen (BCRs) are antibodies anchored in the plasma membrane.
• The affinity of these for an epitope on an antigen may be so high that the B cell can bind and internalize the antigen when it is present in body fluids in concentrations thousands of times smaller than a macrophage would need.
• The remaining steps of antigen processing occur by the same class II pathway described above for macrophages producing
• fragments of antigen displayed at the cell surface nestled in the groove of class II histocompatibility molecules.
• A CD4+ T cell that recognizes the displayed antigen is stimulated to release lymphokines.
• These, in turn, stimulate the B cell to enter the cell cycle.
• Because of the part they play in stimulating B cells, these CD4+ T cells are called Helper T cells ("Th").
• The B cell grows into a clone of cells (called plasma cells)
• These synthesize receptors (BCRs) with the identical binding site for the epitope but without the transmembrane tail.
• These antibodies are secreted into the surroundings.
Lipid and Polysaccharide Antigens
Lipid Antigens
• Lipid antigens are presented to T cells by cell-surface molecules designated CD1 ("cluster of differentiation" 1).
• Antigen-presenting cells express several different forms of CD1 at their surface. Each is probably specialized to bind a particular type of lipid antigen (e.g. lipopeptide vs glycolipid).
• The exposed surface of CD1 molecules forms an antigen-binding groove much like that of MHC molecules except that
• the amino acids in the groove are more hydrophobic than those in MHC molecules.
• Like protein antigens, lipid antigens are also presented as fragments, i.e., as a "hot dog in a bun".
Polysaccharide Antigens
Some bacterial polysaccharides ingested by APCs
• can be degraded in their lysosomes
• and presented to T cells by MHC class II molecules.
The binding of a T cell to an antigen-presenting cell (APC) is by itself not enough to activate the T cell and turn it into an effector cell: one able to, for examples,
• kill the APC (CD8+ cytotoxic T lymphocytes [CTLs])
• carry out cell-mediated immune reactions (CD4+ Th1 cells)
• provide help to B cells (CD4+ Th2 cells)
In order to become activated, the T cell must not only bind to the epitope (MHC-peptide) with its TCR but also receive a second signal from the APC. The receipt of this second signal is called costimulation. Among the most important of these costimulators are molecules on the APC designated B7 and their ligand on the T cell designated CD28. The binding of CD28 to B7 provides the second signal needed to activate the T cell.
Although T cells may encounter self antigens in body tissues, they will not respond unless they receive a second signal. In fact, binding of their TCR ("signal one") without "signal two" causes them to self-destruct by apoptosis. Most of the time, the cells presenting the body's own antigens either
• fail to provide signal two or
• transmit an as-yet-unidentified second signal that turns the T cell into a regulatory T cell (Treg) that suppresses immune responses.
In either case, self-tolerance results. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4L%3A_Bone_Marrow_Transplants.txt |
Immunological responses, both antibody-mediated and cell-mediated involve close contact between a T cell and an antigen-presenting cell (APC).
Examples:
• Helper T cells (Th1) with "professional" antigen-presenting cells like dendritic cells and some macrophages
• Helper T cells (Th2) with B cells
• Cytotoxic T lymphocytes (CTLs) with their targets
• NK cells with their targets
TCR/MHC-peptide pair
• On the T cell, the T-cell receptor for antigen (TCR) bound to
• a major histocompatibility complex (MHC) molecule on the antigen-presenting cell (APC).
• For CD4+ T cells, the MHC molecules are class II, and the binding is aided by CD4.
• For CD8+ T cells (e.g., CTLs), the MHC molecules are class I, and the binding is aided by CD8.
• In both cases, the MHC molecule has an antigenic peptide nestled in an exterior groove (MHC-peptide).
• The TCR molecules are tethered by actin filaments in the cytoplasm.
• Several hundred TCR/MHC-peptide pairs are needed to stimulate a naive T cell to begin mitosis, but only 50 or so are needed to activate a "memory" T cell to do its work; that is, to become an effector T cell.
Costimulatory pairs
The costimulatory molecule CD28 on the T cell bound to its ligand, B7, on the APC.
General adhesion molecules
• Leukocyte Function-associated Antigen-1 (LFA-1), an integrin on the T cell, bound to
• InterCellular Adehesion Molecule-1 (ICAM-1) on the APC.
Cytokine receptors
Cytokine receptors also cluster in the synapse (not shown in the diagram) where they are exposed to cytokines secreted into the synapse.
Formation of an immunological synapse causes the T cell to
• become activated with various signal pathways turning on new gene transcription
• release, by exocytosis, the contents of its vesicles:
• Type 1 helper T cells (Th1): lymphokines like IFN-γ and TNF-β
• Type 2 helper T cells (Th2): lymphokines like IL-4, IL-5, IL-10, and IL-13 that stimulate B cells
• Cytotoxic T Lymphocytes (CTLs): cytotoxic molecules like perforin and granzymes that kill the target
15.4O: Dendritic Cells
Dendritic cells (DCs) get their name from their surface projections (that resemble the dendrites of neurons). They are found in most tissues of the body and are particularly abundant in those that are interfaces between the external and internal environments (e.g., skin, lungs, and the lining of the gastrointestinal tract) where they are ideally placed to encounter extrinsic antigens, including those expressed by invading pathogens.
Although there are several distinct subtypes of DCs, they all share these features:
• They are actively motile.
• They continuously sample their surroundings — ingesting antigens by endocytosis (using phagocytosis, receptor-mediated endocytosis, and pinocytosis).
• Many of these antigens are "self" antigens, e.g., dead cells, proteins in the extracellular fluid.
• But the antigens can also be foreign antigens, for example, bacteria that are resident in the body (e.g., in the colon) or that invade the body.
In either case, the ingested antigens are degraded in lysosomes into peptide fragments that are then displayed at the cell-surface nestled in class II MHC molecules.
Having ingested antigen in the tissue, they migrate to lymph nodes and spleen where they can meet up with T cells bearing the appropriate T-cell receptor for antigen (TCR).
What happens next depends on the nature of the antigen.
• Self antigens are presented to T cells without any costimulatory molecules. This interaction causes the T cells to divide for a brief time, but then they commit suicide by apoptosis and so cannot attack tissues of the body. The animal becomes tolerant to that antigen.
• Foreign antigens produce a different outcome. The dendritic cells becomes "activated' and begin to display not only
• the MHC-peptide complex for the TCR of the T cells but also
• costimulatory molecules, e.g. B7 which binds to CD28 on the T cell
The importance of dendritic cells in developing immunity to pathogens is dramatically shown in those rare infants who lack a functioning gene needed for the formation of dendritic cells. They are so severely immunodeficient that they are at risk of life-threatening infections.
What accounts for the activation of dendritic cells by foreign antigens but not by self antigens? Pathogens, especially bacteria, have molecular structures that
• are not shared with their host
• are shared by many related pathogens
• are relatively invariant; that is, do not evolve rapidly (in contrast, for example, to such pathogen molecules as the hemagglutinin and neuraminidase of influenza viruses).
These are called Pathogen-Associated Molecular Patterns (PAMPs)
Examples:
• the flagellin of bacterial flagella
• the peptidoglycan of Gram-positive bacteria
• the lipopolysaccharide (LPS, also called endotoxin) of Gram-negative bacteria
• double-stranded RNA. (Some viruses of both plants and animals have a genome of dsRNA. And many other viruses of both plants and animals have an RNA genome that in the host cell is briefly converted into dsRNA).
• unmethylated DNA (eukaryotes have many times more cytosines, in the dinucleotide CpG, with methyl groups attached).
Dendritic cells have a set of transmembrane receptors that recognize different types of PAMPs. These are called Toll-like receptors (TLRs) because of their homology to receptors first discovered and named in Drosophila.
TLRs identify the nature of the pathogen and turn on an effector response appropriate for dealing with it. These signaling cascades lead to the expression of various cytokine genes.
• Interleukin 12 (IL-12) drives the nearby T cells to become Th1 cells, which will provide help for cell-mediated immunity including attack against intracellular pathogens.
• IL-23, which promotes differentiation of the T cells into Th17 helper cells, which can deal with extracellular bacteria.
• IL-4, which promotes differentiation of the T cells into Th2 cells which provide help for antibody production by B cells.
Under other circumstances, activated dendritic cells may secrete TGF-β and IL-10, leading to the formation of regulatory T cells (Treg) that dampen immune responses.
Dendritic Cell Subsets
While all DCs share certain features, they actually represent a variety of cell types with different differentiation histories, phenotypic traits and, as outlined above, different effector functions.
Examples:
Myeloid Dendritic Cells
As their name implies, these cells ("mDCs") are derived from the same myeloid progenitors in the bone marrow that give rise to granulocytes and monocytes. They present antigen to T cells and activate the T cells by secreting large amounts of IL-12.
Plasmacytoid Dendritic Cells
These cells ("pDCs") get their name from their extensive endoplasmic reticulum which resembles that of plasma cells. However, unlike plasma cells that are machines for pumping out antibodies, pDCs secrete huge amounts of interferon-alpha especially in response to viral infections.
Plasmacytoid DCs have internal toll-like receptors:
• TLR-7 and TLR-8, which bind to the single-stranded RNA (ssRNA) genomes of such viruses as influenza, measles, and mumps.
• TLR-9, which binds to the unmethylated cytosines in the dinucleotide CpG in the DNA of the pathogen. (The cytosines in the host's CpG dinucleotides often have methyl groups attached.)
CD8+ vs. CD8− Dendritic Cells
These subsets are found in the mouse spleen.
• The CD8 subset presents antigen engulfed from the surroundings — using the class II pathway — to CD4+ helper T cells.
• The CD8+ subset can present extracellular antigens using the class I pathway as well. The peptide/MHC class I molecules are presented to CD8+ T cells which go on to become cytotoxic T lymphocytes (CTL). This phenomenon is called cross-presentation.
Dendritic cells can also present undegraded antigen to B cells; that is, antigen that has not been processed into peptide/MHC complexes.
Monocyte-derived Dendritic Cells (Mo-DCs)
Humans (and mice) have another population of dendritic cells that develop from blood-borne monocytes that have been exposed to Gram-negative bacteria (or their LPS). The LPS is detected by their TLR4 molecules. Mo-DCs can present antigen to both CD4+ T cells and CD8+ T cells (cross-presentation).
Ralph Steinman
Ralph Steinman, the pioneer in the study of dendritic cells, has provided striking visual evidence of the cellular interactions between antigen-presenting dendritic cells, T cells, and B cells. When spleen cells are cultured with antigen, tight clusters of cells form (see figure). The clustering occurs in two phases:
• an early phase (days 0–2) during which only the dendritic cells and T cells need to be present to form clusters
• a later phase (days 2–5) when antigen-primed B cells enter the cluster and differentiate into antibody-secreting cells
Pictures Courtesy of Ralph Steinman from K. Inaba et al., J. Exp. Med. 160:858, 1984
Homing
Some dendritic cells not only activate T cells to respond to a particular antigen but tell them where to go to deal with that antigen.
Two examples:
Antigens in the skin
• Dendritic cells engulf antigens in (or even on!*) the skin and, while doing so, convert calciferol (vitamin D3) present in the skin into calcitriol (1,25[OH]2 Vitamin D3).
• When they activate the appropriate T cells in a nearby lymph node, the calcitriol induces those T cells to express a surface receptor designated CCR10 (a member of the CC chemokine receptor family).
• CCR10 binds the chemokine CCL27 — which is present in the skin.
• So when these T cells reajavascript:void('Remove Anchor')ch the skin, they stop their travels and go to work there.
Antigens in the GI tract
• Dendritic cells in the lining of the intestine are always busy engulfing the many antigens present there.
• While doing so, they convert the abundant amount of retinol (vitamin A) there into retinoic acid.
• When they activate the appropriate T cells in a nearby lymph node, the retinoic acid induces those T cells to express another CC chemokine receptor designated CCR9.
• CCR9 binds the chemokine CCL25 present in the intestine.
• So when these T cells reach the intestine, they stop their travels and go to work. (CCL25 also attracts IgA-secreting B cells.)
Switching Homing Directions
Most vaccines are given by injection into muscle or skin. This works very well for inducing systemic immunity; that is, IgG antibodies in the blood able to attack pathogens (e.g., tetanus) that are present in the blood.
Injected vaccines do not work as well for illnesses caused by intestinal pathogens such as
• typhoid fever (caused by Salmonella typhi)
• cholera (caused by Vibrio cholerae)
However, a group of German immunologists reported in July 2011 that:
• whereas dendritic cells receiving antigens from injections under the skin influence T cells to migrate back to the skin (as we saw above),
• if these subcutaneous injections are accompanied by injections of retinoic acid, the T cells migrate to the intestine instead.
• CCR9+ plasma cells secreting antigen-specific IgA antibodies also appeared in the intestine.
Using this technique, these workers were able to protect their mice from mouse typhoid (Salmonella typhimurium) and cholera toxin.
15.4P: Passive Immunity
Immune Globulin (IG)
Horse and sheep proteins are foreign to the human patient and will, in due course, elicit an active immune response. This may lead to an allergic reaction such as systemic anaphylaxis or serum sickness. To avoid such problems, humans are often used as the source of passive antibodies.
• IG is also used to provide protection to boys with X-linked agammaglobulinemia, who are unable to manufacture antibodies because of a mutation in their single (because on their X chromosome) gene for Bruton's tyrosine kinase.
• hepatitis A ("infectious" hepatitis), measles, and rubella. Some immune globulin (IG) is prepared from the gamma globulin fraction of pooled plasma from the outdated blood of several thousand blood donors on the assumption that this large pool will contain good levels of antibodies against many common diseases such as
• Some preparations of immune globulin are harvested from selected individual donors who have either recently recovered from the disease or who have been deliberately and intensively immunized against it. These are used to provide immediate protection against such diseases as rabies, tetanus, varicella (chicken pox), and complications arising from giving the smallpox vaccine (vaccinia immune globulin or VIG).
• The recent need for an effective treatment for people with inhalational anthrax has led to the use of plasma donated by military personnel previously actively immunized with anthrax vaccine. Soon it should be possible to prepare a purified immune globulin from this plasma. Further down the road will be the use of antianthrax monoclonal antibodies.
• Rh immune globulin (RhIg) or Rhogam is used to prevent Rh-negative mothers from becoming sensitized to the Rh antigen of their newborn child.
Advantages of human immune globulin
The preparation contains fewer irrelevant serum proteins and of those that remain, being human proteins, they are far less immunogenic and are catabolized more slowly than horse proteins. However, care must be (and is) taken to ensure that the preparations are not contaminated with human pathogens such as the AIDS virus (HIV) or hepatitis viruses.
Non-antigen-specific effects of human immune globulin
Intravenous injections of IG have helped patients with such autoimmune disorders as
• immune hemolytic anemia
• immune thrombocytopenic purpura
• myasthenia gravis
The therapeutic effect seems to have nothing to do with the antigen specificities (e.g., antitetanus) of the antibodies in the preparation. Instead it is the C-region portion of the antibody molecules that provides the protection. Animal studies suggest that it does so by binding to a class of receptors on macrophages, which inhibits them from phagocytosing antibody-coated cells, e.g.,
• antibody-coated red cells in immune hemolytic anemia
• antibody-coated platelets in idiopathic thrombocytopenic purpura
The spleen is packed with macrophages and is where most of red blood cell and platelet destruction occurs in these diseases (and explains why removal of the spleen so often helps the patient). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4N%3A_The_Immunological_Synapse.txt |
The ability of a multicellular organism to defend itself against invasion by pathogens (bacteria, fungi, viruses, etc.) depends on its ability to mount immune responses. All metazoans (probably) have inborn defense mechanisms that constitute innate immunity. Vertebrates have not only innate immunity but also are able to mount defense mechanisms that constitute adaptive immunity. This table gives some of the distinguishing features of each type of immunity.
Innate Immunity Adaptive Immunity
Pathogen recognized by receptors encoded in the germline Pathogen recognized by receptors generated randomly
Receptors have broad specificity, i.e., recognize many related molecular structures called PAMPs (pathogen-associated molecular patterns) Receptors have very narrow specificity; i.e., recognize a particular epitope
PAMPs are essential polysaccharides and polynucleotides that differ little from one pathogen to another but are not found in the host. Most epitopes are derived from polypeptides (proteins) and reflect the individuality of the pathogen.
Receptors are PRRs (pattern recognition receptors) In jawed vertebrates, the receptors are B-cell (BCR) and T-cell (TCR) receptors for antigen
Immediate response Slow (3–5 days) response (because of the need for clones of responding cells to develop)
Little or no memory of prior exposure Memory of prior exposure
Occurs in all metazoans Occurs in vertebrates only
The Cells of the Innate Immune System
A variety of different types of cells participate in innate immunity. What they all have in common is that the receptors by which they recognize pathogens are limited in their specificity. This is in contrast to the B cells and T cells of the adaptive immune system that generate receptors — BCRs and TCRs respectively — that are exquisitely specific for the pathogen. The players:
• The several granulocytes of the blood and tissues
• neutrophils
• eosinophils
• basophils and mast cells
• monocytes and macrophages
• dendritic cells
• Innate Lymphoid Cells (ILCs). These are cells that look like lymphocytes but do not have the antigen receptors found on B lymphocytes (BCRs) and T lymphocytes (TCRs). They include cytotoxic Natural Killer (NK) cells and several subsets of non-cytotoxic cells (ILC1, ILC2, ILC3, etc.) each with it own pattern of cytokine secretion and favored targets.
Pathogen-Associated Molecular Patterns (PAMPs)
Pathogens, especially bacteria, have molecular structures that are not shared with their host and are shared by many related pathogens. They are relatively invariant; that is, do not evolve rapidly (in contrast, for example, to such pathogen molecules as the hemagglutinin and neuraminidase of influenza viruses).
Examples:
• the flagellin of bacterial flagella
• the peptidoglycan of Gram-positive bacteria
• the lipopolysaccharide (LPS, also called endotoxin) of Gram-negative bacteria
• double-stranded RNA. (Some viruses of both plants and animals have a genome of dsRNA. And many other viruses of both plants and animals have an RNA genome that in the host cell is briefly converted into dsRNA).
• unmethylated DNA (eukaryotes have many times more cytosines, in the dinucleotide CpG, with methyl groups attached).
Pattern Recognition Receptors (PRRs)
There are three groups:
1. secreted molecules that circulate in blood and lymph;
2. surface receptors on phagocytic cells like macrophages that bind the pathogen for engulfment;
3. cell-surface receptors that bind the pathogen initiating a signal leading to the release of effector molecules (cytokines).
Secreted PRRs
Example: Circulating proteins (e.g., C-reactive protein) that bind to PAMPs on the surface of many pathogens. This interaction triggers the complement cascade leading to the opsonization of the pathogen and its speedy phagocytosis.
Phagocytosis Receptors
Macrophages have cell-surface receptors that recognize certain PAMPs, e.g., those containing mannose. When a pathogen covered with polysaccharide with mannose at its tips binds to these, it is engulfed into a phagosome.
Toll-Like Receptors (TLRs)
Macrophages, dendritic cells, and epithelial cells have a set of transmembrane receptors that recognize different types of PAMPs. These are called Toll-like receptors (TLRs) because of their homology to receptors first discovered and named in Drosophila. Mammals have 12 different TLRs each of which specializes — often with the aid of accessory molecules — in a subset of PAMPs. In this way, the TLRs identify the nature of the pathogen and turn on an effector response appropriate for dealing with it. These signaling cascades lead to the expression of various cytokine genes. Examples:
• TLR-1: Forms a heterodimer with TLR-2 at the cell surface which binds to the peptidoglycan of Gram-positive bacteria like Streptococci and Staphylococci.
• TLR-2: With TLR-1, binds cell-wall components of Gram-positive bacteria.
• TLR-3: Binds to the double-stranded RNA of viruses engulfed in endosomes.
• TLR-4: Activated by the lipopolysaccharide (endotoxin) in the outer membrane of Gram-negative bacteria like Salmonella and E. coli O157:H7
• TLR-5: Binds to the flagellin of motile bacteria like Listeria.
• TLR-6: Forms a heterodimer with TLR-2 and responds to peptidoglycan and certain lipoproteins.
• TLR-7 and TLR-8: Form a heterodimer that binds to the single-stranded RNA (ssRNA) genomes of such viruses as influenza, measles, and mumps that have been engulfed in endosomes.
• TLR-9: Binds to the unmethylated CpG of the DNA of bacteria that have been engulfed in endosomes. (The cytosines in the host's CpG dinucleotides often have methyl groups attached.)
• TLR-11:In mice, it binds proteins expressed by several infectious protozoans (Apicomplexa) as well as, like TLR-5, to flagellin. Humans do not have TLR-11.
In all these cases, binding of the pathogen to the TLR initiates a signaling pathway leading to the activation of NF-κB. This transcription factor turns on many cytokine genes such as those for tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and chemokines, which attract white blood cells to the site. All of these effector molecules lead to inflammation at the site. And even before these late events occur, the binding of Gram-positive bacteria to TLR-2 and Gram-negative bacteria to TLR-4 enhances phagocytosis and the fusion of the phagosomes with lysosomes.
Innate Immunity can trigger Adaptive Immunity
This can occur in several ways:
Macrophages and dendritic cells are phagocytes and are also responsible for "presenting" antigens to T cells to initiate both cell-mediated and antibody-mediated adaptive immune responses.
• Digested fragments of the engulfed pathogen are returned to the cell surface nestled in the cavity of class II histocompatibility molecules.
• Gene transcription turned on by the interaction of PAMPs and TLRs causes transmembrane molecules called B7 to appear at the cell surface.
• T cells have a receptor for B7 called CD28.
• Simultaneous binding of
• CD28 to B7 and
• the antigen/class II complex to TCRs specific for it
• activates the T cell.
• This leads to repeated mitotic divisions producing clones of CD4+ T cells that can carry out cell-mediated immune responses and/or stimulate B cells to secrete antibodies of the appropriate specificity
Dendritic cells also engulf self-antigens, e.g., body cells that have died by apoptosis, but because these have no PAMPs associated with them, there is no second signal to activate the T cells.
The interaction of PAMPs and TLRs on dendritic cells causes them to secrete cytokines, including
• interleukin 12 (IL-12) which stimulates the production of Th1 cells
• interleukin 23 (IL-23) which stimulates the production of Th17 cells
• interleukin 6 (IL-6), which interferes with the ability of regulatory T cells to suppress the responses of effector T cells to antigen. A double-negative is a positive.
B cells are also antigen-presenting cells. They bind antigen with their BCRs and engulf it into lysosomes. They then transport the digested fragments to the cell surface incorporated in class II histocompatibility molecules just as macrophages and dendritic cells do. B cells also have TLRs. When a PAMP such as LPS binds the TLR, it enhances the response of the B cell to the antigen.
It has been known for many years that for vaccines to be effective, the preparation must contain not only the antigen but also materials called adjuvants. Several adjuvants contain PAMPs, and their stimulus to the innate immune system enhances the response of the adaptive immune system to the antigen in the vaccine. Pathogens coated with fragments of the complement protein C3 are not only opsonized for phagocytosis but also bind more strongly to B cells that have bound the pathogen through their BCR. This synergistic effect enables antibody production to occur at doses of antigen far lower than would otherwise be needed. Some workers feel that, in fact, adaptive immunity is not possible without the assistance of the mechanisms of innate immunity.
Antimicrobial Peptides
In addition to their innate pathogen-recognition systems, vertebrates (including ourselves), invertebrates (e.g., Drosophila), even plants and fungi secrete antimicrobial peptides that protect them from invasion by bacteria and other pathogens. In fact, probably all multicellular organisms benefit from this form of innate immunity. For humans, the best-studied antimicrobial peptides are the defensins, hepcidin and the cathelicidins
Defensins
All our epithelial surfaces
• skin
• lining of the GI tract
• lining of the genitourinary tracts
• lining of the nasal passages and lungs
are protected by defensins.
• Some defensins are secreted by the epithelial cells themselves; others by Th17 cells and neutrophils.
• Some are secreted all the time; others only in response to attack by pathogens. (In some cases their genes are turned on by activated TLRs.)
• They are synthesized from larger gene-encoded precursors which are
• cut to produce the active peptide.
• These range in length from 25 to 45 amino acids.
• In humans, they contain 6 invariant cysteines that form 3 disulfide bonds that assist in producing a secondary structure that consists of 3 strands of anti-parallel beta sheet.
• They attack the outer surface of the cell membrane surrounding the pathogen eventually punching lethal holes in it. (Unlike eukaryotes, the phospholipids in the outer membrane of bacteria carry a surplus of negative charges, and the positive charges on the defensins probably enable them to penetrate the bacterial membranes while sparing host membranes.)
Curiously, some defensins (β-defensin) also affect coat color (in dogs and mice) and in other ways mimic the effects of melanocyte-stimulating hormone (MSH).
Hepcidin
Hepcidin is a peptide of 25 amino acids with a secondary structure (beta sheet) like that of the defensins. It is secreted by the liver and controls the level of iron in the blood and ECF by regulating its release from intracellular stores. Hepcidin secretion is increased in response to invasion by pathogens (fungi and bacteria). Many of these require iron for their virulence and by blocking the release of iron into the blood, hepcidin starves them of this essential factor.
Cathelicidins
The best known human cathelicidin is LL37, a peptide of 37 amino acids synthesized by macrophages, neutrophils, adipocytes, and epithelial cells (providing antimicrobial protection to our skin and the lining of our urinary tract). Unlike the defensins, its secondary structure is alpha helix.
Like defensins, the gene for LL37 can be turned on by activated TLRs. In macrophages, for example, cathelicidin synthesis within the cell promotes killing of engulfed bacteria like M. tuberculosis, the agent of TB. Activation of the cathelicidin gene requires the presence of the active form of vitamin D (1,25 [OH]2 vitamin D3). This may explain:
• why people with a deficiency of vitamin D are more susceptible to tuberculosis;
• the physiological basis for the practice of exposing patients to sunlight in TB sanitariums (before the days of antibiotics).
Antimicrobial Peptides and the GI Tract
The contents of the GI tract (especially the colon) are loaded with bacteria. But most of these cause no trouble thanks to a variety of defenses. Among these is the barrier of antimicrobial peptides that exists from mouth to anus.
• The epithelium of the mouth and tongue is protected by a layer of antimicrobial peptides as well as those secreted in the saliva.
• The stomach is also protected by antimicrobial peptides (cut by pepsin from a larger precursor) as well as by the low pH of gastric juice.
• The liquefied contents that leave the stomach are quickly neutralized by the bicarbonate ions in the pancreatic fluid. However, any bacteria that survived the trip through the stomach (e.g., E. coli has a proton pump that enables it to survive the strong acid of the gastric juice) are kept in check by the antimicrobial peptides secreted by the Paneth cells of the small intestine. So, the contents of the small intestine normally contain only a small population of microbes.
• Not so for the large intestine (colon). The colon supports an enormous population (>1013) of microorganisms, but these seldom invade its lining thanks to
• a protective barrier of antimicrobial peptides as well as
• the protective actions of continuous stimulation of
• TLR-2s by Gram-positive commensals and
• TLR-4s by Gram-negative commensals
• The rectum is also protected by an epithelial barrier of antimicrobial peptides. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4Q%3A_Innate_Immunity.txt |
Sometimes the interaction of antibodies with antigen is useful by itself. For example,
• coating a virus or bacterium thus preventing it from binding to — and invading — a host cell (e.g., antipolio antibodies)
• binding to a toxin molecule (e.g., diphtheria or tetanus toxin) thus keeping the toxin from entering a cell where it does its dirty work
But most of the time, the binding of antibodies to antigen performs no useful function until and unless it can activate an effector mechanism. The complement system serves several effector roles. The complement system provides the actual protection from the response while the interaction of antibodies and antigen provides the specificity of the response. Put another way, antibodies "finger" the target, complement destroys it.
Features of the system
• The complement system consists of some 30 proteins circulating in blood plasma.
• Most of these are inactive until they are cleaved by a protease which, in turn, converts them into a protease.
• Thus many components of the system serve as the substrate of a prior component and then as an enzyme to activate a subsequent component.
• This pattern of sequential activation produces an expanding cascade of activity (reminiscent of the operation of the blood clotting system).
The Classical Pathway
The binding of antibody to its antigen often triggers the complement system through the so-called classical pathway. It can occur in solution or — as shown here — when the antibodies have bound to antigens on a cell surface.
The proteins of the classical pathway
C1
C1 exists in blood serum as a molecular complex containing:
• 6 molecules of C1q
• 2 molecules of C1r
• 2 molecules of C1s
The constant regions of mu chains (IgM) and some gamma chains (IgG) contain a binding site for C1q. A single molecule of IgM is enough to initiate the pathway. IgG is far less efficient, requiring several molecules to do so (6 is the optimum — the same as the number of C1q molecules in C1).
• Binding of C1q activates C1s and C1r.
• Activated C1s (a serine protease) cleaves two serum proteins:
• C4 is cleaved into a large fragment
• C4b, which binds covalently to sugar residues on cell-surface glycoproteins, and a smaller, inactive, fragment of
• C4a which diffuses away.
• C2 is cleaved into
• C2b, which binds noncovalently to a site on C4b, leaving a smaller, inactive, fragment of
• C2a which diffuses away.
• The complex of C4b•2b is called "C3 convertase" because it catalyzes the cleavage of C3. (C4b•2b is also a serine protease.)
C3
C3 is the most abundant protein of the complement system (~1.3 mg/ml). Because of its abundance and its ability to activate itself, it greatly magnifies the response.
• C4b•2b cuts C3 into two major fragments:
• C3b, which binds covalently to glycoproteins scattered across the cell surface. Macrophages and neutrophils have receptors for C3b and can bind the C3b-coated cell or particle preparatory to phagocytosis. This effect qualifies C3b as an opsonin.
• C3a This small fragment is released into the surrounding fluids. It can bind to receptors on basophils and mast cells triggering them to release their vasoactive contents (e.g., histamine). Because of the role of these materials in anaphylaxis, C3a is called an anaphylatoxin.
• Some of the C3b binds to molecules of C5 creating an allosteric change that exposes them to cleavage by C4b•2b (which is thus a "C3/C5 convertase".)
C5
Cleavage of C5 by the C3/C5 convertase initiates the assembly of a set of complement proteins that make up the membrane attack complex. (The membrane attack complex can also be formed by another C5 convertase produce by the "alternative pathway".)
The Membrane Attack Complex
Cleavage of C5 by the C3/C5 convertase, produces:
• C5a, which is released into the fluid surroundings where it
• is a potent anaphylatoxin (like C3a)
• is a chemotactic attractant for neutrophils
• C5b, which serves as the anchor for the assembly of a single molecule each of C6, C7 and C8.
• The resulting complex C5b•6•7•8 guides the polymerization of as many as 18 molecules of C9 into a tube inserted into the lipid bilayer of the plasma membrane. This tube forms a channel allowing the passage of ions and small molecules. Water enters the cell by osmosis and the cell lyses.
The electron micrograph (courtesy of Drs. J. H. Humphrey and R. Dourmashkin) shows holes punched through the cell wall of the Gram-negative bacterium Shigella dysenteriae by the terminal components of the complement system. (Some of the holes are larger than expected for C9 channels and probably were enlarged later by the action of lysozyme.)
Effector Functions of Complement
Cell lysis is only one function (and probably not the most important one) of the complement system. The complement system acts in several ways to mobilize defense mechanisms.
• Opsonization by C3b targets foreign particles for phagocytosis.
• Chemotaxis by C5a attracts phagocytic cells to the site of damage.
• This is aided by the increased permeability of the capillary beds mediated by C3a and C5a.
• The early complement components are also important for solubilizing antigen-antibody complexes assisting in their catabolism and elimination from the body. Failure of this function can lead to immune complex disorders.
• Lysis of antibody-coated cells. (In some cases, this causes more harm than good; complement-mediated lysis can cause such serious disorders as
• Rh disease
• immune hemolytic anemia
• immune thrombocytopenic purpura
• Promoting antibody formation. Breakdown of C3b generates a fragment (C3d) that binds to antigens enhancing their uptake by dendritic cells and B cells.
(The C3d.antigen complex binds to the same receptor on B cells that the Epstein-Barr virus (EBV) uses to gain entry into B cells — where it may cause mononucleosis and, sometimes, Burkitt's lymphoma.)
The Alternative Pathway
The complement system can also be triggered without antigen-antibody complexes. Even in their absence, there is a spontaneous conversion of C3 to C3b. Ordinarily the C3b is quickly inactivated: the C3b binds to inhibitory proteins and sialic acid present on the surface of the body's own cells, and the process is aborted. However, bacteria and other foreign materials that may get into the body lack these proteins and have little or no sialic acid.
So the C3b
• binds a protein called Factor B forming a complex of C3b•Bb.
• C3b•Bb is also a C3 convertase acting on more C3 to form:
• C3b•Bb•C3b, which is a C5 convertase and can start the assembly of the membrane attack complex.
• more C3b! This second function (shown here) creates a positive feedback loop, amplifying what might have started as a small reaction (the formation of C3b by either or both the classical and alternative pathways) into a massive production of C3b.
Regulation of Complement Activity
The explosive potential of the complement system requires that it be kept under tight control. At least 12 proteins are known that do this. Three examples:
• Factor H removes Bb from the alternative pathway C3 convertase breaking the positive feedback loop.
• Factor I inactivates C3b.
• C1 inhibitor (C1INH) binds to sites on activated C1r and C1s shutting down their proteolytic activity. So when C1 is activated by antigen-antibody complexes, there is only a brief interval during which it can cleave C4 and C2 before it is deactivated by C1INH.
Disorders of the Complement System
With so many proteins involved, it is not surprising that inherited deficiencies of one or another are sometimes encountered in humans. Four examples:
• C3. An inherited deficiency of C3 predisposes the person to frequent bouts of bacterial infections.
• C2. Curiously, immune complex disorders, not bacterial infections, are the main problem with a deficiency of C2 (or of one of the other "early" components like C1q, C1r, C1s, or C4). This emphasizes the important role of the complement system in clearing away antigen-antibody complexes. A deficiency of C2 (or one of the other early components) is frequently found in patients with the autoimmune disorder systemic lupus erythematosus (SLE).
• C9. Another curiosity: most people who cannot make C9 have no more of a problem with bacterial infections than those who can. Laboratory studies suggest that the C5b•6•7•8 complex by itself is able to lyze bacteria although not as efficiently as C9. And, in fact, a deficiency of C8 is associated with a sharply-increased risk of bacterial meningitis.
• C1INH. A deficiency of C1INH produces hereditary angioedema (HAE). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4R%3A_The_Complement_System.txt |
Inflammation is a response of a tissue to injury, often injury caused by invading pathogens. It is characterized by increased blood flow to the tissue causing increased temperature, redness, swelling, and pain.
A bacterial infection initiates inflammation through several interconnecting mechanisms:
• The "nonself" surface of bacteria allows the complement system to be activated through the "alternative pathway".
• Certain surface molecules of the bacteria, called Pathogen-Associated Molecular Patterns (PAMPs), bind to Toll-like receptors (TLRs) on or in a variety of leukocytes.
Mast Cells
Mast cells are found in the tissues. Their cytoplasm is loaded with granules containing mediators of inflammation. Their surface is coated with a variety of receptors which, when engaged by the appropriate ligand, trigger exocytosis of the granules. Mast cells appear to be key players in the initiation of inflammation.
• Their Toll-like receptors trigger exocytosis when they interact with PAMPs like
• the lipopolysaccharide (LPS or "endotoxin") of Gram-negative bacteria (TLR-4)
• the peptidoglycan of Gram-positive bacteria (bind TLR-2)
• Their receptors for complement fragments trigger exocytosis when they bind C3a and C5a bacteria coated with C3b
Activated mast cells release literally dozens of potent mediators. Some immediately as they discharge their granules, some later as they synthesize them by new gene transcription. These mediators are active in either (or, in some cases, both)
• recruiting all the types of white blood cell to the site
• monocytes that become macrophages when they leave the blood and enter the tissue
• neutrophils
• antigen-presenting dendritic cells
• all kinds of lymphocytes:
• B cells and T cells, leading to an adaptive immune response;
• NK cells (an effector cell in innate immunity).
• eosinophils
• activating many of these recruited cells to produce their own mediators of inflammation.
Tumor Necrosis Factor-alpha (TNF-α)
Large amounts of TNF-α are quickly released by stimulated mast cells. All the cells involved in inflammation have receptors for TNF-α and are activated by it to synthesize more on their own. This positive feedback quickly amplifies the response.
Tryptase
Tryptase is the most abundant protein released by mast cells. It is a serine protease. Like the mammalian enzyme trypsin, tryptase cleaves peptide bonds on the C-terminal side of arginines and lysines. It activates C3 of the complement system and probably supports inflammation in other ways as well.
Chemokines
These are chemotactic cytokines; that is, secreted proteins that attract other leukocytes into the area. Several have been identified.
Reactive Oxygen Species (ROS)
These are produced by activated phagocytes: macrophages and neutrophils. They are toxic for microorganisms but can also lead to tissue injury. ROS are described in detail on another page. Link to it.
Histamine
The granules of mast cells are loaded with histamine and their exocytosis releases this potent mediator. Histamine increases the blood flow to the area and the leakage of fluid and proteins from the blood into the tissue space. Thus the quick release of histamine produces the redness and swelling associated with inflammation.
Interleukin-1 (IL-1)
Macrophages, monocytes, and activated platelets are sources of this cytokine. IL-1 has both
• paracrine effects on cells in the vicinity
• causing them to produce tissue factor and thus triggering the blood clotting cascade
• stimulating the synthesis and secretion of a variety of other interleukins
• helping to activate T cells and thus initiate an adaptive immune response
• endocrine (hormonal) effects as it is carried in the blood throughout the body
• decreasing blood pressure
• inducing fever.
IL-1 causes fever by stimulating the release of prostaglandins, which act on the temperature control center of the hypothalamus.
Inflammasomes
IL-1 is synthesized from a larger precursor that is cleaved by a caspase (caspase-1). Caspase-1 is part of a multi-protein complex in the cytosol of macrophages and neutrophils called an inflammasome. Inflammasomes are activated by several different products produced by invading bacteria. Some of these are first "seen" by toll-like receptors (TLRs) thus providing a link between the innate immune system and inflammation.
Bradykinin
Bradykinin is a nonapeptide (9 amino acids). It is synthesized by proteolytic cleavage of an inactive precursor (a kininogen) that is produced by the liver and circulates at all times in the blood (one of the alpha-globulins).
Bradykinin relaxes the smooth muscle walls of the arterioles lowering blood pressure and increasing blood flow to the tissue and makes the capillaries leakier, allowing blood components to enter the tissue space. These effects (like those of histamine) produce the redness, warmth, and swelling of inflammation.
The process:
• Hageman factor (also known as clotting factor XII [12]) normally circulates in the blood as inactive precursor.
• When tissue damage allows blood to escape into the tissue space, Hageman factor comes in contact with the collagens in the tissue space and becomes activated.
• Activated Hageman factor is a serine protease that cleaves an inactive precursor called prekallikrein into another serine protease — kallikrein.
• Kallikrein then cleaves kininogen forming bradykinin.
Bradykinin also
• stimulates the release of nitric oxide
• stimulates phospholipase to increase the production of prostaglandins
• mediates the closing of the ductus arteriosus when a baby is born
• plays a major role in the dangerous swelling associated with hereditary angioedema (HAE)
Prostaglandins and Leukotrienes
These potent mediators of inflammation are derivatives of arachidonic acid (AA) a 20-carbon unsaturated fatty acid produced from membrane phospholipids. The principal pathways of arachidonic acid metabolism are
• the cyclooxygenase (COX) pathway, which produces prostaglandin H2 (PGH2). PGH2 serves as the substrate for two enzymatic pathways - one leading to the production of several prostaglandins (PG); the other leading to the production of thromboxane (Tx).
• the 5-lipoxygenase pathway, which produces a collection of leukotrienes (LT)
Acute Inflammation: The Good Side of Inflammation
The acute inflammatory response to tissue damage is of great value. By
• isolating the damaged area
• mobilizing effector cells and molecules to the site
• in the late stages - promoting healing,
inflammation protects the body. Its importance is demonstrated by the problems people with inherited defects in components of the process have with infections.
Some examples:
• a failure to produce reactive oxygen species (ROS) leads to chronic granulomatous disease (CGD)
• inherited defects in the ability to produce the later complement components (C5, C6, C7, C8, C9) increase the risk of certain infections.
Chronic Inflammation: The Bad Side of Inflammation
In chronic inflammation, the inflammatory response is out of proportion to the threat it is faced with or is directed against inappropriate targets. In the first case, the result can be more damage to the body than the agent itself would have produced.
Allergies and Autoimmune Diseases
All the many types of allergies and many of the autoimmune diseases are examples of inflammation in response to what should have been a harmless agent.
Some examples:
• Asthma
• Rheumatoid Arthritis (RA)
• Multiple Sclerosis (MS)
• Systemic Lupus Erythematosus (SLE)
In many of these cases, the problem is made worse by the formation of antibodies against self antigens or persistent antigens from smoldering infections. The antibodies complex with the antigens triggering the complement system with all its mediators of inflammation. The result: immune complex disorders.
Treating Inflammation
Inappropriate inflammation can be treated with
• steroids like the glucocorticoid cortisol
• nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen (e.g., Motrin®, Advil®).
• a number of therapeutic proteins produced by recombinant DNA technology.
NonSteroidal Anti-Inflammatory Drugs (NSAIDs)
The NSAIDs achieve their effects by blocking the activity of cyclooxygenases. The body produces two different forms of cyclooxygenase:
• COX-1, which is involved in pain, promoting clotting, and protecting the stomach
• COX-2, which is involved in the pain produced by inflammation.
In addition to reducing the fever and pain of inflammation, NSAIDs also inhibit clotting. They do this by interfering with the synthesis of thromboxane A2 in platelets. This is the reason that aspirin is given to patients undergoing angioplasty. Many people take a baby aspirin a day in the hope of avoiding heart attacks. But regular use of NSAIDs has a downside: a tendency to develop ulcers in the stomach and duodenum. Most of the NSAIDs inhibit both COX-1 and COX-2. However, some newer drugs, the so-called COX-2 inhibitors, such as rofecoxib (Vioxx®) and celecoxib (Celebrex®) are much more active against COX-2 than COX-1.
COX-2 inhibitors are effective against inflammation and avoid damage to the GI tract. But, unfortunately, they increase the risk of blood clots — which can cause heart attacks and strokes — because they do not block the synthesis of thromboxane A2 by platelets (which contain only COX-1). So people depending on NSAIDs for their heart protective effects must monitor any use of COX-2 inhibitors carefully.
In fact, because of the increased risk of heart attacks and strokes, the manufacturer of Vioxx® removed it from the market on 30 September 2004.
Therapeutic Proteins
Recombinant DNA and monoclonal antibody technology have produced some new therapies that are being enlisted in the battle against damaging inflammation.
• an IL-1 antagonist that binds and inactivates the IL-1 receptor.
• etanercept (Embrel®). A soluble version of the TNF-α receptor. It binds TNF-α preventing it from carrying out its many inflammatory actions. Potent but carries a severe risk of allowing infections to develop.
• recombinant protein C. To help the body dissolve the tiny clots that are triggered during inflammation.
• Infliximab (Remicade®). Binds to tumor necrosis factor-alpha (TNF-α). Shows promise against some inflammatory diseases such as rheumatoid arthritis (by blunting the activity of Th1 cells). Side-effects: can convert a latent case of tuberculosis into active disease; can induce the formation of autoantibodies (by promoting the development of Th2 cells).
In fact, all the more powerful anti-inflammatory agents (e.g., glucocorticoids) increase the risk of infection.
Sepsis and Septic Shock
On occasions, for reasons that are not entirely clear, the inflammatory response — usually to an infection by lipopolysaccharide (LPS)-bearing Gram-negative bacteria — spirals out of control progressing until it involves the entire body. This life-threatening development is called sepsis.
The circulatory system loses its integrity:
• There is a breakdown of the adherens junctions between the cells lining the capillaries allowing fluid to leak into the tissue spaces — edema.
• There is a breakdown in the control of blood clotting. What should have been a mechanism to help wall off an infected area and promote healing leads instead to a dangerous deposition of fibrin in small blood vessels throughout the body.
If these responses are massive, they can lead to septic shock
• a failure of many organs: lungs, kidneys, etc.
• a sharp drop in blood pressure
• death
Toxic Shock Syndrome
Some Gram-positive cocci can produce a similar condition, but here the eliciting agent is not LPS but a toxin liberated by the bacteria. In theory, anti-inflammatory agents should be useful in combating sepsis. But so far, only recombinant protein C has shown any promise (by inhibiting the formation of thrombin), and severe bleeding is a dangerous side-effect.
Inflammation and Cancer
Chronic inflammation is also a frequent cause of cancer. Liver cancer is often the sequel to years of inflammation caused by infection by hepatitis B and/or C viruses. Lung cancer often is the end stage of years of chronic inflammation caused by inhaled irritants, of which tobacco smoke is the most reliable. Cervical cancer can follow chronic infection and inflammation caused by papilloma viruses and chlamydiae. Chronic infection with the liver fluke Opisthorchis viverrini is responsible for many cases of bile duct cancer in Thailand and Laos. Bladder, colon, pancreas, stomach, and other cancers may similarly be the final stage of years of inflammation.
The strong link between chronic inflammation and cancer should not be surprising when you consider that the reactive oxygen species (ROS) liberated during inflammation are powerful DNA-damaging agents. There is increased mitosis in response to inflammation puts more cells at risk of mutations as they replicate their DNA during S phase. Apoptosis, the programmed death of damaged cells, is suppressed in inflamed tissue. So cells with precancerous genetic mutations, which should have committed suicide, live on to grow into a full-blown cancer. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4S%3A_Inflammation.txt |
The incidence of asthma in the United States (as well as in many other developed countries) has reached epidemic proportions. In the last two decades, the number of sufferers in the U. S. has doubled to more than 14 million people. In an attack of asthma, the bronchi become constricted, making it difficult to breathe in and — especially — out. The sufferer wheezes and coughs. Severe attacks can be life-threatening.
The Mechanism
An attack of asthma begins when an allergen is inhaled. The allergen binds to IgE antibodies — those that have binding sites for the allergen — on mast cells in the lungs. Binding triggers exocytosis of the mast cells with the release of histamine and leukotrienes. These substances cause the smooth muscle cells of the bronchi to contract narrowing the lumen of the bronchi. This is the early phase. They attract an accumulation of inflammatory cells — especially eosinophils — and the production of mucus. This is the late phase. With repeated attacks, the lining of the bronchi becomes damaged. Although asthma begins as an allergic response, in time attacks can be triggered by nonspecific factors like cold air, exercise, and tobacco smoke.
Some people are predispositioned to develop asthma
For reasons that are not yet understood, some people have a predisposition to respond to antigens by making antibodies of the IgE class. The trait tends to run in families suggesting a genetic component. These people are said to suffer from atopy. The T helper cells of atopic people are largely of the Th2 type rather that Th1. And mice whose genes for a transcription factor (called "T-bet") used to make Th1 cells have been knocked out make fewer Th1 and more Th2 cells and suffer the lung changes typical of human asthma even though not exposed to any known allergen. Th2 helper cells help B cells make IgE antibodies by synthesizing interleukin 4 (IL-4) and interleukin 13 (IL-13), which promote class switching. They also release interleukin 5 (IL-5) which attracts eosinophils and other inflammatory cells to the site, producing the late phase of the response.
Asthma - a disease of developed countries
No one knows for certain. It is certainly not a matter of air pollution. Air pollution can trigger attacks of asthma, but some regions with heavily-polluted air have a much lower incidence of asthma than regions with relatively clean air. One intriguing possibility: sanitation and widespread childhood immunization may enable children to avoid the infections — especially viral — that stimulate the immune system to respond with Th1 helper cells rather than Th2 cells. Children in Europe that give positive DTH responses to tuberculin (a response mediated by Th1 cells) have lower rates of asthma than children who are negative in the tuberculin test. European children growing up on farms where they are exposed to high levels of bacteria and fungi associated with farm animals have a lower incidence of asthma and atopy than their suburban peers. But children in tropical, undeveloped countries, who are often infected with parasitic worms, have high levels of Th2 cells and IgE but a very low incidence of asthma. Perhaps, then, a variety of chronic infections in childhood activate mechanisms (e.g., production of regulatory T cells) that suppress all inflammatory immune responses both Th1- and Th2-mediated.
Treatments
Beta-adrenergic agonists
• These drugs (albuterol is a popular example) mimic the action of adrenaline.
• They relax the smooth muscle of the bronchi.
• They may be inhaled or given by mouth.
• While useful in the early phase of an attack, they provide no protection against the longterm damage produced during the late phase.
Corticosteroids
• These drugs reduce the inflammation of the late phase of the response.
• They may be given in an inhaler (e.g., beclomethasone) or by mouth (e.g., prednisone)
Cromolyn sodium
Cromolyn sodium (disodium cromoglycate)
• inhibits exocytosis of mast cells thus blocking the release of histamine and leukotrienes
• is used mainly to prevent attacks (e.g., triggered by exercise) and is of no use in the early phase of an ongoing attack
Leukotriene inhibitors
Two types of leukotriene inhibitors received FDA approval in 1996.
• Zileuton (Zyflo®) blocks leukotriene synthesis by inhibiting the action of 5-lipoxygenase.
• Montelukast (Singulair®) blocks the leukotriene receptors on the surface of
• smooth muscle cells
• eosinophils
Possible future treatments still under investigation
• Anti-IgE antibodies. These interfere with the binding of IgE to mast cells. Omalizumab (Xolair®), a humanized monoclonal antibody produced by recombinant DNA technology, has been approved for use against allergic asthma (but carrying a "black-box" warning of the slight risk of its precipitating an anaphylactic reaction).
• Drugs that bind to IL-13 keeping it from promoting IgE synthesis.
• Treatments that stimulate the production of Th1 cells by the immune system. Injections of a harmless mycobacterium (a relative of the TB bacillus) might do the trick. Th1 cells secrete interferon-gamma which is a powerful inhibitor of Th2 cells.
Some of these treatments are already in clinical trials. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4U%3A_Asthma.txt |
AIDS stands for Acquired Immune Deficiency Syndrome. It represents the late stages of infection by a retrovirus called Human Immunodeficiency Virus (HIV). The immune deficiency is caused by the loss of the CD4+ T cells that are essential for both cell-mediated immunity and antibody-mediated immunity.
HIV - Human Immunodeficiency Viruses
They are retroviruses. There are two of them:
• HIV-1 — the major cause of AIDS throughout the world;
• HIV-2 — mostly found in West Africa.
Infection
HIV can only enter cells that express
• the transmembrane protein CD4 found on helper T cells
• a second G protein-coupled "coreceptor" (GPCR) on these cells:
• Strains of HIV (designated "R5") bind the coreceptor CCR5. These are the strains that are most infectious.
• Strains of HIV (designated "X4") bind the coreceptor CXCR4.
• Both strains usually coexist in an ongoing infection with X4 tending to dominate in the final stages of AIDS.
The virion binds to both CD4 and either coreceptor by means of molecules on its surface called glycoprotein 120 (gp120). The virion then is swept into the cell by receptor-mediated endocytosis. Fusion of the lipid membranes of the virion and the endosome - Endocytic vesicle, releases the contents of the virion into the cytosol. The fusion is mediated by gp41. When HIV infects a cell its molecules of reverse transcriptase and integrase are carried into the cell attached to the viral RNA molecules. The reverse transcriptase synthesizes DNA copies of the RNA. These enter the nucleus where the integrase catalyzes their insertion into the DNA of the host's chromosomes. The HIV DNA is transcribed into fresh RNA molecules which reenter the cytosol where some are translated by host ribosomes. The env RNA is translated into molecules of the envelope protein (gp160). These pass through the endoplasmic reticulum and then the Golgi apparatus where they become glycosylated by enzymes of the host cell.
Proteases of the host cell then cut gp160 into
• gp120 which sits on the surface of the virions (and is the target of most of the vaccines currently being tested).
• gp41, a transmembrane protein associated with gp120.
• the gag and pol genes are translated into a single protein molecule which is cleaved by the viral protease into
• 6 different capsid proteins
• the protease
• reverse transcriptase
• the integrase
• other RNA molecules become incorporated into fresh virus particles
Disease Transmission
HIV is present in body fluids especially blood and semen, especially in the early and late phases of the disease. Breaks or abrasions in mucous membranes and skin allow the virus in.
In North America, transmission occurs primarily
• between men when one ejaculates into the rectum (or mouth — the adenoids and tonsils are filled with dendritic cells) of the other
• among intravenous drug users who share needles
• in women who are the sexual partners of bisexual men or i.v. drug users
• in the newborn babies of these women
• in recipients of infected blood or blood products. This last category accounted for a devastating epidemic among hemophiliacs in the 1980s who unknowingly used HIV-contaminated preparations of factor 8 (VIII). In some areas, 90% or more of the hemophiliacs developed AIDS. That risk, and the risk from blood transfusions, is now virtually zero because
• all donated blood is now tested to see if the donor has been infected with HIV (as well as some other viruses)
• plasma-derived preparations of factors 8 (VIII) and 9 (IX) are now treated with heat and/or solvents to destroy any viruses that might be present;
• recombinant factor 8 (VIII) and recombinant factor 9 (IX) made by genetic engineering are now available.
Disease Progression
Infection by HIV produces three phases of disease:
• an early phase that
• lasts about 2 weeks
• is accompanied by fever, aches, and other flu-like symptoms
• is accompanied by high levels of virus in the blood.
• a middle phase with these features:
• lasts for months or even years
• produces few, if any, symptoms
• patient's blood contains few viruses, but contains antibodies to the virus which are the basis of the most common test for HIV infection
• continuous infection, death, and replacement of CD4+ T cells
• It is the late phase that is called AIDS. It has these features:
• A rapid decline in the number of CD4+ T cells. When these drop below about 350 per µl (normal is >1000), the patient's immunity is sufficiently weakened that opportunistic infections begin. These are infections caused by organisms that ordinarily do not cause disease symptoms in immunocompetent people. They include:
• viruses, e.g., herpes simplex, herpes varicella-zoster, Epstein-Barr virus (EBV)
• bacteria, e.g., Mycobacterium tuberculosis
• fungi, e.g. Candida albicans (the cause of "thrush"), Pneumocystis jirovecii (causes pneumonia)
• protozoans, e.g., Microsporidia
• When the CD4+ count drops below 200 per µl (mm3), opportunistic infections become more severe and cancer (e.g., lymphoma, Kaposi's sarcoma) may develop. Untreated, these usually kill the patient within a year or so.
Treatment
In affluent countries, the progression of HIV disease has been markedly slowed by the use of HAART (= Highly Active AntiRetroviral Therapy). This refers to combined therapy with three or more drugs, e.g., two that target the reverse transcriptase and one that targets the viral protease.
Reverse Transcriptase Inhibitors
• Nucleoside analogs. Examples:
• zidovudine (AZT)(Retrovir®)
• lamivudine (Epivir®)
• didanosine (Videx®)
Each of these drugs "fools" the reverse transcriptase into incorporating it into the growing DNA strand which then halts further DNA synthesis.
• Other Reverse Transcriptase Inhibitors
• These drugs, e.g., efavirenz (Sustiva®) inhibit the enzyme by other mechanisms.
Protease Inhibitors
These block the viral protease so that the proteins needed for assembly of new viruses cannot be cleaved from the large protein precursor. Examples:
• indinavir (Crixivan®)
• saquinavir (Invirase®)
• ritonavir (Norvir®)
Fusion Inhibitors
Fusion of the virion membrane with the endosome membrane involves noncovalent binding between two segments of the gp41 molecule designated HR1 and HR2. Enfuvirtide (Fuzeon®), a synthetic polypeptide containing 36 of the amino acids present in the HR2 segment, interferes with this process. It probably acts as a kind of competitive inhibitor, binding to HR1 thus preventing HR2 from binding HR1.
Integrase Inhibitors
Raltegravir (Isentress®), a drug that inhibits the HIV-1 integrase, has slowed disease progression in patients for whom other drugs were losing their effectiveness.
Inhibiting Coreceptor Binding
Several drugs — as well as some monoclonal antibodies — that block the binding of HIV to the coreceptors CCR5 and CXCR4 are being tested for safety and efficacy. Maraviroc®, a drug that binds to CCR5, has performed so well that it received FDA approval in 2007. People who have a mutation in their CCR5 gene are resistant to infection. Early clinical trials of gene therapy in which a patient's normal CCR5 gene is deliberately disrupted have shown promise.
Problems with drug treatment
Despite the great advances in slowing the progression of the disease, reversing at least for a time the symptoms of the late stages of the disease and preventing the infection of babies born to infected mothers drug therapy has many drawbacks.
• The drugs are so expensive (\$7,000 to \$10,000 per year) that they not only drain resources in affluent countries but are simply unavailable in the many poor countries where the epidemic rages.
• They have many unpleasant side-effects (e.g., nausea, diarrhea, liver damage).
• They demand a very complicated dosing regimen: over a dozen pills a day (not counting those needed to cope with the accompanying opportunistic infections).
• They have to be continued even after active virus disappears because HIV-1 can integrate into the DNA of resting memory CD4+ T cells as a provirus and emerge as active virus later.
• They often lose effectiveness as they select for the emergence of drug-resistant virions in the patient. This latter problem is particularly serious because of the speed at which mutations occur in HIV (as we shall now see).
Genetic Variability of HIV
Reverse transcription (RNA → DNA) lacks the proofreading capabilities of DNA replication or of normal transcription (DNA → RNA). Therefore errors, i.e., mutations, are frequent. Because of these,
• The population of viruses in a single patient becomes genetically more diverse as time goes by. This can lead to:
• appearance of strains that invade other types of cells such as X4 strains that target T cells and strains that target cells of the brain, etc.
• Development of resistance to the anti-viral drugs being used.
• New strains and subtypes of HIV-1 and HIV-2 arise in the human population.
• These complicate the efforts to develop a vaccine against HIV
• But as we shall now see that these have helped to unravel the origins of the disease.
Origin of HIV
Genome sequencing of different isolates of HIV-1 and HIV-2 shows that each is related to retroviruses that occur in primates in Africa. These are designated simian immunodeficiency viruses (SIV) although they do not cause immune deficiency (or any disease) in their natural host. However, on those occasions when a SIV accidentally infects a primate of a different species, it does cause disease in the new host. The human epidemic is one example.
• HIV-1 is most closely related to a SIV found in chimpanzees (Pan troglodytes troglodytes)
• HIV-2 is most closely related to a SIV that occurs in the sooty mangabey (Cercocebus atys).
Genome analysis also permits the construction of phylogenetic trees which reveal different clades of HIV just as such analysis reveals evolutionary relationship between species. The picture so far:
• HIV-1 appears to have infected humans on at least 4 different occasions giving rise to 4 clades: M, N, O, and P. Groups M and N appear to have jumped at separate times from chimpanzees to humans while O may have jumped from gorillas to humans. Except in parts of West Africa, most human cases are caused by members of Group M.
• HIV-2 appears to have jumped from sooty mangabeys to humans on at least 4 different occasions (there are 4 clades).
• How? These (and other) primates are often slaughtered for food and exposure to their blood and tissues is probably the route of transmission. In fact the chimpanzee SIV that gave rise to HIV-1 appears to be itself the product of recombination between two monkey SIVs that infected chimpanzees. (Chimps often eat monkeys.)
Just as with other evolutionary trees, one can also estimate from genome sequences the time of divergence of two branches. This evidence indicates that the Group M clade of HIV-1 invaded humans sometime very early in the 20th century.
But the worldwide epidemic of AIDS did not get its start until the 1980s. What took so long? An answer to that requires an appreciation of the way in which contagious diseases spread. Their rate of spread depends on:
• The ease of transmission. The transmissibility of HIV is very low. HIV is not like influenza or measles which spread like wildfire.
• The length of time the host remains contagious. Again, HIV is not like influenza or measles where the period of contagiousness is just a few days. For HIV, it can be years.
• The number of susceptible contacts; that is, the proximity of potential new hosts. For sexually-transmitted diseases (STDs), that means the number of sexual contacts.
So diseases like HIV only smolder in isolated populations because they lack the density of susceptible contacts. In crowded populations, the equation changes. There has been a dramatic population shift from rural to urban areas in sub-Saharan Africa since 1950. In the case of STDs, the availability of multiple sexual contacts — perhaps accompanied by changing sexual mores — tips the scales. In any case, the major factor today in the spread of HIV is promiscuity, whether homosexual or heterosexual.
Prevention of AIDS
Vaccines
Many once-feared infectious diseases have been reduced or eliminated by the development of a vaccine to prevent the disease. Over two dozen experimental anti-HIV vaccines have been developed and clinical trials of some of these have been and are presently being undertaken. So far, the results have been disappointing. There are probably several reasons. Some of the vaccines attempt to induce antibodies, e.g., against the outer portion of the envelope protein (called gp120). But antibody-mediated immunity may not give adequate protection. The gene (env) encoding the envelope protein mutates too rapidly. The virus may be able to stay within cells out of the reach of circulating antibodies. High levels of antibodies (the basis of the most common test of infection) persist even while the disease pursues its inexorable course.
So other vaccines have been designed to favor the development of cell-mediated immunity; e.g., cytotoxic T cells.
Many of these are DNA vaccines, for example,
• a live virus such as canarypox (a harmless relative of smallpox) or an adenovirus which serves as a vector for introducing HIV genes (DNA)
• a mixture of plasmids encoding several HIV genes (e.g., gag, pol, and env of several HIV strains).
It is hoped that expression of these genes within the cells (e.g., muscle) of the subject will induce a protective immune response, but to date there has been no success.
Behavior
Because HIV transmission is so difficult, changing behavior could go a long way toward stopping the epidemic.
• Reducing the number of sexual partners.
• If injecting drugs cannot be stopped, then using sterile needles (thus not sharing them) would prevent infection.
• Using condoms and/or other (e.g., chemical) barriers to prevent contact with infectious semen.
In the words of Anthony S. Fauci, Director of the National Institute of Allergy and Infectious Diseases, "Unlike microbial scourges, such as malaria and tuberculosis (among many others), for which there is very little that people can do to prevent infection, HIV infection in adults is entirely preventable by behavior modification". | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4V%3A_AIDS.txt |
Active Immunization or Vaccination
The terms vaccination and vaccine derive from the work of Edward Jenner who, over 200 years ago, showed that inoculating people with material from skin lesions caused by cowpox (L. vaccinus, of cows) protected them from the highly contagious and frequently fatal disease smallpox. Since Jenner's time, the term has been retained for any preparation of dead or weakened pathogens, or their products, that when introduced into the body, stimulates the production of protective antibodies or T cells without causing the disease. In molecular terms, the goal is to introduce harmless antigen(s) with epitopes that are also found on the pathogen.
Vaccination is also called active immunization because the immune system is stimulated to develop its own immunity against the pathogen. Passive immunity, in contrast, results from the injection of antibodies formed by another animal (e.g., horse, human) which provide immediate, but temporary, protection for the recipient.
Kinds of Vaccines
Killed whole organisms
In this relatively crude approach, the vaccine is made from the entire organism, killed to make it harmless. The typhoid and cholera vaccines are examples.
Attenuated organisms
Here, the organism has been cultured so as to reduce its pathogenicity, but still retain some of the antigens of the virulent form. The Bacillus Calmette-Guérin (BCG) is a weakened version of the bacterium that causes tuberculosis in cows. BCG is used as a vaccine against tuberculosis in many European countries but is rarely used in the U.S.
Toxoids
In some diseases, diphtheria and tetanus are notorious examples, it is not the growth of the bacterium that is dangerous, but the protein toxin that is liberated by it. Treating the toxin with, for example, formaldehyde, denatures the protein so that it is no longer dangerous, but retains some epitopes on the molecule that will elicit protective antibodies.
Surface molecules
Antibodies are most likely to be protective if they bind to the surface of the invading pathogen triggering its destruction. Several vaccines employ purified surface molecules:
• Influenza vaccine contains purified hemagglutinins from the viruses currently in circulation around the world.
• The gene encoding a protein expressed on the surface of the hepatitis B virus, called hepatitis B surface antigen or HBsAg, can now be expressed in E. coli cells and provides the material for an effective vaccine. Hepatitis B infection is strongly associated with the development of liver cancer. Here then is a vaccine against a cancer.
• The genes encoding the capsid proteins of 9 strains of human papilloma virus (HPV) can be expressed in yeast and the resulting recombinant proteins are incorporated in a vaccine (Gardasil 9®). Because infection with some of these strains of HPV can lead to cervical cancer, here is another vaccine against cancer.
• Some 80 different strains of Streptococcus pneumoniae cause pneumonia in humans. They differ in the chemistry of the polysaccharide capsule that surrounds them and makes it difficult for phagocytes to engulf them by endocytosis. One current vaccine consists of tiny amounts of the purified capsular polysaccharides of the 23 most common and/or dangerous strains.
Inactivated virus
Like killed bacterial vaccines, these vaccines contain whole virus particles that have been treated (again, often with formaldehyde) so that they cannot infect the host's cells but still retain some unaltered epitopes. The Salk vaccine for polio (IPV) is an example.
Attenuated virus
In these vaccines, the virus can still infect but has been so weakened that it is no longer dangerous. The measles, mumps, and rubella ("German measles") vaccines are examples. The Sabin oral polio vaccine (OPV) is another example. It has advantages over the Salk vaccine in that
• it is given by mouth rather than by injection;
• the viruses it contains can spread to the other members of the vaccinee's family thus immunizing them as well.
It has the disadvantage that on rare occasions one of the strains in the vaccine regains full virulence and causes the disease. For this reason, the Salk vaccine has once again become the preferred vaccine worldwide.
Example: A new method of attenuation
The various attenuated-virus vaccines in current use were developed by rather hit-or-miss methods. However, scientists have been working on a technology exploiting the phenomenon of codon bias - that may make possible the rational development of safer vaccines.
One group, at Stony Brook University (see J.R. Coleman et al., Science, 27 June 2008), has engineered polio virus with hundreds of mutations in the genes encoding its capsid protein. However, every one of these is a "silent" mutation; that is, it simply changes the codon for the amino acid to a different codon for the same amino acid. When they created polio viruses in which pairs of new codons were ones that the wild polio virus avoids using (because its human host does), they found that the new viruses were far less infectious that the original. But note, that this procedure did not introduce any change in the amino acid sequence of the capsid protein. So one would expect that all the epitopes recognized by the immune system would be unchanged. And, indeed, they went on to show that mice immunized with the synthetic virus were protected from disease caused by the wild virus.
As mentioned above, one of problems associated with the attenuated live virus polio vaccine (Sabin) is the rare back mutation to full virulence. Such back mutation in these engineered viruses would be extremely unlikely considering the hundreds of silent mutations that would have to be reversed.
Summary Table: Here is a table describing some of the most widely-used vaccines used in humans.
Disease Agent Preparation Notes
Diphtheria Toxoid Often given in a single preparation
DTaP (Daptacel®) for infants and young children; Tdap for teenagers and adults (the lower case letters signify the smaller amounts of the diphtheria and pertussis antigens in Tdap).
Tetanus Toxoid
Pertussis Killed bacteria ("P") or their purified components (acellular pertussis = "aP")
Polio Inactivated virus Inactivated polio vaccine: IPV (Salk)
Attenuated virus Oral polio vaccine; OPV (Sabin)
Both vaccines trivalent (types 1, 2, and 3)
Hepatitis A Inactivated virus HAVRIX® and VAQTA®; also available in single shot with HBsAg (Twinrix®)
Hepatitis B Protein (HBsAg) from the surface of the virus Made by recombinant DNA technology
Rotavirus Attenuated virus (Rotarix®) or 5 strains of the virus (RotaTeq®) to prevent this serious diarrheal disease in infants
Human Papilloma Virus (HPV) Protein from the capsid of 9 strains of the virus Gardasil 9®; made by recombinant DNA technology
Diphtheria, tetanus, pertussis, polio, and hepatitis B Uses acellular pertussis and IPV (Salk) Pediarix®; combination vaccine given in 3 doses to infants
Diphtheria, tetanus, pertussis, polio, and Hemophilus influenzae type b (Hib) Uses acellular pertussis and IPV (Salk) Pentacel®; combination vaccine given in 4 doses to infants
Measles Attenuated virus Often given as a mixture (MMR)
Do not increase the risk of autism. (Nor do any vaccines containing thimerosal as a preservative.)
Mumps Attenuated virus
Rubella Attenuated virus
Chickenpox (Varicella) Attenuated varicella-zoster virus (VZV) Also available combined with MMR ("MMRV" or ProQuad®)
Cholera Killed bacteria Three oral vaccines available
Influenza Hemagglutinins Contains hemagglutinins from the type A
and type B viruses recently in circulation
Attenuated virus FluMist® — contains weakened viruses of the type B
and two type A strains recently in circulation
Pneumococcal infections Capsular polysaccharides A mixture of the capsular polysaccharides of 23 common types. Works poorly in infants.
13 capsular polysaccharides conjugated to protein ("PCV13") Mobilizes helper T cells; works well in infants.
Meningococcal disease 4 polysaccharides conjugated to protein To prevent outbreaks among new groups of young adults, e.g., college freshmen, military recruits
Hemophilus influenzae, type b (Hib) Capsular polysaccharide conjugated to protein Prevents meningitis in children
Rabies Inactivated virus Vaccine prepared from human diploid cell cultures (HDCV)
or chick embryo cells (PCECV)
Smallpox Vaccinia virus Despite the global eradication of smallpox, is used to protect against a possible bioterrorist attack
Anthrax Extract of attenuated bacteria Primarily for veterinarians and military personnel
Typhoid
Three types are available:
1. killed bacteria
2. live, attenuated bacteria (oral)
3. polysaccharide conjugated to protein
Yellow fever Attenuated virus
Tuberculosis Attenuated bacteria (BCG) Rarely used in the U.S.
Some of the Triumphs of Vaccination
The greatest triumph is the eradication of smallpox from the planet, with no naturally-occurring cases having been found since 1977. "Naturally-occurring" because one case (fatal) occurred later following the accidental release of the virus in a laboratory. As far as the public knows, smallpox virus now exists only in laboratories in the U.S. and Russia. There is currently a vigorous debate as to whether these should be destroyed. If smallpox ever should get back out into the environment, the results could be devastating because smallpox vaccination is no longer given and so the population fully susceptible to the disease grows year by year.
A program to try to eliminate polio from the world is now underway. Except for cases caused by OPV, the disease has now been eliminated from the Western hemisphere. Outbreaks of polio still occur in Africa, the Indian subcontinent, and parts of the Near East. Table 1 compares the number of cases of illness in the U.S. in a representative year (either before a vaccine was available or before it came into widespread use) with the number of cases reported in 1994.
Table 1
Disease Total cases Year Cases in 1994 % Change
Diphtheria 206,939 1921 2 -99.9%
Measles 894,134 1941 963 -99.9%
Mumps 152,209 1968 1537 -99.9%
Pertussis 265,269 1934 4617 -99.9%
Poliomyelitis* 21,269 1952 0 -100%
Rubella 57,686 1969 227 -99.9%
Tetanus 1,560 1923 51 -99.9%
*Since 1979, an average of 8 cases of poliomyelitis have occurred in the U.S. each year that are acquired from the vaccine (OPV, the Sabin vaccine) itself. For this reason, the "killed" virus vaccine (IPV, the Salk vaccine) is being reintroduced. As of June 17, 1999, it is recommended that in the future all children receive 4 doses of the Salk vaccine and — except in special circumstances — none of the Sabin vaccine.
Problems of Vaccine Development
With so many triumphs, why haven't vaccines eliminated other common diseases such as malaria and HIV-1 infection?
One problem is that experimental vaccines often elicit an immune response that does not actually protect against the disease. Most vaccines preferentially induce the formation of antibodies rather than cell-mediated immunity. This is fine for those diseases caused by
• toxins (diphtheria, tetanus)
• extracellular bacteria (pneumococci)
• even viruses that must pass through the blood to reach the tissues where they do their damage (polio, rabies)
But viruses are intracellular parasites, out of the reach of antibodies while they reside within their target cells. They must be attacked by the cell-mediated branch of the immune system, such as by cytotoxic T lymphocytes (CTLs). Most vaccines do a poor job of eliciting cell-mediated immunity (CMI).
Example:
Much of the early and so far unsuccessful work on anti-HIV-1 vaccines has focused on the antibody response of the test animal. Antibodies may have a role in preventing infection or minimizing its spread, but cell-mediated responses will probably turn out to be far more important. Certainly there are thousands of patients dying of AIDS despite their high levels of anti-HIV-1 antibodies. The most widespread test for HIV-1 infection does not detect the presence of the virus but the presence of antibodies against the virus.
DNA Vaccines
With DNA vaccines, the subject is not injected with the antigen but with DNA encoding the antigen.
The DNA is incorporated in a plasmid containing
• DNA sequences encoding one or more protein antigens or, often, simply epitopes of the complete antigen(s);
• DNA sequences incorporating a promoter that will enable the DNA to be efficiently transcribed in the human cells.
• Sometimes DNA sequences encoding costimulatory molecules and sequences that target the expressed protein to specific intracellular locations (e.g., endoplasmic reticulum) are included as well.
The DNA vaccine can be injected into a muscle just as conventional vaccines are. In contrast to conventional vaccines, DNA vaccines elicit cell-mediated as well as antibody- mediated immune responses.
The cell-mediated response
• The plasmid is taken up by an antigen-presenting cell (APC) like a dendritic cell.
• The gene(s) encoding the various components are transcribed and translated.
• The protein products are degraded into peptides.
• These are exposed at the cell surface nestled in class I histocompatibility molecules where
• they serve as a powerful stimulant for the development of cell-mediated immunity.
The antibody-mediated response
• If the plasmid is taken up by other cells (e.g. muscle cells), the proteins synthesized are released and can be engulfed by antigen-presenting cells (including B cells).
• In this case, the proteins are degraded in the class II pathway and presented to helper T cells.
• These secrete lymphokines that aid B cells to produce antibodies.
So far, most of the work on DNA vaccines has been done in mice where they have proved able to protect them against tuberculosis, SARS, smallpox, and other intracellular pathogens. In addition, more than a dozen different DNA vaccines against HIV-1 the cause of AIDS are in clinical trials. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.04%3A_Immune_System/15.4W%3A_Vaccines.txt |
The human kidneys are two bean-shaped organs, one on each side of the backbone. They represent about 0.5% of the total weight of the body, but receive 20–25% of the total arterial blood pumped by the heart. Each contains from one to two million nephrons.
The Nephron
The nephron is a tube closed at one end and open at the other. It consists of:
• Bowman's capsule. Located at the closed end, the wall of the nephron is pushed in forming a double-walled chamber.
• Glomerulus. A capillary network within the Bowman's capsule. Blood leaving the glomerulus passes into a second capillary network surrounding the proximal tubule.
• Proximal convoluted tubule. Coiled and lined with cells carpeted with microvilli and stuffed with mitochondria.
• Loop of Henle. It makes a hairpin turn and returns to the distal convoluted tubule.
• Distal convoluted tubule, which is also highly coiled and surrounded by capillaries.
• Collecting duct. It leads to a calyx, one of many small chambers from which urine drains into the pelvis of the kidney from where it flows through a ureter to the bladder and, periodically, on to the outside world.
The Bowman's capsules are packed in the cortex of the kidney, the tubules and collecting ducts descend into the medulla.
The image above shows a cut section of the cortex of a mouse kidney as seen under the scanning electron microscope. Near the top (center) can be seen a Bowman's capsule with its glomerulus. Directly beneath is another Bowman's capsule with its glomerulus removed. The remainder of the field shows the lumens of both proximal and distal tubules as they have been cut at various angles.
Formation of Urine
The nephron makes urine by filtering the blood of its small molecules and ions and then reclaiming the needed amounts of useful materials. Surplus or waste molecules and ions are left to flow out as urine. In 24 hours the kidneys reclaim: ~1,300 g of NaCl, ~400 g NaHCO3, ~180 g glucose and almost all of the 180 liters of water that entered the tubules. Blood enters the glomerulus under pressure. This causes water, small molecules (but not macromolecules like proteins) and ions to filter through the capillary walls into the Bowman's capsule. This fluid is called nephric filtrate. As the table shows, it is simply blood plasma minus almost all of the plasma proteins. Essentially it is no different from interstitial fluid.
Table 1: Composition of plasma, nephric filtrate, and urine (each in g/100 ml of fluid). These are representative values. The values for salts are especially variable, depending on salt and water intake.
Component Plasma Nephric Filtrate Urine Concentration % Reclaimed
Urea 0.03 0.03 1.8 60X 50%
Uric acid 0.004 0.004 0.05 12X 91%
Glucose 0.10 0.10 None - 100%
Amino acids 0.05 0.05 None - 100%
Total inorganic salts 0.9 0.9 <0.9–3.6 <1–4X 99.5%
Proteins and other macromolecules 8.0 None None - -
Figure 15.5.1.4 Urine formation process
• Nephric filtrate collects within the Bowman's capsule and then flows into the proximal tubule.
• Here all of the glucose and amino acids, >90% of the uric acid, and ~60% of inorganic salts are reabsorbed by active transport.
• The active transport of Na+ out of the proximal tubule is controlled by angiotensin II.
• The active transport of phosphate (PO43-) back into the blood is regulated (suppressed) by both the parathyroid hormone and fibroblast growth factor 23 (FGF-23).
• As these solutes are removed from the nephric filtrate, a large volume of the water follows them by osmosis (80–85% of the 180 liters deposited in the Bowman's capsules in 24 hours).
• As the fluid flows into the descending segment of the loop of Henle, water continues to leave by osmosis because the interstitial fluid is very hypertonic. This is caused by the active transport of Na+ out of the tubular fluid as it moves up the ascending segment of the loop of Henle.
• In the distal tubules, more sodium is reclaimed by active transport, and still more water follows by osmosis.
• Final adjustment of the sodium and water content of the body occurs in the collecting ducts.
Sodium
Although 97% of the sodium has already been removed, it is the last 3% that determines the final balance of sodium and hence water content and blood pressure in the body. The reabsorption of sodium in the distal tubule and the collecting ducts is closely regulated by the synergistic action of the hormones vasopressin and aldosterone.
Water
• The hypertonic interstitial fluid surrounding the collecting ducts provides a high osmotic pressure for the removal of water.
• Transmembrane channels made of proteins called aquaporins are inserted in the plasma membrane greatly increasing its permeability to water. When open, an aquaporin channel allows 3 billion molecules of water to pass through each second.
• Insertion of aquaporin-2 channels requires signaling by vasopressin (also known as arginine vasopressin [AVP] or the antidiuretic hormone [ADH]).
• Vasopressin binds to receptors (called V2 receptors) on the basolateral surface of the cells of the collecting ducts.
• Binding of the hormone triggers a rising level of cAMP within the cell.
• This "second messenger" initiates a chain of events culminating in the insertion of aquaporin-2 channels in the apical surface of the cell.
• The release of vasopressin (from the posterior lobe of the pituitary gland) is regulated by the osmotic pressure of the blood.
• Anything that dehydrates the body, such as perspiring heavily,
• increases the osmotic pressure of the blood
• turns on the vasopressinV2 receptorsaquaporin-2 pathway.
The result:
• As little as 0.5 liter/day of urine may remain of the original 180 liters/day of nephric filtrate.
• The concentration of salts in the urine can be as much as four times that of the blood. (But not high enough to enable humans to benefit from drinking sea water, which is saltier still.)
• If the blood should become too dilute (as would occur after drinking a large amount of water),
• Vasopressin secretion is inhibited.
• The aquaporin-2 channels are taken back into the cell by endocytosis.
• The result: a large volume of watery urine is formed (with a salt concentration as little as one-fourth of that of the blood).
Diabetes insipidus
This disorder is characterized by excretion of large amounts of a watery urine (as much as 30 liters - about 8 gallons each day and unremitting thirst.
It can have several causes:
• Insufficient secretion of vasopressin.
• Inheritance of two mutant genes for the vasopressin receptor (V2) [in females; because the gene is X-linked, only one does it for males].
• Inheritance of one (for dominant mutations) or two (for recessive versions) mutant genes for aquaporin-2.
Liddle's Syndrome
The most obvious effect of this rare inherited disorder is extremely high blood pressure (hypertension). It is caused by a single mutant allele (therefore the syndrome is inherited as a dominant trait) encoding the aldosterone-activated sodium channel in the collecting ducts. The defective channel is always "on" so too much Na+ is reabsorbed and too little is excreted. The resulting elevated osmotic pressure of the blood produces hypertension.
Tubular Secretion
Although urine formation occurs primarily by the filtration-reabsorption mechanism described above, an auxiliary mechanism, called tubular secretion, is also involved. The cells of the tubules remove certain molecules and ions from the blood and deposit these into the fluid within the tubules.
Example: Excess hydrogen ions (H+) are combined with ammonia (NH3) to form ammonium ions (NH4+) and transported to the cells of the collecting ducts. Here the NH4+ dissociates back into ammonia and H+. Both are then secreted into the fluid within the collecting ducts (the protons by active transport).
Tubular secretion of H+ is important in maintaining control of the pH of the blood. When the pH of the blood starts to drop, more hydrogen ions are secreted. If the blood should become too alkaline, secretion of H+ is reduced. In maintaining the pH of the blood within its normal limits of 7.3–7.4, the kidney can produce a urine with a pH as low as 4.5 or as high as 8.5. Excess potassium ions (K+) are also disposed of by tubular secretion.
The Kidney and Homeostasis
While we think of the kidney as an organ of excretion, it is more than that. It does remove wastes, but it also removes normal components of the blood that are present in greater-than-normal concentrations. When excess water, sodium ions, calcium ions, potassium ions, and so on are present, the excess quickly passes out in the urine. On the other hand, the kidneys step up their reclamation of these same substances when they are present in the blood in less-than-normal amounts. Thus the kidney continuously regulates the chemical composition of the blood within narrow limits. The kidney is one of the major homeostatic devices of the body.
Hormones of the Kidneys
The human kidney is also an endocrine gland secreting two hormones:
• Erythropoietin (EPO)
• Calcitriol (1,25[OH]2 Vitamin D3), the active form of vitamin D
as well as the enzyme renin.
The Artificial Kidney
The artificial kidney uses the principle of dialysis to purify the blood of patients whose own kidneys have failed.
The left portion of the figure ("Dialysis unit") shows the mechanism used today in artificial kidneys. Small molecules like urea are removed from the blood because they are free to diffuse between the blood and the bath fluid, whereas large molecules (e.g., plasma proteins) and cells remain confined to the blood. The bath fluid must already have had essential salts added to it to prevent the dangerous loss of these ions from the blood. Note that blood and bath fluid flow in opposite directions across the dialysis membrane. This "counter-current" exchange maintains a diffusion gradient through the entire length of the system. An anticoagulant is added to the blood so it will not clot while passing through the machine. The anticoagulant is neutralized as the blood is returned to the patient.
Artificial kidneys have proved of great benefit in helping patients of acute kidney malfunction survive the crisis until their own kidneys resume operation. They have also enabled people suffering from chronic kidney failure to remain alive, though at an enormous expense of time (often three sessions of 6 or more hours per week), money, and psychological well-being. Furthermore, although dialysis does a good job at removing wastes, it cannot perform the other functions of the kidney:
• providing precise homeostatic control over the concentration of such vital ingredients as glucose and Na+
• secreting its hormones
An Artificial Kidney of the Future?
In an attempt to solve these problems, a research team at the University of Michigan is experimenting with adding a "Bioreactor unit" (above) to the dialysis unit. The bioreactor consists of many hollow, porous tubes on the inner wall of which is attached a monolayer of proximal tubule cells (derived from pigs). The dialysis bath fluid passes through the lumen of the tubes where molecules and ions can be picked up by the apical surface of the cells. Discharge of essential molecules and ions (as well as hormones) at the basolateral surface of the cells places these materials back in the blood (just as the proximal tubule cells in the nephron normally do). So far, all the testing has been done using dogs, but the results seem promising.
Kidney Transplants
The ideal alternative to long-term dialysis is transplantation of a new kidney. The operation is technically quite easy. The recipient's diseased kidneys are usually left in place, but the renal arteries and veins are tied off except for the branches supplying the adrenal glands. The major problems is the shortage of donors suitably matched for histocompatibility molecules so as to avoid the problem of graft rejection by the recipient's immune system - that unless the donor and recipient are identical twins - "sees" the kidney as "foreign". | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.05%3A_Excretion/15.5A%3A_Human_Kidneys.txt |
All vertebrates have kidneys. Like the human kidney, they are made up of many nephrons. However, there are differences in the structure and functioning of various vertebrate kidneys that adapt them to the environment in which the animals live.
Freshwater Vertebrates
All animals that live in fresh water must cope with a continual inflow of water from their hypotonic environment. In order to maintain homeostasis of its extracellular fluid (ECF), the freshwater fish must excrete this excess water. Contraction of its heart (powered by ATP) provides the pressure to force the water, small molecules, and ions into the glomerulus as nephric filtrate. The essential ingredients are then reclaimed by the tubules, returning to the blood in the capillaries surrounding the tubules. The blood in these capillaries comes from the glomerulus (as in humans) and also from the renal portal veins which drain the posterior part of the fish's body.
After solute reabsorption is complete, the urine is little more than water. Most of the nitrogenous wastes (including large amounts of ammonia, NH3) leave by diffusion out of the gills. So, the kidney is mostly a device for maintaining water balance in the animal, rather than an organ of excretion.
Amphibians
The amphibian kidney also functions chiefly as a device for excreting excess water. The permeable skin of the frog provides an easy route for the fresh water of its pond to enter by osmosis. But, as their name suggests, amphibians also spend time on land. Then the problem is to conserve water, not eliminate it.
The frog adjusts to the varying water content of its surroundings by adjusting the rate of filtration at the glomerulus. When blood flow through the glomerulus is restricted, a renal portal system is present to carry away materials reabsorbed through the tubules. The frog is also able to use its urinary bladder to aid water conservation. When in water, the frog's bladder quickly fills up with a hypotonic urine. On land, this water is reabsorbed into the blood helping to replace water lost through evaporation through the skin. The reabsorption is controlled by a hormone similar to mammalian ADH.
Lizards and Snakes
Many reptiles live in dry environments (e.g., rattlesnakes in the desert). Among the many adaptations to such environments is their ability to convert waste nitrogen compounds into uric acid. Uric acid is quite insoluble and so can be excreted using only a small amount of water. Thus we find that reptile glomeruli are quite small and, in fact, some reptiles have no glomeruli at all. Those with glomeruli filter just enough fluid to wash the uric acid, which is secreted by the tubules, into the cloaca. Most of this moisture is reabsorbed in the cloaca. Emptying the cloaca deposits feces (brown) and uric acid (a white paste). The cloaca is a chamber through which the feces and the gametes, as well as urine, pass on the way to the outside. The name comes from the Latin word for sewer. These water conservation mechanisms can allow the reptile to forgo drinking water. The water content of its food plus the water produced by cellular respiration is usually sufficient.
Birds
Bird kidneys function like those of reptiles (from which they are descended). Uric acid is also their chief nitrogenous waste. Most birds have a limited intake of fresh water. However, they need filter only enough to wash a slurry of uric acid into the cloaca where enough additional water is reclaimed to convert the uric acid into a semisolid paste. It is the whitish material that pigeons leave on statues.
Mammals
All mammals share our use of urea as their chief nitrogenous waste. Urea requires much more water to be excreted than does uric acid. Mammals produce large amounts of nephric filtrate but are able to reabsorb most of this in the tubules. But even so, humans lose several hundred ml each day in flushing urea out of the body. Some mammals have more efficient kidneys than ours. The kangaroo rat of the desert can produce a urine 17 times more concentrated that its blood. (The best we can do is 3-4 times as concentrated.) The efficiency of the kangaroo rat kidney enables it to survive without drinking water — simply depending on the water content of its food and that produced by cellular respiration.
We like to think of ourselves as highly advanced. Why don't we have kidneys as efficient as those of the reptiles and birds? It is the luck of our inheritance. The line of vertebrate evolution that produced the mammals split off before the evolution of the diapsids whose ability to convert nitrogenous wastes into uric acid was passed on to all their descendants, including the lizards, snakes, and birds.
Marine Fishes
Marine fishes face just the opposite problem from that of freshwater fishes. The salt content of sea water (~3%) is so hypertonic to that of their extracellular fluid that they are in continual danger of dehydration. The two major groups of marine fishes have solved this dilemma differently.
Cartilaginous Fishes (Chondrichthyes)
The cartilaginous fishes such as sharks, skates, and rays have developed high levels of urea in their blood. Shark's blood may contain 2.5% urea in contrast to the 0.01-0.03% in other vertebrates. This high level makes sharks blood isotonic to sea water, so the shark lives in osmotic balance with its environment and has a kidney that functions like ours with the exception that far more urea is reabsorbed in the shark's tubules than in ours.
Bony Fishes (Osteichthyes)
Marine bony fishes have solved the problem differently. They do lose water continuously but replace it by drinking sea water and then desalting it. The salt is returned to the sea by active transport at the gills. Living in constant danger of dehydration by the hypertonic sea, there is no reason to pump out large amounts of nephric filtrate at the glomerulus. The less water placed in the tubules, the less that has to be reabsorbed. So it is not surprising that many bony fishes have small glomeruli and some have no glomeruli at all. With a reduction in the filtration-reabsorption mechanism, the marine bony fishes rely more on tubular secretion for eliminating excess or waste solutes. Tubular secretion requires a good blood supply to the tubules. Lacking efficient glomeruli, the renal portal system must carry most of the burden. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.05%3A_Excretion/15.5B%3A_Vertebrate_Kidneys.txt |
Urea is the chief nitrogenous waste of mammals. Most of our nitrogenous waste comes from the breakdown of amino acids. This occurs by deamination.
Deamination of amino acids results in the production of ammonia (NH3). Ammonia is an extremely toxic base and its accumulation in the body would quickly be fatal. However, the liver contains a system of carrier molecules and enzymes which quickly converts the ammonia (and carbon dioxide) into urea. This is called the urea cycle.
The Urea Cycle
One turn of the cycle:
• consumes 2 molecules of ammonia
• consumes 1 molecule of carbon dioxide
• creates 1 molecule of urea ((NH2)2CO
• regenerates a molecule of ornithine for another turn.
Although our bodies cannot tolerate high concentrations of urea, it is much less poisonous than ammonia. Urea is removed efficiently by the kidneys.
Problems in Urea Cycle
There are several inherited diseases of the urea cycle caused by mutations in genes encoding one or another of the necessary enzymes. The most common of these is an inherited deficiency of ornithine transcarbamylase, an enzyme needed for the conversion of ornithine to citrulline. It results in elevated levels of ammonia that may be so high as to be life-threatening. It is an X-linked disorder; therefore most commonly seen in males. It can be cured by a liver transplant. It can also be caused by a liver transplant! In 1998, an Austrian woman was given a new liver from a male cadaver who - unknown to the surgeons - had a mutation in his single ornithine transcarbamylase gene. The woman's blood level of ammonia shot up, and she died a few days later.
Uric acid
Humans also excrete a second nitrogenous waste, uric acid. It is the product of nucleic acid, not protein, metabolism. It is produced within peroxisomes. Uric acid is only slightly soluble in water and easily precipitates out of solution forming needlelike crystals of sodium urate. These contribute to the formation of kidney stones and produce the excruciating pain of gout when deposited in the joints.
Curiously, our kidneys reclaim most of the uric acid filtered at the glomeruli. Why, if it can cause problems?
• Uric acid is a potent antioxidant and thus can protect cells from damage by reactive oxygen species (ROS).
• The concentration of uric acid is 100-times greater in the cytosol than in the extracellular fluid. So when lethally-damaged cells release their contents, crystals of uric acid form in the vicinity. These enhance the ability of nearby dendritic cells to "present" any antigens released at the same time to T cells leading to a stronger immune response.
So the risk of kidney stones and gout may be the price we pay for these protections.
Most mammals have an enzyme - uricas - for breaking uric acid down into a soluble product. However, during the evolution of great apes and humans, the gene encoding uricase became inactive. A predisposition to gout is our legacy.
Uric acid is the chief nitrogenous waste of insects, lizards and snakes and birds. It is the whitish material that birds leave on statues. These animals convert the waste products of protein metabolism as well as nucleic acid metabolism into uric acid. Because of its low solubility in water, these animals are able to eliminate waste nitrogen with little loss of water. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.05%3A_Excretion/15.5C%3A_Urea_Cycle.txt |
The essence of multicellularity is the coordinated interaction of the various kinds of cells that make up the body. Cells communicate with each other by chemical signals. Three kinds of chemical signaling can be distinguished:
• autocrine: the cell signals itself through a chemical that it synthesizes and then responds to. Autocrine signaling can occur solely within the cytoplasm of the cell or by a secreted chemical interacting with receptors on the surface of the same cell
• paracrine: chemical signals that diffuse into the area and interact with receptors on nearby cells. Examples include the release of cytokines that cause an inflammatory response in the area and the release of neurotransmitters at synapses in the nervous system.
• endocrine: the chemicals are secreted into the blood and carried by blood and tissue fluids to the cells they act upon.
This page will examine the properties of endocrine signaling.
Kinds of Hormones
There are two major classes of hormone: (1) proteins, peptides, and modified amino acids and (2) steroids.
Proteins, peptides and modified amino acids
These hydrophilic (and mostly large) hormone molecules bind to receptors on the surface of "target" cells; that is, cells able to respond to the presence of the hormone. These receptors are transmembrane proteins. Binding of the hormone to its receptor initiates a sequence of intracellular signals that may alter the behavior of the cell (such as by opening or closing membrane channels) or stimulate (or repress) gene expression in the nucleus by turning on (or off) the promoters and enhancers of the genes
This is the sequence of events:
• The hormone binds to a site on the extracellular portion of the receptor.
• The receptors are transmembrane proteins that pass through the plasma membrane 7 times, with their N-terminal exposed at the exterior of the cell and their C-terminal projecting into the cytoplasm.
• Binding of the hormone to the receptor
• activates a G protein associated with the cytoplasmic C-terminal
• This initiates the production of a "second messenger". The most common of these are
• cyclic AMP, (cAMP) which is produced by adenylyl cyclase from ATP
• inositol 1,4,5-trisphosphate (IP3)
• The second messenger, in turn, initiates a series of intracellular events (shown here as short arrows) such as
• phosphorylation and activation of enzymes
• release of Ca2+ into the cytosol from stores within the endoplasmic reticulum
• In the case of cAMP, these enzymatic changes activate the transcription factor CREB (cAMP response element binding protein).
• Once bound to its response element 5' TGACGTCA 3' in the promoters of genes that are able to respond to the hormone, activated CREB turns on gene transcription.
• The cell begins to produce the appropriate gene products in response to the hormonal signal it had received at its surface.
Steroid Hormones
Steroid hormones, being hydrophobic molecules, diffuse freely into all cells. However, their "target" cells contain cytoplasmic and/or nuclear proteins that serve as receptors of the hormone. The hormone binds to the receptor and the complex binds to hormone response elements — stretches of DNA within the promoters of genes responsive to the hormone. The hormone/receptor complex acts as a transcription factor turning target genes "on" (or "off").
Hormone Regulation
The levels of hormones circulating in the blood are tightly controlled by three homeostatic mechanisms:
1. When one hormone stimulates the production of a second, the second suppresses the production of the first. Example: The follicle stimulating hormone (FSH) stimulates the release of estrogens from the ovarian follicle. A high level of estrogen, in turn, suppresses the further production of FSH.
2. Antagonistic pairs of hormones. Example: Insulin causes the level of blood sugar (glucose) to drop when it has risen. Glucagon causes it to rise when it has fallen.
3. Hormone secretion is increased (or decreased) by the same substance whose level is decreased (or increased) by the hormone. Example: a rising level of Ca2+ in the blood suppresses the production of the parathyroid hormone (PTH). A low level of Ca2+ stimulates it.
Hormone Transport
Although a few hormones circulate simply dissolved in the blood, most are carried in the blood bound to plasma proteins. For example, all the steroid hormones, being highly hydrophobic, are transported bound to plasma proteins.
Summary Table of Human Hormaones
Hormone Structure (1) Principal Source
Thyroid-stimulating hormone (TSH) protein (201) Anterior lobe of pituitary
Follicle-stimulating hormone (FSH) protein (204)
Luteinizing hormone (LH) protein (204)
Prolactin (PRL) protein (198)
Growth hormone (GH) protein (191)
Adrenocorticotropic hormone (ACTH) peptide (39)
Vasopressin peptide (9) Posterior lobe of pituitary
Oxytocin peptide (9)
Thyrotropin-releasing hormone (TRH) peptide (3) Hypothalamus
Gonadotropin-releasing hormone (GnRH) peptide (10)
Growth hormone-releasing hormone (GHRH) peptides (40, 44)
Corticotropin-releasing hormone (CRH) peptide (41)
Somatostatin peptides (14, 28)
Dopamine tyrosine derivative
Melatonin tryptophan derivative Pineal gland
Thyroxine (T4) tyrosine derivative Thyroid Gland
Calcitonin peptide (32)
Parathyroid hormone (PTH) protein (84) Parathyroid glands
protein (251) Bone
Osteocalcin peptide (49)
Erythropoietin (EPO) protein (166)
Glucocorticoids (e.g., cortisol) steroids Adrenal cortex
Mineralocorticoids (e.g., aldosterone) steroids
Androgens (e.g., testosterone) steroids
Adrenaline (epinephrine) tyrosine derivative Adrenal medulla
Noradrenaline (norepinephrine) tyrosine derivative
Estrogens (e.g., estradiol) steroid Ovarian follicle
Progesterone steroid Corpus luteum and placenta
Human chorionic gonadotropin (HCG) protein (237) Trophoblast and placenta
Androgens (e.g., testosterone) steroid Testes
Insulin protein (51) Pancreas (Islets of Langerhans)
Glucagon peptide (29)
Somatostatin peptides (14, 28)
Amylin peptide (37)
Erythropoietin (EPO) protein (166) Kidney
Calcitriol steroid derivative
Calciferol (vitamin D3) steroid derivative Skin
Atrial-natriuretic peptide (ANP) peptides (28, 32) Heart
Gastrin peptides (e.g., 14) Stomach and intestine
Secretin peptide (27)
Cholecystokinin (CCK) peptides (e.g., 8)
Fibroblast Growth Factor 19 (FGF19) protein (216)
Incretins peptides (e.g., 31, 42)
Somatostatin peptides (14, 28)
Neuropeptide Y peptide (36)
Ghrelin peptide (28)
PYY3-36 peptide (34)
Serotonin tryptophan derivative
protein (70) Liver
Angiotensinogen protein (485)
Thrombopoietin protein (332)
Hepcidin peptide (25)
Betatrophin protein (193)
Leptin protein (167) Fat cells (adipocytes)
Retinol Binding Protein 4 protein (~180)
Adiponectin protein (117)
Asprosin protein (140)
Note: Numbers within parentheses indicate the number of amino acids in the protein or peptide(s).
15.6.01: Human Hormones
The Thyroid Gland
The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands. The thyroid gland synthesizes and secretes: thyroxine (T4), and triiodothyronine (T3) and calcitonin.
T4 and T3
Both hormones are derivatives of the amino acid tyrosine with four atoms of iodine in T4 , three in T3. The thyroid secretes mainly (80%) T4 , but when T4 enters target cells, one atom of iodine is removed from it converting it into T3. T3 is the more potent of the two hormones. It has many effects. Among the most prominent of these are:
• an increase in metabolic rate (seen by a rise in body temperature and the uptake of oxygen)
• an increase in the rate and strength of the heart beat
The thyroid cells responsible for the synthesis of T4 and T3 take up circulating iodine from the blood and attach them to tyrosine residues in the protein thyroglobulin. This action, as well as the synthesis of the hormones, is stimulated by the binding of thyroid stimulating hormone (TSH; also known as thyrotropin) to transmembrane receptors at the cell surface.
3
• Cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
• Myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
• Goiter: enlargement of the thyroid gland. Can be caused by:
• inadequate iodine in the diet with resulting low levels of T4 and T3
• an autoimmune attack against the thyroglobulin in the thyroid gland (called Hashimoto's thyroiditis)
Why should a hypothyroid disease produce an enlarged gland? The activity of the thyroid is under negative feedback control:
• The synthesis and release of thyrotropin releasing hormone (TRH) and TSH is normally inhibited as the levels of T4 and T3 rise in the blood.
• When the iodine supply is inadequate, T4 and T3 levels fall.
• This stimulates the hypothalamus and pituitary to increased TRH and TSH activity respectively. This stimulates the thyroid gland to enlarge.
• The symptoms of hypothyroidism can also result from inherited mutations in the genes encoding:
• the receptor for TSH (present on the surface of thyroid cells)
• the receptor for T3 (present in the nucleus of almost all cells)
The T3 receptor is a nuclear protein bound to the thyroid response element in the promoters of the many genes whose expression is influenced by thyroid hormones. When its ligand, T3, binds to it, it becomes a transcription factor turning on the transcription of many genes.
• Graves´ disease: Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.
• Osteoporosis: High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.
Calcitonin
Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca2+) circulating in the blood. A rise in its level, such as would occur with the absorption of calcium from a meal, stimulates the cells to release calcitonin. Calcitonin prevents a sharp rise in blood calcium by inhibiting the uptake of Ca2+ from the small intestine and inhibiting the Ca2+-releasing activity of osteoclasts.
Because it slows the loss of Ca2+ from bones, calcitonin has been examined as a possible treatment for osteoporosis, a weakening of the bones that is a leading cause of hip and other bone fractures in the elderly. Being a polypeptide, calcitonin cannot be given by mouth (it would be digested), and giving by injection is not appealing. However, inhaling calcitonin appears to be an effective way to get therapeutic levels of the hormone into the blood. A synthetic version of calcitonin (trade name = Miacalcin) is now available as a nasal spray.
The Parathyroid Glands
The parathyroid glands are 4 tiny structures embedded in the rear surface of the thyroid gland. They secrete parathyroid hormone (PTH) a polypeptide of 84 amino acids. PTH increases the concentration of Ca2+ in the blood in three ways. PTH promotes
• release of Ca2+ from the huge reservoir in the bones. (99% of the calcium in the body is incorporated in our bones.)
• reabsorption of Ca2+ from the fluid in the tubules in the kidneys
• absorption of Ca2+ from the contents of the intestine (this action is mediated by calcitriol, the active form of vitamin D.)
PTH also regulates the level of phosphate in the blood. Secretion of PTH reduces the efficiency with which phosphate is reclaimed in the proximal tubules of the kidney causing a drop in the phosphate concentration of the blood.
Control of the Parathyroids
The cells of the parathyroid glands have surface G-protein-coupled receptors that bind Ca2+ (the same type of receptor is found on the calcitonin-secreting cells of the thyroid and on the calcium absorbing cells of the kidneys). Binding of Ca2+ to this receptor depresses the secretion of PTH and thus leads to a lowering of the concentration of Ca2+ in the blood. Two classes of inherited disorders involving mutant genes encoding the Ca2+ receptor occur:
• loss-of-function mutations with the mutant receptor always "off". Patients with these mutations have high levels of Ca2+ in their blood and excrete small amounts of Ca2+ in their urine. These mutations cause hyperparathyroidism.
• gain-of-function mutations with the mutant receptor always "on" (as though it had bound Ca2+). People with these mutations have low levels of Ca2+ in their blood and excrete large amounts of Ca2+ in their urine. These mutations cause hypoparathyroidism.
Rare autoimmune disorders can mimic one or the other of these inherited disorders. In each case, autoantibodies bind to the receptors.
• If these inhibit the receptors, they cause hyperparathyroidism.
• If they activate the receptors (like those in Graves' disease), they cause hypoparathyroidism.
Diseases of the thyroid: Hyperparathyroidism
Tumors in the parathyroids elevate the level of PTH causing a rise in the level of blood Ca2+ at the expense of calcium stores in the bones. So much calcium may be withdrawn from the bones that they become brittle and break.
Until recently, treatment has been the removal of most — but not all — of the parathyroid tissue (i.e. the goal is the removal of 3 1/2 glands). Now clinical trials have begun on a drug (designated R-568) that mimics the action of calcium on the parathyroids, resulting in a drop in PTH and blood Ca2+ and sparing the calcium stores in the bone.
Diseases of the thyroid: Hypoparathyroidism
Causes:
• accidental removal of or damage to the parathyroids during neck surgery
• inherited mutations in the PTH gene
• inherited predisposition to an autoimmune attack against the parathyroids (and other glands)
• inherited defect in the embryonic development of the parathyroids (DiGeorge syndrome)
Treatment:
• give calcium supplements
• give calcitriol (1,25[OH]2 vitamin D3)
• give teriparatide (Forteo®), a synthetic (by recombinant DNA) version of PTH (containing only the 34 amino acids at the N-terminal).
For reasons that are not yet clear, this drug when given in daily injections (because it would be digested if taken by mouth), promotes strong bones and thus has been approved as a treatment for osteoporosis. While continuous high levels of PTH weaken bones by removing calcium from them, periodic injections of this drug strengthen bone by increasing the number and activity of osteoblasts. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.01%3A_Thyroid_and_Parathyroids.txt |
Over two dozen hormones have been identified in various parts of the gastrointestinal system. Most of them are peptides and many of them are also found in other tissues, especially the brain. Many act in a paracrine manner as well as being carried in the blood as true hormones. Their importance to health is uncertain as no known deficiency disorders have been found for any of them. We shall look at 8 of them here:
1. gastrin
2. somatostatin
3. secretin
4. cholecystokinin (CCK)
5. fibroblast growth factor 19 (FGF19)
6. incretins
7. ghrelin
8. neuropeptide Y (NPY)
9. peptide YY3-36 (PYY3-36)
The endocrine cells of the small intestine also secrete serotonin and substance P.
Gastrin
Gastrin is a mixture of several peptides, of which the most active contains 14 amino acids. It is secreted by cells in the stomach and duodenum. It stimulates the exocrine cells of the stomach to secrete gastric juice, a mixture of hydrochloric acid and the proteolytic enzyme pepsin.
Somatostatin
This mixture of peptides is secreted by cells in the gastric glands of the stomach and acts on the stomach (thus a paracrine effect) where it inhibits the release of gastrin and hydrochloric acid, the duodenum where it inhibits the release of secretin and cholecystokinin and the pancreas where it inhibits the release of glucagon. Taken together, all of these actions lead to a reduction in the rate at which nutrients are absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and the pancreas.
Secretin
It is a polypeptide of 27 amino acids and is secreted by cells in the duodenum when they are exposed to the acidic contents of the emptying stomach. It stimulates the exocrine portion of the pancreas to secrete bicarbonate into the pancreatic fluid (thus neutralizing the acidity of the intestinal contents).
Cholecystokinin (CCK)
A mixture of peptides, of which an octapeptide (8 amino acids) is the most active. It is secreted by cells in the duodenum and jejunum when they are exposed to food. It acts on the gall bladder stimulating it to contract and force its contents of bile into the intestine and on the pancreas stimulating the release of pancreatic digestive enzymes into the pancreatic fluid. CCK also acts on vagal neurons leading back to the medulla oblongata which give a satiety signal (i.e., "that's enough food for now").
Fibroblast Growth Factor 19 (FGF19)
A protein of 216 amino acids and is secreted by cells in the lower portion of the small intestine (the ileum). Travels in the hepatic portal system to the liver where is stimulates the the synthesis of bile acids, uptake of glucose and its conversion into glycogen. It also travels to the gall bladder where it relaxes its smooth muscle wall allowing filling (in contrast to CCK which contracts the gall bladder).
Incretins
The release of insulin from the pancreas is much greater when glucose is ingested with food rather than injected intravenously. It is due to the fact that the arrival of food in the duodenum stimulates the release of polypeptides called incretins. The two most important are:
• glucagon-like peptide-1 (GLP-1) the most active version of which has 29 amino acids
• glucose-dependent insulinotropic polypeptide (GIP) of 42 amino acids.
Their effects:
• enhancing the ability of glucose to stimulate insulin secretion by the pancreas
• stimulating the ability of the tissues (e.g., liver and muscle) to take up glucose from the blood
• slowing the emptying of the stomach
• suppressing glucagon secretion
• suppressing appetite thus reducing food intake.
All the actions prevent a sharp rise in blood glucose when consuming a sugar-rich meal. Exenatide (Byetta®) and liraglutide (Victoza®) are synthetic peptides that mimic the action of GLP-1 but the effects are longer-lasting. They are being used to treat patients with type 2 diabetes.
Ghrelin
Ghrelin is a lipopeptide consisting of 28 amino acids with a covalently attached 8-carbon fatty acid. Ghrelin is secreted by endocrine cells in the stomach, especially when one is hungry and acts on the hypothalamus to stimulate feeding. This action counteracts the inhibition of feeding by leptin and PYY3-36. Ghrelin increases the deposition of adipose tissue in mice and rats and does not seem to increase their appetite (ghrelin knockout animals don't eat any more food than normal animals).
Neuropeptide Y (NPY)
Neuropeptide Y contains 36 amino acids. It is a potent feeding stimulant and causes increased storage of ingested food as fat. Neuropeptide Y is also secreted by neurons in the hypothalamus where it blocks the transmission of pain signals to the brain and induces a calming effect in laboratory animals exposed to stressful situations. Velneperit is a drug that blocks the action of neuropeptide Y on its receptors. It is in clinical trials for the treatment of obesity.
PYY3-36
Peptide YY3-36 contains 34 amino acids, many of them in the same positions as those in neuropeptide Y. But the action of PYY3-36 is just the reverse of that of NPY, being a potent feeding inhibitor. It is released by cells in the intestine after meals. The amount secreted increases with the number of calories ingested and especially when these are derived from proteins rather than carbohydrates or fats. (This may explain the efficacy of the protein-rich, carbohydrate-poor Atkins diet.) PYY3-36 acts on the hypothalamus to suppress appetite, the pancreas to increase its exocrine secretion of digestive juices, and the gall bladder to stimulate the release of bile.
The appetite suppression mediated by PYY3-36 works more slowly than that of cholecystokinin and more rapidly than that of leptin. In a recent human study, volunteers given PYY3-36 were less hungry and ate less food over the next 12 hours than those who received saline. Neither group knew what they were getting, but one of the side-effects of injected PYY3-36 is a feeling of nausea and a bad taste to food which might account for these results! | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.02%3A_Hormones_of_the_Gut.txt |
The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum after a meal. However, scattered through the pancreas are several hundred thousand clusters of cells called islets of Langerhans. The islets are endocrine tissue containing four types of cells. In order of abundance, they are the:
• beta cells, which secrete insulin and amylin
• alpha cells, which secrete glucagon
• delta cells, which secrete somatostatin
• gamma cells, which secrete pancreatic polypeptide
Beta Cells
Insulin is a small protein consisting of an alpha chain of 21 amino acids linked by two disulfide (S—S) bridges to a beta chain of 30 amino acids. Beta cells have channels in their plasma membrane that serve as glucose detectors. Beta cells secrete insulin in response to a rising level of circulating glucose ("blood sugar"). Insulin affects many organs. It stimulates skeletal muscle fibers totake up glucose and convert it into glycogen. Insulin also take up amino acids from the blood and convert them into protein. It
• acts on liver cells
• stimulating them to take up glucose from the blood and convert it into glycogen while
• inhibiting production of the enzymes involved in breaking glycogen back down ("glycogenolysis") and
• inhibiting "gluconeogenesis"; that is, the conversion of fats and proteins into glucose.
• acts on fat (adipose) cells to stimulate the uptake of glucose and the synthesis of fat
• acts on cells in the hypothalamus to reduce appetite
In each case, insulin triggers these effects by binding to the insulin receptor - a transmembrane protein embedded in the plasma membrane of the responding cells. Taken together, all of these actions result in:
• the storage of the soluble nutrients absorbed from the intestine into insoluble, energy-rich products (glycogen, protein, fat)
• a drop in the level of blood sugar
Diabetes Mellitus
Diabetes mellitus is an endocrine disorder characterized by many signs and symptoms. Primary among these are a failure of the kidney to efficiently reclaim glucose so that glucose spills over into the urine and a resulting increase in the volume of urine because of the osmotic effect of this glucose (it reduces the return of water to the blood).
Diabetes mellitus is a disorder quite distinct from the similarly-named diabetes insipidus. They both result in the production of large amounts of urine (diabetes), but in one the urine is sweet while in the other (caused by ADH deficiency) it is not. Before the days of laboratory tests, a simple taste test ("mellitus" or "insipidus") enabled the doctor to make the correct diagnosis. There are three categories of diabetes mellitus:
• Type 1
• Type 2
• Inherited Forms of Diabetes Mellitus
Type 1 Diabetes Mellitus
(also known as Insulin-Dependent Diabetes Mellitus or IDDM)
• is characterized by little (hypo) or no circulating insulin
• most commonly appears in childhood
• it results from destruction of the beta cells of the islets
• the destruction results from a cell-mediated autoimmune attack against the beta cells
• What triggers this attack is still a mystery. One possibility: peptides derived from insulin may bond to unrelated peptides to form a "neoantigen"; that is, an antigen that was not present when tolerance to self-antigens was being established.
Type 1 diabetes is controlled by carefully-regulated injections of insulin. Insulin cannot be taken by mouth because, being a protein, it would be digested. However, the U.S. FDA has approved [in January 2006] an insulin inhaler that delivers insulin through the lungs and may reduce the number of daily injected doses needed.
Injections of insulin must be done carefully. Injections after vigorous exercise or long after a meal may drive the blood sugar level down to a dangerously low value causing an insulin reaction. The patient becomes irritable, fatigued, and may lose consciousness. If the patient is still conscious, giving a source of sugar (e.g., candy) by mouth usually solves the problem quickly. Injections of glucagon are sometimes used.
Type 2 Diabetes Mellitus
Type 2 is also known as Non Insulin-Dependent Diabetes Mellitus (NIDDM) and adult-onset diabetes. However, this type eventually leads to insulin dependence and also is now appearing in many children so those terms are no longer appropriate. Many people develop Type 2 diabetes mellitus without an accompanying drop in insulin levels (at least at first). In many cases, the problem appears to be a failure to express a sufficient number of glucose transporters in the plasma membrane (and T-system) of their skeletal muscles.
Normally when insulin binds to its receptor on the cell surface, it initiates a chain of events that leads to the insertion in the plasma membrane of increased numbers of a transmembrane glucose transporter (called GLUT4). This transporter forms a channel that permits the facilitated diffusion of glucose into the cell.
Skeletal muscle is the major "sink" for removing excess glucose from the blood (and converting it into glycogen). In type 2 diabetes, the patient's ability to remove glucose from the blood and convert it into glycogen may be only 20% of normal. This is called insulin resistance. Curiously, vigorous exercise seems to increase the expression of the glucose transporter on skeletal muscle and this may explain why type 2 diabetes is more common in people who live sedentary lives.
Type 2 diabetes mellitus usually strikes in adults and, particularly often, in overweight people. However, over the last few years in the U. S., the incidence of type 2 diabetes in children has grown to the point where they now account for 20% of all newly-diagnosed cases (and, like their adult counterparts, are usually overweight). Several drugs, all of which can be taken by mouth, are useful in restoring better control over blood sugar in patients with type 2 diabetes. However, late in the course of disease, patients may have to begin to take insulin. It is as though after years of pumping out insulin in an effort to overcome the patient's insulin resistance, the beta cells become exhausted.
Inherited Forms of Diabetes Mellitus
Some cases of diabetes result from mutant genes inherited from one or both parents. Examples:
• mutant genes for one or another of the transcription factors needed for transcription of the insulin gene (5 mutant versions have been identified).
• mutations in one or both copies of the gene encoding the insulin receptor. These patients usually have extra-high levels of circulating insulin but defective receptors. The mutant receptors
• may fail to be expressed properly at the cell surface
• may fail to transmit an effective signal to the interior of the cell.
• a mutant version of the gene encoding glucokinase, the enzyme that phosphorylates glucose in the first step of glycolysis.
• mutations in the gene encoding part of potassium channels in the plasma membrane of the beta cell. The channels fail to close properly causing the cell to become hyperpolarized and blocking insulin secretion.
• mutations in several mitochondrial genes which reduce insulin secretion by beta cells. These diseases are inherited from the mother as only her mitochondria survive in the fertilized egg.
While symptoms usually appear in childhood or adolescence, patients with inherited diabetes differ from most children with type 2 diabetes in having a history of diabetes in the family and not being obese.
insulin
For many years, insulin extracted from the glands of cows and pigs was used. However, pig insulin differs from human insulin by one amino acid; beef insulin by three. Although both work in humans to lower blood sugar, they are seen by the immune system as "foreign" and induce an antibody response in the patient that blunts their effect and requires higher doses. Two approaches have been taken to solve this problem:
• Convert pig insulin into human insulin by removing the one amino acid that distinguishes them and replacing it with the human version. This approach is expensive, so now the favored approach is to
• Insert the human gene for insulin into E. coli and grow recombinant human insulin in culture tanks. Insulin is not a glycoprotein so E. coli is able to manufacture a fully-functional molecule (trade name = Humulin). Yeast is also used (trade name = Novolin).
Recombinant DNA technology has also made it possible to manufacture slightly-modified forms of human insulin that work faster (Humalog® and NovoLog®) or slower (Lantus®) than regular human insulin.
Amylin
Amylin is a peptide of 37 amino acids, which is also secreted by the beta cells of the pancreas. Some of its actions include inhibits the secretion of glucagon, slows the emptying of the stomach, sends a satiety signal to the brain. All of its actions tend to supplement those of insulin, reducing the level of glucose in the blood. A synthetic, modified, form of amylin (pramlintide or Symlin®) is used in the treatment of type 2 diabetes.
Alpha Cells
The alpha cells of the islets secrete glucagon, a polypeptide of 29 amino acids. Glucagon acts principally on the liver where it stimulates the conversion of glycogen into glucose ("glycogenolysis") and fat and protein into intermediate metabolites that are ultimately converted into glucose ("gluconeogenesis"). In both cases, the glucose is deposited in the blood.
X-Ray Crystal Structure of Glucagon based on PDB 1GCN. (CC BY-SA 3.0; Truthortruth).
Glucagon secretion is
• stimulated by low levels of glucose in the blood;
• inhibited by high levels, and
• inhibited by amylin.
The physiological significance of this is that glucagon functions to maintain a steady level of blood sugar level between meals. Injections of glucagon (which is readily available thanks to recombinant DNA technology) are sometimes given to diabetics suffering from an insulin reaction in order to speed the return of normal levels of blood sugar.
Delta Cells
The delta cells secrete somatostatin, which consists of two polypeptides, one of 14 amino acids and one of 28. Somatostatin has a variety of functions. Taken together, they work to reduce the rate at which food is absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and by the intestine.
Gamma Cells
The gamma cells of the islets secrete a 36-amino-acid pancreatic polypeptide, which reduces appetite.
Contributors and Attributions
John W. Kimball. This content is distributed under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.03%3A_Hormones_of_the_Pancreas.txt |
The pituitary gland is pea-sized structure located at the base of the brain. In humans, it consists of two lobes: the Anterior Lobe and the Posterior Lobe.
Hormones of the Anterior Lobe
The anterior lobe contains six types of secretory cells, all but one of which are specialized to secrete only one of the anterior lobe hormones. All of them secrete their hormone in response to hormones reaching them from the hypothalamus of the brain.
Thyroid Stimulating Hormone (TSH)
TSH (also known as thyrotropin) is a glycoprotein consisting of a beta chain of 118 amino acids and an alpha chain of 92 amino acids. The alpha chain is identical to that found in two other pituitary hormones, FSH and LH as well as in the hormone chorionic gonadotropin. Thus it is its beta chain that gives TSH its unique properties. The secretion of TSH is stimulated by the arrival of thyrotropin releasing hormone (TRH) from the hypothalamus and is inhibited by the arrival of somatostatin from the hypothalamus.
As its name suggests, TSH stimulates the thyroid gland to secrete its hormone thyroxine (T4). It does this by binding to transmembrane G-protein-coupled receptors (GPCRs) on the surface of the cells of the thyroid. Some people develop antibodies against their own TSH receptors. When these bind the receptors, they "fool" the cell into making more T4 causing hyperthyroidism. The condition is called thyrotoxicosis or Graves' disease.
Hormone deficiencies
A deficiency of TSH causes hypothyroidism: inadequate levels of T4 (and thus of T3). Physicians occasionally encounter patients who are homozygous for mutant TSH receptors or mutant TRH receptors. In either case, they suffer from hypothyroidism. A deficiency of TSH, or mutant TSH receptors, have also been implicated as a cause of osteoporosis. Mice, whose TSH receptors have been knocked out, develop increased numbers of bone-reabsorbing osteoclasts.
Follicle-Stimulating Hormone (FSH)
FSH is a heterodimeric glycoprotein consisting of the same alpha chain found in TSH (and LH) and a beta chain of 118 amino acids, which gives it its unique properties. Synthesis and release of FSH is triggered by the arrival from the hypothalamus of gonadotropin-releasing hormone (GnRH). The effect of FSH depends on one's sex. In sexually-mature females, FSH (assisted by LH) acts on the follicle to stimulate it to release estrogens. FSH produced by recombinant DNA technology (Gonal-f®) is available to promote ovulation in women planning to undergo in vitro fertilization (IVF) and other forms of assisted reproductive technology. In sexually-mature males, FSH acts on spermatogonia stimulating (with the aid of testosterone) the production of sperm.
Luteinizing Hormone (LH)
LH is synthesized within the same pituitary cells as FSH and under the same stimulus (GnRH). It is also a heterodimeric glycoprotein consisting of the same 92-amino acid alpha subunit found in FSH and TSH (as well as in chorionic gonadotropin) and a beta chain of 121 amino acids that is responsible for its properties.
The effects of LH also depend on sex. In sexually-mature females, a surge of LH triggers the completion of meiosis I of the egg and its release (ovulation) in the middle of the menstrual cycle; LH also stimulates the now-empty follicle to develop into the corpus luteum, which secretes progesterone during the latter half of the menstrual cycle. Women with a severe LH deficiency can now be treated with human LH (Luveris®) produced by recombinant DNA technology. LH in males acts on the interstitial cells (also known as Leydig cells) of the testes stimulating them to synthesize and secrete the male sex hormone, testosterone. LH in males is also known as interstitial cell stimulating hormone (ICSH).
Prolactin (PRL)
Prolactin is a protein of 198 amino acids. During pregnancy it helps in the preparation of the breasts for future milk production. After birth, prolactin promotes the synthesis of milk. Prolactin secretion is stimulated by TRH and repressed by estrogens and dopamine. In pregnant mice, prolactin stimulates the growth of new neurons in the olfactory center of the brain.
Growth Hormone (GH)
Human growth hormone (HGH; also called somatotropin) is a protein of 191 amino acids. The GH-secreting cells are stimulated to synthesize and release GH by the intermittent arrival of growth hormone releasing hormone (GHRH) from the hypothalamus. GH promotes body growth by:
• binding to receptors on the surface of liver cells.
• This stimulates them to release insulin-like growth factor-1 (IGF-1; also known as somatomedin)
• IGF-1 acts directly on the ends of the long bones promoting their growth
Things that can go wrong:
• In childhood,
• hyposecretion of GH produces a short but normally-proportioned body.
• Growth retardation can also result from an inability to respond to GH. This can be caused by inheriting two mutant genes encoding the receptors for
• GHRH or
• GH (causing Laron syndrome, a form of dwarfism) or
• homozygosity for a disabling mutation in STAT5b, which is part of the "downstream" signaling process after GH binds its receptor.
• hypersecretion leads to gigantism
• In adults, a hypersecretion of GH or GHRH leads to acromegaly.
Hormone-replacement therapy
GH from domestic mammals like cows and pigs does not work in humans. So for many years, the only source of GH for therapy was that extracted from the glands of human cadavers. But this supply was shut off when several patients died from a rare neurological disease attributed to contaminated glands. Now, thanks to recombinant DNA technology, recombinant human GH (rHGH) is available. While a benefit to patients suffering from GH deficiency or the short stature associated with Turner syndrome, there has also been pressure to use it to stimulate growth in youngsters who have no deficiency but whose parents want them to grow up tall. So, in the summer of 2003, the U.S. FDA approved the use of human growth hormone (HGH) for boys predicted to grow no taller than 5′3″ and for girls, 4′11″ even though otherwise perfectly healthy.
ACTH — the adrenocorticotropic hormone
ACTH is a peptide of 39 amino acids. It is cut from a larger precursor proopiomelanocortin (POMC). ACTH acts on the cells of the adrenal cortex, stimulating them to produce
• glucocorticoids, like cortisol
• mineralocorticoids, like aldosterone
• androgens (male sex hormones, like testosterone)
• In the fetus, ACTH stimulates the adrenal cortex to synthesize a precursor of estrogen called dehydroepiandrosterone sulfate (DHEA-S) which helps prepare the mother for giving birth.
Production of ACTH depends on the intermittent arrival of corticotropin-releasing hormone (CRH) from the hypothalamus. Hypersecretion of ACTH is a frequent cause of Cushing's syndrome.
Alpha Melanocyte-Stimulating Hormone (α-MSH )
Alpha MSH is also a cleavage product of proopiomelanocortin (POMC). In fact, α-MSH is identical to the first 13 amino acids at the amino terminal of ACTH. MSH is discussed in a separate page.
Hormones of the Posterior Lobe
The posterior lobe of the pituitary releases two hormones — both synthesized in the hypothalamus — vasopressin and oxytocin into the circulation.
Vasopressin
Vasopressin is a peptide of 9 amino acids (Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly). It is also known as arginine vasopressin (AVP) and the antidiuretic hormone (ADH). Vasopressin acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood. Thus it acts to reduce the volume of urine formed (giving it its name of antidiuretic hormone). A deficiency of vasopressin or inheritance of mutant genes for its receptor (called V2) leads to excessive loss of urine, a condition known as diabetes insipidus. The most severely-afflicted patients may urinate as much as 30 liters (almost 8 gallons!) of urine each day. The disease is accompanied by terrible thirst, and patients must continually drink water to avoid dangerous dehydration.
Another type of receptor for vasopressin (designated V1a) is found in the brain, e.g., in voles and mice (rodents) and in primates like monkeys and humans.
• Male prairie voles (Microtus pinetorum) and marmoset monkeys have high levels of the V1a receptor in their brains, tend to be monogamous, and help with care of their young.
• Male meadow voles (Microtus montanus) and rhesus monkeys have lower levels of the V1a receptor in their brains, are promiscuous, and give little or no help with the care of their young.
Meadow voles whose brains have been injected with a vector causing increased expression of the V1a receptor become more like prairie voles in their behavior. (See Lim, M. M. et al., Nature, 17 June 2004.)
The level of expression of the V1a receptor gene is controlled by a "microsatellite" region upstream (5') of the ORF. This region contains from 178 to 190 copies of a repeated tetranucleotide (e.g., CAGA). Prairie voles have more copies of the repeat than meadow voles, and they express higher levels of the receptor in the parts of the brain associated with these behaviors. A similar microsatellite region is present in the pygmy chimpanzee or bonobo (Pan paniscus) but is much shorter in the less-affectionate common chimpanzee (Pan troglodytes).
Vasopressin and the Circadian Clock
Mice are nocturnal and become active at the start of the night. This is a circadian rhythm that persists for a time even after the lights in the lab are turned off each day 8 hours sooner (like arriving in London after a flight from Los Angeles, California). Only after 8–10 days do the mice overcome their "jet lag", adjusting to the new dark-light schedule. (It also takes us about one day to reset our circadian rhythms for each hour that our day-night schedule is shifted.)
It turns out that arginine vasopressin, acting on the suprachiasmatic nucleus (SCN), plays a role in this resistance to resetting their circadian clock. Mice with their genes for the V1a and V1b receptors knocked out adjust much more quickly (2–4 days) to the change. What evolutionary advantage this resistance to resetting the circadian clock confers is not clear, but understanding the mechanism raises the possibility of using drugs to speed getting over jet lag and also to help those whose work shifts are periodically altered. (Read about this work in Yamaguchi, Y., et al. in the 4 October 2013 issue of Science.).
Oxytocin
Oxytocin is a peptide of 9 amino acids (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly). It acts on certain smooth muscles by stimulating contractions of the uterus at the time of birth and stimulating release of milk when the baby begins to suckle. Oxytocin is often given to prospective mothers to hasten birth.
In rodents, oxytocin also acts on the nucleus accumbens and amygdala in the brain where it enhances bonding between males and females after they have mated and bonding between a mother and her newborn. In mice, oxytocin acts on striated muscle stem cells to promote repair after they have been injured. In humans, oxytocin increases the level of one's trust in other people. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.04%3A_Hormones_of_the_Pituitary.txt |
The hypothalamus is a region of the brain. It contains several types of neurons responsible for secreting different hormones.
• Thyrotropin-releasing hormone (TRH)
• Gonadotropin-releasing hormone (GnRH)
• Growth hormone-releasing hormone (GHRH)
• Corticotropin-releasing hormone (CRH)
• Somatostatin
• Dopamine
All of these are released into the blood in the capillaries and travel immediately – in portal veins – to a second capillary bed in the anterior lobe of the pituitary, where they exert their effects. All of them are released in periodic spurts. In fact, replacement hormone therapy with these hormones does not work unless the replacements are also given in spurts. Two other hypothalamic hormones vasopressin and oxytocin travel in the neurons themselves to the posterior lobe of the pituitary where they are released into the circulation.
Thyrotropin-releasing hormone (TRH)
TRH is a tripeptide (GluHisPro). When it reaches the anterior lobe of the pituitary it stimulates the release there of thyroid-stimulating hormone (TSH) and prolactin (PRL)
Gonadotropin-releasing hormone (GnRH)
GnRH is a peptide of 10 amino acids. Its secretion at the onset of puberty triggers sexual development, and from then on it is essential for normal sexual physiology in both males and females. In both sexes, its secretion occurs in periodic pulses usually occurring every 1–2 hours.
Space-filling model of gonadotropin-releasing hormone. (public domain, Fvasconcellos).
Primary Effects Secondary Effects
FSH and LH Up estrogen and progesterone Up (in females)
testosterone Up (in males)
After puberty, a hyposecretion of GnRH may result from intense physical training or anorexia nervosa. Synthetic agonists of GnRH are used to treat inherited or acquired deficiencies of GnRH secretion and prostate cancer. In this case, continuous high levels of the GnRH agonist
• reduces the number of GnRH receptors in the pituitary, which
• reduces its secretion of FSH and LH, which
• reduces the secretion of testosterone, which
• reduces the stimulation of the cells of the prostate.
Growth hormone-releasing hormone (GHRH): GHRH is a mixture of two peptides, one containing 40 amino acids, the other 44. As its name indicates, GHRH stimulates cells in the anterior lobe of the pituitary to secrete growth hormone (GH).
Corticotropin-releasing hormone (CRH): CRH is a peptide of 41 amino acids. As its name indicates, its acts on cells in the anterior lobe of the pituitary to release adrenocorticotropic hormone (ACTH). CRH is also synthesized by the placenta and seems to determine the duration of pregnancy. It may also play a role in keeping the T cells of the mother from mounting an immune attack against the fetus.
Somatostatin: Somatostatin is a mixture of two peptides, one of 14 amino acids, the other of 28. Somatostatin acts on the anterior lobe of the pituitary to inhibit the release of growth hormone (GH) and inhibit the release of thyroid-stimulating hormone (TSH). Somatostatin is also secreted by cells in the pancreas and in the intestine where it inhibits the secretion of a variety of other hormones.
Dopamine
Dopamine is a derivative of the amino acid tyrosine. It mediates several functions in the brain, including inhibiting the release of prolactin (PRL) from the anterior lobe of the pituitary, modulating motor-control centers (a loss of dopamine-secreting cells produces Parkinson's disease), and activating the reward centers of the brain. Dopamine-secreting cells are also found in other parts of the body where most of its actions are paracrine; that is, acting on nearby cells.
Vasopressin and Oxytocin
These peptides are released from the posterior lobe of the pituitary and are described in the page devoted to the pituitary. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.05%3A_Hormones_of_the_Hypothalamus.txt |
The adrenal glands are two small structures situated one atop each kidney. Both in anatomy and in function, they consist of two distinct regions an outer layer, the drenal ortex, which surrounds the adrenal medulla.
The Adrenal Cortex
Using cholesterol as the starting material, the cells of the adrenal cortex secrete a variety of steroid hormones. These fall into three classes:
• glucocorticoids (e.g., cortisol)
• mineralocorticoids (e.g., aldosterone)
• androgens (e.g., testosterone)
Production of all three classes is triggered by the secretion of ACTH from the anterior lobe of the pituitary.
These hormones achieve their effects by:
• Travelling through the body in the blood. Because they are so hydrophobic, they must be carried bound to a serum globulin.
• Entering from the blood into all cells.
• Binding to their recepto - a protein present in the cytoplasm and/or nucleus of "target" cells.
• The hormone-receptor complex binds to a second to form a homodimer.
• The homodimer migrates into the nucleus (if it did not form there) where it binds to specific hormone response elements in DNA.
• These are specific DNA sequences in the promoter of genes that will be turned on (or off) by the interaction.
• Other transcription factors are recruited to the promoter and gene transcription begins at some genes and is inhibited at others.
Glucocorticoids
The glucocorticoids get their name from their effect of raising the level of blood sugar (glucose). One way they do this is by stimulating gluconeogenesis in the liver: the conversion of fat and protein into intermediate metabolites that are ultimately converted into glucose.
The most abundant glucocorticoid is cortisol (also called hydrocortisone). Cortisol and the other glucocorticoids also have a potent anti-inflammatory effect on the body. They depress the immune response, especially cell-mediated immune responses.
For this reason glucocorticoids are widely used in therapy:
• to reduce the inflammatory destruction of rheumatoid arthritis and other autoimmune diseases
• to prevent the rejection of transplanted organs
• to control asthma
Mineralocorticoids
The mineralocorticoids get their name from their effect on mineral metabolism. The most important of them is the steroid aldosterone. Aldosterone acts on the kidney promoting the reabsorption of sodium ions (Na+) into the blood. Water follows the salt and this helps maintain normal blood pressure.
Aldosterone also
• acts on sweat glands to reduce the loss of sodium in perspiration
• acts on taste cells to increase the sensitivity of the taste buds to sources of sodium.
The secretion of aldosterone is stimulated by:
• a drop in the level of sodium ions in the blood
• a rise in the level of potassium ions in the blood
• angiotensin II
• ACTH (as is that of cortisol)
Androgens
The adrenal cortex secretes precursors to androgens such as testosterone.
In sexually-mature males, this source is so much lower than that of the testes that it is probably of little physiological significance. However, excessive production of adrenal androgens can cause premature puberty in young boys.
In females, the adrenal cortex is a major source of androgens. Their hypersecretion may produce a masculine pattern of body hair and cessation of menstruation.
Addison's Disease: Hyposecretion of the adrenal cortices
Addison's disease has many causes, such as
• destruction of the adrenal glands by infection
• their destruction by an autoimmune attack
• an inherited mutation in the ACTH receptor on adrenal cells
The essential role of the adrenal hormones means that a deficiency can be life-threatening. Fortunately, replacement therapy with glucocorticoids and mineralocorticoids can permit a normal life.
Cushing's Syndrome: Excessive levels of glucocorticoids
In Cushing's syndrome, the level of glucocorticoids, especially cortisol, is too high.
It can be caused by:
• excessive production of ACTH by the anterior lobe of the pituitary
• excessive production by the adrenals themselves (e.g., because of a tumor), or (quite commonly)
• as a result of glucocorticoid therapy for some other disorder such as rheumatoid arthritis or preventing the rejection of an organ transplant
The Adrenal Medulla
The adrenal medulla consists of masses of neurons that are part of the sympathetic branch of the autonomic nervous system. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland.
The adrenal medulla releases adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine). Both are derived from the amino acid tyrosine. Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors — transmembrane proteins in the plasma membrane of many cell types.
Some of the effects are:
• increase in the rate and strength of the heartbeat resulting in increased blood pressure
• blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain
• rise in blood sugar
• increased metabolic rate
• bronchi dilate
• pupils dilate
• hair stands on end ("gooseflesh" in humans)
• clotting time of the blood is reduced
• increased ACTH secretion from the anterior lobe of the pituitary
All of these effects prepare the body to take immediate and vigorous action. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.06%3A_Adrenal_Glands.txt |
The ovaries of sexually-mature females secrete both a mixture of estrogens (of which 17 β-estradiol is the most abundant and most potent) and progesterone.
Estrogens
Estrogens are steroids and are primarily responsible for the conversion of girls into sexually-mature women including:
• development of breasts
• further development of the uterus and vagina
• broadening of the pelvis
• growth of pubic and axillary hair
• increase in adipose (fat) tissue
• participate in the monthly preparation of the body for a possible pregnancy
• participate in pregnancy if it occurs
Estrogens also have non-reproductive effects. For example, they antagonize the effects of the parathyroid hormone, minimizing the loss of calcium from bones and thus helping to keep bones strong. They also promote blood clotting.
Progesterone
Progesterone is also a steroid. It has many effects in the body, some having nothing to do with sex and reproduction. Here we shall focus on the role of progesterone in the menstrual cycle and pregnancy.
How estrogens and progesterone achieve their effects
Steroids like estrogens and progesterone are small, hydrophobic molecules that are transported in the blood bound to a serum globulin.
• In "target" cells, i.e., cells that change their gene expression in response to the hormone, they bind to receptor proteins located in the cytoplasm and/or nucleus.
• The hormone-receptor complex enters the nucleus (if it formed in the cytoplasm) and binds to specific sequences of DNA, called the estrogen (or progesterone) response elements.
• Response elements are located in the promoters of genes.
• The hormone-receptor complex acts as a transcription factor (often recruiting other transcription factors to help) which turns on (sometimes off) transcription of those genes.
• Gene expression in the cell produces the response.
Some "target" cells also have other types of estrogen and progesterone receptors that are embedded in a membrane (endoplasmic reticulum and plasma membrane respectively). Binding of the hormone to them produces more rapid effects than those of the nuclear receptors. For example, human sperm have receptors that within a second of being exposed to progesterone activate the sperm to increased motility.
Regulation of Estrogen and Progesterone
The synthesis and secretion of estrogens is stimulated by follicle-stimulating hormone (FSH), which is, in turn, controlled by the hypothalamic gonadotropin releasing hormone (GnRH).
Hypothalamus GnRH Pituitary FSH Follicle Estrogens
High levels of estrogens suppress the release of GnRH (bar) providing a negative-feedback control of hormone levels.
It works like this: Secretion of GnRH depends on certain neurons in the hypothalamus which express a gene (KISS-1) encoding a protein of 145 amino acids. From this are cut several short peptides collectively called kisspeptin. These are secreted and bind to G-protein-coupled receptors on the surface of the GnRH neurons stimulating them to release GnRH. However, high levels of estrogen (or progesterone or testosterone) inhibit the secretion of kisspeptin and suppress further production of those hormones.
Progesterone production is stimulated by luteinizing hormone (LH), which is also stimulated by GnRH.
Hypothalamus GnRH Pituitary LH Corpus luteum Progesterone
Elevated levels of progesterone control themselves by the same negative feedback loop used by estrogen (and testosterone).
The Menstrual Cycle
About every 28 days, some blood and other products of the disintegration of the inner lining of the uterus (the endometrium) are discharged from the uterus, a process called menstruation. During this time a new follicle begins to develop in one of the ovaries. After menstruation ceases, the follicle continues to develop, secreting an increasing amount of estrogen as it does so.
• The rising level of estrogen causes the endometrium to become thicker and more richly supplied with blood vessels and glands.
• A rising level of LH causes the developing egg within the follicle to complete the first meiotic division (meiosis I), forming a secondary oocyte.
• After about two weeks, there is a sudden surge in the production of LH.
• This surge in LH triggers ovulation: the release of the secondary oocyte into the fallopian tube.
• Under the continued influence of LH, the now-empty follicle develops into a corpus luteum (hence the name luteinizing hormone for LH).
• Stimulated by LH, the corpus luteum secretes progesterone which
• continues the preparation of the endometrium for a possible pregnancy
• inhibits the contraction of the uterus
• inhibits the development of a new follicle
• If fertilization does not occur (which is usually the case)
• the rising level of progesterone inhibits the release of GnRH which, in turn, inhibits further production of progesterone
• As the progesterone level drops
• the corpus luteum begins to degenerate
• the endometrium begins to break down, its cells committing programmed cell death
• the inhibition of uterine contraction is lifted
• the bleeding and cramps of menstruation begin.
Fertilization
Fertilization of the egg is also influenced by progesterone. Sperm swim towards the egg by chemotaxis following a gradient of progesterone secreted by cells surrounding the egg. Progesterone opens CatSper ("cation sperm") channels in the plasma membrane surrounding the anterior portion of the sperm tail. This allows an influx of Ca2+ ions which causes the flagellum to beat more rapidly and vigorously.
Pregnancy
As the fertilized egg passes down the fallopian tube, it undergoes its first mitotic divisions. By the end of the week, the developing embryo has become a hollow ball of cells called a blastocyst. At this time, the blastocyst reaches the uterus and embeds itself in the endometrium, a process called implantation. With implantation, pregnancy is established.
The blastocyst has two parts:
• the inner cell mass, which will become the baby
• the trophoblast, which will
• develop into the placenta and umbilical cord
• begin to secrete human chorionic gonadotropin (HCG).
HCG is a glycoprotein. It is a heterodimer of the same alpha subunit (of 92 amino acids) used by TSH, FSH, and LH and a unique beta subunit (of 145 amino acids). HCG behaves much like FSH and LH with one crucial exception: it is NOT inhibited by a rising level of progesterone. Thus HCG prevents the deterioration of the corpus luteum at the end of the fourth week and enables pregnancy to continue beyond the end of the normal menstrual cycle. Because only the implanted trophoblast makes HCG, its early appearance in the urine of pregnant women provides the basis for the most widely used test for pregnancy (which can provide a positive signal even before menstruation would have otherwise begun). As pregnancy continues, the placenta becomes a major source of progesterone, and its presence is essential to maintain pregnancy. Mothers at risk of giving birth too soon can be given a synthetic progestin to help them retain the fetus until it is full-term.
Birth
Toward the end of pregnancy,
• The placenta releases large amounts of CRH which stimulates the pituitary glands of both mother and her fetus to secrete.
• ACTH, which acts on their adrenal glands causing them to release the estrogen precursor dehydroepiandrosterone sulfate (DHEAS).
• This is converted into estrogen by the placenta.
• The rising level of estrogen causes the smooth muscle cells of the uterus to
• synthesize connexins and form gap junctions. Gap junctions connect the cells electrically so that they contract together as labor begins.
• express receptors for oxytocin.
• Oxytocin is secreted by the posterior lobe of the pituitary as well as by the uterus.
• Prostaglandins are synthesized in the placenta and uterus.
• The normal inhibition of uterine contraction by progesterone is turned off by several mechanisms while
• both oxytocin and prostaglandins cause the uterus to contract and labor begins.
Three or four days after the baby is born, the breasts begin to secrete milk. Milk synthesis is stimulated by the pituitary hormone prolactin (PRL), and its release from the breasts is stimulated by oxytocin. Milk contains an inhibitory peptide. If the breasts are not fully emptied, the peptide accumulates and inhibits milk production. This autocrine action thus matches supply with demand.
Other Hormones
• Relaxin
As the time of birth approaches in some animals (e.g., pigs, rats), this polypeptide has been found to:
• relax the pubic ligaments
• soften and enlarge the opening to the cervix
Relaxin is found in pregnant humans but at higher levels early in pregnancy than close to the time of birth. Relaxin promotes angiogenesis, and in humans it probably plays a more important role in the development of the interface between the uterus and the placenta that it does in the birth process.
• Activins, Inhibins, Follistatin.
These proteins are synthesized within the follicle. Activins and inhibins bind to follistatin. Activins increase the action of FSH; inhibins, as their name suggests, inhibit it. How important they are in humans remains to be seen. However the important role that activin and follistatin play in the embryonic development of vertebrates justifies mentioning them here.
Oral contraceptives - the "pill"
The feedback inhibition of GnRH secretion by estrogens and progesterone provides the basis for the most widely-used form of contraception. Dozens of different formulations of synthetic estrogens or progestins (progesterone relatives or both are available. Their inhibition of GnRH prevents the mid-cycle surge of LH and ovulation. Hence there is no egg to be fertilized. Usually the preparation is taken for about three weeks and then stopped long enough for normal menstruation to occur. The main side-effects of the pill stem from an increased tendency for blood clots to form (estrogen enhances clotting of the blood).
RU-486
RU-486 (also known as mifepristone) is a synthetic steroid related to progesterone. Unlike the synthetic progestins used in oral contraceptives that mimic the actions of progesterone, RU-486 is a progesterone antagonist; that is, it blocks the action of progesterone. It does this by binding more tightly to the progesterone receptor than progesterone itself but without the normal biological effects:
• The RU-486/receptor complex is not active as a transcription factor.
• Thus genes that are turned on by progesterone are turned off by RU-486.
• The proteins needed to establish and maintain pregnancy are no longer synthesized.
• The endometrium breaks down.
• The embryo detaches from it and can no longer make chorionic gonadotropin (HCG).
• Consequently the corpus luteum ceases its production of progesterone.
• The inhibition on uterine contraction is lifted.
• Soon the embryo and the breakdown products of the endometrium are expelled.
These properties of RU-486 have caused it to be used to induce abortion of an unwanted fetus. In practice, the physician assists the process by giving a synthetic prostaglandin (e.g., misoprostol [Cytotec®]) 36–48 hours after giving the dose of RU-486. Use of RU-486 is generally limited to the first seven weeks of pregnancy. RU-486 has been used for many years in some countries. However, the controversies surrounding abortion in the United States kept it from being authorized for use here until September 2000.
Menopause
The menstrual cycle continues for many years. But eventually, usually between 42 and 52 years of age, the follicles become less responsive to FSH and LH. They begin to secrete less estrogen. Ovulation and menstruation become irregular and finally cease. This cessation is called menopause.
With levels of estrogen now running one-tenth or less of what they had been, the hypothalamus is released from their inhibitory influence (bar). As a result it now stimulates the pituitary to increased activity. The concentrations of FSH and LH in the blood rise to ten or more times their former values. These elevated levels may cause a variety of unpleasant physical and emotional symptoms.
Hormone Replacement Therapy (HRT)
Many menopausal women elect to take a combination of estrogen and progesterone after they cease to make their own. The benefits are (1) reduction in the unpleasant symptoms of the menopause and (2) a reduction in the loss of calcium from bones and thus a reduction in osteoporosis and the fractures that accompany it. It was also believed that HRT reduced the risk of cardiovascular disease. However, a recent study of 16,000 menopausal women was stopped 3 years early when it was found that, in fact, HRT increased (albeit only slightly) not decreased the incidence of cardiovascular disease. Perhaps synthetic selective estrogen response modulators or SERMs (raloxifene is an example) will provide the protective effects without the harmful ones.
Environmental estrogens
Some substances that find their way into the environment, such as
• DDE, a breakdown product of the once widely-used insecticide DDT
• DDT itself — still used in some countries (e.g., Mexico)
• PCBs, chemicals once used in a wide variety of industrial applications
can bind to the estrogen (and androgen) receptors and mimic (weakly) the effects of the hormone. This has created anxiety that they may be responsible for harmful effects such as cancer and low sperm counts.
However, there is as yet little evidence to support these worries. No epidemiological relationship has been found between the incidence of breast cancer and the levels of these compounds in the body. As for laboratory studies that found a synergistic effect of two of these substances on receptor binding (findings that created the great alarm), these have not been replicated in other laboratories, and the authors of the original report have since withdrawn it as invalid.
Males
The principal androgen (male sex hormone) is testosterone. This steroid is manufactured by the interstitial (Leydig) cells of the testes. Secretion of testosterone increases sharply at puberty and is responsible for the development of the so-called secondary sexual characteristics (e.g., beard) of men.
Testosterone is also essential for the production of sperm. Production of testosterone is controlled by the release of luteinizing hormone (LH) from the anterior lobe of the pituitary gland, which is in turn controlled by the release of GnRH from the hypothalamus. LH is also called interstitial cell stimulating hormone (ICSH).
Hypothalamus GnRH Pituitary LH Testes Testosterone
The level of testosterone is under negative-feedback control: a rising level of testosterone suppresses the release of GnRH from the hypothalamus. This is exactly parallel to the control of estrogen secretion in females.
In mice, osteocalcin, a hormone secreted by osteoblasts of the bone, stimulates the synthesis of testosterone by Leydig cells even more powerfully than LH. Whether this effect occurs in humans remains to be seen.
Males need estrogen, too!
In 1994, a man was described who was homozygous for a mutation in the gene encoding the estrogen receptor. A single nonsense mutation had converted a codon (CGA) for arginine early in the protein into a STOP codon (TGA). Thus no complete estrogen receptor could be synthesized.
This man was extra tall, had osteoporosis and "knock-knees", but was otherwise well. His genetic defect confirms the important role that estrogen has in both sexes for normal bone development. It is not known whether this man was fertile or not. However, mutations in their estrogen receptor gene have been found in other men who are sterile, and male mice whose estrogen receptor gene has been "knocked out" are sterile.
Another function of estrogen in males. The accumulation of fat in the abdomen, so characteristic of aging males (including yours truly), is caused by declining levels of estrogen.
Anabolic steroids
A number of synthetic androgens are used for therapeutic purposes. These drugs promote an increase in muscle size with resulting increases in strength and speed. This has made them popular with some athletes, e.g., weight lifters, cyclists, runners, swimmers, professional football players. Usually these athletes (females as well as males) take doses far greater than those used in standard therapy. Such illicit use carries dangers (besides being banned from an event because of a positive drug test): acne, a decrease in libido, and — in males — testicle size and sperm counts to name a few.
Genetic abnormalities of gonadal function
Many things can go wrong with sexual development in both males and females, fortunately rarely. Let's look at a few that clearly result from the inheritance of single-gene mutations.
• Inherited mutations in both copies of the gene encoding the GnRH receptor result in failure to develop at puberty.
• Mutations in the gene encoding the LH receptor prevent normal sexual development in both sexes.
• Mutations in the gene encoding the FSH receptor block development of the gonads in both males and females.
• Mutations in any of the genes encoding the enzymes for synthesis and metabolism of testosterone interfere with normal sexual function in males.
• A similar spectrum of disorders in males can be caused by mutations in the genes encoding the androgen receptor. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.07%3A_Sex_Hormones.txt |
Progesterone is one of the steroid hormones. It is secreted by the corpus luteum and by the placenta and is responsible for preparing the body for pregnancy and, if pregnancy occurs, maintaining it until birth.
• Corpus luteum: Progesterone secretion by the corpus luteum occurs after ovulation and continues the preparation of the endometrium for a possible pregnancy. It also inhibits contraction of the uterus and inhibits development of a new follicle. If pregnancy does not occur, secretion wanes toward the end of the menstrual cycle, and menstruation begins.
• Placenta: If pregnancy does occur, the placenta begins to secrete progesterone which supplements that of the corpus luteum. In fact, by the fifth month of pregnancy, the placenta secretes sufficient progesterone by itself that the corpus luteum is no longer essential to maintain pregnancy.
Progesterone, like all steroids, is a small hydrophobic molecule. Thus it diffuses freely through the plasma membrane of all cells. However, in target cells, like those of the endometrium it becomes tightly bound to a cytoplasmic protein the progesterone receptor. The complex of receptor and its hormone moves into the nucleus. There it binds to a progesterone response element, which is a specific sequence of DNA in the promoters of certain genes that is needed to turn those genes on (or off). Thus the complex of progesterone with its receptor forms a transcription factor.
Progestins
Progestins are synthetic modifications of the progesterone molecule. Several different ones are prescribed
• for birth control pills
• for hormone replacement therapy (HRT) to reduce the unpleasant symptoms of the menopause
• to treat young women who cease to menstruate normally
• to prevent premature birth
Some examples:
• norgestrel (Trade name = Orvette®) - used as an oral contraceptive
• levonorgestrel
• the ingredient in "Plan B", an oral contraceptive taken after unprotected intercourse.
• released from Mirena®, an intrauterine device (IUD).
• norethindrone (Trade name = Aygestin®; used in HRT (hormone replacement therapy)
RU-486
RU-486 (also known as mifepristone) is a synthetic steroid related to progesterone. Unlike the progestins discussed above, that mimic the action of progesterone, RU-486 blocks the action of progesterone. Synthetic molecules that mimic the action of a natural molecule are called agonists; those that oppose it are antagonists. RU-486 is a progesterone antagonist. It binds to the progesterone receptor, and in so doing prevents progesterone itself from occupying its receptor. Thus the gene transcription normally turned on by progesterone is blocked, and the proteins necessary to begin and maintain pregnancy are not synthesized. If RU-486 is taken shortly after intercourse, it prevents pregnancy. If taken early in pregnancy, it causes the embryo to be aborted. This result has caused RU-486 to be widely used in Europe to terminate early pregnancy. It has not found widespread acceptance in the U.S.
15.6.1.09: Melatonin and the Pineal Gland
The pineal gland is a tiny structure located at the base of the brain. Its principal hormone is melatonin, a derivative of the amino acid tryptophan.
Synthesis and release of melatonin is
• stimulated by darkness
• inhibited by light.
But even without visual cues, the level of melatonin in the blood rises and falls on a daily (circadian) cycle with peak levels occurring in the wee hours of the morning. However, this cycle tends to drift in people who are totally blind — often making them sleepy during the day and wide awake at night. Giving melatonin at bedtime has proved helpful in a number of cases
Melatonin is readily available in drug stores and health food stores, and it has become quite popular. Ingesting even modest doses of melatonin raises the melatonin level in the blood to as much as 100 times greater than normal. These levels appear:
• to promote going to sleep and thus help insomnia
• to hasten recovery from jet lag
• not to have dangerous side effects | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.08%3A_Progesterone.txt |
Kidney
The human kidney secretes two hormones:
• Erythropoietin (EPO)
• Calcitriol (1,25[OH]2 Vitamin D3)
as well as the enzyme renin.
Erythropoietin (EPO)
Erythropoietin is a glycoprotein that acts on the bone marrow to increase the production of red blood cells. Stimuli such as bleeding or moving to high altitudes (where oxygen is scarcer) trigger the release of EPO. People with failing kidneys can be kept alive by dialysis, which only cleanses the blood of wastes. Without a source of EPO, these patients suffer from anemia. Now, thanks to recombinant DNA technology, recombinant human EPO is available to treat these patients.
Some other uses for recombinant EPO:
• Some of the drugs used to treat AIDS, zidovudine (AZT) for example, cause anemia as a side effect. Recombinant EPO helps AIDS patients cope with this one of the many problems that the disease creates.
• Recombinant EPO improves the anemia that is such a frequent side effect of cancer chemotherapy.
• Severe blood loss in Jehovah's Witnesses, whose religion forbids them to receive blood transfusions, can also be helped with recombinant EPO.
Because EPO increases the hematocrit, it enables more oxygen to flow to the skeletal muscles. Some cyclists (and distance runners) have used recombinant EPO to enhance their performance. Although recombinant EPO has exactly the same sequence of amino acids as the natural hormone, the sugars attached by the cells used in the pharmaceutical industry differ from those attached by the cells of the human kidney. This difference can be detected by a test of the athlete's urine.
Another problem: since recombinant EPO became available, over two dozen young competitive cyclists have died unexpectedly (usually during the night). Perhaps an EPO-induced increase in their hematocrit — leading to a reduction in heart rate — is responsible. Prolonged exposure to reduced oxygen levels (e.g., living at high altitude) leads to increased synthesis of EPO. In mice, and perhaps in humans, this effect is mediated by the skin. Mouse skin cells can detect low levels of oxygen ("hypoxia") and if this persists, blood flow to the kidneys diminishes leading to increased synthesis of EPO by them.
EPO is also synthesized by osteoblasts in mice that have been made anemic and in the brain when oxygen becomes scarce there (e.g., following a stroke), and helps protect neurons from damage. Perhaps recombinant human EPO will turn out to be useful for stroke victims as well.
Calcitriol
Calcitriol is 1,25[OH]2 Vitamin D3, the active form of vitamin D. It is derived from
• calciferol (vitamin D3) which is synthesized in skin exposed to the ultraviolet rays of the sun
• precursors ("vitamin D") ingested in the diet.
Calciferol in the blood is converted into the active vitamin in two steps:
• calciferol is converted in the liver into 25[OH] vitamin D3
• this is carried to the kidneys (bound to a serum globulin) where it is converted into calcitriol. This final step is promoted by the parathyroid hormone (PTH).
Calcitriol Action
Calcitriol acts on
• the cells of the intestine to promote the absorption of calcium and phosphate from food
• bone to mobilize calcium from the bone to the blood
Calcitriol enters cells and, if they contain receptors for it (intestine cells do), it binds to them. The calcitriol receptors are zinc-finger transcription factors. The receptor-ligand complex bind to its response element, the DNA sequence:
5' AGGTCAnnnAGGTCA 3'
This sequence of nucleotides (n can be any nucleotide) is found in the promoters of genes that are turned on by calcitriol. Once the hormone-receptor complex is bound to its response element, other transcription factors are recruited to the promoter and transcription of the gene(s) begins.
Deficiency disorders
Insufficient calcitriol prevents normal deposition of calcium in bone.
• In childhood, this produces the deformed bones characteristic of rickets.
• In adults, it produces weakened bones causing osteomalacia.
Angiotensin II modulates all of the below actions to increase in blood pressure:
• constricts the walls of arterioles closing down capillary beds
• stimulates the proximal tubules in the kidney to reabsorb sodium ions
• stimulates the adrenal cortex to release aldosterone. Aldosterone causes the kidneys to reclaim still more sodium and thus water.
• increases the strength of the heartbeat
• stimulates the pituitary to release the vasopressin
Skin
When ultraviolet radiation strikes the skin, it triggers the conversion of dehydrocholesterol (a cholesterol derivative) into calciferol (vitamin D3). Calciferol travels in the blood to the liver where it is converted into 25[OH] vitamin D3. This compound travels to the kidneys where it is converted into calcitriol (1,25 [OH]2 vitamin D3). This final step is promoted by the parathyroid hormone (PTH). Although called a vitamin, calciferol and its products fully qualify as hormones because they are
• made in certain cells
• carried in the blood
• affect gene transcription in target cells
Heart
Natriuretic Peptides
In response to a rise in blood pressure, the heart releases two peptides:
• A-type Natriuretic Peptide (ANP)
This hormone of 28 amino acids is released from stretched atria (hence the "A").
• B-type Natriuretic Peptide (BNP)
This hormone (29 amino acids) is released from the ventricles. (It was first discovered in brain tissue; hence the "B".)
Both hormones lower blood pressure by
• relaxing arterioles
• inhibiting the secretion of renin and aldosterone
• inhibiting the reabsorption of sodium ions by the kidneys.
The latter two effects reduce the reabsorption of water by the kidneys. So the volume of urine increases as does the amount of sodium excreted in it. The net effect of these actions is to reduce blood pressure by reducing the volume of blood in the circulatory system. These effects give ANP and BNP their name (natrium = sodium; uresis = urinate). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.10%3A_Hormones_of_Kidney_Skin_and_Heart.txt |
Leptin
Several strains of laboratory mice are homozygous for single-gene mutations that causes them to become grossly obese.
These fall into two classes:
• When ob/ob mice are treated with injections of leptin they lose their excess fat and return to normal body weight.
• db/db = mutations in the gene that encodes the receptor for leptin
Study of these animals has led to an understanding of the action of leptin in humans.
Human leptin is a protein of 167 amino acids. It is manufactured in the adipocytes (fat cells) of white adipose tissue, and the level of circulating leptin is directly proportional to the total amount of fat in the body.
Leptin acts on receptors in the hypothalamus of the brain where it:
• counteracts the effects of neuropeptide Y (a potent feeding stimulant secreted by cells in the gut and in the hypothalamus)
• counteracts the effects of anandamide (another potent feeding stimulant that binds to the same receptors as THC, the active ingredient of marijuana)
• promotes the synthesis of α-MSH, an appetite suppressant
• the result is the inhibition of food intake
This inhibition is long-term, in contrast to
• the rapid inhibition of eating by cholecystokinin (CCK)
• the slower suppression of hunger between meals mediated by PPY3-36
The absence of a functional hormone (or its receptor) leads to uncontrolled food intake and resulting obesity.
Leptin also acts on hypothalamic neurons responsible for
• the conversion of white adipose tissue (WAT) into "beige" adipose tissue. "Beige" cells metabolize food into heat rather than storing it as fat. In mice, leptin promotes weight loss even with normal food intake.
• the secretion of gonadotropin-releasing hormone (GnRH). Women who are very thin from limited food intake or intense physical training may cease to menstruate because of their lack of leptin-secreting fat cells. Treating them with recombinant human leptin can sometimes restore normal menstruation.
• stimulating the sympathetic nervous system to trigger the breakdown of fat in adipose tissue.
In addition to its effect on the hypothalamus, leptin acts directly on the cells of the liver and skeletal muscle where it stimulates the oxidation of fatty acids in the mitochondria. This reduces the storage of fat in those tissues. T cells where it enhances the production of Th1 cells promoting inflammation. Mice without leptin are protected from autoimmune disease (which may account for the reports that restricting food intake helps humans with rheumatoid arthritis).
Mutations in the gene for leptin, or in its receptor, are rarely found in obese people.
The rare cases:
• Extreme obesity in five members of two families that are homozygous for mutations (frameshift in one family, missense in the other) in their leptin gene; i.e., they are like ob/ob mice.
• Extreme obesity among three members of a family that are homozygous for mutations in their leptin receptor gene; i.e., they are like db/db mice.
• Only moderate obesity in people who are heterozygous (one mutant and one normal) for their leptin genes.
Recombinant human leptin is now available, but trials to see if it can reduce obesity in humans as it does in ob/ob mice have been disappointing.
However, the 16 September 1999 issue of The New England Journal of Medicine reported the results of a year-long trial of recombinant human leptin in a 9-year-old girl who is homozygous for a frameshift mutation in her leptin genes. The findings:
• She began the trial weighing 208 pounds (94.4 kg), of which 123 lbs (55.9 kg) was fat (adipose tissue).
• She was given daily injections of recombinant leptin for one year.
• At the end of that time,
• she had lost 36 lbs (16.4 kg), most of it fat.
• Her appetite and thus food intake had decreased.
• Her immune system made antileptin antibodies but these did not seem to interfere with the action of the hormone.
But trials of recombinant leptin in obese humans who do not have mutations in both their leptin genes have not shown any great benefit in weight reduction. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.11%3A_Leptin_-_the_Fat_Hormone.txt |
The liver synthesizes and secretes at least four important hormones:
• Insulin-like Growth Factor-1 (IGF-1)
• Angiotensinogen
• Thrombopoietin
• Hepcidin
• Betatrophin
Insulin-like Growth Factor-1
This protein of 70 amino acids was once called somatomedin because it is not growth hormone but is the immediate stimulus for growth of the body. Growth hormone released from the anterior lobe of the pituitary binds to receptors on the surface of liver cells which stimulates the synthesis and release of IGF-1 from them. Many cells have receptors for IGF-1, especially cells in the bone marrow and in the cartilaginous growing regions of the long bones.
Binding of IGF-1 to cells with receptors for it stimulates them to move from G1 of the cell cycle to S phase and on to mitosis.
Mice with one of their Igf-1receptor genes "knocked out" live 25% longer than normal mice. This may result from an increase in their resistance to the damaging effects of reactive oxygen species (ROS) or to an increased efficiency at clearing away clumped proteins in their cells (or both). These heterozygous mice appear to be normal in every other respect.
The levels of IGF-1 in the blood are highest during the years of puberty which is, of course, a time of rapid growth. Occasionally children are found that have stunted growth because they have inherited mutant genes for the growth hormone (GH) receptor. Recombinant human IGF-1 has been successfully used to treat them.
Angiotensinogen
This protein is released into the blood where it serves as the precursor for angiotensin. How angiotensin is manufactured, and the role it plays in maintaining blood pressure, is described in the discussion of renin.
Thrombopoietin (TPO)
Thrombopoietin is a protein of 332 amino acids. It stimulates precursor cells in the bone marrow to differentiate into megakaryocytes. Megakaryocytes generate platelets, essential to blood clotting.
The production of megakaryocytes — and thus platelets — is under homeostatic control. It works like this:
• Circulating platelets are covered with receptors for TPO.
• So are megakaryocytes and their precursors, but there are fewer of them.
• When platelet counts are high, most of the circulating TPO is bound to the platelets and less is left to stimulate megakaryocytes.
• When platelet counts drop, more TPO becomes available to stimulate megakaryocytes to replenish the platelet supply.
• Humans manufacture about 1011 platelets each day under normal conditions.
15.6.1.13: Melanocyte Stimulating Hormone (MSH)
Melanocyte-stimulating hormone gets its name because of its effect on melanocytes: skin cells that contain the black pigment, melanin. In humans, melanocytes are responsible for moles, freckles, and suntan (and, if they turn cancerous, melanoma). In most vertebrates, MSH is produced by an intermediate lobe of the pituitary gland. Its secretion causes a dramatic darkening of the skin of fishes, amphibians, and reptiles. The darkening occurs as granules of melanin spread through the branches of specialized melanocytes called melanophores.
The photomicrograph on the right shows melanophores in the skin of a frog with the melanin dispersed throughout the branches of the cells. This effect is produced by MSH. When the pigment retreats to the center of the cells, the skin lightens.
• The granules are carried outward along microtubules using kinesin as the motor.
• They assemble at the actin-rich periphery of the cell carried by a myosin.
• The granules are carried back to the center of the cell along microtubules using dynein as the motor.
The above photo was taken a few moments after the frog on the right was injected with a small dose of MSH. The response to MSH does not occur during mitosis; presumably the microtubules with their dyneins and kinesins are needed for operation of the mitotic spindle.
Tanning
Proopiomelanocortin (POMC), the same precursor molecule from which the adrenocorticotropic hormone (ACTH) is synthesized, also produces two forms of MSH. One of them, α-MSH, is identical to the first 13 amino acids at the amino terminal of ACTH. α-MSH is responsible for tanning in humans.
• When ultraviolet light strikes skin cells (keratinocytes), it activates the transcription factor p53.
• p53 turns on transcription of the gene encoding POMC.
• Cleavage of the POMC protein produces
• α-MSH. This is secreted from the cells and stimulates nearby melanocytes (thus a paracrine effect) to synthesize melanin in packets called melanosomes. The melanosomes are transferred to the skin cells where they form a protective cap over the nucleus. This cap helps protect the DNA within the nucleus from the damaging effects of ultraviolet radiation.
• ACTH. This is secreted into the blood and may help reduce skin inflammation by stimulating the release of glucocorticoids from the adrenal cortex.
• β-endorphin. This may help suppress the pain of sunburn. In mice (and perhaps humans) the rise in the level of β-endorphin upon exposure to uv light activates mu (µ) receptors — the same ones that opiates bind to. This leads to similar addictive behavior - the mice seek uv exposure and show signs of withdrawal when β-endorphin binding is blocked (by naloxone).
Injections of a synthetic version of α-MSH called Melanotan I also darkens the skin. This raises the possibility of using melanotan to get a suntan without the risks of exposure to ultraviolet light. A second synthetic version of MSH dubbed Melanotan II also darkened the skin of male volunteers. Unexpectedly, it also caused many of them to develop penile erections. This has raised the possibility of using MSH to cure impotence.
MSH and appetite
Humans have no intermediate lobe in their pituitary gland, and MSH may not be a circulating hormone for us. However, α-MSH is produced by POMC neurons in the brain where it acts to suppress appetite. Some cases of extreme obesity have been traced to mutations in the brain receptor for α-MSH. Presumably these people are unable to respond to the appetite-suppressing effect of their α-MSH. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.01%3A_Human_Hormones/15.6.1.12%3A_Hormones_of_the_Liver.txt |
Because of their rigid exoskeleton, insects can only grow by periodically shedding their exoskeleton - called molting. Molting occurs repeatedly during larval development. At the final molt, the adult emerges. In several insect orders, notably
• ants and bees (Hymenoptera) — example the honeybee
• flies (Diptera) — example Drosophila melanogaster
• butterflies and moths (Lepidoptera) such as the silkworm moth
the adult looks entirely different from the larva that preceded it. This marked transformation is called complete metamorphosis.
Complete metamorphosis takes place during a seemingly-dormant stage called the pupa. In fact, intense biological activity is going on within the pupal case. The cells of virtually all the differentiated larval structures muscles, salivary glands, gut, etc. die by apoptosis. The nutrients they release are then available for the further development of nests of cells - the imaginal discs - that have been quietly developing within the larval body. Their differentiation produces the structures of the adult - legs, wings, compound eyes, etc.
The sequence ending in the center panel (B) shows the larval, pupal, and adult stages during normal development of the domestic silkworm moth, Bombyx mori.
Prothoracicotropic Hormone (PTTH)
Molting and pupation require the hormone, PTTH, secreted by a two pairs of cells in the brain of the larva. If these cells are cut out of the brain of a full-grown larva, pupation does not occur. This is not because of the trauma of surgery; if transplanted somewhere else in the caterpillar's body, pupation occurs normally. PTTH is a homodimer of two polypeptides of 109 amino acids. PTTH does not drive pupation directly but, as its name suggests, acts on the prothoracic glands.
Ecdysone
There are two prothoracic glands located in the thorax. Under the influence of PTTH, they secrete the steroid hormone ecdysone. Acting together, PTTH and ecdysone trigger every molt: larva-to-larva as well as pupa-to-adult. What, then, accounts for the dramatic changes of metamorphosis?
Juvenile Hormone (JH)
Juvenile hormone is secreted by two tiny glands behind the brain, the corpora allata. As long as there is enough JH, ecdysone promotes larva-to-larva molts. With lower amounts of JH, ecdysone promotes pupation. Complete absence of JH results in formation of the adult.
So if the corpora allata are removed from an immature silkworm, it immediately spins a cocoon and becomes a small pupa. A miniature adult eventually emerges (shown in panel (A) above). Conversely, if the corpora allata of a young silkworm are place in the body of a fully-mature larva, metamorphosis does not occur. The next molt produces an extra-large caterpillar (panel (C) above).
JH affects gene expression
Adult insects do not normally molt, but if extra amounts of PTTH are given to an adult Rhodnius (the "kissing bug"), it is forced into an extra molt. The English insect physiologist V. B. Wigglesworth showed that if juvenile hormone is first applied to the insect's exoskeleton, the regions affected by it revert to larval type after this extra molt.
What a beautiful example of the power of a single molecule to unleash a different pattern of gene expression! Presumably, JH interacts with hormone receptors in the cells to produce a new set of transcription factors.
Insect hormones and pest control
Knowledge of insect hormones has provided a number of opportunities to enlist them or molecules related to them in the battle against insect pests. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.06%3A_Hormones/15.6.02%3A_Insect_Hormones.txt |
Sexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes plants as well as animals the two gametes differ in structure ("anisogamy") and are contributed by different parents. Gametes need motility to be able to meet and unite and food to nourish the developing embryo. In animals (and some plants), these two rather contrasting needs are met by anisogametes: sperm that are motile (and small) and eggs that contain food.
15.07: Sexual Reproduction
Sexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes plants as well as animals the two gametes differ in structure ("anisogamy") and are contributed by different parents. Gametes need motility to be able to meet and unite and food to nourish the developing embryo. In animals (and some plants), these two rather contrasting needs are met by anisogametes: sperm that are motile (and small) and eggs that contain food.
Sex Organs of the Human Male
The reproductive system of the male has two major functions: (1) production of sperm and (2) delivery of these to the reproductive tract of the female. Sperm production - spermatogenesis - takes place in the testes. Each testis is packed with seminiferous tubules (laid end to end, they would extend more than 20 meters) where spermatogenesis occurs.
Spermatogenesis
The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of sperm. Spermatogonia divide by mitosis to produce more spermatogonia or differentiate into spermatocytes. Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete. Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process.
For simplicity, the figure shows the behavior of just a single pair of homologous chromosomes with a single crossover. With 22 pairs of autosomes and an average of two crossovers between each pair, the variety of gene combinations in sperm is very great.
Sperm
Sperm cells are little more than flagellated nuclei. Each consists of a head, which has an acrosome at its tip and contains a haploid set of chromosomes in a compact, inactive, state, a midpiece containing mitochondria and a single centriole, and a tail which is a flagellum. This electron micrograph (courtesy of Dr. Don W. Fawcett and Susumu Ito) shows the sperm cell of a bat. Note the orderly arrangement of the mitochondria. They supply the ATP to power the whiplike motion of the tail.
An adult male manufactures over 100 million sperm cells each day. These gradually move into the epididymis where they undergo further maturation. The acidic environment in the epididymis keeps the mature sperm inactive. In addition to making sperm, the testis is an endocrine gland. Its principal hormone, testosterone, is responsible for the development of the secondary sex characteristics of men such as the beard, deep voice, and masculine body shape. Testosterone is also essential for making sperm. Testosterone is made in the interstitial cells (also called Leydig cells) that lie between the seminiferous tubules.
LH
Interstitial cells are, in turn, the targets for a hormone often called interstitial cell stimulating hormone (ICSH). It is a product of the anterior lobe of the pituitary gland. However, ICSH is identical to the luteinizing hormone (LH) found in females, and I prefer to call it LH.
FSH
Follicle-stimulating hormone (also named for its role in females) acts directly on spermatogonia to stimulate sperm production (aided by the LH needed for testosterone synthesis).
Sex Organs of the Human Female
The responsibility of the female mammal for successful reproduction is considerably greater than that of the male.
She must
• manufacture eggs
• be equipped to receive sperm from the male
• provide an environment conducive to fertilization and implantation
• nourish the developing baby not only before birth but after too
Oogenesis
Egg formation takes place in the ovaries. In contrast to males, the initial steps in egg production occur prior to birth. Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed (*). By the time she is born, only about 1 million of these remain (the others eliminated by apoptosis). Each has begun the first steps of the first meiotic division stopping at the diplotene stage of meiosis I.
No further development occurs until years later when the girl becomes sexually mature. Then the primary oocytes recommence their development, usually one at a time and once a month. The primary oocyte grows much larger and completes meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.
In humans (and most vertebrates), the first polar body does not go on to meiosis II, but the secondary oocyte does proceed as far as metaphase of meiosis II and then stops.
Only if fertilization occurs will meiosis II ever be completed. Entry of the sperm restarts the cell cycle breaking down MPF (M-phase promoting factor) and turning on the anaphase-promoting complex (APC). Completion of meiosis II converts the secondary oocyte into a fertilized egg or zygote (and also a second polar body). As in the diagram for spermatogenesis, the behavior of the chromosomes is greatly simplified.
The photomicrograph (courtesy of Turtox) shows polar body formation during oogenesis in the whitefish. Even allowing for the fact that fish eggs are larger than mammalian eggs, you can readily see how the polar body gets little more than one set of chromosomes.
These events take place within a follicle, a fluid-filled envelope of cells surrounding the developing egg. The ripening follicle also serves as an endocrine gland. Its cells make a mixture of steroid hormones collectively known as estrogen. Estrogen is responsible for the development of the secondary sexual characteristics of a mature woman, e.g.,
• a broadening of the pelvis
• development of the breasts
• growth of hair around the genitals and in the armpits
• development of adipose tissue leading to the more rounded body contours of adult women.
Estrogen continues to be secreted throughout the reproductive years of women During this period, it plays an essential role in the monthly menstrual cycle.
Ovulation
Ovulation occurs about two weeks after the onset of menstruation. In response to a sudden surge of LH, the follicle ruptures and discharges a secondary oocyte. This is swept into the open end of the fallopian tube and begins to move slowly down it.
Copulation and Fertilization
For fertilization to occur, sperm must be deposited in the vagina within a few (5) days before or on the day of ovulation. Sperm transfer is accomplished by copulation. Sexual excitation dilates the arterioles supplying blood to the penis. Blood accumulates in three cylindrical spongy sinuses that run lengthwise through the penis. The resulting pressure causes the penis to enlarge and erect and thus able to penetrate the vagina. Movement of the penis back and forth within the vagina causes sexual tension to increase to the point of ejaculation. Contraction of the walls of each vas deferens propels the sperm along. Fluid is added to the sperm by the seminal vesicles, Cowper's glands, and the prostate gland. These fluids provide
• a source of energy (fructose)
• an alkaline environment to activate the sperm
• perhaps in other ways provide an optimum chemical environment for them
The mixture of sperm and accessory fluids is called semen. It passes through the urethra and is expelled into the vagina. Physiological changes occur in the female as well as the male in response to sexual excitement, although these are not as readily apparent. In contrast to the male, however, such responses are not a prerequisite for copulation and fertilization to occur. Once deposited within the vagina, the sperm proceed on their journey into and through the uterus and on up into the fallopian tubes. It is here that fertilization may occur if an "egg" is present (strictly speaking, it is still a secondary oocyte until after completion of meiosis II).
Although sperm can swim several millimeters each second, their trip to and through the fallopian tubes may be assisted by muscular contraction of the walls of the uterus and the tubes. The trip is fraught with heavy mortality. An average human ejaculate contains over one hundred million sperm, but only a few dozen complete the journey, arriving within 15 minutes of ejaculation. And of these, only one will succeed in fertilizing the egg.
Sperm swim towards the egg by chemotaxis following a gradient of progesterone secreted by cells surrounding the egg. Progesterone opens CatSper ("cation sperm") channels in the plasma membrane surrounding the anterior portion of the sperm tail. This allows an influx of Ca2+ ions which causes the flagellum to beat more rapidly and vigorously. Fertilization begins with the binding of a sperm head to the glycoprotein coating of the egg (called the zona pellucida). Exocytosis of the acrosome at the tip of the sperm head releases enzymes that digest a path through the zona and enable the sperm head to bind to the plasma membrane of the egg. The binding is mediated by the binding of two membrane proteins:
• Izumo1 on the surface of the sperm
• Juno, its receptor on the egg surface
Fusion of their respective membranes allows the entire contents of the sperm to be drawn into the cytosol of the egg. Even though the sperm's mitochondria enter the egg, they are almost always destroyed by autophagy and do not contribute their genes to the embryo. So human mitochondrial DNA is almost always inherited from mothers only. Within moments, enzymes released from the egg cytosol act on the zona making it impermeable to other sperm that arrive. Soon the nucleus of the successful sperm enlarges into the male pronucleus. At the same time, the egg (secondary oocyte) completes meiosis II forming a second polar body and the female pronucleus.
The male and female pronuclei move toward each other while duplicating their DNA in S phase. Their nuclear envelopes disintegrate. A spindle is formed (following replication of the sperm's centriole), and a full set of dyads assembles on it. The fertilized egg or zygote is now ready for its first mitosis. When this is done, 2 cells each with a diploid set of chromosomes are formed.
In sea urchins, the block to additional sperm entry and the fusion of the pronuclei are triggered by nitric oxide generated in the egg by the sperm acrosome.
Pregnancy
Development begins while the fertilized egg is still within the fallopian tube. Repeated mitotic divisions produces a solid ball of cells called a morula. Further mitosis and some migration of cells converts this into a hollow ball of cells called the blastocyst. Approximately one week after fertilization, the blastocyst embeds itself in the thickened wall of the uterus, a process called implantation, and pregnancy is established.
Fig.15.7.1.7 Blastocyst
The blastocyst produces two major collections of cells:
• Three or four blastocyst cells develop into the inner cell mass, which will form 3 extraembryonic membranes: amnion, yolk sac, and (a vestigial) allantois and in about 2 months, become the fetus and, ultimately, the baby.
• The remaining 100 or so cells form the trophoblast, which will develop into the chorion that will go on to make up most of the placenta. All the extraembryonic membranes play vital roles during development but will be discarded at the time of birth.
The placenta grows tightly fused to the wall of the uterus. Its blood vessels, supplied by the fetal heart, are literally bathed in the mother's blood. Although there is normally no mixing of the two blood supplies, the placenta does facilitate the transfer of a variety of materials between the fetus and the mother such as
• receiving food
• receiving oxygen and discharging carbon dioxide
• discharging urea and other wastes
• receiving antibodies (chiefly of the IgG class). These remain for weeks after birth, protecting the baby from the diseases to which the mother is immune.
But the placenta is not simply a transfer device. Using raw materials from the mother's blood, it synthesizes large quantities of proteins and also some hormones. The metabolic activity of the placenta is almost as great as that of the fetus itself. The umbilical cord connects the fetus to the placenta. It receives deoxygenated blood from the iliac arteries of the fetus and returns oxygenated blood to the liver and on to the inferior vena cava.
Because its lungs are not functioning, circulation in the fetus differs dramatically from that of the baby after birth. While within the uterus, blood pumped by the right ventricle bypasses the lungs by flowing through the foramen ovale and the ductus arteriosus.
Although the blood in the placenta is in close contact with the mother's blood in the uterus, intermingling of their blood does not normally occur. However, some of the blood cells of the fetus usually do escape into the mother's circulation — where they have been known to survive for decades. This raises the possibility of doing prenatal diagnosis of genetic disorders by sampling the mother's blood rather than having to rely on the more invasive procedures of amniocentesis and chorionic villus sampling (CVS).
Fragments of fetal DNA (~ 300 bp long) from apoptotic cells of the placenta are also found in the mother's plasma as early as 5 weeks after implantation. These can be tested for various forms of aneuploidy, e.g. the trisomy 21 of Down syndrome. Far rarer is the leakage of mother's blood cells into the fetus. However, it does occur. A few pregnant women with leukemia or lymphoma have transferred the malignancy to their fetus. Some babies have also acquired melanoma from the transplacental passage of these highly-malignant cells from their mother.
During the first 2 months of pregnancy, the basic structure of the baby is being formed. This involves cell division, cell migration, and the differentiation of cells into the many types found in the baby. During this period, the developing baby - called an embryo - is very sensitive to anything that interferes with the steps involved. Virus infection of the mother, e.g., by rubella ("German measles") virus or exposure to certain chemicals may cause malformations in the developing embryo. Such agents are called teratogens ("monster-forming"). The tranquilizer, thalidomide, taken by many pregnant European women between 1954 and 1962, turned out to be a potent teratogen and was responsible for the birth of several thousand deformed babies.
After about two months, all the systems of the baby have been formed, at least in a rudimentary way. From then on, development of the fetus, as it is now called, is primarily a matter of growth and minor structural modifications. The fetus is less susceptible to teratogens than is the embryo. Pregnancy involves a complex interplay of hormones.
The placenta is an allograft
One of the greatest unsolved mysteries in immunology is how the placenta survives for 9 months without being rejected by the mother's immune system. Every cell of the placenta carries the father's genome (a haploid set of his chromosomes); including one of his #6 chromosomes where the genes for the major histocompatibility antigens (HLA) are located.
One partial exception: none of the genes on the father's X chromosome are expressed. While X-chromosome inactivation is random in the cells of the fetus, it is NOT random in the cells of the trophoblast. In every cell of the trophoblast — and its descendants — it is the paternal X chromosome that is inactivated. But this does not solve our problem because the genes for all the major histocompatibility antigens are located on chromosome 6, which is not inactivated.
Thus the placenta is immunologically as foreign to the mother as a kidney transplant would be. Yet it thrives. Despite a half-century of research, the mechanism for this immunologically privileged status remains uncertain. But one thing is clear - The mother is not intrinsically tolerant of the father's antigens. She will promptly reject a skin transplant from the father. She develops antibodies against his histocompatibility antigens expressed by the fetus. In fact, women who have borne several children by the same father are often excellent sources of anti-HLA serum for use in tissue typing. So what accounts for the phenomenon? Some possibilities:
• The placenta does not express class II histocompatibility antigens.
• Nor does it express the strongly-immunogenic class I histocompatibility antigens (HLA-A, HLA-B). It does express HLA-C, but this is only weakly immunogenic.
• The cells of the placenta secrete progesterone, which is immunosuppressive.
• In lab rats the embryos (and the mother's endometrium) secrete corticotropin-releasing hormone (CRH). This hormone induces the expression of Fas ligand (FasL) on the cells of the placenta. Activated T cells express Fas so any threatening T cells would commit suicide by apoptosis when they encounter FasL on their target.
• In laboratory mice the cells of the placenta degrade the amino acid tryptophan. Tryptophan is essential for T-cell function. When mice are treated with an inhibitor of the Trp-degrading enzyme, their fetuses are promptly aborted by the action of the mother's lymphocytes. (D. H. Munn, et. al., Science, 281: 1191, 21 Aug 1998.)
• In mice, expression of genes encoding cytokines needed to attract effector T cells (e.g., CTLs) into a tissue is suppressed in the cells of the placenta.
• Perhaps most important of all is the increased production in the mother of immunosuppressive regulatory T cells (Treg).
• Depletion of Treg cells in pregnant mice leads to spontaneous abortion while
• injection of Treg cells into mice that are otherwise prone to abortion enables them to carry their fetuses to term.
• In humans, the number of Treg cells rises during pregnancy (in the fetus as well as the mother).
In vitro fertilization
On July 25, 2013 Louise Brown celebrated her 35th birthday. She was the first of what today number some four million (worldwide) "test tube babies"; that is, she developed from an egg that was fertilized outside her mother's body - the process called in vitro fertilization (IVF). IVF involves
• Harvesting mature eggs from the mother. This is not an easy process. The mother must undergo hormonal treatments to produce multiple eggs, which then must be removed (under anesthesia) from her ovaries.
• Harvesting sperm from the father. Harvesting is usually no problem, but often the sperm are defective in their ability to fertilize (so setting the stage for ICSI).
• Mixing sperm and eggs in a culture vessel ("in vitro").
• Culturing the fertilized eggs for several days until they have developed to at least the 8-cell stage.
• Placing two or more of these into the mother's uterus (which her hormone treatments have prepared for implantation).
• Keeping one's fingers crossed as only about one-third of the attempts result in a successful pregnancy.
Intra cytoplasmic Sperm Injection (ICSI)
Successful IVF assumes the availability of healthy sperm. But many cases of infertility arise from defects in the father's sperm. Often these can be overcome by directly injecting a single sperm into the egg. In the U.S. today, some two-thirds of ART procedures employ ICSI (even though as many as half of these do not involve male infertility).
Ooplasmic Transfer
Infertility in some cases may stem from defects in the cytoplasm of the mother's egg. To circumvent these, cytoplasm can be removed from the egg of a young, healthy woman ("Donor egg") and injected along with a single sperm into the prospective mother's egg. Although a few healthy children have been born following ooplasmic transfer, the jury is still out on its safety, and it is not approved for use in the U.S. One reason for concern is that ooplasmic transfer results in an egg carrying both the mother's mitochondria and mitochondria from the donor (in normal fertilization, all the mitochondria in the father's sperm are destroyed in the egg). This condition called heteroplasmy creates a child having two different mitochondrial DNA genomes in all of its cells.
Fig.15.7.1.9 ooplasmic transfer
In rare, but important, cases, the defect in the prospective mother's cytoplasm is the result of her having mitochondria with a mutant gene. Ooplasmic transfer is of no help in these cases because the fertilized egg will still contain a preponderance of the mother's defective mitochondria. But three techniques worked out on laboratory animals show promise of being adapted to aid such women to produce healthy young.
Three Possible Ways to Prevent Transmission of Mitochondrial Diseases
Maternal Spindle Transfer
Researchers in Oregon reported in the 17 September 2009 issue of Nature that they had been able to produce 4 healthy rhesus monkeys with no mitochondria from their biological mother.
Their procedure:
• Remove the spindle with all its attached chromosomes from the mother's oocyte at metaphase II of meiosis. They managed to do this without any of her mitochondria being withdrawn as well.
• Enucleate the oocyte of the mitochondria donor and then insert the mother's chromosomes — still attached to the spindle — into it. Then inject a sperm from the father.
• Allow the fertilized egg to develop into a blastocyst.
• Implant this in the uterus of a surrogate mother.
• The result: 4 healthy babies each with the nuclear genes of their mother and father but none of the mitochondria of their mother.
Pronuclear Transfer
In this procedure,
• An egg containing mutant mitochondria is fertilized by IVF.
• The two pronuclei are removed and injected into a fertilized egg with healthy mitochondria and whose own pronuclei have been removed.
• Allow the egg to develop into a blastocyst.
• Implant this in the uterus.
Both procedures (1) and (2) are under investigation for use in humans.
Polar Body Transfer
Both the first polar body (formed before fertilization) and the second polar body (formed after fertilization) contain a genome equivalent to that of the secondary oocyte and zygote respectively. However, they contain few, if any, mitochondria. In mice, transfer of either polar body to an enucleated recipient egg (with healthy mitochondria) yield young mice with few, if any, of the donor mother's mitochondria with their defective mtDNA. So in mice, at least, this technique produces offspring with less heteroplasmy than the other two techniques.
If any of these techniques can be applied to humans (there are safety and regulatory hurdles still to be overcome), it would allow women carrying defective mitochondria to bear babies free of the ailment.
The Upside of ART
• It has allowed some four million previously-infertile couples to have children.
• It permits screening (on one cell removed from the 8-celled morula) for the presence of genetic disorders - thus avoiding starting a pregnancy if a disorder is found.
• One can use frozen sperm allowing fatherhood for a man who is no longer able to provide fresh sperm.
• Because a number of morulas are created, the extras can be frozen, stored, and used later
• if the initial attempt fails (the prospective mother must still receive hormones to prepare her uterus for implantation and the success rate is lower with thawed morulas).
• Where regulations permit, the extras can be used as a source of embryonic stem (ES) cells.
The Downside of ART
• Although improving, the success rate is still sufficiently low (~35%) that the process often has to be repeated (which is physically demanding as well as very expensive).
• Because several morulas are usually transferred, multiple births are common (about 50%), and as is often the case with multiple births, the babies are born early and weigh less (~one-third of all ART babies in the U.S. are born early). To reduce the number of twins, triplets, etc., more ART centers are turning to "single-embryo transfer" (SET). Some ART centers find that they can increase the success rate and thus rely more on SET by culturing the morulas for 5–6 days, instead of the usual 2–3 days, before transferring them (by now they have become blastocysts) to the mother.
• The risk of birth defects may be increased slightly (from ~6% in "normal" pregnancies to ~8% in ART pregnancies).
• ART procedures in experimental animals often result in a failure of correct gene imprinting. Whether this will pose a problem for humans remains to be seen.
Birth and Lactation
Exactly what brings about the onset of labor is still not completely understood. Probably a variety of integrated hormonal controls are at work. A growing body of evidence implicates a rise in the level of fetal DNA in the mother's blood as a trigger for the onset of labor. The first result of labor is the opening of the cervix. With continued powerful contractions, the amnion ruptures and the amniotic fluid (the "waters") flows out through the vagina. The baby follows, and its umbilical cord can be cut. The infant's lungs expand, and it begins breathing. This requires a major switchover in the circulatory system. Blood flow through the umbilical cord, ductus arteriosus, and foramen ovale ceases, and the adult pattern of blood flow through the heart, aorta, and pulmonary arteries begins. In some infants, the switchover is incomplete, and blood flow through the pulmonary arteries is inadequate. Failure to synthesize enough nitric oxide (NO) is one cause. Shortly after the baby, the placenta and the remains of the umbilical cord (the "afterbirth") are expelled.
At the time of birth, and for a few days after, the mother's breasts contain a fluid called colostrum. It is rich in calories and proteins, including antibodies that provide passive immunity for the newborn infant. Three or four days after delivery, the breasts begin to secrete milk.
• Its synthesis is stimulated by the pituitary hormone prolactin (PRL).
• Its release is stimulated by a rise in the level of oxytocin when the baby begins nursing.
• Milk also contains an inhibitory peptide. If the breasts are not fully emptied, the peptide accumulates and inhibits milk production. This autocrine action thus matches supply with demand.
Birth Control
The following table summarizes the various birth control method available and in use today.
Popularity (% using the method) and relative effectiveness of several methods of birth control among U. S. women using contraceptives. The pregnancy rate is the number of pregnancies per 100 women in the first year of using the method.
Method Popularity Pregnancy Rate
Natural family planning (rhythm) 1% 25
Male condom 16% 17
Oral contraceptives ("the pill") 28% 0.3–8.7*
Intrauterine devices (IUD) 8.5% 0.1–1.0*
Implants, e.g., Implanon® ~1% 0.05–1.0*
DMPA injections ~3.5% 6.7
Male + Female Sterilization 37% <<1%
None 85
* The lower value is found under ideal conditions; i.e., among highly-motivated women receiving good medical care. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.07%3A_Sexual_Reproduction/15.7A%3A_Sexual_Reproduction.txt |
Asexual reproduction is the formation of new individuals from the cell(s) of a single parent. It is very common in plants; less so in animals.
Asexual Reproduction in Plants
All plant organs have been used for asexual reproduction, but stems are the most common.
Stems
In some species, stems arch over and take root at their tips, forming new plants. The horizontal above-ground stems (called stolons) of the strawberry produce new daughter plants at alternate nodes. Underground stems such as rhizomes, bulbs, corms and tubers are used for asexual reproduction as well as for food storage. Irises and day lilies, for example, spread rapidly by the growth of their rhizomes.
Leaves
The above photo shows the leaves of the common ornamental plant Bryophyllum (also called Kalanchoë). Mitosis at meristems along the leaf margins produce tiny plantlets that fall off and can take up an independent existence.
Roots
Some plants use their roots for asexual reproduction. The dandelion is a common example. Trees, such as the poplar or aspen, send up new stems from their roots. In time, an entire grove of trees may form - all part of a clone of the original tree.
Plant Propagation
Commercially-important plants are often deliberately propagated by asexual means in order to keep particularly desirable traits (e.g., flower color, flavor, resistance to disease). Cuttings may be taken from the parent and rooted.
Grafting
Grafting is widely used to propagate a desired variety of shrub or tree. All apple varieties, for example, are propagated this way.
Apple seeds are planted only for the root and stem system that grows from them. After a year's growth, most of the stem is removed and a twig (scion) taken from a mature plant of the desired variety is inserted in a notch in the cut stump (the stock). So long the cambiums of scion and stock are united and precautions are taken to prevent infection and drying out, the scion will grow. It will get all its water and minerals from the root system of the stock. However, the fruit that it will eventually produce with be identical (assuming that it is raised under similar environmental conditions) to the fruit of the tree from which the scion was taken.
Apomixis
Citrus trees and many other species of angiosperms use their seeds as a method of asexual reproduction; a process called apomixis.
• In one form, the egg is formed with 2n chromosomes and develops without ever being fertilized.
• In another version, the cells of the ovule (2n) develop into an embryo instead of - or in addition to - the fertilized egg.
Hybridization between different species often yields infertile offspring. But in plants, this does not necessarily doom the offspring. Many such hybrids use apomixis to propagate themselves. The many races of Kentucky bluegrass growing in lawns across North America and the many races of blackberries are two examples of sterile hybrids that propagate successfully by apomixis. Recently, an example of apomixis in gymnosperms was discovered (see Pichot, C., et al, in the 5 July 2001 issue of Nature). In a rare cypress, the pollen grains are diploid, not haploid, and can develop into an embryo when they land on either
• the female cones of their own species (rare) or
• those of a much more common species of cypress.
Is this paternal apomixis in a surrogate mother a desperate attempt to avoid extinction?
Breeding apomictic crop plants
Many valuable crop plants (e.g., corn) cannot be propagated by asexual methods like grafting.
Agricultural scientists would dearly love to convert these plants to apomixis: making embryos that are genetic clones of themselves rather than the product of sexual reproduction with its inevitable gene reshuffling. After 20 years of work, an apomictic corn (maize) has been produced, but it does not yet produce enough viable kernels to be useful commercially.
Asexual Reproduction in Animals
Budding
Here, offspring develop as a growth on the body of the parent. In some species, e.g., jellyfishes and many echinoderms, the buds break away and take up an independent existence. In others, e.g., corals, the buds remain attached to the parent and the process results in colonies of animals. Budding is also common among parasitic animals, e.g., tapeworms.
Fragmentation
As certain tiny worms grow to full size, they spontaneously break up into 8 or 9 pieces. Each of these fragments develops into a mature worm, and the process is repeated.
Parthenogenesis
In parthenogenesis ("virgin birth"), the females produce eggs, but these develop into young without ever being fertilized. Parthenogenesis occurs in some fishes, several kinds of insects, and a few species of frogs and lizards. It does not normally occur in mammals because of their imprinted genes. However, using special manipulations to circumvent imprinting, laboratory mice have been produced by parthenogenesis.
In a few nonmammalian species it is the only method of reproduction, but more commonly animals turn to parthenogenesis only under certain circumstances. Examples:
• Aphids use parthenogenesis in the spring when they find themselves with ample food. In this species, reproduction by parthenogenesis is more rapid than sexual reproduction, and the use of this mode of asexual reproduction permits the animals to quickly exploit the available resources.
• Female Komodo dragons (the largest lizard) can produce offspring by parthenogenesis when no male is available for sexual reproduction. Their offspring are homozygous at every locus including having identical sex chromosomes. Thus the females produce all males because, unlike mammals, females are the heterogametic sex (ZW) while males are homogametic (ZZ).
Parthenogenesis is forced on some species of wasps when they become infected with bacteria (in the genus Wolbachia). Wolbachia can pass to a new generation through eggs, but not through sperm, so it is advantageous to the bacterium for females to be made rather that males. In these wasps (as in honeybees), fertilized eggs (diploid) become females and unfertilized (haploid) eggs become males. However, in Wolbachia-infected females, all their eggs undergo endoreplication producing diploid eggs that develop into females without fertilization, by parthenogenesis. Treating the wasps with an antibiotic kills off the bacteria and "cures" the parthenogenesis!
Apis mellifera capensis
Occasionally worker honeybees develop ovaries and lay unfertilized eggs. Usually these are haploid, as you would expect, and develop into males. However, workers of the subspecies Apis mellifera capensis (the Cape honeybee) can lay unfertilized diploid eggs that develop into females (who continue the practice). The eggs are produced by meiosis, but then the polar body nucleus fuses with the egg nucleus restoring diploidy (2n). The phenomenon is called automictic thelytoky.
Cape honey bees gorging on honey. (CC BY-SA 3.0; Discott)
Why Choose Asexual Reproduction?
Perhaps the better question is: Why not? After all, asexual reproduction would seem a more efficient way to reproduce. Sexual reproduction requires males but they themselves do not produce offspring. Two general explanations for the overwhelming prevalence of sexually-reproducing species over asexual ones are:
• Perhaps sexual reproduction has kept in style because it provides a mechanism to weed out (through the recombination process of meiosis) harmful mutations that arise in the population reducing its fitness. Asexual reproduction leads to these mutations becoming homozygous and thus fully exposed to the pressures of natural selection.
• Perhaps it is the ability to adapt quickly to a changing environment that has caused sex to remain the method of choice for most living things.
Purging Harmful Mutations
Most mutations are harmful - changing a functional allele to a less or nonfunctional one. An asexual population tends to be genetically static. Mutant alleles appear but remain forever associated with the particular alleles present in the rest of that genome. Even a beneficial mutation will be doomed to extinction if trapped along with genes that reduce the fitness of that population. But with the genetic recombination provided by sex, new alleles can be shuffled into different combinations with all the other alleles available to the genome of that species. A beneficial mutation that first appears alongside harmful alleles can, with recombination, soon find itself in more fit genomes that will enable it to spread through a sexual population.
Evidence (from Paland and Lynch in the 17 February 2006 issue of Science): Some strains of the water flea Daphnia pulex (a tiny crustacean) reproduce sexually, others asexually. The asexual strains accumulate deleterious mutations in their mitochondrial genes four times as fast as the sexual strains.
Evidence (from Goddard et al. in the 31 March 2005 issue of Nature): Budding yeast missing two genes essential for meiosis adapt less rapidly to growth under harsh conditions than an otherwise identical strain that can undergo genetic recombination. Under good conditions, both strains grow equally well.
Evidence (from Rice and Chippindale in the 19 October 2001 issue of Science):
Using experimental Drosophila populations, they found that a beneficial mutation introduced into chromosomes that can recombine did over time increase in frequency more rapidly than the same mutation introduced into chromosomes that could not recombine.
So sex provides a mechanism for testing new combinations of alleles for their possible usefulness to the phenotype:
• deleterious alleles weeded out by natural selection
• useful ones retained by natural selection
Some organisms may still gain the benefits of genetic recombination while avoiding sex. Many mycorrhizal fungi use asexual reproduction only. However, at least two species have been shown to have multiple — similar — copies of the same gene; that is, are polyploid. Perhaps recombination between these (during mitosis?) enables these organisms to avoid the hazards of accumulating deleterious mutations. (See the paper by Pawlowska and Taylor in the 19 Feb 2004 issue of Nature.) But there are many examples of populations that thrive without sex, at least while they live in a stable environment.
Red Queen hypothesisnment
As we have seen (above), populations without sex are genetically static. They may be well-adapted to a given environment, but will be handicapped in evolving in response to changes in the environment. One of the most potent environmental forces acting on a species environment is its parasites. The speed with which parasites like bacteria and viruses can change their virulence may provide the strongest need for their hosts to have the ability to make new gene combinations. So sex may be virtually universal because of the never-ending need to keep up with changes in parasites.
Evidence:
• Some parasites interfere with sexual reproduction in their host:
• Wolbachia-induced parthenogenesis discussed above is an example.
• Several types of fungi block wind pollination of their grass hosts forcing them to inbreed with its resulting genetic uniformity.
• There is some evidence that genetically uniform populations are at increased risk of devastating epidemics and population crashes.
• Flour beetles (Tribolium castaneum) parasitized by the microsporidium Nosema whitei increase the rate of recombination during meiosis.
• Drosophila females parasitized by bacteria produce more recombinant offspring than non-infected mothers do.
The idea that a constantly-changing environment, especially with respect to parasites, drives evolution is often called the Red Queen hypothesis. It comes from Lewis Carroll's book Through the Looking Glass, where the Red Queen says "Now here, you see, it takes all the running you can do to keep in the same place".
The possibilities outlined above are not mutually exclusive and a recent study [see Morran, L. T., et al., in Nature, 462:350, 19 November 2009] suggests that both forces are at work in favoring sexual reproduction over its alternatives.
The organism for testing these theories was Caenorhabditis elegans. While C. elegans does not reproduce asexually, most worms are hermaphrodites and usually reproduce by self-fertilization with each individual fertilizing its own eggs. This quickly results in its genes becoming homozygous and thus fully-exposed to natural selection just as they are in asexually-reproducing species. Hermaphrodites have two X chromosomes and self-fertilization ("selfing") usually produces more of the same; that is, hermaphrodites produce more hermaphrodites. However, an occasional nondisjunction generates an embryo with a single X chromosome and this develops into a male. These males can mate with hermaphrodites (their sperm is preferred over the hermaphrodites own) and, in fact, such "outcrossing" produces a larger number of offspring. It also produces 50% hermaphrodites and 50% males.
Testing the role of outcrossing vs. self-fertilization in maintaining fitness in the face of an increased mutation rate
These workers developed six strains of worms:
• two that could reproduce only by selfing
• two that could reproduce only by crossing a male with an hermaphrodite ("outcrossing")
• "wild-type" worms
All the strains were exposed to a chemical mutagen that increased the spontaneous mutation rate some fourfold.
The results: After 50 generations, the
• the strains of worms that could reproduce only by selfing suffered a serious decline in fitness
• the strains of worms that could reproduce only by outcrossing suffered no decline
• the wild-type worms with intermediate levels of outcrossing (20–30%) suffered only moderate declines in fitness
Fitness was measured by placing the worms in a petri dish with a barrier that they had to cross to reach their food (E. coli).
The conclusion: the genetic recombination provided by outcrossing protected the worms from loss of fitness even in the face of an increase in mutation rate.
Testing the role of outcrossing vs. self-fertilization in the speed of adaptation to a changed environment
For these tests, one of each category of mating types was exposed over 40 generations to a pathogenic bacterium (Serratia marcescens) that killed most worms when eaten by them.
The results: After 40 generations,
• the strain of worms that could reproduce only by selfing were just as susceptible to the pathogen as they were at the start while
• the strain of worms that could reproduce only by outcrossing had evolved a high degree of resistance to the pathogen
• the wild-type worms only developed a modest increase in their resistance to the bacteria.
Since these studies were reported, the same team has expanded their experiments to examine the effects of evolution in the pathogen (Serratia marcescens), that is, to look for evidence of coevolution of host and parasite. (Reported by Morran, L. T., et al., in Science, 333: 216, 8 July 2011.)
Over 30 generations of worms, they harvested and tested the bacteria recovered from the bodies of worms that had died within 24 hours of infection. They found that:
• Worms that can maintain genetic variability by outcrossing suffered substantially lower mortality from the coevolved parasite that did worms from the starting population (kept frozen until used).
• Worms that could only reproduce by selfing became so susceptible to the evolving strain of Serratia marcescens that they died out within 20 generations.
• Curiously, the selection pressure of the increasing virulence of Serratia marcescens caused wild-type worms to increase their rate of outcrossing from the normal 20–30% to over 80%. So one response to the pressure of this coevolving parasite was to promote sex in its host.
Reproduction in Rotifers
Rotifers are microscopic invertebrates. They are assigned a phylum of their own (not discussed elsewhere in these pages). The phylum includes:
• a class of ~1,500 species called monogonont rotifers (they have only a single gonad). The monogonont rotifers can choose either asexual or sexual reproduction as circumstances warrant.
• a class of ~350 species called bdelloid rotifers. The bdelloid rotifers are limited to asexual reproduction only. Even after years of study, neither males nor haploid eggs have ever been found in any members of this group. It looks as though they gave up sexual reproduction millions of years ago.
A bdelloid rotifer. (CC BY-SA 3.0; Bob Blaylock)
Laboratory studies show that monogonont rotifers favor asexual reproduction when they are living in a stable environment but shift to more sexual reproduction when placed in a varied or unfavorable environment. As they adapt to the new environment, they gradually switch back to asexual reproduction.
But how have the bdelloid rotifers that never engage in sexual reproduction managed to survive? How have they avoided the demands of the Red Queen; that is, avoided extinction at the hands of parasites?
One study (Wilson, C. G. and Sherman, P. W., Science, 327:574, 29 January 2010) reveals a mechanism. These tiny animals can be completely desiccated (dried out) and remain in suspended animation for years. In the desiccated state, they can be blown vast distances (some species are worldwide in their distribution). Once deposited in a moist environment (a few drops of water are sufficient), they resume an active life. Wilson and Sherman have shown that the desiccation that is harmless to the rotifers is lethal to their fungal parasite. So once dried, they are not only cured of their parasite, but can then be blown to a spot where they can resume an active life with no parasites present.
Another way in which these rotifers can avoid the evolutionary dead-end expected of asexually-reproducing organisms has been revealed by DNA sequencing of their genome. It turns out that they can purge their genome of deleterious alleles by gene conversion (during mitosis).
In any case despite its disadvantages sexual reproduction is here to stay
• reducing the effect of harmful mutations
• increasing the speed with which populations can adapt to changes in their environment | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.07%3A_Sexual_Reproduction/15.7B%3A_Asexual_Reproduction_in_Animals.txt |
Mechanical and/or Chemical Barriers
Male Condom
• a sheath of thin, flexible material (e.g., latex) worn over the penis
• effective if used carefully
• also protects against sexually-transmitted disease (STD) agents such as
• HIV, the cause of AIDs
• herpes virus
• human papilloma virus (HPV)
• chlamydiae
• Neisseria gonorrhoeae, the cause of gonorrhea
Female Condom
• a thin-walled pouch inserted into the vagina
• protects against sexually-transmitted diseases (STDs)
• less effective than the male condom
Diaphragm
• rubber dome placed at the upper end of the vagina
• may be used along with spermicides
Cervical Cap
• impermeable cap fitted over the cervix
• best used with spermicides
Spermicides
• chemicals, such as nonoxynol 9, that inactivate sperm. Inserted into the vagina — often incorporated in a sponge — prior to intercourse.
• not very effective if used alone
Hormonal Contraception
Oral Contraceptives - the "Pill"
• Many formulations combining varying amounts of a synthetic estrogen and a synthetic progestin (progesterone-like steroid)
• taken for 3 weeks, then stopped to allow menstruation
• most widely-used reversible method
Skin ("Transdermal") Patch
• The Ortho Evra® patch releases an estrogen and a progestin through the skin.
• A fresh patch is applied each week for 3 weeks, and then a week without allows menstruation.
• The failure rate is about 9%.
Vaginal Ring
• a small plastic ring inserted into the vagina
• NuvaRing® releases both an estrogen and a progestin.
• It is left in place for 3 weeks and then removed for a week to allow menstruation.
• The failure rate is about 9%.
Injectable Contraceptive
• An injection containing a synthetic progestin (depot medroxyprogesterone acetate or "DMPA")(Depo-Provera®)
• One injection given every three months inhibits the release of GnRH, thus inhibiting the synthesis of FSH and LH and blocking ovulation.
• One of the most reliable methods of birth control.
Contraceptive Implant
• Implanon® and Nexplanon®, each a tiny (40 x 2 mm) flexible plastic rod that releases a synthetic progestin is inserted under the skin (requiring a local anesthetic).
• Prevents pregnancy for up to 3 years.
• If pregnancy is desired sooner, is easily removed (again requiring a small incision and a local anesthetic) and normal fertility quickly returns.
• Although used by only ~1% of U.S. women, implants are among the most effective of the birth control methods.
"Morning After" Pill
The most popular formulation in the U.S., called Plan B One-Step®, contains a high dose of a progestin. If taken within 72 hours after unprotected intercourse, the drug interferes with ovulation and, if ovulation has occurred, with fertilization.
If so many days have elapsed that implantation has occurred, RU-486 may be used. RU-486 is a synthetic steroid related to progesterone. Unlike the progestins discussed above, that mimic the action of progesterone, RU-486 blocks the action of progesterone. (Synthetic molecules that mimic the action of a natural molecule are called agonists; those that oppose it are antagonists.) RU-486 (also known as mifepristone) is a progesterone antagonist. It binds to the progesterone receptor, and in so doing prevents progesterone itself from occupying its receptor. Thus the gene transcription normally turned on by progesterone is blocked, and the proteins necessary to begin and maintain pregnancy are not synthesized. Because RU-486 is used after implantation, it is causing an early abortion and thus has been subjected to controversy.
Intrauterine Devices (IUD)
• The intrauterine device (IUD) is a device, usually T-shaped, inserted into the uterus by a physician.
• Two types available in the U.S.:
• ParaGard®, which is coated with copper wire and can be left in place for 10 years;
• Mirena®, which releases a progestin and can be left in place for up to 5 years.
• Although used by only 8.5% of U.S. women, IUDs are among the most effective of the birth control methods. Only about 1 woman in a thousand becomes pregnant during her first year of using Mirena®.
Natural Family Planning - Rhythm Methods
• An egg must be fertilized on the day of ovulation.
• Sperm can live in the female reproductive tract for up to 6 days.
• So copulation that takes place more than 5 days before or a day after ovulation is unlikely to lead to pregnancy.
• Abstinence during this period is called natural family planning or the rhythm method.
• Its success (which is low) depends upon being able to determine accurately just when ovulation occurs.
• Highly-motivated women can do this by
• monitoring their body temperature (which rises slightly at ovulation)
• the amount and consistency of the mucus secreted by their uterus, and more recently
• measuring the concentration of estrogen and/or progesterone in the urine (which mirrors the level in the blood).
• It is favored by those who do not currently want a baby, but do not wish to use contraceptive devices (about 1% of U.S. couples).
Abortion
• the deliberate removal of the embryo or fetus before it is ready for birth
• Done
• mechanically
• using a suction device (during the first 3 months of pregnancy)
• using surgery (later in pregnancy)
or
• chemically (using RU-486 and prostaglandins) during the first 7 weeks of pregnancy
All methods of birth control have been the subject of controversy (except for natural family planning).
• In general, the controversy over a given method is proportional to the lateness of the stage of the reproductive process.
• So not surprisingly abortion is a particularly controversial procedure especially when it is induced in the later stages of pregnancy.
• Nevertheless, worldwide some 46 million pregnancies are terminated each year by induced abortion.
Sterilization
Roughly one-third of U.S. couples still in their reproductive years have chosen for one or the other to be sterilized.
Tubal Ligation
• Both fallopian tubes (oviducts) are cut and tied so that no egg can be fertilized.
• Requires incision(s) and so must be done under anesthesia.
Vasectomy
• Each vas deferens is cut near the top of the scrotum.
• Can be done in the doctor's office, with a local anesthetic, in 30-40 minutes.
• Testosterone secretion by the testes is not inhibited.
• Does not stop the production of the various glandular secretions that make up the bulk of the semen.
• Copulation and ejaculation proceed normally.
• Sometimes the operation can be reversed, but don't count on it.
Quinacrine Sterilization (QS)
• Pellets of the antimalarial drug quinicrine are placed (by a physician) in the uterus
• Done twice, a month apart.
• Causes scarring of the fallopian tubes
• Clinical trials are in progress in the U.S.
Summary
Popularity (% using the method) and relative effectiveness of several methods of birth control among U. S. women using contraceptives. The pregnancy rate is the number of pregnancies per 100 women in the first year of using the method.
Method Popularity Pregnancy Rate
Natural family planning (rhythm) 1% 25
Male condom 16% 17
Oral contraceptives ("the pill") 28% 0.3–8.7*
Intrauterine devices (IUD) 8.5% 0.1–1.0*
Implants, e.g., Implanon® ~1% 0.05–1.0*
DMPA injections ~3.5% 6.7
Male + Female Sterilization 37% <<1%
None 85
* The lower value is found under ideal conditions; i.e., among highly-motivated women receiving good medical care.
The bottom line: The failure rate of the pill, patch, and vaginal ring, as they are commonly used, is 20 times that in women using an IUD, or implant.
Future Prospects
• Not too bright because pharmaceutical houses are reluctant to invest the huge amounts of money and time needed to develop products that expose them to controversy, put them at risk of lawsuits and whose largest market is in countries too poor to afford them.
• Research on possible male birth control pills is going on. However, it is not yet easy to see how spermatogenesis can be blocked without causing undesirable side-effects.
• Some research is proceeding on contraceptive vaccines; that is, using the immune system to block one or another step in the process (e.g. fertilization). Examples: vaccines to raise antibodies
• against gonadotropin-releasing hormone, GnRH (for males)
• against human chorionic gonadotropin, HCG (for females)
• to immobilize sperm (also for females)
But, what risks such antibodies might present is not at all clear. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.07%3A_Sexual_Reproduction/15.7C%3A_Birth_Control.txt |
Many tests are now available to detect genetic diseases such as sickle cell disease, cystic fibrosis and phenylketonuria (PKU). Most of these tests can not only be performed on cells removed from adults but also on cells removed from the fetus and even from a pre-implantation embryo.
Amniocentesis
During its development, the fetus sheds cells into the amniotic fluid. After 14–22 weeks of pregnancy, a small volume of this fluid can be removed (using a needle inserted through the abdominal wall).
Using ultrasound to locate the position of the placenta prior to amniocentesis. These sonograms are made by recording the echoes received from structures within the abdomen. A, amniotic cavity; B, urinary bladder; F, part of the fetus; P, placenta. Both longitudinal (left) and transverse (right) scans are needed for accurate localization of the placenta. (Courtesy of the Downstate Medical Center of the State University of New York.)
Separating the cells and culturing them enables the clinician to look for
• chromosome abnormalities (e.g., the three number 21 chromosomes of Down syndrome)
• certain enzymatic defects (e.g., an inability to metabolize galactose, hence milk)
• the sex of the fetus
Over 100 genetic abnormalities can be diagnosed by amniocentesis and the pregnancy deliberately ended if the parents wish it.
Chorionic Villus Sampling (CVS)
This is an alternate method of prenatal diagnosis. A small amount of placental tissue is sucked out by a tube inserted through the abdominal wall or through the vagina (the latter avoiding the need for an incision). For some tests the fetal cells can be examined immediately without the need to culture them. Another advantage of CVS is that it can be performed earlier in pregnancy (after only 10–12 weeks) than amniocentesis. If an abortion is to be performed, it is a simpler process early in pregnancy.
Non-Invasive Prenatal Genetic Testing (NIPT)
Although the blood vessels of the placenta are in close contact with the mother's blood vessels in the uterus, intermingling of their blood does not normally occur. However some of cells of the fetus do manage to get into the mother's circulation where they may represent 1 in a million of her white blood cells (so only some 2–6 cells per ml of blood). Fragments of fetal DNA (~ 300 bp long) from apoptotic cells of the placenta are also found in the mother's plasma as early as 5 weeks after implantation. This raises the possibility of using genetic tests (e.g., PCR) to identify mutations or chromosomal abnormalities in the fetus using a small (~10 ml) sample of blood drawn from the mother.
Two home blood test kits for determining the sex of the fetus are already on the market. The collected drops of blood are sent to a laboratory to determine whether any Y-chromosome-specific DNA (e.g., SRY) is present. Tests of fetal DNA for Down syndrome (trisomy 21), as well as for trisomy 13 and 18, have both higher sensitivity (false negatives <0.1%) and specificity (false positives <0.2%) than amniocentesis and CVS.
In several European countries, Rh-negative mothers can now have their blood screened for the presence of an Rh-positive fetus. The time will surely come when such NIPT screening will become available for many genetic disorders. The procedure is also called non-invasive prenatal diagnosis - NIPD.
The level of fetal DNA in the mother's blood rises to a peak at the time of birth and some evidence suggests that this rise may be a trigger to start the birth process.
Preimplantation Genetic Diagnosis (PGD)
Genetic Analysis of Blastomeres
One of the remarkable facts about mammalian development is that all the cells in the early (e.g., 8-cell) embryo are not needed to produce a healthy fetus (which is why a single fertilized egg can on occasions produce identical twins, triplets, etc.). So couples using in vitro fertilization (IVF) also can take advantage of genetic screening. While the embryo is in culture, one or two cells can safely be removed and tested for their genotype. For example:
• The sex of the embryo can be determined with a probe for Y-specific DNA. This permits prospective mothers carrying a severe X-linked trait like hemophilia A to choose a female rather than a male embryo for attempted implantation.
• Fluorescent probes specific for the DNA of particular chromosomes can detect (by FISH) if there is an abnormal number (aneuploidy) such as the three #21 chromosomes of Down syndrome.
• In fact the entire karyotype of the embryo can be determined. Random fragments of DNA prepared by the polymerase chain reaction (PCR) of all the DNA of a cell from the embryo can be
• given a fluorescent label
• applied to the metaphase chromosomes of a standard reference cell that has a normal karyotype along with
• DNA fragments from the reference cell labelled with a different color.
Comparing the intensity of the two colors from each chromosome shows whether the embryo has the normal amount of DNA for that chromosome or is aneuploid containing either:
• too much (e.g. 3 copies of #21 — trisomy)
• too little (only a single copy of #14 — monosomy)
Genetic Analysis of Polar Bodies
As meiosis I is completed, one set of duplicated chromosomes (dyads) is extruded into the first polar body. The DNA of the polar body can be amplified by the polymerase chain reaction (PCR) and tested.
If the mother is heterozygous for a trait, and no crossover has occurred, and the polar body contains the mutant alleles (see figure), the egg can be safely fertilized. (For simplicity, the figure shows only the pair of homologues carrying the locus of concern.)
However, if crossing over has occurred, the first polar body would contain one mutant and one healthy allele. In that case, there is a 50:50 chance that, after fertilization, the other copy of the mutant allele will end up in the egg (instead of in the second polar body). So the second polar body should also be tested to see if it also contains the mutant allele. Only if it does can the egg be safely used.
15.7E: Extraembryonic Membranes and the Physiology of the Placenta
The embryos of reptiles, birds, and mammals produce 4 extraembryonic membranes - amnion, yolk sac, chorion and allantois. In birds and most reptiles, the embryo with its extraembryonic membranes develops within a shelled egg.
• The amnion protects the embryo in a sac filled with amniotic fluid.
• The yolk sac contains yolk — the sole source of food until hatching. Yolk is a mixture of proteins and lipoproteins.
• The chorion lines the inner surface of the shell (which is permeable to gases) and participates in the exchange of O2 and CO2 between the embryo and the outside air.
• The allantois stores metabolic wastes (chiefly uric acid) of the embryo and, as it grows larger, also participates in gas exchange.
With these four membranes, the developing embryo is able to carry on essential metabolism while sealed within the egg. Surrounded by amniotic fluid, the embryo is kept as moist as a fish embryo in a pond. Although (most) mammals do not make a shelled egg, they do also enclose their embryo in an amnion. For this reason, the reptiles, birds, and mammals are collectively referred to as the amniota.
Amniotic Egg groups in Mammals
Mammals fall into three groups that differ in the way they use the amniotic egg.
• These primitive mammals produce a shelled egg like their reptilian ancestors. Only four species exist today: three species of spiny anteater (echidna) and the duckbill platypus.
• Marsupials
Marsupials do not produce a shelled egg. The egg, which is poorly supplied with yolk, is retained for a time within the reproductive tract of the mother. The embryo penetrates the wall of the uterus. The yolk sac provides a rudimentary connection to the mother's blood supply from which it receives food, oxygen, and other essentials. However, this interface between the tissues of the uterus and the extraembryonic membranes never becomes elaborately developed, and the young are born in a very immature state.
The photo shows 18 newborn baby opossums fitting easily into a teaspoon. Despite their tiny size, they are able to crawl into a pouch on the mother's abdomen, attach themselves to nipples, and drink milk from her mammary glands. Marsupials are still abundant in Australia, but only the opossum is found in North America.
• Placental mammals
In placental mammals, the extraembryonic membranes form a placenta and umbilical cord, which connect the embryo to the mother's uterus in a more elaborate and efficient way. The blood supply of the developing fetus is continuous with that of the placenta. The placenta extracts food and oxygen from the uterus. Carbon dioxide and other wastes (e.g., urea) are transferred to the mother for disposal by her excretory organs.
Humans are placental mammals. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.07%3A_Sexual_Reproduction/15.7D%3A_Prenatal_Screening.txt |
A genetic mosaic is a creature whose body is built of a mixture of cells of two or more different genotypes. In mammals they arise by several different mechanisms:
• The fusion of two different zygotes, or early embryos, into one. (The reverse of the process that produces identical twins!) The resulting animal is called a chimera (after the monster in Greek mythology with a lion's head, goat's body, and serpent's tail). The tetraparental mouse is a chimera formed this way. But on rare occasions, the same process can occur spontaneously in humans (especially those using in vitro fertilization).
• The sharing of blood supplies by separate embryos. This occurs with the occasional fraternal cattle twins and also — less often — with human fraternal twins who have shared the same placenta. Blood stem cells of each twin seed the bone marrow of the other. Only their blood cells are mosaic.
• During early development, errors during mitosis can produce stem cells that go on to populate a tissue or organ with, for example, a chromosomal aberration (e.g., aneuploid).
Example: Occasionally a baby is born with blood cells that have three copies of chromosome 21 (the same set responsible for Down syndrome). This can produce a leukemia-like illness that, fortunately, often disappears as that cell population declines.
• All female mammals are mosaic for the genes on the X chromosome because of the random inactivation of one or the other X chromosome in all their somatic cells.
• Anyone unlucky enough to have a cancer is a genetic mosaic because all cancers are made up of the descendants of cells carrying a suite of mutations not found in normal cells.
• Recent advances have enabled the coding portions of the genome of single cells to be sequenced. Early results indicate than even normal cells in an adult have accumulated a suite of somatic mutations that differs from cell to cell. So all of us are genetic mosaics! However, the rate of somatic mutations in these normal cells is only a fourth of that in cancer cells.
The Tetraparental Mouse
As the name suggests, tetraparental mice have four parents: two fathers and two mothers (not including the foster mother that gives birth to them!). This is how they are made:
• Early embryos at the 8-cell stage are removed from two different pregnant mice and placed in tissue culture medium.
• Two different embryos are gently pushed together and, often, will fuse into a single embryo.
• After a period of further growth in culture, the fused embryo is implanted in a foster mother (whose uterus has been prepared for implantation by mating her with a vasectomized male).
• The mouse that is born is a chimera, all (usually) of whose organs are made of some cells derived from one pair of parents and some cells derived from the other pair.
The photograph shows a tetraparental mouse derived from a pair of inbred mice with black fur and a pair with white fur. Note the intermingling of black and white patches. This mouse is not the same as an F1 hybrid produced by mating a white mouse with a black one. In that case, all the cells would be of the same genotype, and the coat would have been a uniform brown.
A Tetragametic Human
A report by Yu, et. al. in the May 16, 2002 issue of The New England Journal of Medicine documents the discovery of a tetragametic woman; that is a woman derived from four different gametes, not just two. She came to the doctors' attention because she needed a kidney transplant.
• Tissue typing, which is done with blood cells, showed her to have inherited the "1" HLA region of her father (who was 1,2) and the "3" region of her mother (who was 3,4).
• She had two brothers,
• One who inherited 1 from their father and 3 from their mother
• The other who inherited 2 from their father and 3 from their mother.
• Her husband typed 5,6
• Of her three sons,
• One was 1,6 which was to be expected
• the other two were both 2,5. The 5 they got from their father, but where did the 2 come from?
• The first thought was that she could not have been their mother, but clearly she knew better. (Paternity may sometimes be in doubt, but not maternity.)
• A clue came from typing other tissues. DNA analysis of her skin cells, hair follicles, thyroid cells, bladder cells, and cells scraped from inside her mouth revealed not only 1 and 3 but also 2 and 4. It is not clear why her bone marrow was an exception - containing only 1,3 stem cells.
• How were these results possible? The most reasonable explanation is that
• Her mother had simultaneously ovulated two eggs one containing a chromosome 6 with HLA 3 and the other with HLA 4.
• Her father would, of course, have produced equal numbers of 1-containing and 2-containing sperm.
• A 1-sperm fertilized the 3-egg.
• A 2-sperm fertilized the 4-egg.
• Soon thereafter the resulting early embryos fused into a single embryo.
• As this embryo developed into a fetus, both types of cells participated in constructing her various organs including her oogonia (but not, apparently, the blood stem cells in her bone marrow).
• Although she was a mosaic for the HLA (and other) genes on chromosome 6, all her cells were XX. So both the father's successful sperm cells had carried his X chromosome.
However, tetraparental humans have been found that were mosaic for sex chromosomes as well; that is, some of their cells were XX; the other XY. In some cases this mosaic pattern results in a hermaphrodite - a person with a mixture of male and female sex organs.
So what are her chances for finding a suitable kidney donor?
The HLA region on chromosome 6 carries a set of genes that encode the major transplantation antigens; that is, the antigens that trigger graft rejection. Ordinarily, there is only a 1 in 4 chance that two siblings share the same transplantation antigens if both parents were heterozygous as in her case. But because this woman has all four sets of transplantation antigens, she can accept a kidney from any one of her brothers as well as her mother (her father was dead) without fear of rejecting it.Laboratory tests confirmed that she was unable to generate T cells able to react against the cells of either brother or her mother.
Rat-Mouse Chimeras
In the 3 September 2010 issue of Cell, Kobayashi et al. report the creation of healthy rat-mouse chimeras:
• mice with functioning rat tissues
• rats with functioning mouse tissues.
Their procedure:
• Generate induced pluripotent stem cells (iPSCs) from embryonic fibroblasts of each species.
• Inject:
• mouse iPSCs into rat blastocysts
• rat iPSCs into mouse blastocysts.
• Implant these blastocysts into the uterus of pseudopregnant foster mothers of the same species as the blastocyst.
The Pdx-1−/− Mouse
Pdx-1 encodes a transcription factor that is essential for the development of the pancreas. Transgenic mice lacking a functioning Pdx-1 gene (Pdx-1−/−) die shortly after birth.
However, Kobayashi et al. found that injecting rat induced pluripotent stem cells (iPSCs) into mouse Pdx-1−/− blastocysts produced a few viable mouse chimeras complete with a pancreas made up almost exclusively of rat cells. The pancreas was fully functional producing both exocrine secretions (e.g., pancreatic amylase) and endocrine secretions (e.g., insulin, glucagon, and somatostatin). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.07%3A_Sexual_Reproduction/15.7F%3A_Genetic_Mosaics.txt |
In February 1997, a research team at the Roslin Institute in Edinburgh, Scotland, headed by Dr. I. Wilmut, reported (in the 27 February 1997 issue of Nature) that they had succeeded in producing a healthy lamb, named Dolly, from the nucleus of a cell taken from an adult sheep.
In February 1997, a research team at the Roslin Institute in Edinburgh, Scotland, headed by Dr. I. Wilmut, reported (in the 27 February 1997 issue of Nature) that they had succeeded in producing a healthy lamb, named Dolly, from the nucleus of a cell taken from an adult sheep.
Why has this achievement created such a stir?
After all, all the cells in an adult are
• descended from the fertilized egg
• have been produced by mitosis
Many years earlier, the German embryologist Hans Spemann showed that even after 5 divisions of the fertilized egg, the nuclei retained the potential to program the complete development of an adult. Using strands of baby hair, he tied loops around fertilized amphibian (newt) eggs so that they were constricted into two halves with the nucleus confined to one half and a narrow bridge of cytoplasm connecting the two halves.
He found that:
• At first only the half containing the zygote nucleus would divide by mitosis.
• Eventually a nucleus would cross into the other half and it, too, would begin dividing.
• So long as both halves contained some of a cytoplasmic region called the gray crescent, the second half would then go on to develop into a second perfectly-formed embryo.
• Enucleate the eggs produced by Scottish Blackface ewes (female sheep).
• Treat the ewes with gonadotropin-releasing hormone (GnRH) to cause them to produce oocytes ready to be fertilized. Like all mammals, these are arrested at metaphase of the second meiotic division (meiosis II).
• Plunge a micropipette into the egg over the polar body and suck out not only the polar body but the haploid pronucleus within the egg.
• Fuse each enucleated egg with a diploid cell growing in culture.
• Cells from the mammary gland of an adult Finn Dorset ewe (they have white faces) are grown in tissue culture.
• Five days before use, the nutrient level in the culture is reduced so that the cells stop dividing and enter G0 of the cell cycle.
• Donor cells and enucleated recipient cells are placed together in culture.
• The cultures are exposed to pulses of electricity to
• cause their respective plasma membranes to fuse;
• stimulate the resulting cell to begin mitosis (by mimicking the stimulus of fertilization).
• Culture the cells until they have grown into a morula (solid mass of cells) or even into a blastocyst (6 days).
• Transfer several of these into the uterus of each (of 13, in this case) Scottish Blackface ewes (previously treated with GnRH to prepare them for implantation.
• Wait (with your fingers crossed).
The result: One ewe gave birth (148 days later) to Dolly.
What made Dolly different?
The Wilmut group also used the same technique to produce healthy lambs using cells from lamb embryos (9 days after fertilization) and lamb fetuses (26 days after fertilization). But in these experiments, there was no way to know the phenotype of the nuclear donor because it had not yet been born. So, too, the recent cloning of monkeys from embryo nuclei represents simply an expansion of nature's ability to produce identical twins, etc. whose traits we will not know until they are born and grow up. But the nucleus that made Dolly came from an adult animal whose phenotypic traits were there to be seen.
How do we know that Dolly is not the progeny of an unsuspected mating of the foster mother?
• She has a white face and the foster mother is a Scottish Blackface
• DNA fingerprinting reveals bands found in Finn Dorset sheep (the breed that supplied the mammary cells), not those of Scottish Blackface sheep
What accounts for this remarkable achievement?
Besides years of hard work, we do not know. Perhaps:
• Using cells in G0 demethylates inactive genes and makes it possible once again for them to be expressed.
• The mammary gland cells were not truly differentiated epithelial cells but primitive stem cells present in the mammary gland. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.07%3A_Sexual_Reproduction/15.7G%3A_Human_Cloning.txt |
Excitable cells are those that can be stimulated to create a tiny electric current. Muscle fibers and nerve cells (neurons) are excitable. The color photo is of a single interneuron in the retina of a rabbit. The cell has been injected with a fluorescent dye to reveal all its branches. Each of the small knobs at the tips of the branches makes a synapse with another cell in the retina.
The electric current in neurons is used to rapidly transmit signals through the animal, while the current in muscles is used to initiate contraction.
The Resting Potential
All cells (not just excitable cells) have a resting potential: an electrical charge across the plasma membrane, with the interior of the cell negative with respect to the exterior. The size of the resting potential varies, but in excitable cells runs about −70 millivolts (mv). The resting potential arises from two activities:
• The sodium/potassium ATPase: This pump pushes only two potassium ions (K+) into the cell for every three sodium ions (Na+) it pumps out of the cell so its activity results in a net loss of positive charges within the cell.
• Some potassium channels in the plasma membrane are "leaky" allowing a slow facilitated diffusion of K+ out of the cell (red arrow).
Ionic Relations in the Cell
The sodium/potassium ATPase produces
• a concentration of Na+ outside the cell that is some 10 times greater than that inside the cell
• a concentration of K+ inside the cell some 20 times greater than that outside the cell.
The concentrations of chloride ions (Cl) and calcium ions (Ca2+) are also maintained at greater levels outside the cell EXCEPT that some intracellular membrane-enclosed compartments may also have high concentrations of Ca2+ (green oval).
Depolarization
Certain external stimuli reduce the charge across the plasma membrane.
• mechanical stimuli (e.g., stretching, sound waves) activate mechanically-gated sodium channels
• certain neurotransmitters (e.g., acetylcholine) open ligand-gated sodium channels
In each case, the facilitated diffusion of sodium into the cell reduces the resting potential at that spot on the cell creating an excitatory postsynaptic potential or EPSP. If the potential is reduced to the threshold voltage (about −50 mv in mammalian neurons), an action potential is generated in the cell.
Action Potentials
If depolarization at a spot on the cell reaches the threshold voltage, the reduced voltage now opens up hundreds of voltage-gated sodium channels in that portion of the plasma membrane. During the millisecond that the channels remain open, some 7000 Na+ rush into the cell. The sudden complete depolarization of the membrane opens up more of the voltage-gated sodium channels in adjacent portions of the membrane. In this way, a wave of depolarization sweeps along the cell. This is the action potential. (In neurons, the action potential is also called the nerve impulse.)
The nerve impulse: (Figure 15.8.1.3) In the resting neuron, the interior of the axon membrane is negatively charged with respect to the exterior (A). As the action potential passes (B), the polarity is reversed. Then the outflow of K+ ions quickly restores normal polarity (C). At the instant pictured in the diagram, the moving spot, which has traced these changes on the oscilloscope as the impulse swept past the intracellular electrode, is at position C.
The refractory period
A second stimulus applied to a neuron (or muscle fiber) less than 0.001 second after the first will not trigger another impulse. The membrane is depolarized (position B), and the neuron is in its refractory period. Not until the −70 mv polarity is reestablished (position C) will the neuron be ready to fire again. Repolarization is first established by the facilitated diffusion of potassium ions out of the cell. Only when the neuron is finally rested are the sodium ions that came in at each impulse actively transported back out of the cell. In some human neurons, the refractory period lasts only 0.001–0.002 second. This means that the neuron can transmit 500–1000 impulses per second.
The action potential is all-or-none
The strength of the action potential is an intrinsic property of the cell. So long as they can reach the threshold of the cell, strong stimuli produce no stronger action potentials than weak ones. However, the strength of the stimulus is encoded in the frequency of the action potentials that it generates.
Myelinated Neurons
The axons of many neurons are encased in a fatty sheath called the myelin sheath. It is the greatly expanded plasma membrane of an accessory cell called the Schwann cell. Where the sheath of one Schwann cell meets the next, the axon is unprotected. The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier).
The inrush of sodium ions at one node creates just enough depolarization to reach the threshold of the next. In this way, the action potential jumps from one node to the next. This results in much faster propagation of the nerve impulse than is possible in nonmyelinated neurons.
Multiple sclerosis
This autoimmune disorder results in the gradual destruction of myelin sheaths. Despite this, transmission of nerve impulses continues for a period as the cell inserts additional voltage-gated sodium channels in portions of the membrane formerly protected by myelin.
Hyperpolarization
Despite their name, some neurotransmitters inhibit the transmission of nerve impulses. They do this by opening chloride channels and/or potassium channels in the plasma membrane. In each case, opening of the channels increases the membrane potential by letting negatively-charged chloride ions (Cl) IN and positively-charged potassium ions (K+) OUT. This hyperpolarization is called an inhibitory postsynaptic potential (IPSP) because it counteracts any excitatory signals that may arrive at that neuron. Although the threshold voltage of the cell is unchanged, it now requires a stronger excitatory stimulus to reach threshold.
Example: Gamma amino butyric acid (GABA). This neurotransmitter is found in the brain and inhibits nerve transmission by both mechanisms:
• binding to GABAA receptors opens chloride channels in the neuron
• binding to GABAB receptors opens potassium channels
Integrating Signals
A single neuron, especially one in the central nervous system (see color photo at top), may have thousands of other neurons synapsing on it. Some of these release activating (depolarizing) neurotransmitters; others release inhibitory (hyperpolarizing) neurotransmitters.
The receiving cell is able to integrate these signals. The diagram shows how this works in a motor neuron.
• The EPSP created by a single excitatory synapse is insufficient to reach the threshold of the neuron.
• EPSPs created in quick succession, however, add together ("summation"). If they reach threshold, an action potential is generated.
• The EPSPs created by separate excitatory synapses (A + B) can also be added together to reach threshold.
• Activation of inhibitory synapses (C) makes the resting potential of the neuron more negative. The resulting IPSP may also prevent what would otherwise have been effective EPSPs from triggering an action potential.
Normally, the number of EPSPs needed to reach threshold is greater than shown here.
One might expect that depolarization at one point on the plasma membrane would generate an action potential irrespective of inhibitory signals elsewhere. However, this is avoided in many neurons by the axon hillock and the axon initial segment (the AIS). This is the region where the axon emerges from the cell body and is unmyelinated. The portion of the plasma membrane in this region has few or no synapses of its own and a lower threshold than elsewhere on the cell.
Neurons can establish such distinctive domains on their plasma membrane by anchoring (with actin filaments) transmembrane proteins as barriers to block the free diffusion of membrane proteins from the cell body to the axon. The action potential is usually generated in the axon initial segment. Having neither excitatory nor inhibitory synapses of its own, it is able to evaluate the total picture of EPSPs and IPSPs created in the dendrites and cell body. Only if, over a brief interval, the sum of depolarizing signals minus the sum of the hyperpolarizing signals exceeds the threshold of the axon initial segment will an action potential be generated.
This method for the neuron to evaluate a mix of positive and negative signals occurs rapidly. It turns out, however, that neurons also have a long-term way to integrate a mix of positive and negative signals converging on them. This long-term response involves changes in gene activity leading to changes in the number and activity of the cell's many synapses. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8A%3A_Neurons.txt |
The coordination of cellular activities in animals is usually considered to involve the endocrine system (where the response is to hormones: chemicals secreted into the blood by endocrine glands and carried by the blood to the responding cell) and a nervous system (response to electrical impulses passing from the central nervous system to muscles and glands). However, in fact, coordination by the nervous system is also chemical. Most neurons achieve their effect by releasing chemicals, the neurotransmitters, on a receiving cell: another neuron (a "postsynaptic" neuron), a muscle cell and a gland cell. So the real distinction between nervous and endocrine coordination is that nervous coordination is faster and more localized. Neurotransmitters are chemicals that act in a paracrine fashion.
The junction between the axon terminals of a neuron and the receiving cell is called a synapse. Synapses at muscle fibers are also called neuromuscular junctions or myoneural junctions.
• Action potentials travel down the axon of the neuron to its end(s), the axon terminal(s).
• Each axon terminal is swollen forming a synaptic knob.
• The synaptic knob is filled with membrane-enclosed vesicles containing a neurotransmitter.
• Arrival of an action potential at the synaptic knob opens Ca2+ channels in the plasma membrane.
• The influx of Ca2+ triggers the exocytosis of some of the vesicles.
• Their neurotransmitter is released into the synaptic cleft.
• The neurotransmitter molecules bind to receptors on the postsynaptic membrane.
• These receptors are ligand-gated ion channels.
Excitatory synapses
The neurotransmitter at excitatory synapses depolarizes the postsynaptic membrane (of a neuron in this diagram). Example: acetylcholine (ACh)
• Binding of acetylcholine to its receptors on the postsynaptic cell opens up ligand-gated sodium channels.
• These allow an influx of Na+ ions, reducing the membrane potential.
• This reduced membrane potential is called an excitatory postsynaptic potential or EPSP.
• If depolarization of the postsynaptic membrane reaches threshold, an action potential is generated in the postsynaptic cell.
Inhibitory synapses
The neurotransmitter at inhibitory synapses hyperpolarizes the postsynaptic membrane. Example: gamma aminobutyric acid (GABA) at certain synapses in the brain.
The GABAA receptor is a ligand-gated chloride channel. Binding of GABA to the receptors increases the influx of chloride (Cl) ions into the postsynaptic cell raising its membrane potential and thus inhibiting it. (The neurotransmitter glycine acts in the same way at synapses in the spinal cord and the base of the brain.) This is a fast response - taking only about 1 millisecond. Binding of GABA to GABAB receptors activates an internal G protein and a "second messenger" that leads to the opening of nearby potassium (K+) channels. As you might expect, this is a slower response, taking as long as 1 second.
In both cases, the resulting facilitated diffusion of ions (chloride IN; potassium OUT) increases the membrane potential (to as much as −80 mv). This increased membrane potential is called an inhibitory postsynaptic potential (IPSP) because it counteracts any excitatory signals that may arrive at that neuron. A hyperpolarized neuron appears to have an increased threshold. Actually, the threshold voltage (about −50 mv) has not changed. It is simply a question of whether the depolarization produced by excitatory synapses on the cell minus the hyperpolarizing effect of inhibitory synapses can reach this value or not.
Neurotransmitters
Acetylcholine (ACh)
Widely used at synapses in the peripheral nervous system. Released at the terminals of
• all motor neurons activating skeletal muscle.
• all preganglionic neurons of the autonomic nervous system.
• the postganglionic neurons of the parasympathetic branch of the autonomic nervous system.
Also mediates transmission at some synapses in the brain. These include synapses involved in the acquisition of short-term memory. Drugs that enhance ACh levels - acetylcholinesterase inhibitors - are now used in elderly patients with failing memory (e.g., Alzheimer's patients).
Nicotinic vs. Muscarinic Acetylcholine Receptors
ACh acts on two different types of receptor:
• Nicotinic receptors are
• found at the neuromuscular junction of skeletal (only) muscles
• on the post-ganglionic neurons of the parasympathetic nervous system
• on many neurons in the brain (e.g. neurons in the hypothalamus whose activation by nicotine suppresses appetite)
• Nicotine is an agonist (hence the name)
• Curare is an antagonist (hence its ability to paralyze skeletal muscles)
• Muscarinic receptors are
• Found at the neuromuscular junctions of cardiac and smooth muscle as well as on glands and on the post-ganglionic neurons of the sympathetic nervous system.
• Muscarine (a toxin produced by certain mushrooms) is an agonist.
• Atropine is an antagonist (hence its use in acetylcholinesterase poisoning).
Amino acids
• Glutamic acid (Glu); used at excitatory synapses in the central nervous system (CNS). Essential for long term potentiation (LTP), a form of memory.
Like GABA, Glu acts on two types of CNS synapses:
• FAST (~1 msec) with Glu opening ligand-gated Na+ channels;
• SLOW (~1 sec) with Glu binding to G-protein-coupled receptors receptors that turn on a "second messenger" cascade of biochemical changes that open channels allowing Na+ into the cell.
• Gamma aminobutyric acid (GABA); used at inhibitory synapses in the CNS.
• Glycine (Gly). Also used at inhibitory synapses in the CNS. In fact, both GABA and glycine are released together at some inhibitory synapses.
Catecholamines
Synthesized from tyrosine (Tyr).
• Noradrenaline (also called norepinephrine). Released by postganglionic neurons of the sympathetic branch of the autonomic nervous system. Also used at certain synapses in the CNS.
• Dopamine. Used at certain synapses in the CNS.
Other monoamines
• Serotonin (also known as 5-hydroxytryptamine or 5HT). Synthesized from tryptophan (Trp).
• Histamine
Both of these neurotransmitters are confined to synapses in the brain. However, serotonin is also secreted from the duodenum, where it acts in a paracrine manner to stimulate intestinal peristalsis, and as a circulating hormone, where it is taken up by platelets and also suppresses bone formation.
Peptides
A selection of 9 of the 40 or more peptides that are suspected to serve as neurotransmitters in the brain. The first six also serve as hormones.
• Vasopressin
• Oxytocin
• Somatostatin
• Gonadotropin-releasing hormone (GnRH)
• Angiotensin II
• Cholecystokinin (CCK)
• Substance P
• Two enkephalins
• Met-enkephalin (Tyr-Gly-Gly-Phe-Met)
• Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu)
ATP
ATP - probably along with another neurotransmitter is released at some synapses in the brain.
Synaptic Plasticity
Most neurons release a single neurotransmitter at the synapses at their axon terminals. However, some exceptions have been found.
• neurons that release one transmitter at some of their terminals, a different one at others
• neurons that switch from one neurotransmitter to a different one when the stimulus reaching them changes. Example: interneurons in the hypothalamus of the rat that release dopamine when the rats are exposed to short-day photoperiods (which these nocturnal animals like) but switch to releasing somatostatin when the rats are exposed to long days (which they don't like).
Turning Synapses Off
Once its job is done, the neurotransmitter must be removed from the synaptic cleft to prepare the synapse for the arrival of the next action potential. Two methods are used:
• Reuptake. The neurotransmitter is taken back into the synaptic knob of the presynaptic neuron by active transport. All the neurotransmitters except acetylcholine use this method.
• Acetylcholine is removed from the synapse by enzymatic breakdown into inactive fragments. The enzyme used is acetylcholinesterase.
Nerve gases used in warfare (e.g., sarin) and the organophosphate insecticides (e.g., parathion) achieve their effects by inhibiting acetylcholinesterase thus allowing ACh to remain active. Atropine is used as an antidote because it blocks ACh muscarinic receptors.
Drugs and Synapses
Many drugs that alter mental state achieve at least some of their effects by acting at synapses.
GABA Receptors
The GABAA receptor is a ligand-gated chloride channel. Activation of the receptors increases the influx of chloride (Cl) ions into the postsynaptic cell raising its membrane potential and thus inhibiting it. A number of drugs bind to the GABAA receptor. They bind at sites different from the spot where GABA itself binds, but increase the strength of GABA's binding to its site. Thus they enhance the inhibitory effect of GABA in the CNS. These drugs include sedatives like phenobarbital and anti-anxiety drugs like Valium, Librium, Halcion (all members of a group called benzodiazepines). In view of their common action, it is not surprising that they act additively; taken together these drugs can produce dangerous overdoses. The recreational (and illegal) drug γ-hydroxybutyrate binds to the GABAB receptor.
Catecholamine synapses
Many antidepressant drugs (the so-called tricyclic antidepressants like amitriptyline ["Elavil"]) interfere with the reuptake of noradrenaline and serotonin from their synapses and thus enhance their action at the synapse. The popular antidepressant fluoxetine ("Prozac"), seems to block only the reuptake of serotonin.
Dopamine synapses
One class of dopamine receptor is bound by such drugs as chlorpromazine and haloperidol. Binding of these drugs leads to increased synthesis of dopamine at the synapse and eases some of the symptoms of schizophrenia.
Synapses blocking pain signals
The two enkephalins are released at synapses on neurons involved in transmitting pain signals back to the brain. The enkephalins hyperpolarize the postsynaptic membrane thus inhibiting it from transmitting these pain signals. The ability to perceive pain is vital. However, faced with massive, chronic, intractable pain, it makes sense to have a system that decreases its own sensitivity. Enkephalin synapses provide this intrinsic pain-suppressing system.
Opiates such as heroin, morphine, codeine, and methadone bind these same receptors. This makes them excellent pain killers. However, they are also highly addictive.
• By binding to enkephalin receptors, they enhance the pain-killing effects of the enkephalins.
• A homeostatic reduction in the sensitivity of these synapses compensates for continued exposure to opiates.
• This produces tolerance, the need for higher doses to achieve the prior effect.
• If use of the drug ceases, the now relatively insensitive synapses respond less well to the soothing effects of the enkephalins, and the painful symptoms of withdrawal are produced.
Electrical Synapses
Electrical synapses are a rare exception to the general rule that neurons signal other neurons by release of chemical neurotransmitters. Some properties of electrical synapses:
• The two neurons are connected by gap junctions, the space between them being much smaller (~2 nm) than that at chemical synapses (~20 nm).
• In many cases, transmission of action potentials can pass in either direction.
• Transmission between neurons is as much as ten times faster than in chemical synapses.
• Thus electrical synapses can mediate, for example, the very rapid escape response of crayfish that encounter a threat.
• Electrical synapses in the CNS permit groups of interneurons to fire together.
• They are found in vertebrates (e.g. in the hippocampus of the brain) as well as in invertebrates like the crayfish. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8B%3A_Synapses.txt |
The central nervous system is made up of the spinal cord and brain. The spinal cord conducts sensory information from the peripheral nervous system (both somatic and autonomic) to the brain. It also conducts motor information from the brain to our various effectors: skeletal muscles, cardiac muscle, smooth muscle, glands, and serves as a minor reflex center. The brain receives sensory input from the spinal cord as well as from its own nerves (e.g., olfactory and optic nerves) and devotes most of its volume (and computational power) to processing its various sensory inputs and initiating appropriate and coordinated motor outputs
White Matter vs. Gray Matter
Both the spinal cord and the brain consist of white matter (bundles of axons each coated with a sheath of myelin) and gray matter (masses of the cell bodies and dendrites each covered with synapses). In the spinal cord, the white matter is at the surface, the gray matter inside. In the brain of mammals, this pattern is reversed. However, the brains of "lower" vertebrates like fishes and amphibians have their white matter on the outside of their brain as well as their spinal cord.
The Meninges
Both the spinal cord and brain are covered in three continuous sheets of connective tissue, the meninges. From outside in, these are thedura mater — pressed against the bony surface of the interior of the vertebrae and the cranium, the arachnoid, nd the pia mater. The region between the arachnoid and pia mater is filled with cerebrospinal fluid (CSF).
The Interstitial Fluid of the Central Nervous System
The cells of the central nervous system are bathed in a fluid, called cerebrospinal fluid (CSF), that differs from that serving as the interstitial fluid (ISF) of the cells in the rest of the body. Cerebrospinal fluid leaves the capillaries in the choroid plexus of the brain. It contains far less protein than "normal" because of the blood-brain barrier, a system of tight junctions between the endothelial cells of the capillaries. This barrier creates problems in medicine as it prevents many therapeutic drugs from reaching the brain. CSF flows uninterrupted throughout the central nervous system through the central cerebrospinal canal of the spinal cord and through an interconnected system of four ventricles in the brain.
CSF returns to the blood through lymphatic vessels draining the brain.In mice, the flow of CSF increases by 60% when they are asleep. Perhaps one function of sleep is to provide the brain a way of removing potentially toxic metabolites accumulated during waking hours.
The Spinal Cord
31 pairs of spinal nerves arise along the spinal cord. These are "mixed" nerves because each contain both sensory and motor axons. However, within the spinal column, all the sensory axons pass into the dorsal root ganglion where their cell bodies are located and then on into the spinal cord itself. All the motor axons pass into the ventral roots before uniting with the sensory axons to form the mixed nerves.
The spinal cord carries out two main functions:
• It connects a large part of the peripheral nervous system to the brain. Information (nerve impulses) reaching the spinal cord through sensory neurons are transmitted up into the brain. Signals arising in the motor areas of the brain travel back down the cord and leave in the motor neurons.
• The spinal cord also acts as a minor coordinating center responsible for some simple reflexes like the withdrawal reflex.
The interneurons carrying impulses to and from specific receptors and effectors are grouped together in spinal tracts.
Crossing Over of the Spinal Tracts
Impulses reaching the spinal cord from the left side of the body eventually pass over to tracts running up to the right side of the brain and vice versa. In some cases this crossing over occurs as soon as the impulses enter the cord. In other cases, it does not take place until the tracts enter the brain itself.
The Brain
The brain of all vertebrates develops from three swellings at the anterior end of the neural tube of the embryo. From front to back these develop into the
• forebrain (also known as the prosencephalon — shown in light color)
• midbrain (mesencephalon — gray)
• hindbrain (rhombencephalon — dark color) The human brain is shown from behind so that the cerebellum can be seen.
The human brain receives nerve impulses from the spinal cord and 12 pairs of cranial nerves:
• Some of the cranial nerves are "mixed", containing both sensory and motor axons
• Some, e.g., the optic and olfactory nerves (numbers I and II) contain sensory axons only
• Some, e.g. number III that controls eyeball muscles, contain motor axons only.
The Hindbrain
The main structures of the hindbrain (rhombencephalon) are the medulla oblongata, pons and cerebellum.
Medulla oblongata
The medulla looks like a swollen tip to the spinal cord. Nerve impulses arising here rhythmically stimulate the intercostal muscles and diaphragm thus making breathing possible. It also regulate heartbeats and regulate the diameter of arterioles thus adjusting blood flow. The neurons controlling breathing have mu (µ) receptors, the receptors to which opiates, like heroin, bind. This accounts for the suppressive effect of opiates on breathing. Destruction of the medulla causes instant death.
Pons
The pons seems to serve as a relay station carrying signals from various parts of the cerebral cortex to the cerebellum. Nerve impulses coming from the eyes, ears and touch receptors are sent on the cerebellum. The pons also participates in the reflexes that regulate breathing.
The reticular formation is a region running through the middle of the hindbrain (and on into the midbrain). It receives sensory input (e.g., sound) from higher in the brain and passes these back up to the thalamus. The reticular formation is involved in sleep, arousal (and vomiting).
Cerebellum
The cerebellum consists of two deeply-convoluted hemispheres. Although it represents only 10% of the weight of the brain, it contains as many neurons as all the rest of the brain combined. Its most clearly-understood function is to coordinate body movements. People with damage to their cerebellum are able to perceive the world as before and to contract their muscles, but their motions are jerky and uncoordinated. So the cerebellum appears to be a center for learning motor skills (implicit memory). Laboratory studies have demonstrated both long-term potentiation (LTP) and long-term depression (LTD) in the cerebellum.
The Midbrain
The midbrain (mesencephalon) occupies only a small region in humans (it is relatively much larger in "lower" vertebrates). We shall look at only three features:
• the reticular formation: collects input from higher brain centers and passes it on to motor neurons.
• the substantia nigra: helps "smooth" out body movements; damage to the substantia nigra causes Parkinson's disease.
• the ventral tegmental area (VTA): packed with dopamine-releasing neurons that are activated by nicotinic acetylcholine receptors and whose projections synapse deep within the forebrain.The VTA seems to be involved in pleasure: nicotine, amphetamines and cocaine bind to and activate its dopamine-releasing neurons and this may account at least in part for their addictive qualities.
The midbrain along with the medulla and pons are often referred to as the "brainstem".
The Forebrain
The human forebrain (prosencephalon) is made up of a pair of large cerebral hemispheres, called the telencephalon. Because of crossing over of the spinal tracts, the left hemisphere of the forebrain deals with the right side of the body and vice versa. A group of structures located deep within the cerebrum make up the diencephalon.
Diencephalon
We shall consider four of its structures:
• Thalamus.
• All sensory input (except for olfaction) passes through these paired structures on the way up to the somatic-sensory regions of the cerebral cortex and then returns to them from there.
• Signals from the cerebellum pass through them on the way to the motor areas of the cerebral cortex.
• Lateral geniculate nucleus (LGN). All signals entering the brain from each optic nerve enter a LGN and undergo some processing before moving on the various visual areas of the cerebral cortex.
• Hypothalamus.
• The seat of the autonomic nervous system. Damage to the hypothalamus is quickly fatal as the normal homeostasis of body temperature, blood chemistry, etc. goes out of control.
• The source of 8 hormones, two of which pass into the posterior lobe of the pituitary gland.
• Posterior lobe of the pituitary.
Receives vasopressin and oxytocin from the hypothalamus and releases them into the blood.
The Cerebral Hemispheres
Each hemisphere of the cerebrum is subdivided into four lobes visible from the outside:
• frontal
• parietal
• occipital
• temporal
Hidden beneath these regions of each cerebral cortex is
• An olfactory bulb; they receive input from the olfactory epithelia.
• A striatum; they receive input from the frontal lobes and also from the limbic system (below). At the base of each striatum is a nucleus accumbens (NA).
The pleasurable (and addictive) effects of amphetamines, cocaine, and perhaps other psychoactive drugs seem to depend on their producing increasing levels of dopamine at the synapses in the nucleus accumbens (as well as the VTA).
• a limbic system; they receives input from various association areas in the cerebral cortex and pass signals on to the nucleus accumbens. Each limbic system is made up of a:
• hippocampus. It is essential for the formation of long-term memories.
• The amygdala appears to be a center of emotions (e.g., fear). It sends signals to the hypothalamus and medulla which can activate the flight or fight response of the autonomic nervous system. In rats, at least, the amygdala contains receptors for
• vasopressin whose activation increases aggressiveness and other signs of the flight or fight response
• oxytocin whose activation lessens the signs of stress
The amygdala receives a rich supply of signals from the olfactory system, and this may account for the powerful effect that odor has on emotions (and evoking memories).
Mapping the Functions of the Brain
It is estimated that the human brain contains some 86 billion (8.6 x 1010) neurons averaging 10,000 synapses on each; that is, almost 1015 connections. How to unravel the workings of such a complex system?
Several methods have been useful.
Histology
Microscopic examination with the aid of selective stains has revealed many of the physical connections created by axons in the brain.
The Electroencephalograph (EEG)
This device measures electrical activity (brain "waves") that can be detected at the surface of the scalp. It can distinguish between, for example, sleep and excitement. It is also useful in diagnosing brain disorders such as a tendency to epileptic seizures.
Damage to the Brain
Many cases of brain damage from, for example,
• strokes (interruption of blood flow to a part of the brain)
• tumors in the brain
• mechanical damage (e.g., bullet wounds)
have provided important insights into the functions of various parts of the brain.
Example 1:
Battlefield injury to the left temporal lobe of the cerebrum interferes with speech.
Example 2: Phineas P. Gage
In 1848, an accidental explosion drove a metal bar completely through the frontal lobes of Phineas P. Gage. Not only did he survive the accident, he never even lost consciousness or any of the clearly-defined functions of the brain. However, over the ensuing years, he underwent a marked change in personality. Formerly described as a reasonable, sober, conscientious person, he became — in the words of those observing him — "thoughtless, irresponsible, fitful, obstinate, and profane". In short, his personality had changed, but his vision, hearing, other sensations, speech, and body coordination were unimpaired. (Similar personality changes have since been often observed in people with injuries to their prefrontal cortex.)
The photograph (courtesy of the Warren Anatomical Museum, Harvard University Medical School) shows Gage's skull where the bar entered (left) and exited (right) in the accident (which occurred 12 years before he died of natural causes in 1861).
Stimulating the exposed brain with electrodes
There are no pain receptors on the surface of the brain, and some humans undergoing brain surgery have volunteered to have their exposed brain stimulated with electrodes during surgery. When not under general anesthesia, they can even report their sensations to the experimenter. Experiments of this sort have revealed a band of cortex running parallel to and just in front of the fissure of Rolando that controls the contraction of skeletal muscles. Stimulation of tiny spots within this motor area causes contraction of the muscles.
The area of motor cortex controlling a body part is not proportional to the size of that part but is proportional to the number of motor neurons running to it. The more motor neurons that activate a structure, the more precisely it can be controlled. Thus the areas of the motor cortex controlling the hands and lips are much larger than those controlling the muscles of the torso and legs. A similar region is located in a parallel band of cortex just behind the fissure of Rolando. This region is concerned with sensation from the various parts of the body. When spots in this sensory area are stimulated, the patient reports sensations in a specific area of the body. A map can be made based on these reports. When portions of the occipital lobe are stimulated electrically, the patient reports light. However, this region is also needed for associations to be made with what is seen. Damage to regions in the occipital lobe results in the person's being perfectly able to see objects but incapable of recognizing them.
The centers of hearing — and understanding what is heard — are located in the temporal lobes.
CT = X-ray C omputed T omography
This is an imaging technique that uses a series of X-ray exposures taken from different angles. Computer software can integrate these to produce a three-dimensional picture of the brain (or other body region). CT scanning is routinely used to quickly diagnose strokes.
PET = P ositron-E mission T omography
This imaging technique requires that the subject be injected with a radioisotope that emits positrons.
• Water labeled with oxygen-15 (H215O) is used to measure changes in blood flow (which increases in parts of the brain that are active). The short half-life of 15O (2 minutes) makes it safe to use.
• Deoxyglucose labeled with fluorine-18. The brain has a voracious appetite for glucose (although representing only ~2% of our body weight, the brain receives ~15% of the blood pumped by the heart and consumes ~20% of the energy produced by cellular respiration when we are at rest). When supplied with deoxyglucose, the cells are tricked into taking in this related molecule and phosphorylating it in the first step of glycolysis. But no further processing occurs so it accumulates in the cell. By coupling a short-lived radioactive isotope like 18F to the deoxyglucose and using a PET scanner, it is possible to visualize active regions of the brain.
The images in fig. 15.8.3.8 (courtesy of Michael E. Phelps from Science 211:445, 1981) were produced in a PET scanner. The dark areas are regions of high metabolic activity. Note how the metabolism of the occipital lobes (arrows) increases when visual stimuli are received.
Similarly, sounds increase the rate of deoxyglucose uptake in the speech areas of the temporal lobe.
The image in fig. 15.8.3.9 on the right (courtesy of Gary H. Duncan from Talbot, J. D., et. al., Science 251: 1355, 1991) shows activation of the cerebral cortex by a hot probe (which the subjects describe as painful) applied to the forearm.
Most cancers consume large amounts of glucose (cellular respiration is less efficient than in normal cells so they must rely more on the inefficient process of glycolysis). Therefore PET scanning with 18F-fluorodeoxyglucose is commonly used to monitor both the primary tumor and any metastases.
MRI = M agnetic R esonance I maging
This imaging technique uses powerful magnets to detect magnetic molecules within the body. These can be endogenous molecules or magnetic substances injected into a vein.
fMRI = Functional Magnetic Resonance Imaging
fMRI exploits the changes in the magnetic properties of hemoglobin as it carries oxygen. Activation of a part of the brain increases oxygen levels there increasing the ratio of oxyhemoglobin to deoxyhemoglobin.
The probable mechanism:
• The increased demand for neurotransmitters must be met by increased production of ATP.
• Although this consumes oxygen (needed for cellular respiration),
• it also increases the blood flow to the area.
• So there is an increase and not a decrease in the oxygen supply to the region, which provides the signal detected by fMRI.
Magnetoencephalography (MEG)
MEG detects the tiny magnetic fields created as individual neurons "fire" within the brain. It can pinpoint the active region with a millimeter, and can follow the movement of brain activity as it travels from region to region within the brain. MEG is noninvasive requiring only that the subject's head lie within a helmet containing the magnetic sensors. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8C%3A_The_Human_Central_Nervous_System.txt |
The nervous system is divided into the peripheral nervous system (PNS) and the central nervous system (CNS).
The PNS consists of
• sensory neurons running from stimulus receptors that inform the CNS of the stimuli
• motor neurons running from the CNS to the muscles and glands - called effectors - that take action.
The CNS consists of the spinal cord and the brain.
The peripheral nervous system is subdivided into the
• sensory-somatic nervous system and the
• autonomic nervous system
The Sensory-Somatic Nervous System
The sensory-somatic system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.
The Cranial Nerves
Nerves Type Function
I
Olfactory
sensory olfaction (smell)
II
Optic
sensory vision
(Contain 38% of all the axons connecting to the brain.)
III
Oculomotor
motor* eyelid and eyeball muscles
IV
Trochlear
motor* eyeball muscles
V
Trigeminal
mixed Sensory: facial and mouth sensation
Motor: chewing
VI
Abducens
motor* eyeball movement
VII
Facial
mixed Sensory: taste
Motor: facial muscles and
salivary glands
VIII
Auditory
sensory hearing and balance
IX
Glossopharyngeal
mixed Sensory: taste
Motor: swallowing
X
Vagus
mixed main nerve of the
parasympathetic nervous system (PNS)
XI
Accessory
motor swallowing; moving head and shoulder
XII
Hypoglossal
motor* tongue muscles
The autonomic nervous system consists of sensory neurons and motor neurons that run between the central nervous system (especially the hypothalamus and medulla oblongata) and various internal organs such as the heart, lungs, viscera and the glands (both exocrine and endocrine). It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. The contraction of both smooth muscle and cardiac muscle is controlled by motor neurons of the autonomic system. The actions of the autonomic nervous system are largely involuntary (in contrast to those of the sensory-somatic system). It also differs from the sensory-somatic system as it is using two groups of motor neurons to stimulate the effectors instead of one. First, the preganglionic neurons arise in the CNS and run to a ganglion in the body. Here they synapse with postganglionic neurons, which run to the effector organ (cardiac muscle, smooth muscle, or a gland).
The autonomic nervous system has two subdivisions:
• sympathetic nervous system
• parasympathetic nervous system.
The Sympathetic Nervous System
The preganglionic motor neurons of the sympathetic system (shown in black) arise in the spinal cord. They pass into sympathetic ganglia which are organized into two chains that run parallel to and on either side of the spinal cord. The preganglionic neuron may do one of three things in the sympathetic ganglion:
• synapse with postganglionic neurons (shown in white) which then reenter the spinal nerve and ultimately pass out to the sweat glands and the walls of blood vessels near the surface of the body.
• pass up or down the sympathetic chain and finally synapse with postganglionic neurons in a higher or lower ganglion
• leave the ganglion by way of a cord leading to special ganglia (e.g. the solar plexus) in the viscera. Here it may synapse with postganglionic sympathetic neurons running to the smooth muscular walls of the viscera. However, some of these preganglionic neurons pass right on through this second ganglion and into the adrenal medulla. Here they synapse with the highly-modified postganglionic cells that make up the secretory portion of the adrenal medulla.
The neurotransmitter of the preganglionic sympathetic neurons is acetylcholine (ACh). It stimulates action potentials in the postganglionic neurons. The neurotransmitter released by the postganglionic neurons is noradrenaline (also called norepinephrine). The action of noradrenaline on a particular gland or muscle is excitatory is some cases, inhibitory in others. At excitatory terminals, ATP may be released along with noradrenaline.
The release of noradrenaline
• stimulates heartbeat
• raises blood pressure
• dilates the pupils
• dilates the trachea and bronchi
• stimulates glycogenolysis — the conversion of liver glycogen into glucose
• shunts blood away from the skin and viscera to the skeletal muscles, brain, and heart
• inhibits peristalsis in the gastrointestinal (GI) tract
• inhibits contraction of the bladder and rectum
• and, at least in rats and mice, increases the number of AMPA receptors in the hippocampus and thus increases long-term potentiation (LTP).
In short, stimulation of the sympathetic branch of the autonomic nervous system prepares the body for emergencies: for "fight or flight" (and, perhaps, enhances the memory of the event that triggered the response).
Activation of the sympathetic system is quite general because
• a single preganglionic neuron usually synapses with many postganglionic neurons
• the release of adrenaline from the adrenal medulla into the blood ensures that all the cells of the body will be exposed to sympathetic stimulation even if no postganglionic neurons reach them directly.
The Parasympathetic Nervous System
The Nobel Prize winning physiologist Otto Loewi discovered (in 1920) that the effect of both sympathetic and parasympathetic stimulation is mediated by released chemicals. He removed the living heart from a frog with its sympathetic and parasympathetic nerve supply intact. As expected, stimulation of the first speeded up the heart while stimulation of the second slowed it down.
Loewi found that these two responses would occur in a second frog heart supplied with a salt solution taken from the stimulated heart. Electrical stimulation of the vagus nerve leading to the first heart not only slowed its beat but, a short time later, slowed that of the second heart also. The substance responsible was later shown to be acetylcholine. During sympathetic stimulation, adrenaline (in the frog) is released.
Parasympathetic stimulation causes
• slowing down of the heartbeat (as Loewi demonstrated)
• lowering of blood pressure
• constriction of the pupils
• increased blood flow to the skin and viscera
• peristalsis of the GI tract
In short, the parasympathetic system returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the danger is over. The vagus nerves also help keep inflammation under control. Inflammation stimulates nearby sensory neurons of the vagus. When these nerve impulses reach the medulla oblongata, they are relayed back along motor fibers to the inflamed area. Release of acetylcholine suppresses the release of inflammatory cytokines, e.g., tumor necrosis factor (TNF), from macrophages in the inflamed tissue.
Although the autonomic nervous system is considered to be involuntary, this is not entirely true. A certain amount of conscious control can be exerted over it as has long been demonstrated by practitioners of Yoga and Zen Buddhism. During their periods of meditation, these people are clearly able to alter a number of autonomic functions including heart rate and the rate of oxygen consumption. These changes are not simply a reflection of decreased physical activity because they exceed the amount of change occurring during sleep or hypnosis. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8D%3A_The_Peripheral_Nervous_System.txt |
The activity of the nervous system is mediated by many kinds of interneurons releasing one or another neurotransmitter such as
• noradrenaline
• gamma aminobutyric acid (GABA)
• dopamine
• glutamate (Glu)
• acetylcholine (ACh)
• serotonin
Presynaptic neurons synthesize and package their neurotransmitter in vesicles for release (by exocytosis) at the synapse. They often have "reuptake" transporters that reclaim the transmitter back into the cell when it has done its job. Postsynaptic neurons display receptors to which the neurotransmitter binds. All of this machinery provides many targets for alteration by exogenous chemicals; that is, psychoactive chemicals introduced into the body. These drugs fall into several distinct families.
Stimulants
The most widely used stimulants are
• caffeine (in coffee, tea, and cola beverages)
• nicotine (in cigarettes)
• amphetamines
• cocaine
All of these drugs mimic the stimulation provided by the sympathetic nervous system.
Nicotine binds to a subset of acetylcholine (ACh) receptors. ACh is a neurotransmitter at synapses early in the pathways of sympathetic stimulation. Although a weak drug in one sense, nicotine is strongly addictive. The use of e-cigarettes, chewing gum and skin patches containing nicotine is designed to satisfy the craving for nicotine while avoiding the serious health effects of other ingredients in cigarette smoke.
Amphetamines and cocaine bind to — thus blocking — transporters used for the reuptake of dopamine (and noradrenaline) into presynaptic neurons. This causes the level of dopamine to rise in the synapses. High levels of dopamine in an area of the brain called the nucleus accumbens appear to mediate the pleasurable effects associated with these (as well as other) psychoactive drugs.
Table 1: Some amphetamines
Generic name Trade name
dextroamphetamine sulfate Dexedrine
methylphenidate Ritalin
pemoline Cylert
mixture of 4 amphetamines Adderall
The chief medical uses for amphetamines and amphetamine-like drugs are to help people lose weight (because they suppress appetite) and to help children with attention deficit/hyperactivity disorder (ADHD) to perform better in school. At first glance, this second use seems counterproductive. This controversial procedure seems to work by increasing the alertness of the child so that it can focus its energies more effectively on the tasks in front of it.
Fen-Phen
Fen-Phen refers to a mixture of two amphetamine-like drugs fenfluramine and phentermine that were prescribed for losing weight. Because of reports of occasional very serious side effects, the mixture is no longer available and fenfluramine has been removed from the U.S. market.
Cocaine
Cocaine has been used for thousands of years by certain tribes in the Andes of South America. Cocaine and some of its relatives have legitimate medical uses as local anesthetics (e.g., lidocaine). However, the widespread recreational use of cocaine has created serious social problems. In order to achieve its effects, cocaine must cross the so-called blood-brain barrier. If antibodies are bound to the cocaine molecule, it cannot cross. This has raised the possibility of immunizing people against cocaine. It works in mice.
Sedatives
Sedatives induce sleep. They include
• ethanol (beverage alcohol)
• barbiturates, such as
• phenobarbital
• secobarbital (Seconal®)
• meprobamate (Miltown®, Equanil®)
Ethanol
Ethyl alcohol (ethanol) is, by a wide margin, the most widely used drug in most of the world. Its popularity comes not from its sedative effect but from the sense of well-being that it induces at low doses. Perhaps low doses sedate those parts of the brain involved with, for example, tension and anxiety and in this way produce a sense of euphoria. However, higher doses depress brain centers involved in such important functions as pain sensation, coordination, and balance. At sufficiently high doses, the reticular formation can be depressed enough to cause loss of consciousness.
Ethanol increases the release of the neurotransmitter GABA activating GABAA receptors and directly inhibits NMDA receptors.
Barbiturates
Barbiturates are often prescribed as sleeping pills and also to prevent seizures. Barbiturates mimic some of the action of ethanol, particularly in their ability to depress the reticular formation (thus promoting sleep) and, in high doses, the medulla oblongata (thus stopping breathing).
Barbiturates bind to a subset of GABA receptors designated GABAA receptors. These are ligand-gated channels that enhance the flow of chloride ions (Cl) into the postsynaptic neuron, thus increasing its resting potential and making it less likely to fire. By binding to the GABAA receptor, barbiturates (and perhaps ethanol) increase the natural inhibitory effect of GABA synapses. Barbiturates and alcohol act additively — the combination producing a depression greater than either one alone. The combination is a frequent cause of suicide, both accidental and planned.
Meprobamate
Meprobamate is prescribed as a tranquilizer, but its action is quite different from the tranquilizers discussed below. Its molecular activity is like that of other sedatives and in combination with them can produce a lethal overdose. All sedatives produce two related physiological effects:
• tolerance — the necessity for a steadily-increasing dose to achieve the same physiological and psychological effects
• physical dependence — withdrawal of the drug precipitates unpleasant physical and psychological symptoms.
These traits are also shared with nicotine, opioids, and other psychoactive drugs.
Local Anesthetics
These chemical relatives of cocaine act by blocking the voltage-gated Na+ channels of sensory neurons preventing them from generating action potentials. They are injected or applied topically and block transmission not only in pain-conducting neurons but in others as well (causing general numbness).
Examples:
• lidocaine (Xylocaine®)
• procaine (Novocaine®)
Inhaled Anesthetics
Most of these are volatile hydrocarbons or ethers. Diethyl ether and chloroform are seldom used today, having been replaced by safer alternatives such as isofluorane, a fluorinated ether. Some, like isofluorane, bind to inhibitory GABA receptors) in the brain hyperpolarizing, and thus decreasing the sensitivity of, postsynaptic neurons. Others, like ketamine, block the activity of excitatory glutamate receptors.
Other Hydrocarbons
1,4-Butanediol is a common solvent. When ingested, it is converted into γ-hydroxybutyrate, an increasingly-popular (and illegal) "club drug". γ-Hydroxybutyrate acts on GABAB receptors. Conversion of 1,4-butanediol to γ-hydroxybutyrate requires the enzyme alcohol dehydrogenase, the same enzyme used to metabolize ethanol. Ingesting both ethanol and 1,4-butanediol delays the effects of the latter.
Opioids
These are substances isolated from the opium poppy or synthetic relatives. (They are also called opiates.)
Examples:
• morphine
• codeine
• heroin
• fentanyl (a synthetic that is ~80 times more potent than morphine)
• methadone
• oxycodone
Opioids depress nerve transmission in sensory pathways of the spinal cord and brain that signal pain. This explains why opioids are such effective pain killers. Opioids also inhibit brain centers controlling coughing, breathing, and intestinal motility. Both morphine and codeine are used as pain killers, and codeine is also used in cough medicine. Opioids are exceedingly addictive, quickly producing tolerance and dependence. Although heroin is even more effective as a painkiller than morphine and codeine, it is so highly addictive that its use is illegal. Methadone is a synthetic opioid that is used to break addiction to heroin (and replace it with addiction to methadone).
Opioids bind to so-called mu (µ) receptors . These G-protein-coupled receptors are located on the subsynaptic membrane of neurons involved in the transmission of pain signals. Their natural ligands are two pentapeptides (containing five amino acids):
• Met-enkephalin (Tyr-Gly-Gly-Phe-Met-COO-)
• Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu-COO-)
Release of enkephalins suppresses the transmission of pain signals. Little is to be gained by having the perception of pain increase indefinitely in proportion to the amount of damage done to the body. Beyond a certain point, it makes sense to have a system that decreases its own sensitivity in the face of massive, intractable pain.
By binding to mu (µ) receptors, opioids like morphine enhance the pain-killing effects of enkephalin neurons. Opioid tolerance can be explained, at least in part, as a homeostatic response that reduces the sensitivity of the system to compensate for continued exposure to high levels of morphine or heroin. When the drug is stopped, the system is no longer as sensitive to the soothing effects of the enkephalin neurons and the pain of withdrawal is produced.
Mu (µ) receptors are also found on the cells in the medulla oblongata that regulate breathing. This accounts for the suppressive effect opioids have on breathing.
Opioid antagonists
Opioid antagonists such as naloxone (Narcan®) and naltrexone (ReVia®) bind to µ receptors but instead of activating them, they prevent the binding of the opioids themselves. In fact, if the receptors are already occupied by, for example, heroin molecules, naloxone will push the heroin molecules off and quickly rescue the patient from a drug overdose. Naltrexone is used to help recovering heroin addicts stay drug-free.
Antipsychotics
Antipsychotics (also called "neuroleptics") are used to treat schizophrenia, a common and devastating mental disease. They act by binding to one class of receptors for the neurotransmitter dopamine. There are two groups currently in use:
• "Typical" antipsychotics (sometimes referred to as "major tranquilizers"). Examples:
• chlorpromazine (Thorazine®)
• haloperidol (Haldol®)
• "Atypical" antipsychotics (also referred to as "second generation" antipsychotics). Examples:
• risperidone (Risperdal®)
• olanzapine (Zyprexa®)
• quetiapine (Seroquel®)
Tranquilizers
Tranquilizers act like sedatives in reducing anxiety and tensions. Most belong to a group called benzodiazepines and include such commonly-prescribed drugs as Xanax® and Klonopin®. The benzodiazepines act on interneurons that use the inhibitory neurotransmitter GABA. By binding to GABAA receptors on the postsynaptic membrane, they enhance the action of GABA at the synapse.This is the same receptor to which barbiturates (and perhaps ethanol) bind. Thus although benzodiazepines seem safe enough when used alone, combining them with ethanol or barbiturates can be (and often has been) lethal.
Antidepressants
Antidepressants fall into four chemical categories (of which we shall examine three). Most share a common property: they increase the amount of serotonin at synapses that use it as a neurotransmitter.
Monoamine oxidase inhibitors (MAOIs)
These drugs act on a mitochondrial enzyme that breaks down monoamines such as noradrenaline and serotonin. By inhibiting the enzyme in presynaptic serotonin-releasing neurons, more noradrenaline and serotonin is deposited in the synapse. Some examples: Parnate®, Nardil®, Marplan®. For several reasons, MAO inhibitors are not used much anymore.
Tricyclic antidepressants (TCAs)
These drugs block the reuptake of noradrenaline, dopamine, and serotonin causing an increase in the level of these neurotransmitters in the synapse.
Examples:
Generic name Trade name
imipramine Tofranil®
clomiprimine Anafranil®
amitriptyline Elavil®
Although tricyclics are still prescribed for pain relief, their role as antidepressants has largely been taken over by the serotonin reuptake inhibitors (SRIs).
Selective serotonin reuptake inhibitors (SSRIs)
These drugs inhibit the reuptake of serotonin but not of noradrenaline.
Examples:
Generic name Trade name
fluoxetine Prozac®
paroxetine Paxil®
sertraline Zoloft®
Although all these drugs quickly increase the amount of serotonin in the brain, there is more to the story than that. Unlike most psychoactive drugs, antidepressants do not relieve the symptoms of depression until a week or more after dosing begins. During this period, the number of serotonin receptors on the postsynaptic membranes decreases. How this translates into relief of symptoms is not yet understood.
Serotonin and norepinephrine reuptake inhibitors (SNRIs)
Because they act on the reuptake of both serotonin and noradrenaline (norepinephrine), this category of antidepressants is also known as dual reuptake inhibitors.
Examples: venlafaxine (Effexor®) and duloxetine (Cymbalta®).
Bupropion
Bupropion (Wellbutrin®) is a novel drug that blocks the reuptake of noradrenaline and dopamine. Although it does not interfere with the uptake of serotonin, it also appears to be an effective antidepressant.
Atomoxetine
This drug (Strattera®) selectively interferes with the reuptake of noradrenaline. It is used in children with attention deficit/hyperactivity disorder (ADHD).
Psychedelics
Psychedelic drugs distort sensory perceptions, especially sight and sound. Some such as mescaline, psilocybin and dimethyltryptamine (DMT) are natural plant products.
The photograph shows the peyote cactus in flower. The cactus head contains several psychedelic chemicals, of which mescaline is the most important. Dried cactus heads ("mescal buttons") have been used since pre-Columbian times in the religious ceremonies of native peoples in Mexico. About a century ago, this religious use spread to some tribes in the United States and Canada who, in 1922, became incorporated into the Native American Church. Other psychedelic drugs are synthetic. These include
• lysergic acid diethylamide (LSD)
• dimethoxymethylamphetamine (DOM or "STP")
• methylenedioxymethamphetamine (MDMA or "ecstasy")
As their name suggests, DOM and MDMA also share the stimulant qualities of amphetamines. All the psychedelics have a molecular structure that resembles serotonin and probably bind to serotonin receptors on the postsynaptic membrane.
Phencyclidine (PCP)
PCP is used as an anesthetic in veterinary medicine. Used (illicitly) by humans (called "crystal" or "angel dust"), it can produce a wide variety of powerful reactions resembling those of stimulants as well as psychedelics. Unlike the other psychedelics, it binds to (and inhibits) NMDA receptors (in the hippocampus and other parts of the forebrain).
Marijuana
The main psychoactive ingredient in marijuana is delta-9-tetrahydrocannabinol9-THC). It binds to
• CB1 receptors (G-protein-coupled receptors) that are present on presynaptic membranes in many parts of the brain.
• CB2 receptors are also found in the brain as well as being highly-expressed on cells of the immune system (e.g., B cells and T cells).
THC produces the drowsiness of sedatives like alcohol, the dulling of pain (like opioids) and in high doses, the perception-distorting effects of the psychedelics. Unlike sedatives and opioids, however, tolerance to THC does not occur. In fact, the drug is excreted so slowly from the body that, with repeated use, a given response is achieved by a lower dose.
The natural ligands of the CB receptors are the endocannabinoids - anandamide and 2-arachidonylglycerol (2-AG). Both of these compounds are produced from phospholipids.
What are these natural ligands doing? They probably will turn out to have multiple effects, but the clearest ones so far are their effects on
• appetite. Mice given anandamide eat more than normal while those whose genes for the CB1 receptor have been "knocked out" eat less than normal.These findings will be no surprise to the ill humans (e.g., with cancer or AIDS) who find that marijuana improves their appetite. Rimonabant (Acomplia®), a drug that blocks the ability of the body's natural CB1 ligands to bind the CB1 receptor was sold for a time in Europe as an appetite suppressant. (Because of its side effects, it was never approved for use in the U.S. and was removed from the European market in 2008.)
• development of correct synaptic connections in the embryonic brain. Mice whose genes for the CB1 receptor have been knocked out develop defects in the wiring pattern of interneurons in their brain (which may account for the cognitive defects that have been reported in the children of women who used marijuana during pregnancy).
• neuronal activity in the adult brain. Mice whose genes for the CB1 receptor have been knocked out are more susceptible to epileptic seizures. Marijuana has been used for centuries to control epileptic seizures in humans.
• suppressing contact dermatitis. Knockout mice lacking CB1 and CB2 receptors mount a more vigorous allergic inflammatory response to agents (like nickel) that elicit contact sensitivity. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8E%3A_Drugs_and_the_Nervous_System.txt |
Nitric oxide is a gas. It is highly reactive; that is, it participates in many chemical reactions. (It is one of the nitrogen oxides ("NOx") in automobile exhaust and plays a major role in the formation of photochemical smog.) But NO also has many physiological functions.
They share these features:
• NO is synthesized within cells by an enzyme NO synthase (NOS).
• The human (and mouse) genome contains 3 different genes encoding NO synthases.
• nNOS (or NOS-1): found in neurons (hence the "n").
• eNOS (or NOS-3): found in the endothelial (hence the "e") cells that line the lumen of blood vessels.
• iNOS (or NOS-2): found in macrophages. (the "i" stands for "inducible"). Whereas the levels of nNOS and eNOS are relatively steady, expression of iNOS genes awaits an appropriate stimulus (e.g., invasion by a pathogen).
• All types of NOS produce NO from arginine with the aid of molecular oxygen and NADPH.
• NO diffuses freely across cell membranes.
• There are so many other molecules with which it can interact, that it is quickly consumed close to where it is synthesized.
• Thus NO acts in a paracrine or even autocrine fashion - affecting only cells near its point of synthesis.
This page examines some of the functions of NO.
Blood Flow
NO relaxes the smooth muscle in the walls of the arterioles. At each systole, the endothelial cells that line the blood vessels release a puff of NO. This diffuses into the underlying smooth muscle cells causing them to relax and thus permit the surge of blood to pass through easily. Mice whose genes for the NO synthase found in endothelial cells (eNOS) has been "knocked out" suffer from hypertension.
Nitroglycerine, which is often prescribed to reduce the pain of angina, does so by generating nitric oxide, which relaxes the walls of the coronary arteries and arterioles. NO also inhibits the aggregation of platelets and thus keeps inappropriate clotting from interfering with blood flow.
Kidney Function
Release of NO around the glomeruli of the kidneys increases blood flow through them thus increasing the rate of filtration and urine formation.
Penile Erection
The erection of the penis during sexual excitation is mediated by NO released from nerve endings close to the blood vessels of the penis. Relaxation of these vessels causes blood to pool in the blood sinuses producing an erection. Three popular prescription drugs enhance this effect by the mechanism described below.
Recent evidence suggests that NO's job in reproduction is not finished with producing an erection. At the moment of contact, release of NO by the acrosome of the sperm activates the egg to complete meiosis II and the other steps of fertilization.
Other Actions on Smooth Muscle
Peristalsis
The wavelike motions of the gastrointestinal tract are aided by the relaxing effect of NO on the smooth muscle in its walls.
Birth
NO also inhibits the contractility of the smooth muscle wall of the uterus. As the moment of birth approaches, the production of NO decreases. Nitroglycerine has helped some women who were at risk of giving birth prematurely to carry their baby to full term.
NO and Inflammation
The NO produced by eNOS (NOS-3) inhibits inflammation in blood vessels. It does this by blocking the exocytosis of mediators of inflammation from the endothelial cells. NO may also block exocytosis in other types of cells such as macrophages and cytotoxic T lymphocytes (CTL).
Effects on Secretion
NO affects secretion from several endocrine glands.
For examples, it stimulates
• the release of Gonadotropin-releasing hormone (GnRH) from the hypothalamus
• the release of pancreatic amylase from the exocrine portion of the pancreas
• the release of adrenaline from the adrenal medulla
NO and the Nervous System
NO and the Autonomic Nervous System
Some motor neurons of the parasympathetic branch of the autonomic nervous system release NO as their neurotransmitter. The actions of NO on penile erection and peristalsis are probably mediated by these nerves.
NO and the Medulla Oblongata
Hemoglobin transports NO at the same time it carries oxygen. When it unloads oxygen in the tissues, it also unloads NO. In severe deoxygenation, NO-sensitive cells in the medulla oblongata respond to this release by increasing the rate and depth of breathing.
NO and the Brain
In laboratory animals (mice and rats), NO is released by neurons in the CA1 region of the hippocampus and stimulates the NMDA receptors there that are responsible for long-term potentiation (LTP) - a type of memory (and learning). The ease with which NO diffuses away from the synapse where it is generated enables it to affect nearby synapses. So what may have begun as a localized action becomes magnified.
Laboratory rats treated with inhibitors of NOS synthesis fail to develop and/or retain learned responses such as the conditioned response. Mice whose genes for nNOS have been knocked out are healthy but display abnormal behavior, e.g., they kill other males and try to mate with nonreceptive females.
NO and Fertilization
The acrosome at the tip of sperm heads activates its NO synthase when it enters the egg. The resulting release of NO in the egg is essential (at least in sea urchins) for triggering the next steps in the process :
• blocking the entry of additional sperm
• orienting the pronuclei for fusion.
Killing Pathogens
NO aids in the killing of engulfed pathogens (e.g., bacteria) within the lysosomes of macrophages.
Mice whose genes for the NO synthase found in macrophages (iNOS) have been knocked out are more susceptible to infections by intracellular bacteria like Listeria monocytogenes. Th1 cells, the ones responsible for an inflammatory response against invaders, secrete NO. Harmless bacteria, living as commensals at the rear of our throat, convert nitrates in our food into nitrites. When these reach the stomach, the acidic gastric juice (pH ~1.4) generates NO from them. This NO kills almost all the bacteria that have been swallowed in our food.
Since the dawn of recorded human history, nitrites have been used to preserve meat from bacterial spoilage.
NO and Longevity
Mice whose genes for eNos have been knocked out
• show signs of premature aging
• have a shortened life span
• fail to benefit from the life-extending effect of a calorie-restricted (CR) diet
Mechanisms of NO Action
The signaling functions of NO begin with its binding to protein receptors on or in the cell. The binding sites can be either:In either case, binding triggers an allosteric change in the protein which, in turn, triggers the formation of a "second messenger" within the cell. The most common protein target for NO seems to be guanylyl cyclase, the enzyme that generates the second messenger cyclic GMP (cGMP).
Three prescription drugs sildenafil (Viagra®), vardenafil (Levitra®) and tadalafil (Cialis®) enhance the effects of NO by inhibiting the enzyme that normally breaks down cGMP.
Bioluminescence
Plants Also Use NO
NO has been implicated in many plant activities. It is a weapon against invading pathogens. Infection of the plant triggers the formation of a NOS that, like the animal versions, makes NO from arginine. Release of NO by the infected cell induces a number of defense responses. A gradient of NO may also guide the pollen tube to its destination in the ovule. The popular prescription drug sildenafil citrate (Viagra®) enhances the effect of NO on pollination (just as it does on penile erection). NO inhibits flowering. NO promotes recovery from etiolation. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8F%3A_Nitric_Oxide_%28NO%29.txt |
Prion diseases are transmissible from host to host of a single species and, sometimes, even from one species to another (such as a laboratory animal). They destroy brain tissue giving it a spongy appearance. For these reasons, prion diseases are also called transmissible spongiform encephalopathies or TSEs.
Some examples:
Creutzfeldt-Jakob Disease CJD humans
variant Creutzfeldt-Jakob Disease vCJD humans; acquired from cattle with BSE
Bovine Spongiform Encephalopathy BSE "mad cow disease"
Kuru infectious; in humans who practiced cannibalism in Papua New Guinea
Gerstmann-Sträussler-Scheinker disease GSS inherited disease of humans
Fatal Familial Insomnia FFI inherited disease of humans
Scrapie infectious disease of sheep and goats
other animal TSEs cats, mink, elk, mule deer
Before the victim dies of a TSE, the damage to the brain is reflected in such signs as loss of coordination and in humans dementia. Injections of ground-up brain tissue from an animal or human patient with a prion disease into another animal (of the appropriate species) transmits the disease. This suggests that the disease is caused by an infectious agent such as a virus. But viruses have a genome and despite intense efforts no evidence of a virus has ever been found in these brain extracts. In fact, treating the extracts with agents (e.g., ultraviolet light) that destroy DNA does not reduce their infectiousness.
To date, the evidence indicates that the infectious agent in the TSEs is a protein. Stanley Prusiner who pioneered in the study of these proteins and was awarded the Nobel Prize in 1997 for his efforts has named them prion proteins (designated PrP) or simply prions. It turns out that prions are molecules of a normal body protein that have changed their three-dimensional configuration.
PrPC
The normal protein
• is called PrPC (for cellular)
• is a glycoprotein normally anchored to the surface of cells.
• has its secondary structure dominated by alpha helices (probably 3 of them)
• is easily soluble
• is easily digested by proteases
• is encoded by a gene designated (in humans) PRNP located on our chromosome 20.
PrPSc
The abnormal, disease-producing protein
• is called PrPSc (for scrapie)
• has the same amino acid sequence as the normal protein; that is, their primary structures are identical but
• its secondary structure is dominated by beta conformation
• is insoluble in all but the strongest solvents
• is highly resistant to digestion by proteases
• When PrPSc comes in contact with PrPC, it converts the PrPC into more of itself (even in the test tube).
• These molecules bind to each other forming aggregates.
• It is not yet clear if these aggregates are themselves the cause of the cell damage or are simply a side effect of the underlying disease process.
Inherited Prion Diseases
Creutzfeldt-Jakob Disease (CJD)
10–15% of the cases of CJD are inherited; that is, the patient comes from a family in which the disease has appeared before. The disease is inherited as an autosomal dominant. The patients have inherited at least one copy of a mutated PRNP gene. Some of the most common mutations are:
• a change in codon 200 converting glutamic acid (E) at that position to lysine (K) (thus designated "E200K")
• a change from aspartic acid (D) at position 178 in the protein to asparagine (D178N) when it is accompanied by a polymorphism in both PRNP genes that encodes valine at position 129. When the polymorphism at codon 129 is Met on both genes, the D178N mutation produces Fatal Familial Insomnia instead.
• a change from valine (V) at position at position 210 to isoleucine (V210I)
Extracts of autopsied brain tissue from these patients can transmit the disease to
• apes (whose PRNP gene is probably almost identical to that of humans).
• transgenic mice who have been given a Prnp gene that contains part of the human sequence.
These results lead to the important realization that prion diseases can only be transmitted to animals that already carry a PRNP gene with a sequence that is at least similar to the one that encoded the PrPSc. In fact, knockout mice with no Prnp genes at all cannot be infected by PrPSc.
Gerstmann-Sträussler-Scheinker disease (GSS)
This prion disease is caused by the inheritance of a PRNP gene with a mutations encoding most commonly
• leucine instead of proline at position 102 (P102L) or
• valine instead of alanine at position 117 (A117V)
Again, the disease is also strongly associated with homozygosity for a polymorphism at position 129 (both residues being methionine).
Brain extracts from patients with GSS can transmit the disease to
• monkeys and apes
• transgenic mice containing a portion of the human PRNP gene.
Transgenic mice expressing the P102L gene develop the disease spontaneously.
Fatal Familial Insomnia (FFI)
People with this rare disorder have inherited
• a PRNP gene with asparagine instead of aspartic acid encoded at position 178 (D178N)
• the susceptibility polymorphism of methionine at position 129 of the PRNP genes.
Extracts from autopsied brains of FFI victims can transmit the disease to transgenic mice.
Infectious Prion Diseases
Kuru
Kuru was once found among the Fore tribe in Papua New Guinea whose rituals included eating the brain tissue of recently deceased members of the tribe. Since this practice was halted, the disease has disappeared. Before then, the disease was studied by transmitting it to chimpanzees using injections of autopsied brain tissue from human victims.
Scrapie
This disease of sheep (and goats) was the first TSE to be studied. It seems to be transmitted from animal to animal in feed contaminated with nerve tissue. It can also be transmitted by injection of brain tissue.
Bovine Spongiform Encephalopathy (BSE) or "Mad Cow Disease"
An epidemic of this disease began in Great Britain in 1985 and before it was controlled, some 800,000 cattle were sickened by it. Its origin appears to have been cattle feed that contained brain tissue from sheep infected with scrapie and had been treated in a new way that no longer destroyed the infectiousness of the scrapie prions.
The use of such food was banned in 1988 and after peaking in 1992, the epidemic declined quickly.
Creutzfeldt-Jakob Disease (CJD)
A number of humans have acquired CJD through accidental exposure to material contaminated with CJD prions.
• Grafts of dura mater taken from patients with inherited CJD have transmitted the disease to 228 recipients.
• Corneal transplants have also inadvertently transmitted CJD.
• Instruments used in brain surgery on patients with CJD have transmitted the disease to other patients. Two years after their supposed sterilization, these instruments remained infectious.
• 226 people have acquired CJD from injections of human growth hormone (HGH) or human gonadotropins prepared from pooled pituitary glands that inadvertently included glands taken from humans with CJD.
Now that both HGH and human gonadotropins are available through recombinant DNA technology, such disastrous accidents need never recur.
Variant Creutzfeldt-Jakob Disease (vCJD)
This human disorder appeared some years after the epidemic of BSE (Mad Cow Disease) swept through the cattle herds in Great Britain. Even though the cow and human PRNP genes differ at 30 codons, the sequence of their prions suggests that these patients (155 by 2005) acquired the disease from eating contaminated beef.
All the patients are homozygous for the susceptibility polymorphism of methionine at position 129. The BSE epidemic has waned, and slaughter techniques that allow cattle nervous tissue in beef for human consumption have been banned since 1989. Now we must wait to see whether more cases of vCJD are going to emerge or whether the danger is over.
Miscellaneous Infectious Prion Diseases
A number of TSEs have been found in other animals. Cats are susceptible to Feline Spongiform Encephalopathy (FSE). Mink are also susceptible to a TSE. Even though mad cow disease has not been seen in North America, a similar disease is found in elk and mule deer in the Rocky Mountains of the U.S.
Sporadic Prion Diseases
CJD and FFI occasionally occur in people who have no family history of the disease and no known exposure to infectious prions. The cause of their disease is uncertain.
• Perhaps a spontaneous somatic mutation has occurred in one of the PRNP genes in a cell.
• Perhaps their normal PrPC protein has spontaneously converted into the PrPSc form.
• Or perhaps the victims were simply unknowingly exposed to infectious prions, and sporadic prion diseases do not exist!
Whatever the answer, all the cases are found in people with a susceptibility polymorphism in their PRNP genes.
Prions in Yeast
Two proteins in yeast (Saccharomyces cerevisiae)
• the Sup35 protein ("Sup35p") and
• the Ure2 protein (Ure2p)
are able to form prions; that is, they can exist either
• in a PrPC-like form that is functional or
• in a PrPSc-like form that is not.
The greater ease with which yeast can be studied has proved that only protein is involved in prion formation and provided insight into the need for PrPSc to find PrPC molecules of a similar primary structure in order to be able to convert them into the PrPSc form.
Evidence that prions are a "protein-only" phenomenon
• A few molecules of a PrPSc form of the Sup35 protein, when introduced into yeast cells, convert the yeast cell's own Sup35 protein into prion aggregates. The resulting "disease" phenotype is then passed on to the cell's daughters.
The introduced protein was synthesized in bacteria making it unlikely that it could be contaminated by any gene-containing infectious agent of yeast.
• Yeast can be "cured" of their prion "disease" by increasing the activity of their chaperones. Presumably the chaperone helps maintain the folded state (with alpha helices) of the protein.
• When the gene for the glucocorticoid receptor is altered to include sequences coding for one part (domain) of the Sup35 protein, the resulting protein forms prions and produces an entirely new phenotype.
Possible basis of species specificity of prions
• A particular PrPSc can only convert PrPC molecules of the same or at least similar primary structure.
• This requirement of "like-with-like" resides in a short sequence at the N-terminal of the protein (rather like an antibody epitope).
• Yeasts engineered to form two types of prion form two types of "pure" aggregates within the cell.
• Even in the test tube, each type of prion finds and aggregates with others of its own type.
So the picture that emerges is that a molecule of PrPSc acts as a "seed" providing a template for converting PrPC to more PrPSc. These interact with each other to form small soluble aggregates. These interact with each other to form large insoluble deposits. Although only a small portion of the prion protein is responsible for its specificity, other parts of the molecule are needed for flipping the molecule from the alpha-helical to the beta conformation. All prion proteins contain tracts of repeated Gln-Asn residues which appear to be essential for the conversion process.
Other Pathogenic Prion-like Proteins
The deposits of PrPScin the brain are called amyloid. Amyloid deposits are also found in other diseases.
Examples:
• Alzheimer's disease is characterized by amyloid deposits of
• the peptide amyloid-beta (Aβ)
• the protein tau
in the brain.
• The brains of Parkinson's disease patients have deposits of α-synuclein.
• Deposits of the protein huntingtin are found in the brains of victims of Huntington's disease.
• Amyloid deposits of the protein transthyretin are found in peripheral nerves, the kidney, and other organs.
With all of these diseases there is evidence that their amyloid-forming proteins, like PrPSc, can act as a "seed" converting a correctly-folded protein into an incorrectly-folded one and have this effect spread from cell to cell. However, they do not seem to be able to be spread from person to person (unlike the TSEs). Perhaps this is because they are not so incredibly resistant to degradation as PrPSc is.
Most cells, including neurons in the brain, contain proteasomes that are responsible for degrading misfolded or aggregated proteins. In the various brain diseases characterized by a build-up of amyloid deposits, it appears that as the small insoluble amyloid precursors accumulate, they bind to proteasomes but cannot be degraded by them. Furthermore, this binding blocks the ability of the proteasomes to process other proteins that are normal candidates for destruction. Because of the critical role of proteasomes in many cell functions, such as mitosis, it is easy to see why this action leads to death of the cell.
Prion-like proteins not always harmful
Evidence:
• Yeast are not harmed when Sup35p and Ure2p form prions.
• The role of CPEB.
CPEB ("cytoplasmic polyadenylation element binding protein") is a protein that
• is found in neurons of the central nervous system (as well as elsewhere)
• stimulates messenger RNA (mRNA) translation
• is needed for long-term facilitation (LTF)
• accumulates at activated (by serotonin) synapses
• has the ability to undergo a change in tertiary structure that
• persists for long periods
• induces the same conformational change in other molecules of CPEB forming prion-like aggregates
Perhaps the accumulation of these aggregates at a stimulated synapse causes a long-term change in its activity (memory). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.08%3A_Nervous_System/15.8G%3A_Prion_Diseases.txt |
• 15.9A: Mechanoreceptors
We and other animals have several types of receptors of mechanical stimuli. Each initiates nerve impulses in sensory neurons when it is physically deformed by an outside force. Mechanoreceptors enable us to detect touch monitor the position of our muscles, bones, and joints — the sense of proprioception detect sounds and the motion of the body.
• 15.9B: Hearing
The sense of hearing is the ability to detect the mechanical vibrations we call sound. Sound waves pass down the auditory canal of the outer ear and strike the eardrum (tympanic membrane) causing it to vibrate. These vibrations are transmitted across the middle ear by three tiny linked bones, the ossicles.
• 15.9C: Vision
• 15.9D: Processing Visual Information
• 15.9E: Vision in Arthropods
The arthropod (e.g., insects, crustaceans) eye is built quite differently from the vertebrate eye (and mollusk eye). Arthropod eyes are called compound eyes because they are made up of repeating units, the ommatidia, each of which functions as a separate visual receptor.
• 15.9F: Heat, Cold, and Pain Receptors
• 15.9G: Taste
• 15.9H: Olfaction - The Sense of Smell
Smell depends on sensory receptors that respond to airborne chemicals. In humans, these chemoreceptors are located in the olfactory epithelium — a patch of tissue about the size of a postage stamp located high in the nasal cavity. The olfactory epithelium is made up of three kinds of cells: sensory neurons each with a primary cilium supporting cells between them basal cells that divide regularly producing a fresh crop of sensory neurons to replace those that die.
• 15.9I: Electric Organs and Electroreceptors
Electric organs are masses of flattened cells, called electrocytes, which are stacked in regular rows along the sides of certain fishes, e.g., the electric eel of South America. The posterior surface of each electrocyte is supplied with a motor neuron, the anterior surface is not.
• 15.9J: Magnetoreceptors
Evidence for an ability to alter their behavior in response to the earth's magnetic field has been found in many animals, including sea turtles, birds, fish (especially common in those that migrate), honeybees, mice as well as in some bacteria.
15.09: Senses
We and other animals have several types of receptors of mechanical stimuli. Each initiates nerve impulses in sensory neurons when it is physically deformed by an outside force such as: touch, pressure, stretching, sound waves, and motion. Mechanoreceptors enable us to detect touch, monitor the position of our muscles, bones, and joints — the sense of proprioception and detect sounds and the motion of the body.
Touch
Light touch is detected by receptors in the skin. Many of these are found next to hair follicles so even if the skin is not touched directly, movement of the hair is detected. Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as 100 per cm2; the back of the hand fewer than 10 per cm2. This can be demonstrated with the two-point threshold test. With a pair of dividers like those used in mechanical drawing, determine (in a blindfolded subject) the minimum separation of the points that produces two separate touch sensations. The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back. The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body.
Proprioception
Proprioception is our "body sense". It enables us to unconsciously monitor the position of our body. It depends on receptors in the muscles, tendons, and joints. If you have ever tried to walk after one of your legs has "gone to sleep," you will have some appreciation of how difficult coordinated muscular activity would be without proprioception.
Four Mechanoreceptors
1: The Pacinian Corpuscle
Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron. Because of its relatively large size, a single Pacinian corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and frequency is applied to the corpuscle by the stylus. The electrical activity is detected by electrodes attached to the preparation.
Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at the first node of Ranvier of the sensory neuron. Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses generated in the neuron. So the more massive or rapid the deformation of a single corpuscle, the higher the frequency of nerve impulses generated in its neuron.
2: Adaptation
When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron. However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops. This is the phenomenon of adaptation. Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing. Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated. This is why Pacinian corpuscles respond especially well to vibrations.
The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception such as spindle fibers adapt slowly if at all.
3: Meissner Corpuscles
Meissner corpuscles, like Pacinian corpuscles, adapt quickly to a sustained stimulus but are activated again when the stimulus is removed. Thus they are especially sensitive to movement across the skin.
4: Merkel Cells
Merkel cells are transducers of light touch, responding to the texture and shape of objects indenting the skin. Unlike Pacinian and Meissner corpuscles, they do not adapt rapidly to a sustained stimulus; that is, they continue to generate nerve impulses so long as the stimulus remains. They are found in the skin often close to hairs. They form synapses with Aβ sensory neurons leading back to the CNS.
In the rat, light movement of a hair triggers a generator potential in a Merkel cell. If this reaches threshold, an influx of Ca++ ions through voltage-gated calcium channels generate action potentials in the Merkel cell. These cause the release of neurotransmitters at the synapse with its Aβ sensory neuron. (This neuron may also have its own mechanically-gated ion channels able to directly generate action potentials more rapidly than Merkel cells can.)
The knee jerk Reflex
The knee jerk is a stretch reflex. Your physician taps you just below the knee with a rubber-headed hammer. You respond with an involuntary kick of the lower leg.
• The hammer strikes a tendon that inserts an extensor muscle in the front of the thigh into the lower leg.
• Tapping the tendon stretches the thigh muscle.
• This activates stretch receptors within the muscle called muscle spindles. Each muscle spindle consists of
• sensory nerve endings wrapped around
• special muscle fibers called spindle fibers (also called intrafusal fibers)
• Stretching a spindle fiber initiates a volley of impulses in the sensory neuron (called an "I-a" neuron) attached to it.
• The impulses travel along the sensory axon to the spinal cord where they form several kinds of synapses:
• Some of the branches of the I-a axons synapse directly with alpha motor neurons (Pacinian Corpuscle). These carry impulses back to the same muscle causing it to contract. The leg straightens.
• Some of the branches of the I-a axons synapse with inhibitory interneurons in the spinal cord (Meissner Corpuscles). These, in turn, synapse with motor neurons leading back to the antagonistic muscle, a flexor in the back of the thigh. By inhibiting the flexor, these interneurons aid contraction of the extensor.
• Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the cerebellum, that coordinate body movements (Merkel Cells). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9A%3A_Mechanoreceptors.txt |
The sense of hearing is the ability to detect the mechanical vibrations we call sound. Sound waves pass down the auditory canal of the outer ear and strike the eardrum (tympanic membrane) causing it to vibrate. These vibrations are transmitted across the middle ear by three tiny linked bones, the ossicles:
• hammer (malleus)
• anvil (incus)
• stirrup (stapes)
• The ossicles also magnify the amplitude of the vibrations.
The middle ear is filled with air and is connected to the outside air by the eustachian tube, which opens into the nasopharynx. Opening of the tube during swallowing or yawning equalizes the air pressure on either side of the eardrum. Allergies or a head cold may inflame the walls of the eustachian tubes making them less easily opened. Rapid changes in pressure at such times — such as descending in an aircraft or during a SCUBA dive, may be quite painful because of the unequal pressure against the eardrums.
The Inner Ear
Vibrations of the innermost ossicle, the stirrup, are transmitted through a flexible membrane, the oval window to the cochlea of the inner ear. The cochlea is a tube, about 3.5 cm long, that is coiled like a snail shell and filled with a special fluid called endolymph. The most dramatic difference in the composition of endolymph from other lymph in the body is its high concentration of potassium (K+) ions. Running through the cochlea for its entire length is a plate of bone and an inner tube that is also filled with endolymph. These structures divide the outer tube of the cochlea into two separate chambers.
Because liquids are practically incompressible, it is necessary to have some way of relieving the pressures created when the oval window is pushed in and out. The flexible round window does this by moving in the opposite direction.
The organ of Corti
The organ of Corti lies within the middle chamber of the cochlea. It contains thousands of hair cells, which are the actual vibration receptors. The apical surface of the hair cells contains an array of stereocilia, which give the hair cells their name. Stereocilia are not built from the "9+2" arrangement of microtubules that are found in true cilia. The hair cells are located between the basilar and tectorial membranes. Vibrations of the endolymph cause vibrations of the basilar membrane. This moves stereocilia at the tips of the hair cells against the tectorial membrane and open potassium channels in them. The influx of K+ from the endolymph depolarizes the cell.
You should note that hair cells differ from most "excitable cells" (neurons and muscle fibers) in their use of potassium ions, not sodium ions, to depolarize the cell. Depolarization of the hair cell causes the release of a neurotransmitter (probably glutamate) at its basal surface and the initiation of nerve impulses in a sensory neuron that synapses with it. These impulses travel back along the auditory nerve (the 8th cranial nerve) to the brain.
Many people, especially when young, can hear sounds with frequencies (pitches) from as low as 16 to as high as 20,000 hertz (cycles per second). Detection of a given frequency is a function of the location of the hair cells along the organ of Corti with the highest frequencies detected near the base of the cochlea, and the remainder of the sound spectrum detected in a progressive fashion with the lowest frequencies detected by hair cells near the tip.
Deafness
Deafness may be acquired or inherited.
Acquired deafness
If a laboratory animal is exposed to very intense, pure tones, it eventually becomes deaf to those frequencies, but its ability to hear other pitches is unimpaired. Examination of its organ of Corti reveals destroyed hair cells in a single area whose location can be easily correlated with the pitch of the destructive sound. Similar deficits occur in humans who are exposed to intense noises for long periods. A trained audiologist can tell by looking at the frequency response whether a patient flies private aircraft.
Inherited deafness
About 1 newborn in a thousand is born deaf because of a genetic defect. As the years go by, many of us (~16%) suffer a progressive loss of hearing because of genetic defects. Literally scores of genes have been identified in recent years whose mutant versions result in hearing loss.
Mutations in a transcription factor have been associated with a stirrup (stapes) that cannot move freely and thus cannot transmit vibrations to the oval window. The proper organization of the stereocilia involves actin, a form of myosin (called myosin VIIA), and cadherins.also cause deafness. Mutations in the gene encoding a protein that helps with actin polymerization cause deafness. Mutations in the myosin VIIA gene and mutations in the gene encoding cadherin 23.
The potassium that enters the hair cells must be removed from them and recycled back to the endolymph for hearing to continue. Scores of mutations in the necessary transport molecules have been linked to inherited deafness. Mutations in genes encoding the K+ channels that allows K+ to leave the hair cell through its basolateral surface (shown in green ). (These same channels are found in the loops of Henle in the kidneys so the mutations can produce defects in kidney function as well as deafness.) Mutations in the connexins (magenta) that form the gap junctions through which the K+ passes from cell to cell on its way back to the secretory cells that will deposit it back in the endolymph. Mutations in the sodium-potassium-chloride cotransporter (shown in yellow) that actively transports K+ against its concentration gradient into the secretory cells. Mutations in the K+ channels (shown in lavender) that allow for the facilitated diffusion of K+ out of the secretory cells and into the endolymph.
Equilibrium
The inner ear also detects:
• the position of the body with respect to gravity
• the motion of the body.
Just above the cochlea are two interconnecting chambers filled with endolymph, the sacculus and utriculus. On their inner surface are patches of hair cells to which are attached thousands of tiny spheres of calcium carbonate (CaCO3). Gravity pulls these downward. As the head is oriented in different directions, these ear stones or otoliths shift their position. The action potentials initiated in the hair cells are sent back to the brain.
Motion of the body is detected in the three semicircular canals at the top of each inner ear, each one oriented in a different plane. There is a small chamber at one end of each canal containing hair cells. Whenever the head is moved, the fluid within the canals lags in its motion so that there is relative motion between the walls and the endolymph. This stimulates the hair cells to send impulses back to the brain.
When the hair cells send messages that are incongruent with what the eyes are seeing and our body is feeling, as may occur in a boat or aircraft during rough weather, motion sickness can result. Some people also suffer severe dizziness because otoliths have become dislodged from their utriculus (e.g. following a blow to the head) and settled in a semicircular canal.
Echolocation
Bats can hear frequencies as high as 150,000 hertz. Sound at these ultrasonic (to us) frequencies travels in fairly straight lines. Bats flying in complete darkness are able to locate obstacles and even insect prey by emitting pulses of this ultrasonic sound and then adjusting the course of flight to the echo that returns to their ears. Such a system of echolocation works on the same principle as sonar for submarine detection.
A blindfolded bat can fly between the wires touching them only rarely. A bat whose ears are plugged collides repeatedly with the wires.
Hunter and hunted. In the top photo, a moth (bright streak) takes successful evasive action upon detecting the approach of a bat (broad streak across the photo). (The diffuse image is a tree in the background.) In the bottom photo, the two streaks intersect, indicating that this time the moth was unable to escape capture by the bat. (Photos by Frederic A. Webster, courtesy of the late Prof. Kenneth D. Roeder.) | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9B%3A_Hearing.txt |
The human eye is wrapped in three layers of tissue:
• the sclerotic coat. This tough layer creates the "white" of the eye except in the front where it forms the transparent cornea. The corneaThe surface of the cornea is kept moist and dust-free by secretions from the tear glands.
• admits light to the interior of the eye and
• bends the light rays to that they can be brought to a focus.
• the choroid coat. This middle layer is deeply pigmented with melanin. It reduces reflection of stray light within the eye. The choroid coat forms the iris in the front of the eye. This, too, is pigmented and is responsible for eye "color". The size of its opening, the pupil, is variable and under the control of the autonomic nervous system. In dim light (or when danger threatens), the pupil opens wider letting more light into the eye. In bright light the pupil closes down. This not only reduces the amount of light entering the eye but also improves its image-forming ability (as does "stopping down" the iris diaphragm of a camera).
• the retina The retina is the inner layer of the eye. It contains the light receptors, the rods and cones (and thus serves as the "film" of the eye). The retina also has many interneurons that process the signals arising in the rods and cones before passing them back to the brain. (Note: the rods and cones are not at the surface of the retina but lie underneath the layer of interneurons.)
The blind spot
All the nerve impulses generated in the retina travel back to the brain by way of the axons in the optic nerve (above). At the point on the retina where the approximately 1 million axons converge on the optic nerve, there are no rods or cones. This spot, called the blind spot, is thus insensitive to light.
You can demonstrate the presence of the blind spot. Cover your right eye with your hand and stare at the red circle as you move closer to the screen (the square will disappear). Or cover your left eye and stare at the red square as you move.
The Lens
The lens is located just behind the iris. It is held in position by zonules extending from an encircling ring of muscle. When this ciliary muscle is relaxed, its diameter increases, the zonules are put under tension, and the lens is flattened and when contracted, its diameter is reduced, the zonules relax, and the lens becomes more spherical. These changes enable the eye to adjust its focus between far objects and near objects.
Farsightedness. If the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye — particularly those from nearby objects — will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. Farsightedness is called hypermetropia.
Nearsightedness. If the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye. Nearsightedness is called myopia.
Cataracts One or both lenses often become cloudy as one ages. When a cataract seriously interferes with seeing, the cloudy lens is easily removed and a plastic one substituted. The entire process can be done in a few minutes as an outpatient under local anesthesia.
The iris and lens divide the eye into two main chambers:
• the front chamber is filled with a watery liquid, the aqueous humor
• the rear chamber is filled with a jellylike material, the vitreous body
The Retina
Four kinds of light-sensitive receptors are found in the retina:
• rods
• three kinds of cones, each "tuned" to respond best to light from a portion of the spectrum of visible light
• cones that absorb long-wavelength light (red)
• cones that absorb middle-wavelength light (green)
• cones that absorb short-wavelength light (blue)
This scanning electron micrograph (courtesy of Scott Mittman and David R. Copenhagen) shows rods and cones in the retina of the tiger salamander. Each type of receptor has its own special pigment for absorbing light. Each consists of a transmembrane protein called opsin coupled to the prosthetic group retinal. Retinal is a derivative of vitamin A (which explains why night blindness is one sign of vitamin A deficiency) and is used by all four types of receptors.
The amino acid sequence of each of the four types of opsin are similar, but the differences account for their differences in absorption spectrum. The retina also contains a complex array of interneurons:
• bipolar cells and ganglion cells that together form a path from the rods and cones to the brain
• a complex array of other interneurons that form synapses with the bipolar and ganglion cells and modify their activity.
Ganglion cells are always active. Even in the dark they generate trains of action potentials and conduct them back to the brain along the optic nerve. Vision is based on the modulation of these nerve impulses. There is not the direct relationship between visual stimulus and an action potential that is found in the senses of hearing, taste, and smell. In fact, action potentials are not even generated in the rods and cones.
Rod Vision
Rhodopsin is the light-absorbing pigment of the rods. This G-protein-coupled receptor (GPCR) is incorporated in the membranes of disks that are neatly stacked (some 1000 or more of them) in the outer portion of the rod. This arrangement is reminiscent of the organization of thylakoids, another light-absorbing device.
Fig.15.9.3.5 Rod cells of kangaroo bat
The electron micrograph (courtesy of Keith Porter) shows the rod cells of the kangaroo rat. The outer segments of the rods contain the orderly stacks of membranes which incorporate rhodopsin. The inner portion contain many mitochondria. The two parts of the rod are connected by a stalk (arrow) that has the same structure as a primary cilium. Although the disks are initially formed from the plasma membrane, they become separated from it. Thus signals generated in the disks must be transmitted by a chemical mediator (a "second messenger" called cyclic GMP (cGMP) to alter the potential of the plasma membrane of the rod. Rhodopsin consists of an opsin of 348 amino acids coupled to retinal. Like all G-protein-coupled receptors, opsin has 7 segments of alpha helix that pass back and forth through the lipid bilayer of the disk membrane.
Retinal
Retinal consists of a system of alternating single and double bonds. In the dark, the hydrogen atoms attached to the #11 and #12 carbon atoms of retinal (red arrows) point in the same direction producing a kink in the molecule. This configuration is designated cis. When light is absorbed by retinal, the molecule straightens out forming the all-trans isomer.
This physical change in retinal triggers the following chain of events culminating in a change in the pattern of impulses sent back along the optic nerve.
1. Formation of all-trans retinal activates its opsin.
2. Activated rhodopsin, in turn, activates many molecules of a G protein called transducin.
3. Transducin activates an enzyme that breaks down cyclic GMP.
4. The drop in cGMP closes Na+ channels in the plasma membrane of the rod. Because these positive ions can no longer enter, the interior of the cell becomes more negative (hyperpolarized) increasing its membrane potential from about −30 to some −70 mV.
5. This slows the release of the neurotransmitter glutamate at synapses between the rod and interneurons (e.g., bipolar cells).
6. This reduction in glutamate release activates some interneuron pathways, inhibits others.
7. The interplay of excited and inhibited interneurons modulates the spontaneous firing of the ganglion cells to which they are connected and gives rise to the ability of the retina to discriminate shapes.
So the retina is not simply a sheet of photocells, but a tiny brain center that carries out complex information processing before sending signals back along the optic nerve. In fact, the retina really is part of the brain and grows out from it during embryonic development.
Rod vision is acute but coarse
Rods do not provide a sharp image for several reasons.
• Adjacent rods are connected by gap junctions and so share their changes in membrane potential.
• Several nearby rods often share a single circuit to one ganglion cell.
• A single rod can send signals to several different ganglion cells.
So if only a single rod is stimulated, the brain has no way of determining exactly where on the retina it was. However, rods are extremely sensitive to light. A single photon (the minimum unit of light) absorbed by a small cluster of adjacent rods is sufficient to send a signal to the brain. So although rods provide us with a relatively grainy, colorless image, they permit us to detect light that is over a billion times dimmer than what we see on a bright sunny day.
Cone Vision
Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colors. Most of the 3 million cones in each retina are confined to a small region just opposite the lens called the fovea. So our sharpest and colorful images are limited to a small area of view. Because we can quickly direct our eyes to anything in view that interests us, we tend not to be aware of just how poor our peripheral vision is.
The three types of cones provide us the basis of color vision. Cones are "tuned" to different portions of the visible spectrum.
• red absorbing cones; those that absorb best at the relatively long wavelengths peaking at 565 nm
• green absorbing cones with a peak absorption at 535 nm
• blue absorbing cones with a peak absorption at 440 nm.
Retinal is the prosthetic group for each pigment. Differences in the amino acid sequence of their opsins accounts for the differences in absorption. The response of cones is not all-or-none. Light of a given wavelength (color), say 500 nm (green), stimulates all three types of cones, but the green-absorbing cones will be stimulated most strongly. Like rods, the absorption of light does not trigger action potentials but modulates the membrane potential of the cones.
Color Blindness
The term color blindness is something of a misnomer. Very few (~1 in 105) people cannot distinguish colors at all. Most "color-blind" people actually have abnormal color vision such as confusing the red and green of traffic lights. As high as 8% of the males in some populations have an inherited defect in their ability to discriminate reds and greens. These defects are found almost exclusively in males because the genes that encode the red-absorbing and green-absorbing opsins are on the X chromosome.
The X chromosome normally carries a cluster of from 2 to 9 opsin genes. The minimum basis for normal red-green vision is one gene whose opsin absorbs efficiently in the red and one that absorbs well in the green (chromosome 1 in the figure). Multiple copies of these genes are also fine (2 and 3). Males with either a "green gene" or "red gene" missing are severely color blind (4 and 5). However, if all the red genes carry mutations (this seldom seems to be the case for the green genes — nobody knows why), then they may have red-green color blindness that ranges from mild to severe depending on the particular mutations involved (6). The rule seems to be that the more the mutations shift the pigment towards green, the more serious the defect. However, a large number of mutations don't always produce serious defects. Multiple mutations in a single gene may offset each other producing only mild defects. And as long as one normal copy of each gene is present, the presence of additional mutated genes seldom produce a serious problem (7).
Why do some males have as many as 9 copies of genes encoding the red and green opsins, when two are enough? The sequences of the red and green genes are the same at 98% of their nucleotides. This high degree of similarity creates the risk of mismatches in synapsis during meiosis with unequal crossing over.
Blue vision
Abnormal blue sensitivity occasionally occurs in humans but is much rarer than abnormalities in red-green vision. The gene for the blue-cone opsin is located on chromosome 7. Thus this trait shows an autosomal pattern of inheritance being found in females as often as in males. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9C%3A_Vision.txt |
By inserting an electrode in a single ganglion cell, it was shown (by Stephen W. Kuffler) that
• Even in the dark, ganglion cells have a slow, steady rate of firing.
• Diffuse light directed on the retina has little effect on this rate.
• But a tiny spot of light falling on a small circular area of the retina can greatly increase the firing rate of some ganglion cells (left) while
• a spot directed around the perimeter of such an "on" area suppresses that ganglion cell (center).
• Light shining on both areas produces no effect (right).
• Other ganglion cells have a central "off" area surrounded by an "on" area.
Two associates of Kuffler, David H. Hubel and Torsten N. Wiesel inserted electrodes in these areas but instead of directing light into the eye, they projected images on a screen in front of the animal (an anesthetized cat or monkey). Using this procedure, they found that
• cells of the lateral geniculate nucleus (LGN) respond about the same way that ganglion cells do; that is, to circular spots of light.
• But the cells in the visual cortex receiving input from the LGN no longer respond to circles of light but only to bars of light (or dark) or to straight-line edges between dark and light areas.
• One of these "simple cortical cells" will only respond when the stimulus is directed at a particular area of the screen and at a specific angle. However, an ineffective position for one of these cortical cells is an effective position for another.
The diagram shows this mechanism by which the circular response areas of ganglion and LGN cells can be converted into the rectangular response areas found in the cells of the visual cortex. Other cells ("complex cortical cells") still want their edges oriented in one direction, but the edges can now be moved across the screen. As the figure shows, this can be explained if a set of simple cortical cells all responding to an edge of the same slope but each responsible for a different part of the visual field converge on a single "complex cortical cell". Thus these complex cortical cells continue to respond to the stimulus even though its absolute position on the retina changes.
While these studies provide only the tiniest glimpse into the workings of the brain, they provide some clues of what will be found:
• At each step of processing, the inputs of a number of interneurons are funneled into a single output.
• So, at each step, some of the information is selectively destroyed.
• A simple cortical cell, for example, fires only if a number of LGN cells converging on it are simultaneously active. Otherwise, the excitation dies out at the synapses.
• In this way, each level of the brain acts as a filtering device and, in doing so, provides a mechanism by which certain features of what might be a very complex stimulus can be discriminated..
• So instead of responding to particular impulses in particular circuits, the mammalian brain seems to respond to the spatial and temporal organization of many impulses passing along many converging circuits.
The importance of these studies was recognized by the award of a Nobel Prize in 1981 to Hubel and Wiesel (too late for Kuffler, who died in 1980). | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9D%3A_Processing_Visual_Information.txt |
The arthropod (e.g., insects, crustaceans) eye is built quite differently from the vertebrate eye (and mollusk eye). Arthropod eyes are called compound eyes because they are made up of repeating units, the ommatidia, each of which functions as a separate visual receptor. Each ommatidium consists of
• a lens (the front surface of which makes up a single facet)
• a transparent crystalline cone
• light-sensitive visual cells arranged in a radial pattern like the sections of an orange
• pigment cells which separate the ommatidium from its neighbors.
The pigment cells ensure that only light entering the ommatidium parallel (or almost so) to its long axis reaches the visual cells and triggers nerve impulses. Thus each ommatidium is pointed at just a single area in space and contributes information about only one small area in the field of view. There may be thousands of ommatidia in a compound eye with their facets spread over most of the surface of a hemisphere.
The photo, courtesy Carolina Biological Supply Company, shows the compound eye of Drosophila melanogaster. The composite of all their responses is a mosaic image — a pattern of light and dark dots rather like the halftone illustrations in a newspaper or magazine. And just as in those media, the finer the pattern of dots, the better the quality of the image.
Grasshopper eyes, with relatively few ommatidia must produce a coarse, grainy image. The honeybee and dragonfly have many more ommatidia and a corresponding improvement in their ability to discriminate ("resolve") detail. Even so, the resolving ability of the honeybee eye is poor in comparison with that of most vertebrate eyes and only 1/60 as good as that of the human eye; that is, two objects that we could distinguish between at 60 feet (18 m) could only be discriminated by the bee at a distance of one foot (0.3 m).
Flicker effect
The compound eye is excellent at detecting motion. As an object moves across the visual field, ommatidia are progressively turned on and off. Because of the resulting "flicker effect", insects respond far better to moving objects than stationary ones. Honeybees, for example, will visit wind-blown flowers more readily than still ones.
Resolution and Sensitivity
Arthropods that are apt to be active in dim light (e.g., crayfish, praying mantis) concentrate the screening pigments of their ommatidia into the lower ends of the pigment cells. This shift enables light entering a single ommatidium at an angle to pass into adjacent ommatidia and stimulate them also. With many ommatidia responding to a single area in the visual field, the image becomes coarser. The praying mantis probably can do little more than distinguish light and dark in the evening.
The shift in pigments does, however, make it more sensitive to light than it is in the daytime as more ommatidia can detect a given area of light.
Color vision
Some insects are able to distinguish colors. This requires two or more pigments, each of which absorbs best at a different wavelength. In the honeybee:
• four of the visual cells in each ommatidium respond best to yellow-green light (544 nm)
• two respond maximally to blue light (436 nm)
• the remaining two respond best to ultraviolet light (344 nm)
This system should enable the honeybee to distinguish colors (except red) and Figure \(2\) shows - behavioral studies verify this.
The above picture shows a demonstration of color vision in honeybees. After a period of feeding from a dish placed on blue cardboard, the bees return to an empty dish on a clean blue card. They are able to distinguish the blue card from others of varying shades of gray.
Ultraviolet vision
Why ultraviolet vision? Television camera tubes are also sensitive to ultraviolet, as well as visible light, but their glass lens is opaque to ultraviolet. (This is why you cannot get tanned or synthesize calciferol Vitamin D2) from the sunlight passing through window glass.) Using a special ultraviolet-transmitting lens (Figure \(3\)), many insect-pollinated flowers appear to the honeybee quite different from the way they appear to us. The sharp contrasts between flowers that appear similar to us partly explains the efficiency with which honeybees secure nectar from only one species of flower at a time even when other species are also in bloom.
Monarch butterflies, which can migrate as much as 2500 miles (> 4000 km), navigate by ultraviolet light from the sun. When their view of the sun is through a filter that blocks out only its ultraviolet rays, their flight path becomes disoriented. Ultraviolet vision is not limited to animals with compound eyes. A few marsupials, rodents, a bat that feeds on nectar, and many birds have also been shown to have ultraviolet vision. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9E%3A_Vision_in_Arthropods.txt |
This page examines the detection of heat, cold, and pain. Why pain? Because at least some of the receptors of heat and cold, when the stimulus exceeds a certain threshold, transmit signals that the brain interprets as pain.
The Receptors
Few, if any, of the receptors of heat, cold, and pain are specialized transducers (in the way that, for example, the Pacinian corpuscle is). Rather they are sensory neurons whose plasma membrane contains transmembrane proteins that are ion channels that open in response to particular stimuli. A single neuron may contain several types of these ion channels and thus be able to respond to several types of stimuli. Like all sensory spinal neurons, their axons travel to a dorsal root ganglion of the spinal cord, where their cell bodies reside, and then on in to the gray matter of the spinal cord.
Three types of sensory neurons are found in the skin.
• Aδ ("A-delta") fibers
• These are thinly-myelinated.
• They transmit signals in response to heat and touch. If the stimulus exceeds a certain threshold, the brain interprets these as acute pain. This is "good pain" because it warns you to do something to take care of the problems, e.g., a hot saucepan.
• C fibers
• These are unmyelinated and thus conduct impulses slowly.
• C fibers also respond to heat and touch. If the stimulus exceeds a certain threshold, the brain interprets these as diffuse, dull, chronic pain. This is "bad pain" because it cannot be alleviated simply by removing the stimulus. It is pain generated by such things as damaged tissue or pain that remains after the stimulus that caused acute pain has been removed.
• ("A-beta") fibers
• These are thickly-myelinated fibers.
• They mostly respond to painless stimuli such as light touch.
Heat
There are several types of ion channels in the skin that respond to temperature. They are all transmembrane proteins in the plasma membrane that open to let in both calcium ions and sodium ions (the latter the source of the action potential). Between them, they cover a range of temperatures.
• TRPV4
Warm (~27–34°C)
• TRPV3
Warmer (~34–39°C)
• TRPV1
Hot (≥43°C). Also activated by capsaicin, the active ingredient of hot chili peppers, by camphor, by acids (protons), and by pain-inducing products of inflammation.
• TRPV2
Painfully hot (>52°C)
Knockout mice lacking the TRPV1 receptor not only do not avoid water with capsaicin in it but have a diminished response to heat and to substances that normal elicit itching. Birds also have TRPV1 receptors. Theirs also respond to heat (and acids), but do not respond to capsaicin. This must explain why birds happily eat hot chili peppers (and so disperse their seeds). The vampire bat, Desmodus rotundus, expresses normal TRPV1 receptors in the sensory neurons leading to the dorsal root ganglia, and these respond normally to painful heat (> 43°C). However, these bats express a shortened version of TRPV1 (produced by alternative splicing) in their trigeminal nerves that run from the bat's upper lip and nose. The shortened receptors respond to a lower temperature (~30°C) enabling the bats to detect the warmth radiating from the skin of their victims.
Cold
Two candidate receptors:
• One, designated TRPM8, is a channel that admits Ca2+ and Na+ in response to moderate cold (<28°C) or menthol (the ingredient that gives mint its "cool" touch and taste). Knockout mice lacking the gene encoding the TRPM8 receptor do not avoid cold places as normal mice do.
• A second, designated TRPA1, responds to lower temperatures (<18°C). It also responds to several irritant chemicals eliciting signals that the brain interprets at pain. TRPA1 is found in the hair cells of the inner ear that respond to sound and changes in position.) However, TRPA1 knockout mice respond normally to cold and seem to have normal hearing so the precise role of these receptors is still uncertain for those stimuli.
TRPA1 channels serve a different function in pit vipers like rattlesnakes. These cold-blooded animals detect warm-blooded prey using temperature-sensitive neurons at the base of pits in their head. The neurons contain TRPA1 channels that open wide when radiant heat entering the pit raises their temperature above 27°C.
Pain
When sensory nerve fibers are exposed to extremes, they signal pain. Pain receptors are also called nociceptors.
Processing Pathways
All the neurons in the skin are part of the sensory-somatic branch of the peripheral nervous system. Their axons pass into the dorsal root ganglion, where their cell body is located, and then on in to the gray matter of the spinal cord where they synapse with interneurons.
Several different neurotransmitters have been implicated in pain pathways. Three of them:
• glutamate. This seems to be the dominant neurotransmitter when the threshold to pain is first crossed. It is associated with acute ("good") pain.
• substance P. This peptide (containing 11 amino acids) is released by C fibers. It is associated with intense, persistent, chronic - thus "bad" pain.
• glycine. It suppresses the transmission of pain signals in the dorsal root ganglion. Prostaglandins potentiate the pain of inflammation by blocking its action.
Neuropathic Pain
This is pain caused by injury to the nerves themselves such as by mechanical damage, massive inflammation, and growing tumors.
Visceral Pain
The brain can also register pain from stimuli originating in sensory neurons of the autonomic nervous system. This so-called visceral pain is not felt in a discrete location as pain signals transmitted by the sensory-somatic system are.
Treating pain with drugs
The weapons presently available to reduce pain are many in number but few in types. They are
• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Opioids (also called opiates)
NSAIDs
Inflammation is caused by tissue damage and, among other things, causes pain. Damaged tissue releases prostaglandins and these are potent triggers of pain. Prostaglandins are 20-carbon organic acids synthesized from unsaturated fatty acids.
There are at least three key enzymes that synthesize prostaglandins:
• Cyclooxygenase 1 (Cox-1)
• Cyclooxygenase 2 (Cox-2)
• Cyclooxygenase 3 (Cox-3)
Most NSAIDs block the action of all three cyclooxygenases. They include:
• aspirin
• ibuprofen (Advil®, Motrin®)
• naproxen (Aleve®)
• and many others
Two NSAIDs celecoxib (Celebrex®) and rofecoxib (Vioxx®) were introduced in 1999 that selectively inhibit Cox-2 while leaving Cox-1 untouched. It was hoped that these would provide pain relief without the gastrointestinal side effects associated with the broad spectrum NSAIDs. However, the manufacturer of Vioxx® removed it from the market on 30 September 2004 because it increases the risk of heart attacks and strokes.
Acetaminophen (Tylenol®)
This is also a nonsteroidal anti-inflammatory drug but its mode of action is different from the others. It selectively inhibits Cox-3 and provides pain relief without irritating the stomach. It is particularly useful for
• people allergic to aspirin and its relatives
• avoiding the risk of Reye's syndrome that has been associated with giving aspirin to children with viral infections.
Opioids
Opioids are extremely effective pain killers but are also addictive so their use is surrounded by controversy and regulation.
Some examples:
• morphine
• codeine
• heroin
• methadone
• oxycodone
Opioids bind to receptors on interneurons in the pain pathways in the central nervous system. The natural ligands for these receptors are two enkephalins — each a pentapeptide (5 amino acids):
• Met-enkephalin (Tyr-Gly-Gly-Phe-Met)
• Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu)
The drawing shows how this mechanism might work. The activation of enkephalin synapses suppresses the release of the neurotransmitter (substance P) used by the sensory neurons involved in the perception of chronic and/or intense pain. The ability to perceive pain is vital. However, faced with massive, chronic, intractable pain, it makes sense to have a system that decreases its own sensitivity. Enkephalin synapses provide this intrinsic pain-suppressing system.
Morphine and the other opioids bind these same receptors. This makes them excellent pain killers.
However, they are also highly addictive.
• By binding to enkephalin receptors, they enhance the pain-killing effects of the enkephalins.
• A homeostatic reduction in the sensitivity of these synapses compensates for continued exposure to opioids.
• This produces tolerance, the need for higher doses to achieve the prior effect.
• If use of the drug ceases, the now relatively insensitive synapses respond less well to the soothing effects of the enkephalins, and the painful symptoms of withdrawal are produced. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9F%3A_Heat_Cold_and_Pain_Receptors.txt |
There are five primary taste sensations:
• salty
• sour
• sweet
• bitter
• umami
Properties of the taste system
• A single taste bud contains 50–100 taste cells representing all 5 taste sensations (so the classic textbook pictures showing separate taste areas on the tongue are wrong).
• Each taste cell has receptors on its apical surface. These are transmembrane proteins which
• admit the ions that give rise to the sensation of salty
• bind to the molecules that give rise to the sensations of sweet, bitter, and umami
• A single taste cell seems to be restricted to expressing only a single type of receptor (except for bitter receptors).
• A stimulated taste receptor cell triggers action potentials in a nearby sensory neuron leading back to the brain.
• However, a single sensory neuron can be connected to several taste cells in each of several different taste buds.
• The sensation of taste like all sensations resides in the brain.
• And in mice, at least, the sensory neurons for four of the tastes (not sour) transmit their information to four discrete areas of the brain.
Salty
In mice, perhaps humans, the receptor for table salt (NaCl) is an ion channel that allows sodium ions (Na+) to enter directly into the cell depolarizing it and triggering action potentials in a nearby sensory neuron.
In lab animals, and perhaps in humans, the hormone aldosterone increases the number of these salt receptors. This makes good biological sense:
• The main function of aldosterone is to maintain normal sodium levels in the body.
• An increased sensitivity to sodium in its food would help an animal suffering from sodium deficiency (often a problem for ungulates, like cattle and deer).
Sour
Sour receptors detect the protons (H+) liberated by sour substances (acids). This closes transmembrane K+ channels which leads to depolarization of the cell, and the release of the neurotransmitter serotonin into its synapse with a sensory neuron.
Sweet
Sweet substances (like table sugar sucrose) bind to G-protein-coupled receptors (GPCRs) at the cell surface.
• Each receptor contains 2 subunits designated T1R2 and T1R3 and is coupled to G proteins.
• The complex of G proteins has been named gustducin because of its similarity in structure and action to the transducin that plays such an essential role in rod vision.
• Activation of gustducin triggers a cascade of intracellular reactions:
• production of the second messengers inositol trisphosphate (IP3) and diacylglycerol (DAG)
• this releases intracellular stores of Ca++
• this allows in the influx of Na+ ions depolarizing the cell and causing the release of ATP
• this triggers action potentials in a nearby sensory neuron.
The hormone leptin inhibits sweet cells by opening their K+ channels. This hyperpolarizes the cell making the generation of action potentials more difficult. Could leptin, which is secreted by fat cells, be a signal to cut down on sweets?
Bitter
The binding of substances with a bitter taste, e.g., quinine, phenylthiocarbamide [PTC], also takes place on G-protein-coupled receptors that are coupled to gustducin and the signaling cascade is the same as for sweet (and umami).
Humans have genes encoding 25 different bitter receptors ("T2Rs"), and each taste cell responsive to bitter expresses a number (4–11) of these genes. (This is in sharp contrast to the system in olfaction where a single odor-detecting cell expresses only a single type of odor receptor.)
Despite this — and still unexplained — a single taste cell seems to respond to certain bitter-tasting molecules in preference to others.
The sensation of taste — like all sensations — resides in the brain. Transgenic mice that
• express T2Rs in cells that normally express T1Rs (sweet) respond to bitter substances as though they were sweet
• express a receptor for a tasteless substance in cells that normally express T2Rs (bitter) are repelled by the tasteless compound
So it is the activation of hard-wired neurons that determines the sensation of taste, not the molecules nor the receptors themselves.
Umami
Umami is the response to salts of glutamic acid — like monosodium glutamate (MSG) a flavor enhancer used in many processed foods and in many Asian dishes. Processed meats and cheeses (proteins) also contain glutamate. The binding of amino acids, including glutamic acid, takes place on G-protein-coupled receptors that are coupled to heterodimers of the protein subunits T1R1 and T1R3. The signaling cascade that follows is the same as it is for sweet and bitter.
Taste Receptors in Other Locations
Taste receptors have been found in several other places in the body. Examples:
• Bitter receptors (T2Rs) are found on the cilia and smooth muscle cells of the trachea and bronchi where they probably serve to expel inhaled irritants;
• Sweet receptors (T1Rs) are found in cells of the duodenum. When sugars reach the duodenum, the cells respond by releasing incretins. These cause the beta cells of the pancreas to increase the release of insulin.
So the function of "taste" receptors appears to be the detection of chemicals in the environment - a broader function than simply taste. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9G%3A_Taste.txt |
Smell depends on sensory receptors that respond to airborne chemicals. In humans, these chemoreceptors are located in the olfactory epithelium — a patch of tissue about the size of a postage stamp located high in the nasal cavity. The olfactory epithelium is made up of three kinds of cells: sensory neurons each with a primary cilium supporting cells between them basal cells that divide regularly producing a fresh crop of sensory neurons to replace those that die.
Smell depends on sensory receptors that respond to airborne chemicals. In humans, these chemoreceptors are located in the olfactory epithelium — a patch of tissue about the size of a postage stamp located high in the nasal cavity. The olfactory epithelium is made up of three kinds of cells:
• sensory neurons each with a primary cilium
• supporting cells between them
• basal cells that divide regularly producing a fresh crop of sensory neurons to replace those that die (and providing an exception to the usual rule that neurons seldom are replaced)
The sequence of events
• The cilia of the sensory neurons are immersed in a layer of mucus. Odorant molecules (molecules that we can smell) dissolve in the mucus.
• These then bind to receptors on the cilia. These are "7-pass" transmembrane proteins.
• Binding of the odorant activates a G protein coupled to the receptor on its cytoplasmic side.
• This activates adenylyl cyclase, an enzyme embedded in the plasma membrane of the cilia.
• Adenylyl cyclase catalyzes the conversion of ATP to the "second messenger" cyclic AMP (cAMP) in the cytosol.
). Some other examples of GPCRs:
• receptors of peptide hormones
• taste receptors
• the light receptor rhodopsin
• GABAB receptors at certain synapses in the brain
• cAMP opens up ligand-gated sodium channels for the facilitated diffusion of Na+ into the cell
• The influx of Na+ reduces the potential across the plasma membrane.
• If this depolarization reaches threshold, it generates an action potential.
• The action potential is conducted back along the olfactory nerve to the brain.
• The brain evaluates this and other olfactory signals reaching it as a particular odor.
How can one kind of cell enable us to discriminate among so many different odors?
Humans can discriminate between hundreds, perhaps thousands, of different odorant molecules, each with its own structure. How can one kind of cell provide for this?
• The mechanism:
Although a single olfactory neuron contains over a thousand receptor genes, there is only a single enhancer capable of binding to the promoters of these genes and turning them on. There are, of course, two alleles of the enhancer but only one is active (one is methylated; the other is not). Presumably, when the active enhancer encounters the promoter of an olfactory gene, it turns it on and ceases its search. Thus only one olfactory receptor gene gets to be expressed in a single cell, but which one is a matter of chance.
Now we have a mechanism for discriminating among a thousand or so odorants. However,
• Each receptor is probably capable of binding to several different odorants — some more tightly than others. (The cells described above also responded — although more weakly — to 3 related odorants.)
• Each odorant is capable of binding to several different receptors.
This provides the basis for combinatorial diversity. It would work like this:
Assume that
• Odorant A binds to receptors on neurons #3, #427, and #886.
• Odorant B binds to receptors on neurons #2, #427, and #743.
The brain then would interpret the two different patterns of impulses as separate odors.
This mechanism appears capable of discriminating between as many as a trillion different mixtures of odorants. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9H%3A_Olfaction_-_The_Sense_of_Smell.txt |
At rest, the interior of each electrocyte, like a nerve or muscle cell, is negatively charged with respect to the two exterior surfaces. The potential is about 0.08 volt, but because the charges alternate, no current flows. When a nerve impulse reaches the posterior surface, the inflow of sodium ions momentarily reverses the charge just as it does in the action potential of nerves and muscles. (In most fishes, electrocytes are, in fact, modified muscle cells.) Although the posterior surface is now negative, the anterior surface remains positive. The charges now reinforce each other and a current flows just as it does through an electric battery with the cells wired in "series".
With its several thousand electrocytes, the South American electric eel (Electrophorus electricus) produces voltages as high as 600 volts. The flow (amperage) of the current is sufficient (0.25–0.5 ampere) to stun, if not kill, a human. The pulse of current can be repeated several hundred times each second.
Powerful electric organs like those of the electric eel are used as weapons to stun prey as well as potential predators.
The Mechanism
In the 5 December 2014 issue of Science, Kenneth Catania describes his experiments that revealed how the electric eel captures its prey.
While exploring its environment, the eel emits a continuous series of low-voltage discharges. Periodically it interrupts these with a discharge of 2 or 3 high-voltage pulses. These cause nearby prey, e.g. a fish, to twitch. Within a tiny fraction of a second (20–40 ms) of detecting the twitch, the eel unleashes a volley (~400 per second) of high-voltage discharges that stun the prey enabling the eel to capture it.
Remarkably, both the twitch response and the immobilization are triggered by the prey's own motor neurons. A pair of pulses induces a brief contraction while a volley of discharges induces tetanus.
Although action potentials in the prey's motor neurons were not measured directly, two pieces of evidence support this mechanism.
1. The responses remained intact even when the brain and spinal cord of the prey were destroyed thus eliminating the possibility that the prey was relying on a sensory→cns→motor reflex.
2. Curare, which blocks the transmission of action potentials across the neuromuscular junction did block the prey's responses.
So hunting by the electric eel involves a preliminary 2 or 3 powerful pulses to - in Catania's words - answer the question "Are you living prey?". If the answer is "yes", the prey is quickly stunned and ready to eat.
Weak Electric Organs
The electric organs of many fishes are too weak to be weapons. Instead they are used as signaling devices.
Many fishes, besides the electric eel, emit a continuous train of electric signals in order to detect objects in the water around them. The system operates something like an underwater radar and requires that the fishes also have electroreceptors (which are located in the skin). The presence of objects in the water distorts the electric fields created by the fish, and this alteration is detected by the electroreceptors.
Electric fishes use their system of transmitter and receiver for such functions as
• navigating in murky water and/or at night
• locating potential mates
• defense of their territory against rivals of the same species
• attracting other members of their species into schools | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9I%3A_Electric_Organs_and_Electroreceptors.txt |
Evidence for an ability to alter their behavior in response to the earth's magnetic field has been found in many animals, including sea turtles, birds, fish (especially common in those that migrate), honeybees, mice as well as in some bacteria.
Some examples:
• Homing pigeons become disoriented when magnets are placed at the sides of their head. However, this disorientation occurs only on cloudy days, suggesting that their ability to navigate by magnetic cues is a backup system.
• Woodmice, taken from their home territory to a new location 40 meters away, normally orient toward home (left side of figure). But if, while they are being moved, they are exposed to a magnetic field that is just the reverse of the earth's magnetic field — and they are not allowed to see the surrounding terrain — they orient away from their home (right side of figure).
Left: orientation taken by individual woodmice after being removed from their home in a closed box. Right: same experiment except that the mice were subjected to a reversed magnetic field as they were moved from their home. Each dot represents the orientation taken by one mouse. The arrow within each circle indicates the average for all the mice. Mice transported in an open box so they can see landmarks orient correctly whether or not they are exposed to an abnormal magnetic field. (Based on the work of Mather and Baker, Nature, 291:152, 1981.)
Thrush nightingales (Luscinia luscinia) migrate in the fall from northern Europe to equatorial Africa. They interrupt their migration with a stopover in northern Egypt where they feed and gain weight. This stopover presumably provides them with the energy stores they need to fly without feeding across the Sahara Desert.
Fransson and colleagues report in the 1 November 2001 issue of Nature that when they confined naive birds (born in Sweden that spring) in Sweden but exposed them to a magnetic field characteristic of northern Egypt, the birds proceeded to put on weight as though they had arrived in Egypt (and three times more than control birds kept in the normal magnetic field of Sweden).
Receptors that detect magnetic fields
The location and mechanism of action of the receptors in these animals is still a puzzle. Microscopic grains of magnetite (FeO.Fe2O3), a magnetic material, have been found in honeybees and pigeons, but whether and how these might function as receptors is not known.
Certain bacteria orient themselves in magnetic fields as weak as those of the earth and this is mediated by grains of magnetite within the cell. There is also evidence that birds and amphibians can supplement their magnetic sense using the interaction of light and magnetic fields on cryptochrome molecules in their retina. The ability of Drosophila to respond to magnetic fields depends on blue light and cryptochrome.
• In the absence of blue light, the flies do not respond to a magnetic field.
• Mutant flies that lack cryptochrome are likewise insensitive to magnetic fields.
• However, mutant flies whose own cryptochrome genes have been replaced by the human gene respond normally to a magnetic field while exposed to blue light.
How humans detect magnetic fields
The jury is still out. There is some evidence that humans can detect the orientation of magnetic fields. Both cryptochrome and magnetite are found in humans, but their presence may have nothing to do with magnetoreception. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.09%3A_Senses/15.9J%3A_Magnetoreceptors.txt |
The vertebrate body is supported by an endoskeleton made of cartilage and bone. (Sharks and their relatives use only cartilage.) The bones of the human skeleton perform several functions: support, protection of internal organs from mechanical damage (e.g., skull, ribs), reservoir of calcium and phosphate, and source of all the blood cells
The vertebrate body is supported by an endoskeleton made of cartilage and bone. (Sharks and their relatives use only cartilage.)
The bones of the human skeleton perform several functions:
• support
• protection of internal organs from mechanical damage (e.g., skull, ribs)
• reservoir of calcium and phosphate
• source of all the blood cells
Structure
• Periosteum. A tissue covering the bone that brings blood and lymph vessels, as well as nerves, to it.
• Compact bone (also known as cortical bone). Dense deposits of minerals - mainly calcium phosphate and Type I collagen. These are arranged in concentric circles around a central Haversian canal through which blood and lymph vessels as well as nerves pass.
• Spongy bone (also known as trabecular or cancellous bone). The mineral deposits are arranged as a system of struts. Bone marrow fill the spaces between.
• Bone marrow. Some bones, such as the femur, also contain a central cavity filled with bone marrow. Bone marrow contains the stem cells that gives rise to all the types of blood cells.
• Epiphyseal plate. Until the end of puberty, this disk of cartilage produces more cartilage which then is converted into more bone. In this way, the bone grows lengthwise.
Physiology
Looking at a skeleton, bone seems an inert thing. But in the living body it is anything but. The size and shape of bones not only changes during growth, but for the rest one's life it is continuously being remodeled in response to the stresses put on it. Approximately 10% of your bone mass is removed and replaced each year.
The remodeling of bone requires the coordinated activity of two types of cells:
• Osteoclasts are derived from stem cells in the bone marrow - the same ones produce monocytes and macrophages.
Excess activity of osteoclasts (common after menopause in women) produces osteoporosis. The bones become weakened as cortical bone gets thinner and the spaces in spongy bone get larger.
Reduced activity of osteoclasts produces osteopetrosis. This occurs when a person inherits a mutant version(s) of one or another of the genes needed for normal osteoclast function. Inhibition of osteoclast function causes too much bone to form leading to
• extra-dense bone which actually is more brittle than normal bone thus leading to fractures
• filling-in of the bone marrow cavity with bone thus interfering with the normal production of blood cells
Hormones and Bone
Many hormones affect bone growth and remodeling.
• Growth hormone (GH). As its name suggests, GH drives the growth of bones until the adult size is reached.
• Parathyroid hormone (PTH). It promotes the number and activity of osteoblasts.
• Estrogens. Until the end of puberty, estrogens are needed for maturation of the skeleton (in males as well as females). In women, after the menopause, taking supplemental estrogen slows up the bone loss that so often leads to osteoporosis. The estrogen induces FasL in osteoclasts causing them to self-destruct by apoptosis and in this way slows up the destruction of bone.
• Calcitonin and thyroid stimulating hormone (TSH), both of which inhibit the activity of osteoclasts.
• Calcitriol (1,25[OH]2 vitamin D3. Needed for the deposition of calcium into bone.
• Osteoprotegerin is a protein secreted by osteoblasts and their precursors (thus a cytokine) that also inhibits the production and activity of osteoclasts. Clinical trials of a recombinant version (made in E. coli) are underway as a possible treatment for various bone-weakening disorders like osteoporosis and multiple myeloma.
• Leptin, which regulates the balance between osteoblast and osteoclast activity.
• Serotonin. Secreted by the duodenum, serotonin suppresses osteoblasts (at least in mice). This may account for the bone-weakening effect in humans of prolonged use of selective serotonin reuptake inhibitors (SSRIs).
Bone also produces hormones thus is itself an endocrine organ.
• Osteoblasts and their progeny produce a hormone — fibroblast growth factor 23 (FGF-23) — which reduces the reabsorption of phosphate by the kidneys thus allowing more phosphate to pass out in the urine and lowering the phosphate level in the blood. FGF-23 is also called phosphatonin.
• Osteoblasts also secrete osteocalcin, a hormone that stimulates the beta cells of the pancreas to release insulin. This increases the uptake of glucose by skeletal muscles thus enhancing exercise capacity. Calcitonin also stimulates the Leydig cells of the testes to release testosterone. Osteocalcin crosses the blood-brain barrier and affects neurotransmitter levels, memory, and behavior in mice and may do so in humans. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.10%3A_Muscles/15.10A%3A_Bones.txt |
Animals use muscles to convert the chemical energy of ATP into mechanical work. Three different kinds of muscles are found in vertebrate animals.
• Heart muscle also called cardiac muscle makes up the wall of the heart. Throughout our life, it contracts some 70 times per minute pumping about 5 liters of blood each minute.
• Smooth muscle is found in the walls of all the hollow organs of the body (except the heart). Its contraction reduces the size of these structures. Thus it
• regulates the flow of blood in the arteries
• moves your breakfast along through your gastrointestinal tract
• expels urine from your urinary bladder
• sends babies out into the world from the uterus
• regulates the flow of air through the lungs
The contraction of smooth muscle is generally not under voluntary control.
• Skeletal muscle, as its name implies, is the muscle attached to the skeleton. It is also called striated muscle. The contraction of skeletal muscle is under voluntary control.
Anatomy of Skeletal Muscle
A single skeletal muscle, such as the triceps muscle, is attached at its
• origin to a large area of bone; in this case, the humerus.
• at its other end, the insertion, it tapers into a glistening white tendon which, in this case, is attached to the ulna, one of the bones of the lower arm.
As the triceps contracts, the insertion is pulled toward the origin and the arm is straightened or extended at the elbow. Thus the triceps is an extensor. Because skeletal muscle exerts force only when it contracts, a second muscle - a flexor - is needed to flex or bend the joint. The biceps muscle is the flexor of the lower arm. Together, the biceps and triceps make up an antagonistic pair of muscles. Similar pairs, working antagonistically across other joints, provide for almost all the movement of the skeleton.
The Muscle Fiber
Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. The fibers are bound together by connective tissue through which run blood vessels and nerves.Each muscle fibers contains:
• an array of myofibrils that are stacked lengthwise and run the entire length of the fiber;
• mitochondria;
• an extensive smooth endoplasmic reticulum (SER);
• many nuclei (thus each skeletal muscle fiber is a syncytium).
The multiple nuclei arise from the fact that each muscle fiber develops from the fusion of many cells (called myoblasts). The number of fibers is probably fixed early in life. This is regulated by myostatin, a cytokine that is synthesized in muscle cells (and circulates as a hormone later in life). Myostatin suppresses skeletal muscle development. (Cytokines secreted by a cell type that inhibit proliferation of that same type of cell are called chalones.) Cattle and mice with inactivating mutations in their myostatin genes develop much larger muscles. Some athletes and other remarkably strong people have been found to carry one mutant myostatin gene. These discoveries have already led to the growth of an illicit market in drugs supposedly able to suppress myostatin.
In adults, increased muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue. In the mouse, at least, fibers increase in size by attracting more myoblasts to fuse with them. The fibers attract more myoblasts by releasing the cytokine interleukin 4 (IL-4). Anything that lowers the level of myostatin also leads to an increase in fiber size.
Because a muscle fiber is not a single cell, its parts are often given special names such as
• sarcolemma for plasma membrane
• sarcoplasmic reticulum for endoplasmic reticulum
• sarcosomes for mitochondria
• sarcoplasm for cytoplasm
Although this tends to obscure the essential similarity in structure and function of these structures and those found in other cells.
The nuclei and mitochondria are located just beneath the plasma membrane. The endoplasmic reticulum extends between the myofibrils. Seen from the side under the microscope, skeletal muscle fibers show a pattern of cross banding, which gives rise to the other name: striated muscle. The striated appearance of the muscle fiber is created by a pattern of alternating dark A bands and light I bands.
• The A bands are bisected by the H zone running through the center of which is the M line.
• The I bands are bisected by the Z disk.
Each myofibril is made up of arrays of parallel filaments.
• The thick filaments have a diameter of about 15 nm. They are composed of the protein myosin.
• The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins - troponin and tropomyosin.
The anatomy of a sarcomere
The entire array of thick and thin filaments between the Z disks is called a sarcomere.
• The thick filaments produce the dark A band.
• The thin filaments extend in each direction from the Z disk. Where they do not overlap the thick filaments, they create the light I band.
• The H zone is that portion of the A band where the thick and thin filaments do not overlap.
• The M line runs through the exact center of the sarcomere. Molecules of the giant protein, titin, extend from the M line to the Z disk. One of its functions is to provide elasticity to the muscle. It also provides a scaffold for the assembly of a precise number of myosin molecules in the thick filament (294 in one case). It may also dictate the number of actin molecules in the thin filaments.
Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the muscle fiber of which it is a part. [This electron micrograph of a single sarcomere was kindly provided by Dr. H. E. Huxley.]
Activation of Skeletal Muscle
The contraction of skeletal muscle is controlled by the nervous system. The Dying Lioness (an Assyrian relief dating from about 650 B.C.) shows this vividly. Injury to the spinal cord has paralyzed the otherwise undamaged hind legs. In this respect, skeletal muscle differs from smooth and cardiac muscle. Both cardiac and smooth muscle can contract without being stimulated by the nervous system. Nerves of the autonomic branch of the nervous system lead to both smooth and cardiac muscle, but their effect is one of moderating the rate and/or strength of contraction.
The Neuromuscular Junction
Nerve impulses (action potentials) traveling down the motor neurons of the sensory-somatic branch of the nervous system cause the skeletal muscle fibers at which they terminate to contract. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. (The neuromuscular junction is also called the myoneural junction.)
The terminals of motor axons contain thousands of vesicles filled with acetylcholine (ACh). Many of these can be seen in the electron micrograph on the left (courtesy of Prof. B. Katz).
When an action potential reaches the axon terminal, hundreds of these vesicles discharge their ACh onto a specialized area of postsynaptic membrane on the muscle fiber (the folded membrane running diagonally upward from the lower left). This area contains a cluster of transmembrane channels that are opened by ACh and let sodium ions (Na+) diffuse in.
The interior of a resting muscle fiber has a resting potential of about −95 mV. The influx of sodium ions reduces the charge, creating an end plate potential. If the end plate potential reaches the threshold voltage (approximately −50 mV), sodium ions flow in with a rush and an action potential is created in the fiber. The action potential sweeps down the length of the fiber just as it does in an axon. No visible change occurs in the muscle fiber during (and immediately following) the action potential. This period, called the latent period, lasts from 3–10 msec.
Before the latent period is over,
• the enzyme acetylcholinesterase
• breaks down the ACh in the neuromuscular junction (at a speed of 25,000 molecules per second)
• the sodium channels close, and
• the field is cleared for the arrival of another nerve impulse.
• the resting potential of the fiber is restored by an outflow of potassium ions.
The brief (1–2 msec) period needed to restore the resting potential is called the refractory period.
Tetanus
The process of contracting takes some 50 msec; relaxation of the fiber takes another 50–100 msec. Because the refractory period is so much shorter than the time needed for contraction and relaxation, the fiber can be maintained in the contracted state so long as it is stimulated frequently enough (e.g., 50 stimuli per second). Such sustained contraction is called tetanus.
In the above figure:
• When shocks are given at 1/sec, the muscle responds with a single twitch.
• At 5/sec and 10/sec, the individual twitches begin to fuse together, a phenomenon called clonus.
• At 50 shocks per second, the muscle goes into the smooth, sustained contraction of tetanus.
Clonus and tetanus are possible because the refractory period is much briefer than the time needed to complete a cycle of contraction and relaxation. Note that the amount of contraction is greater in clonus and tetanus than in a single twitch.
As we normally use our muscles, the individual fibers go into tetanus for brief periods rather than simply undergoing single twitches.
The Sliding-Filament Model
Each molecule of myosin in the thick filaments contains a globular subunit called the myosin head. The myosin heads have binding sites for the actin molecules in the thin filaments and ATP. Activation of the muscle fiber causes the myosin heads to bind to actin. An allosteric change occurs which draws the thin filament a short distance (~10 nm) past the thick filament. Then the linkages break (for which ATP is needed) and reform farther along the thin filament to repeat the process. As a result, the filaments are pulled past each other in a ratchetlike action. There is no shortening, thickening, or folding of the individual filaments.
Electron microscopy supports this model.
As a muscle contracts,
• the Z disks come closer together
• the width of the I bands decreases
• the width of the H zones decreases
• there is no change in the width of the A band
Conversely, as a muscle is stretched,
• the width of the I bands and H zones increases
• but there is still no change in the width of the A band
Coupling Excitation to Contraction
Calcium ions (Ca2+) link action potentials in a muscle fiber to contraction.
• In resting muscle fibers, Ca2+ is stored in the endoplasmic (sarcoplasmic) reticulum.
• Spaced along the plasma membrane (sarcolemma) of the muscle fiber are inpocketings of the membrane that form "T-tubules". These tubules plunge repeatedly into the interior of the fiber.
• The T-tubules terminate near the calcium-filled sacs of the sarcoplasmic reticulum.
• Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T-tubules.
• The arrival of the action potential at the ends of the T-tubules triggers the release of Ca2+.
• The Ca2+ diffuses among the thick and thin filaments where it
• binds to troponin on the thin filaments.
• This turns on the interaction between actin and myosin and the sarcomere contracts.
• Because of the speed of the action potential (milliseconds), the action potential arrives virtually simultaneously at the ends of all the T-tubules, ensuring that all sarcomeres contract in unison.
• When the process is over, the calcium is pumped back into the sarcoplasmic reticulum using a Ca2+ ATPase.
Isotonic versus Isometric Contractions
If a stimulated muscle is held so that it cannot shorten, it simply exerts tension. This is called an isometric ("same length") contraction. If the muscle is allowed to shorten, the contraction is called isotonic ("same tension").
Motor Units
All motor neurons leading to skeletal muscles have branching axons, each of which terminates in a neuromuscular junction with a single muscle fiber. Nerve impulses passing down a single motor neuron will thus trigger contraction in all the muscle fibers at which the branches of that neuron terminate. This minimum unit of contraction is called the motor unit.
The size of the motor unit is small in muscles over which we have precise control. Examples:
• a single motor neuron triggers fewer than 10 fibers in the muscles controlling eye movements
• the motor units of the muscles controlling the larynx are as small as 2–3 fibers per motor neuron
• In contrast, a single motor unit for a muscle like the gastrocnemius (calf) muscle may include 1000–2000 fibers (scattered uniformly through the muscle).
Although the response of a motor unit is all-or-none, the strength of the response of the entire muscle is determined by the number of motor units activated. Even at rest, most of our skeletal muscles are in a state of partial contraction called tonus. Tonus is maintained by the activation of a few motor units at all times even in resting muscle. As one set of motor units relaxes, another set takes over.
Fueling Muscle Contraction
ATP is the immediate source of energy for muscle contraction. Although a muscle fiber contains only enough ATP to power a few twitches, its ATP "pool" is replenished as needed. There are three sources of high-energy phosphate to keep the ATP pool filled.
• creatine phosphate
• glycogen
• cellular respiration in the mitochondria of the fibers.
Creatine phosphate
The phosphate group in creatine phosphate is attached by a "high-energy" bond like that in ATP. Creatine phosphate derives its high-energy phosphate from ATP and can donate it back to ADP to form ATP.
Creatine phosphate + ADP creatine + ATP
The pool of creatine phosphate in the fiber is about 10 times larger than that of ATP and thus serves as a modest reservoir of ATP.
Glycogen
Skeletal muscle fibers contain about 1% glycogen. The muscle fiber can degrade this glycogen by glycogenolysis producing glucose-1-phosphate. This enters the glycolytic pathway to yield two molecules of ATP for each pair of lactic acid molecules produced. Not much, but enough to keep the muscle functioning if it fails to receive sufficient oxygen to meet its ATP needs by respiration.
However, this source is limited and eventually the muscle must depend on cellular respiration.
Cellular respiration
Cellular respiration not only is required to meet the ATP needs of a muscle engaged in prolonged activity (thus causing more rapid and deeper breathing), but is also required afterwards to enable the body to resynthesize glycogen from the lactic acid produced earlier (deep breathing continues for a time after exercise is stopped). The body must repay its oxygen debt.
Type I vs. Type II Fibers
Several different types of muscle fiber can be found in most skeletal muscles: Type I and and 3 subtypes of Type II fibers. Each type differs in the myosin it uses and also in its structure and biochemistry.
Type I Fibers
• loaded with mitochondria
• depend on cellular respiration for ATP production
• fatty acids the major energy source
• resistant to fatigue
• rich in myoglobin and hence red in color (the "dark" meat of the turkey)
• activated by small-diameter, thus slow-conducting, motor neurons
• also known as "slow-twitch" fibers
• dominant in muscles used in activities requiring endurance (leg muscles) and those that depend on tonus, e.g., those responsible for posture
Type IIb Fibers
• few mitochondria
• rich in glycogen
• depend on creatine phosphate and glycolysis for ATP production
• fatigue easily with the production of lactic acid
• low in myoglobin hence whitish in color (the white meat of the turkey)
• activated by large-diameter, thus fast-conducting, motor neurons
• also known as "fast-twitch" fibers
• dominant in muscles used for rapid movement, e.g., those moving the eyeballs.
The other subtypes of Type II fibers have properties intermediate between those of Type IIb and Type I.
Most skeletal muscles contain some mixture of Type I and Type II fibers, but a single motor unit always contains one type or the other, never both.
In mice, the number of Type I vs Type II fibers can be changed with exercise and drug treatment. Whether the same holds true for humans remains to be seen. Perhaps training in humans does not alter the number of fibers of a particular type but may increase the diameter of one type (e.g., Type I in marathoners, Type IIb in weight lifters) at the expense of the other types.
Cardiac Muscle
Cardiac or heart muscle resembles skeletal muscle in some ways: it is striated and each cell contains sarcomeres with sliding filaments of actin and myosin. However, cardiac muscle has a number of unique features that reflect its function of pumping blood.
• The myofibrils of each cell (and cardiac muscle is made of single cells — each with a single nucleus) are branched.
• The branches interlock with those of adjacent fibers by adherens junctions. These strong junctions enable the heart to contract forcefully without ripping the fibers apart.
This electron micrograph (reproduced with permission from Keith R. Porter and Mary A. Bonneville, An Introduction to the Fine Structure of Cells and Tissues, 4th ed., Lea & Febiger, Philadelphia, 1973) shows an adherens junction and several of the other features listed here.
• The action potential that triggers the heartbeat is generated within the heart itself. Motor nerves (of the autonomic nervous system) do run to the heart, but their effect is simply to modulate — increase or decrease — the intrinsic rate and the strength of the heartbeat. Even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat.
• The action potential that drives contraction of the heart passes from fiber to fiber through gap junctions.
• Significance: all the fibers contract in a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart. Anything that interferes with this synchronous wave (such as damage to part of the heart muscle from a heart attack) may cause the fibers of the heart to beat at random — called fibrillation. Fibrillation is the ultimate cause of most deaths, and its reversal is the function of defibrillators that are part of the equipment in ambulances, hospital emergency rooms, and even on U.S. air lines.
• The refractory period in heart muscle is longer than the period it takes for the muscle to contract (systole) and relax (diastole). Thus tetanus is not possible (a good thing, too!).
• Cardiac muscle has a much richer supply of mitochondria than skeletal muscle. This reflects its greater dependence on cellular respiration for ATP.
• Cardiac muscle has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited.
• Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage - even death - of the affected part. This is what happens in heart attacks.
Smooth Muscle
Smooth muscle is made of single, spindle-shaped cells. It gets its name because no striations are visible in them. Nonetheless, each smooth muscle cell contains thick (myosin) and thin (actin) filaments that slide against each other to produce contraction of the cell. The thick and thin filaments are anchored near the plasma membrane (with the help of intermediate filaments).
Smooth muscle (like cardiac muscle) does not depend on motor neurons to be stimulated. However, motor neurons (of the autonomic system) reach smooth muscle and can stimulate it or relax it depending on the neurotransmitter they release (e.g. noradrenaline or nitric oxide, NO).
Smooth muscle can also be made to contract
• by other substances released in the vicinity (paracrine stimulation)
• Example: release of histamine causes contraction of the smooth muscle lining our air passages (triggering an attack of asthma)
• by hormones circulating in the blood
• Example: oxytocin reaching the uterus stimulates it to contract to begin childbirth.
The contraction of smooth muscle tends to be slower than that of striated muscle. It also is often sustained for long periods. This, too, is called tonus but the mechanism is not like that in skeletal muscle.
Muscle Diseases
The Muscular Dystrophies (MD)
Together myosin, actin, tropomyosin, and troponin make up over three-quarters of the protein in muscle fibers. Some two dozen other proteins make up the rest. These serve such functions as attaching and organizing the filaments in the sarcomere and connecting the sarcomeres to the plasma membrane and the extracellular matrix. Mutations in the genes encoding these proteins may produce defective proteins and resulting defects in the muscles.
Among the most common of the muscular dystrophies are those caused by mutations in the gene for dystrophin. The gene for dystrophin is huge, containing 79 exons spread out over 2.4 million base pairs of DNA. Thus this single gene represents about 0.1% of the entire human genome (3 x 109 bp) and is almost half the size of the entire genome of E. coli!
• Duchenne muscular dystrophy (DMD)
Deletions or nonsense mutations that cause a frameshift usually introduce premature termination codons (PTCs) in the resulting mRNA. Thus at best only a fragment of dystrophin is synthesized and DMD, a very severe form of the disease, results.
• Becker muscular dystrophy (BMD).
If the deletion simply removes certain exons but preserves the correct reading frame, a slightly-shortened protein results that produces BMD, a milder form of the disease.
The gene for dystrophin is on the X chromosome, so these two diseases strike males in a typical X-linked pattern of inheritance.
A treatment for DMD
Deletions of one or more exons in the huge dystrophin gene are the cause of most of the cases of DMD. Exon 50 is a particularly notorious offender. When it is deleted, splicing of the pre-mRNA introduces a frameshift which then introduces a premature termination codon resulting in no functional dystrophin synthesized ("B"). However, an antisense oligonucleotide targeted to exon 51 causes the splicing mechanism to skip over it resulting in the stitching together of exons 49 and 52. This restores the correct reading frame so that only a slightly-altered version of dystrophin is produced, i.e., a BMD-type dystrophin ("C"). Seventeen weeks of weekly injections of 12 young DMD patients in the Netherlands with the oligonucleotide caused their muscles to synthesize sufficient amounts of dystrophin to enable 8 of them to walk better than before. (See Goemans, N., et al., in the 21 April 2011 issue of The New England Journal of Medicine.
Three research groups have used the CRISPR-Cas9 genome editing system to remove a mutated exon in DMD mice. The treatment restored dystrophin synthesis and improved skeletal and cardiac muscle function in the mice.
Myasthenia Gravis
Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction. Patients have smaller end plate potentials (EPPs) than normal. With repeated stimulation, the EPPs become too small to trigger further action potentials and the fiber ceases to contract. Administration of an inhibitor of acetylcholinesterase temporarily can restore contractility by allowing more ACh to remain at the site.
Patients with myasthenia gravis have only 20% or so of the number of ACh receptors found in normal neuromuscular junctions. This loss appears to be caused by antibodies directed against the receptors. Some evidence:
• A disease resembling myasthenia gravis can be induced in experimental animals by immunizing them with purified ACh receptors.
• Anti-ACh receptor antibodies are found in the serum of human patients.
• Experimental animals injected with serum from human patients develop the signs of myasthenia gravis.
• Newborns of mothers with myasthenia gravis often show mild signs of the disease for a short time after their birth. This is the result of the transfer of the mother's antibodies across the placenta during gestation.
The reason some people develop autoimmune antibodies against the ACh receptor is unknown.
The Cardiac Myopathies
Cardiac muscle, like skeletal muscle, contains many proteins in addition to actin and myosin. Mutations in the genes for these may cause the wall of the heart to become weakened and, in due course, enlarged. Among the genes that have been implicated in these diseases are those encoding:
• actin
• two types of myosin
• troponin
• tropomyosin
• myosin-binding protein C (which links myosin to titin)
The severity of the disease varies with the particular mutation causing it (over 100 have been identified so far) . Some mutations are sufficiently dangerous that they can lead to sudden catastrophic heart failure in seemingly healthy and active young adults.
15.10C: Testing the Sliding-Filament Hypothesis
The sliding-filament model postulates that when skeletal (or cardiac) muscle contracts, the thin and thick filaments in each sarcomere slide along each other without their shortening, thickening, or folding and the strength of the relative motion between the thick and thin filaments is determined by the number of cross-bridges that can form between the two.
The Testing
This diagram shows apparatus with which to test the model.
• An isolated muscle (e.g., the calf muscle of a frog) placed in this apparatus cannot shorten when stimulated by an electric shock.
• Thus stimulation of the muscle produces an isometric ("same length") contraction.
• The muscle is placed in the apparatus and stretched to the desired length.
• It is then given a series of tetanizing shocks to measure its "active" tension; that is, the tension produced when it is stimulated.
• The strain gauge measures the tension exerted by the muscle.
• (Muscles are elastic, so if we stretch the muscle in putting it in the apparatus, it will already be exerting a "resting" tension.)
• Now let us plot the effect of muscle length on the active tension that is produced.
The Results
• The muscle produces the highest tension when held in the apparatus at the length it normally has in the intact animal.
• If held at longer (or shorter) lengths, the active tension produced is less.
You may conclude from this that nature knows best. And, in fact she does. If a muscle is surgically reattached to an animal so that its length is changed, the muscle gradually adapts to its new length and, after a few weeks, is able to exert its maximum isometric contractions at the new length.
The Interpretation
• A muscle stretched beyond its normal length has less overlap between the thick and thin filaments.
• Thus fewer cross-bridges can form to slide the filaments against each other.
• In fact, if the muscle is stretched so far that the thin filaments are pulled entirely away from the thick filaments, the muscle exerts no tension at all.
• As for the effect of holding the muscle at shorter than normal length, the thin filaments extend so far across the sarcomere that they interact with cross-bridges exerting force the opposite way — reducing the tension generated.
These electron micrographs (courtesy of Dr. H. E. Huxley) show the pattern of striations in stretched muscle and resting muscle. In the stretched muscle, there is less overlap of the thick and thin filaments.
Consequently
• The light I bands and the H zone become wider.
• The width of the dark A band remains unchanged.
• We would not expect this pattern if sarcomere length involved a change in the length of the thick filaments. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.10%3A_Muscles/15.10B%3A_Muscles.txt |
Behavior is action that alters the relationship between an organism and its environment. Behavior may occur as a result of
• an external stimulus (e.g., sight of a predator)
• internal stimulus (e.g., hunger)
• or, more often, a mixture of the two (e.g., mating behavior)
It is often useful to distinguish between
• innate behavior = behavior determined by the "hard-wiring" of the nervous system. It is usually inflexible, a given stimulus triggering a given response. A salamander raised away from water until long after its siblings begin swimming successfully will swim every bit as well as they the very first time it is placed in the water. Clearly this rather elaborate response is "built in" in the species and not something that must be acquired by practice.
• learned behavior = behavior that is more or less permanently altered as a result of the experience of the individual organism (e.g., learning to play baseball well).
• However, careful analysis often reveals that any particular behavior is a combination of innate and learned components.
Examples of innate behavior:
• taxes
• reflexes
• instincts
Taxes
Some organisms respond to a stimulus by automatically moving directly toward or away from or at some defined angle to it. These responses are called taxes. They are similar to tropisms in plants except that actual locomotion of the entire organism is involved. Link to a detailed discussion.
Reflexes
The Withdrawal Reflex
When you touch a hot object, you quickly pull you hand away using the withdrawal reflex.
These are the steps:
• The stimulus is detected by receptors in the skin.
• These initiate nerve impulses in sensory neurons leading from the receptors to the spinal cord.
• The impulses travel into the spinal cord where the sensory nerve terminals synapse with interneurons.
• Some of these synapse with motor neurons that travel out from the spinal cord entering mixed nerves that lead to the flexors that withdraw your hand.
• Others synapse with inhibitory interneurons that suppress any motor output to extensors whose contraction would interfere with the withdrawal reflex.
The Stretch Reflex
The stretch reflex is examined (with a diagram) on a separate page. Link to it.
Instincts
Instincts are complex behavior patterns which, like reflexes, are inborn, rather inflexible, and valuable at adapting the animal to its environment. They differ from reflexes in their complexity. The entire body participates in instinctive behavior, and an elaborate series of actions may be involved.
The scratching behavior of a dog and a European bullfinch, shown here, is part of their genetic heritage. The widespread behavior of scratching with a hind limb crossed over a forelimb in common to most birds, reptiles, and mammals. (Picture courtesy of Rudolf Freund and Scientific American, 1958.) So instincts are inherited just as the structure of tissues and organs is. Another example.
• The African peach-faced lovebird carries nesting materials to the nesting site by tucking them in its feathers.
• Its close relative, the Fischer's lovebird, uses its beak to transport nesting materials.
• The two species can hybridize. When they do so, the offspring succeed only in carrying nesting material in their beaks. Nevertheless, they invariably go through the motions of trying to tuck the materials in their feathers first.
Foraging Behavior
Foraging for food is a crucial behavior for animals. Like all behavior, it requires the interaction of many components. Nonetheless, it turns out that in some animals, at least, foraging behavior can be altered by a single gene.
Drosophila melanogaster
The discovery of the genetic control of foraging in Drosophila began with the observations of Marla Sokolowski when she was an undergraduate biology student at the University of Toronto.
She noticed that Drosophila larvae, feeding in her culture vessels, displayed one of two distinct feeding patterns:
• "rovers" who moved rapidly over the surface of the culture medium
• "sitters who fed at a much more leisurely pace
She went on to find that this "bimodal" pattern of behavior
• continued when the larvae became adults
• was present in populations of wild fruit flies, not just in her laboratory colonies
After further years of research, she has shown that the behavior is under the control of a single gene, named for ("foraging"). Two alleles are present, at almost equal frequencies, that is, for is polymorphic.
• forR, which is dominant
• fors, the recessive
• About 70% of natural populations are rovers being either homozygous for forR or heterozygous (forR/fors).
• Sitters are homozygous for fors
Both alleles encode a PKG, a protein kinase (an enzyme that attaches phosphate groups to target proteins) that is activated by the "second messenger" cyclic GMP (cGMP). The enzyme encoded by the forR allele is more active than that encoded by fors. She and her colleagues have succeeded in inserting forR DNA into sitters who promptly become rovers.
Why this polymorphism? Why should alleles for two such different behaviors be maintained at such high frequency in the population?
One possible answer: it permits the population to thrive under varying food conditions:
• sitters are favored when food is abundant
• rovers are favored when competition for food is strong, such as in crowded cultures
Honeybees
Honeybees have their version of the for gene, called Amfor ("Apis mellifera for"). It, too, encodes a cGMP-dependent protein kinase (PKG). When worker bees first hatch, they remain in the hive tending to various housekeeping chores, such as feeding the larvae. But when they are 2–3 weeks old, they leave the hive and begin foraging for nectar and pollen. This change in behavior coincides with the increased expression of Amfor. When newly-hatched workers are treated with cGMP, the amount of PKG in their brains goes up and they quickly begin foraging instead of doing housekeeping.
Interaction of Internal and External Stimuli
Instinctive behavior often depends on conditions in the internal environment. In many vertebrates courtship and mating behavior will not occur unless sex hormones (estrogens in females, androgens in males) are present in the blood. The target organ is a small region of the hypothalamus. When stimulated by sex hormones in its blood supply, the hypothalamus initiates the activities leading to mating. The level of sex hormones is, in turn, regulated by the activity of the anterior lobe of the pituitary gland.
The above figure outlines the interactions of external and internal stimuli that lead an animal, such as a rabbit, to see a sexual partner and mate with it.
Releasers of Instinctive Behavior
Once the body is prepared for certain types of instinctive behavior, an external stimulus may be needed to initiate the response. N. Tinbergen (who shared the 1973 Nobel Prize with Konrad Lorenz and Karl von Frisch) showed that the stimulus need not necessarily be appropriate to be effective.
• During the breeding season, the female three-spined stickleback normally follows the red-bellied male (a in the figure) to the nest that he has prepared.
• He guides her into the nest (b) and then
• prods the base of her tail (c).
• She then lays eggs in the nest.
• After doing so, the male drives her from the nest, enters it himself, and fertilizes the eggs (d).
• Although this is the normal pattern, the female will follow almost any small red object to the nest, and once within the nest, neither the male nor any other red object need be present.
• Any object touching her near the base of her tail will cause her to release her eggs.
It is as though she were primed internally for each item of behavior and needed only one specific signal to release the behavior pattern. For this reason, signals that trigger instinctive acts are called releasers. Once a particular response is released, it usually runs to completion even though the stimulus has been removed. One or two prods at the base of her tail will release the entire sequence of muscular actions involved in liberating her eggs.
Chemical signals (e.g., pheromones) serve as important releasers for the social insects: ants, bees, and termites. Many of these animals emit several different pheromones which elicit, for example, alarm behavior, mating behavior, and foraging behavior in other members of their species.
The mammary glands of domestic rabbit mothers emit a pheromone that releases immediate nursing behavior by their babies (pups). A good thing, too, as mothers devote only 5–7 minutes a day to feeding their pups so they had better be quick about it.
The studies of Tinbergen and others have shown that animals can often be induced to respond to inappropriate releasers. For example, a male robin defending its territory will repeatedly attack a simple clump of red feathers instead of a stuffed robin that lacks the red breast of the males.
Although such behavior seems inappropriate to our eyes, it reveals a crucial feature of all animal behavior: animals respond selectively to certain aspects of the total sensory input they receive. Animals spend their lives bombarded by myriad sights, sounds, odors, etc. But their nervous system filters this mass of sensory data, and they respond only to those aspects that the evolutionary history of the species has proved to be significant for survival. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.01%3A_Innate_Behavior.txt |
Some organisms respond to a stimulus by automatically moving directly toward or away from or at some defined angle to it. These responses are called taxes. They are similar to tropisms in plants except that actual locomotion of the entire organism is involved.
Chemotaxis
When a capillary tube filled with glucose is placed in a medium containing E. coli, the bacteria alter their locomotion so that they congregate near the opening of the tube. This chemotactic response does not depend on the bacteria being able to metabolize the substance although presumably that is the value under normal conditions. E. coli responds strongly to a number of organic molecules besides glucose, including galactose and the amino acids serine and aspartic acid. Figure Figure 15.11.2.1 shows how the bacteria have congregated at the opening of a capillary tube filled with a weak solution of an amino acid.
Some of the motile cells of the immune system also display chemotaxis. The substances that attract them are called chemokines (for chemotactic cytokines).
Phototaxis
Photosynthetic microorganisms often display phototaxis.
In the left picture, randomly oriented tracks formed during 1/3 sec by the algae swimming about in red light to which the are insensitive. In the right picture, upon adding a beam of blue-green light from one side, the tracks become oriented in its direction.
Magnetotaxis
Several species of bacteria swim in the direction of magnetic lines of force. Because of the inclination of the earth's magnetic lines of force, this behavior causes the bacterium to swim downward and thus to return to the sediments in which it lives. For an organism as tiny as a bacterium, gravity is of no consequence. So here is an alternate mechanism by which dislodged bacteria can find their way back into their normal habitat.
Magnetotactic bacteria in the Southern Hemisphere achieve the same result by swimming toward the South Pole. Particles resembling these magnetite particles have been found in the Martian meteorite ALH84001. If they were not formed by a purely chemical process (as some believe), they would be another entry in the hunt for evidence that Mars can (or could) support life.
15.11.03: Learned Behavior
Habituation
Habituation is a reduction in a previously-displayed response when no reward or punishment follows. If you make an unusual sound in the presence of the family dog, it will respond usually by turning its head toward the sound. If the stimulus is given repeatedly and nothing either pleasant or unpleasant happens to the dog, it will soon cease to respond. This lack of response is not a result of fatigue or sensory adaptation and is long-lasting; when fully habituated, the dog will not respond to the stimulus even though weeks or months have elapsed since it was last presented.
Sensitization
Sensitization is an increase in the response to an innocuous stimulus when that stimulus occurs after a punishing stimulus.
Sensitization in sea slugs
When the siphon of the sea slug Aplysia is gently touched, the animal withdraws its gill for a brief period. However, if preceded by an electrical shock to its tail, the same gentle touch to the siphon will elicit a longer period of withdrawal. The sensitization response to a single shock (blue bar) dies out after about an hour, and returns to baseline after a day (yellow). So it is an example of short-term memory.
However, if the animal is sensitized with multiple shocks given over several days, its subsequent response to a gentle touch on the siphon is much larger and is retained longer (tan and gray bars). This is an example of long-term memory and requires protein synthesis. (These findings are the work of Eric R. Kandel, who was awarded a Nobel Prize in 2000.)
Imprinting
If newly-hatched geese are exposed to a moving object of reasonable size and emitting reasonable sounds, they will begin to follow it just as they would normally follow their mother. This is called imprinting. The time of exposure is quite critical. A few days after hatching, imprinting no longer occurs. Prior to this time, though, the results can be quite remarkable. A gosling imprinted to a moving box or clucking person will try to follow this object for the rest of its life. In fact, when the gosling reaches sexual maturity, it will make the imprinted object - rather than a member of its own species - the goal of its sexual drive.
Much of our knowledge of imprinting was learned from the research of Konrad Lorenz, shown here with some of his imprinted goslings. Lorenz shared a Nobel Prize in 1973 for his discoveries. (Photo by Tom McAvoy; courtesy of LIFE Magazine, ©1955, Time, Inc.)
Male mice become imprinted with the odor of littermates during the first three weeks of life. When they reach sexual maturity, they avoid mating with close relatives. The odor is controlled by the major histocompatibility complex (MHC).
The Conditioned Response (CR)
The conditioned response is probably the simplest form of learned behavior. It is a response that as a result of experience comes to be caused by a stimulus different from the one that originally triggered it. The Russian physiologist Ivan Pavlov found that placing meat powder in a dog's mouth would cause it to salivate.
The meat powder, an unconditioned stimulus (US), triggers a simple inborn reflex involving taste receptors, sensory neurons, networks of interneurons in the brain, and autonomic motor neurons running to the salivary glands thus producing an unconditioned response (UR).
Pavlov found that if he rang a bell every time he put the meat powder in the dog's mouth, the dog eventually salivated upon hearing the bell alone. This is the conditioned response (CR). The dog has learned to respond to a substitute stimulus, the conditioned stimulus (CS).
We assume that the physiological basis of the conditioned response is the transfer, by appropriate neurons, of nervous activity in the auditory areas of the brain to the motor neurons controlling salivation. This involves the development and/or strengthening of neural circuits, which - we may also assume - is characteristic of all forms of learning.
The conditioned response has proved to be an excellent tool for determining the sensory capabilities of other animals. For example, honeybees can be conditioned to seek food on a piece of blue cardboard. By offering other colors to a blue-conditioned bee, Karl von Frisch (who shared the 1973 Nobel Prize with Lorenz) found that honeybees can discriminate between yellow-green, blue-green, blue-violet, and ultraviolet.
Instrumental Conditioning
Pavlov's dogs were restrained and the response being conditioned (salivation) was innate. But the principles of conditioning can also be used to train animals to perform tasks that are not innate. In these cases, the animal is placed in a setting where it can move about and engage in different activities. The experimenter chooses to reward only one, e.g., turning to the left. By first rewarding (e.g., with a pellet of food) even the slightest movement to the left and then only more complete turns, a skilled experimenter can in about 2 minutes train a naive pigeon to make a complete turn. A little more work and the pigeon will pace out a figure eight.
In the example shown here, the pigeon - presented with two spots of light - pecks at the brighter and reaches down to pick up the grain of food that is its reward.
Such training is known as instrumental conditioning or operant conditioning. The latter term was coined by B. F. Skinner, whose skill with the technique enabled him to train pigeons to play ping-pong and even a toy piano! It is also called trial-and-error learning because the animal is free to try various responses before finding the one that is rewarded.
Maze problems are a form of instrumental conditioning in which the animal is faced with a sequence of alternatives.
In this photo (Courtesy of B. Rensch), Julia, a chimpanzee, uses a magnet to move an iron ring through a maze. Julia is able to solve mazes like this on her first attempt most (86%) of the time and sometimes faster than biology students can!
Concepts
Although most animals solve mazes and other problems by trial and error, Julia (and biology students) usually make only one or two random attempts at solving a problem and then, all of a sudden, "get it". They have made an abstract generalization about the specific problem; that is, have formed a concept. Oddity problems are an example. This young rhesus monkey has learned that food will be found - not under any particular object - but under whichever object is different from the others.
In monkeys (and probably humans as well), concept formation depends on activity in the prefrontal cortex of the brain. Recent research suggests that honeybees can also solve simple oddity problems! | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.02%3A_Taxis.txt |
All learning depends on memory. The formation of memories appears to occur in two separate phases, first short-term memory. (e.g., Humans undergoing electroshock treatment (to alleviate their depression) are unable to remember events that occurred just prior to the treatment, but their memory of earlier events is unimpaired) that followed by formation of long-term memory. Damage to the temporal lobes of the brain can result in the loss of the ability to remember new learning for more than about an hour. Two systems that have been particularly useful in working out the cellular and molecular basis for memory formation are sensitization in the sea slug Aplysia and the study of long-term potentiation (LTP).
Long-Term Potentiation (LTP)
Rats and mice can be trained to solve simple tasks. For example, if a mouse is placed in a pool of murky water, it will swim about until it finds a hidden platform to climb out on. With repetition, the mouse soon learns to locate the platform more quickly. Presumably it does so with the aid of visual cues placed around the perimeter of the pool because it cannot see or smell the platform itself.
Rats or mice who have had a part of their brain called the hippocampus damaged, cannot learn this task, although they continue to solve it quickly if they were trained before their brain damage. This suggests that neurons in the hippocampus are needed for this type of learning. In contrast to the rest of the brain, new neurons are produced in the hippocampus throughout life. They arise from a pool of stem cells in the brain. The integration of newly-formed neurons into existing hippocampal circuitry facilitates the learning of new memories (as well as the forgetting of old ones).
Demonstrating Long-Term Potentiation
The behavior of certain synapses in the "CA1" region of the hippocampus of the rat (or mouse) is consistent with their being essential for this form of long-term memory. Slices of the hippocampus can be removed and its CA1 neurons studied in vitro with recording electrodes. Rapid, intense stimulation of presynaptic neurons evokes action potentials in the postsynaptic neuron. This is just what we would expect from the properties of synapses.
What is remarkable about this system is that over time these synapses become increasingly sensitive so that a constant level of presynaptic stimulation becomes converted into a larger postsynaptic output (graph on right). This long-term potentiation can last for weeks. Treatment of a slice of hippocampus with a drug called aminophosphonovaleric acid ("APV") blocks the formation of LTP. This is because APV blocks the action of NMDA receptors, a subset of postsynaptic receptors that normally respond to the excitatory neurotransmitter glutamate (Glu). NMDA receptors (synapse B above) are distinguished from other Glu-activated receptors in being stimulated by the drug, N-methyl-D-aspartate (NMDA).
NMDA receptors contain a transmembrane channel that allows for the facilitated diffusion of calcium ions (Ca2+) across the plasma membrane of the synapse. Binding of Glu (or NMDA) and D-serine released from a nearby astrocyte to these receptors opens the channel allowing Ca2+ to flow in if — and only if — the same postsynaptic cell has been simultaneously depolarized by other synapses on it (synapse A above). (The drawing is vastly-oversimplified: each CA1 neuron has tens of thousands of synapses on it.)
The influx of Ca2+ into the neuron activates an enzyme called calcium-calmodulin-dependent kinase II (CaMKII). Kinases attach phosphate groups to proteins and, in so doing, alter their functioning. In this case, CaMKII phosphorylates a second type of Glu receptor called AMPA receptors, which makes them more permeable to sodium ions (Na+) thus lowering the resting potential of the cell and making it more sensitive to incoming impulses. In addition, there is evidence that the activity of CaMKII increases the number of AMPA receptors at the synapse.
The ability to make transgenic mice has provided tools to test this model of LTP.
Mutant mice
Mice that are homozygous for a mutant CaMKII transgene fail to develop LTP. This was shown (by A. J. Silver, et. al., in Science 257:206, 1992) in two ways:
• by measuring the current in the postsynaptic cell of normal mice and mutant mice. The graph above(left) shows that mutant mice do not develop the increase in current flow that normal mice (graph above) do.
• The same failure of LTP occurs when the mice are tested on the hidden platform (graph right).
Transgenic mice
Transgenic mice that make extra-large amounts of NMDA receptors show enhanced LTP as shown by
• greater postsynaptic currents in their hippocampus
• their enhanced performance on the hidden-platform test
• and enhanced performance in other tests of learning and memory
These findings were reported in the 2 September 1999 issue of Nature by Tang, Y-P, et al. The experiments described above show that manipulations that affect the postsynaptic electrical response (EPSPs) of neurons stimulated electrically also affect learned behavior. They do not show that learning induces increased EPSPs in postsynaptic neurons of the hippocampus. Now, researchers at MIT have done just that.
They used rats in which they implanted an array of closely-spaced recording electrodes in the CA1 region of the hippocampus. These rats were then placed in a training apparatus where they learned in a single trial that moving from a lighted chamber to a dark one would give them a shock.
In just 30 minutes some, but never all, of the recording electrodes picked up increased EPSPs in the CA1 neurons and the number of AMPA receptors increased in the CA1 cells. So learning this conditioned response produced the electrical and synaptic changes of LTP but only in certain regions of the hippocampus. Presumably other types of learning would produce LTP in other parts of the CA1 region. (You can read the report of their work in Whitlock, J. R., et al., Science, 25 August 2006.)
Early LTP vs. Late LTP
LTP occurs in two phases:
• an early one (in the first hour or so) which involves increased sensitivity of the synapse without any new gene transcription or mRNA translation occurring
• a late one which requires new gene transcription and mRNA translation and results in an increase in the number of AMPA receptors accompanied by an increase in the size of the synaptic connection. These changes persist for many hours and even many days. However, increased AMPA receptor formation seems to require continuous stimulation because (in rats, at least) interfering with the process erases late LTP (and memory) even a month later.
This is yet more evidence that memories are acquired in two phases; early and late. Late LTP may involve not only the addition of AMPA receptors to existing synapses but the formation of entirely new synapses. Researches in Geneva, Switzerland have demonstrated that formation of LTP in rat brains coincides with the formation of additional synapses (at least one more) between the presynaptic axon terminal and the dendrite it synapses with. (Report by Toni, N., et al, Nature, 25 Nov 99). Presumably this, too, increases the efficiency of synaptic transmission.
Summary
• Rapid, intense stimulation of CA1 neurons in the hippocampus depolarizes them.
• Binding of Glu and D-serine to their NMDA receptors opens them.
• Ca2+ ions flow into the cell through the NMDA receptors and bind to calmodulin.
• This activates calcium-calmodulin-dependent kinase II (CaMKII).
• CaMKII phosphorylates AMPA receptors making them more permeable to the inflow of Na+ ions and thus increasing the sensitivity of the cell to depolarization.
• In time CaMKII also increases the number of AMPA receptors at the synapse.
• Increased gene expression (i.e., protein synthesis — perhaps of AMPA receptors) also occurs during the development of LTP.
• Enlargement of the synaptic connections and perhaps the formation of additional synapses occur during the formation of LTP.
LTP has also been demonstrated in neurons of the cerebellum.
Long-Term Depression (LTD)
Slow, weak electrical stimulation of CA1 neurons also brings about long-term changes in the synapses, in this case, a reduction in their sensitivity. This is called long-term depression or LTD. It reduces the number of AMPA receptors at the synapse. Long-term depression also occurs in isolated preparations of neurons from the sea slug, Aplysia and the cerebellum of mice during the development of an conditioned response (CR) | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.04%3A_Long-Term_Potentiation_%28LTP%29.txt |
The domestic honeybee, Apis mellifera, is a colonial insect living in hives containing one queen — a fertile female, a few drones (males), and thousands of workers (infertile females). The workers are responsible for keeping the hive clean, building the wax combs of the hive, tending the young and, when they get older (and when their for gene gets turned on), and foraging for food - nectar and pollen.
ome 5–25% of the workers in the hive are scouts. Their job is to search for new sources of food for the other workers, the foragers, to harvest. While both scouts and foragers look alike, recent research suggests that they represent stable subpopulations with distinctive patterns of gene expression in their brains. (See Liang, Z. S., et al., in the 9 March 2012 issue of Science.)
When food is discovered by scouts, they return to the hive. Shortly after their return, many foragers leave the hive and fly directly to the food. The remarkable thing about this is that the foragers do not follow the scouts back. The scouts may remain in the hive for hours and those that leave continue to hunt for new sources of food even though the foragers are continuing to bring back ample supplies of food from the sites the scouts discovered earlier. So the scout bees have communicated to the foragers the necessary information for them to find the food on their own.
It turns out that the scouts can convey to the foragers information about
• the odor of the food
• its direction from the hive
• its distance from the hive
Distance
When food is within 50–75 meters of the hive, the scouts dance the "round dance" on the surface of the comb (left). But when the food is farther than 75 meters from the hive, the scouts dance the "waggle dance" (right). The waggle dance has two components: (1) a straight run - the direction of which conveys information about the direction of the food and (2) the speed at which the dance is repeated which indicates how far away the food is.
The graph shows the relationship between the speed of the dance and the distance to the food. It is based on data collected by the German ethologist Karl von Frisch. It was he who discovered much of what we know today about honeybee communication (and was honored with a Nobel Prize in 1973).
How do the bees calculate distance? von Frisch thought they measured the distance by assessing the amount of energy it took to get there. But the mechanism turns out to be quite different. The bees measure distance by the motion of images received by their eyes as they fly.
Flicker Effect
Honeybees, like all insects, have compound eyes. These give little information about depth but are very sensitive to "flicker effect". It has long been known to bee keepers that honeybees respond better to flowers
• moving in the breeze
• with complex petals
The importance of flicker effect can be demonstrated by training honeybees to visit food placed on cards with patterns. For example, the bees can distinguish any figure in the top row from any figure in the bottom row more easily than they can distinguish between any of the figures in either row.
Evidence of distance measuring
Working at the University of Notre Dame (Indiana), Esch and Burns found that when the hive and food were placed on top of tall (50 m) buildings, the speed of the waggle dance indicated a distance only half of the distance indicated by bees travelling the same distance (230 m) at ground level. Features of the scenery pass the retina faster when near than when far (compare the changes in your view from an airliner at cruising altitude and as it approaches the runway).
Tests of Searching Behavior
Working at the Australian National University, Srinivasan and his colleagues built tunnels decorating the interior walls with patterns to create flicker. In these three experiments, the bees were first fed in the middle of the tunnel of standard diameter. Then the food was removed.
1. Using the same tunnel, foragers came to same spot in the tunnel.
2. Using a narrower tunnel, foragers began searching for food near its entrance. Forced to fly closer to the vertical stripes, the images moved by faster.
3. Using a tunnel with a larger diameter, foragers searched for food at its far end. Flying farther from the stripes, the images moved by more slowly.
Tests of Dancing Behavior
All tunnels were 6 meters long and located 35 meters from the hive — well within the distance (50–75 m) that normally elicits the round dance.
1. Bees were fed at the entrance. Back at the hive, they danced the round dance as you would expect.
2. Bees fed at end of tunnel. Back at the hive, they danced the waggle dance. Even though the total distance from the hive (41 m) was still well within the "round" range, the complexity of the scenery passed in the last 6 m, elicited the waggle dance.
3. Bees fed at end of tunnel decorated with horizontal stripes. These created no flicker as the bees flew, and on their return to the hive they danced the round dance.
You can read about this second set of experiments in Srinivasan, et. al., Science, 4 February 2000.
Recruits respond to the misinformation given by returning scouts
More recently, the Esch and Srinivasan groups have teamed up to show that naive foragers are fooled by the misinformation that the returning scouts give them. When scouts were fed only 11 m from the hive but at the far end of a striped tunnel, they danced the waggle dance back at the hive as though the food had been 70 m away. Most of the workers they recruited flew out (bypassing the tunnel) 70 m looking for food. You can read about these experiments in Ungless, et. al., Nature, 31 May 2001.
Direction
By itself, the knowledge that food is 6 kilometers (3.7 miles) away is not very useful. But von Frisch also noted that the direction of the straight portion of the waggle dance varied with the direction of the food source from the hive and the time of day.
• At any one time, the direction changes with the location of the food.
• With a fixed source of food, the direction changes by the same angle as the sun during its passage through the sky.
But
• The sun is not visible within the hive.
• The scouts dance on the vertical surface of the combs.
How, then, do they translate flight angles in the darkened hive?
The picture shows the relationship between the angle of the dance on the vertical comb and the bearing of the sun with respect to the location of food. When the food and sun are in the same direction, the straight portion of the waggle dance is directed upward. When the food is at some angle to the right (blue) or left (red) of the sun, the bee orients the straight portion of her dance at the same angle to the right or left of the vertical.
Using radar to track individual bees recruited by the waggle dance, Riley, J. R., et al., (Nature, 12 May 2005) have shown that the recruits do fly in the indicated direction. They even adjust their flight path to compensate for being blown off course by the wind. However, their course is seldom so precise that they can find the food without the aid of vision and/or smell as they neared it.
Other features
Time Sense
When scouts remain in the hive for a long period, they shift the direction of the straight portion of the waggle dance as the day wears on (and the direction of the sun shifts). But they cannot see the sun in the darkened hive. Evidently, they are "aware" of the passing time and make the necessary corrections.
The time sense of honeybees has long been known to people who have sweet snacks in their garden at a set time every day. Within minutes of the regular time, foraging bees arrive for their share of the jam. The speed of the bee's clock seems to be related to its metabolic rate. If normally punctual bees are chilled (to lower their metabolic rate) or exposed to an anesthetizing concentration of carbon dioxide they arrive late to the picnic table (graph above).
Polarized light
von Frisch also discovered that scouts (and foragers) don't actually have to see the sun to navigate. As long as they can see a small patch of clear blue sky, they get along fine. This is because sky light is partially polarized, and the plane of polarization in any part of the sky is determined by the location of the sun. Try it by rotating a pair of polaroid® sun glasses!
Swarming
Before a new queen emerges, the old queen leaves the hive, taking many of the workers with her. The swarm usually settles somewhere, e.g., on a tree branch, while scouts go searching for a new home. Each scout that finds a promising site, returns to the swarm and dances on it just as though she had found food. Eventually, the swarm departs for the location promoted most vigorously. Once the new hive is established, many of the scouts that found the site will become food scouts.
Grooming
Workers have another type of dance — rapidly vibrating from side to side — that tells other workers that she needs help removing dust, pollen, etc. from hard-to-reach places on her body. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.05%3A_Honeybee_Navigation.txt |
Most animals get their food from preying on other organisms, and much of the life of animals involves eating and avoiding being eaten. So it is not surprising to find many examples of adaptations that increase the effectiveness of predation and minimize the risk of being preyed upon. We shall examine some of the devices that help their owners avoid being eaten.
Camouflage (cryptic coloration)
Many animals are patterned to blend in with their surroundings. Some examples include the peppered moth and the winter flounder.
Masquerade
Masquerading animals resemble some inanimate object and thereby escape detection by potential predators (or prey).
The motionless twig caterpillar shown here (courtesy of Muriel V. Williams) complete with "buds" and "lenticels" escapes detection by birds (but pays for its cleverness by occasionally having some other insect lay eggs on it by mistake).
Chemical Defenses
Many plants and animals use repellent chemicals to deter predation. Millipedes secrete hydrocyanic acid when disturbed. Some beetles squirt potential predators with such mixtures as 85% acetic acid or 40% formic acid. The discharge of the skunk is another familiar example.But what if you have a powerful defensive weapon but no potential predator notices until it has launched an attack? One solution to this dilemma is the evolution of warning coloration (also called aposematic coloration).
Aposematic Coloration
This is the larva of the monarch butterfly; an example of aposematic coloration. There is no question of camouflage here. Rather this creature is advertising its presence. The milkweed leaves on which it is feeding contain cardiac glycosides that are toxic to vertebrates because they block the activity of the Na+/K+ ATPase that is essential for many cell functions. The larva stores these within its body and thus becomes unpalatable to vertebrate predators. The chemicals remain in the body even after metamorphosis, so that adults are unpalatable as well.
In these photographs (provided by Lincoln P. Brower) a blue jay eats a portion of a monarch butterfly (left) that had fed (in its larval stage) on poisonous milkweed. A short time later, the blue jay vomits (right). Following this episode, the blue jay refused to eat any other monarch offered to it.
Mimicry
Batesian Mimicry
If an animal is not noxious to potential predators, why not look like an animal that is? Some examples:
• A number of harmless snakes closely mimic the bright warning coloration of the coral snake — the most poisonous snake in the United States.
• The harmless robber fly (right) resembles the bumblebee (left) even though the two are not closely related. The robber fly is a dipteran, with only a single pair of wings, while the bumblebee is a hymenopteran with two pairs.
• The viceroy butterfly (bottom) contains no toxic substances in its body and presumably is quite palatable (one entomologist declared it tastes like dried toast). If so, the viceroy's striking resemblance to the monarch (top) enables it to capitalize on the monarch's unpalatability. (Photos by, and courtesy of, Howard Hoople.)
This phenomenon is called Batesian mimicry (named after Henry W. Bates, a nineteenth-century naturalist who studied many such cases).
Müllerian Mimicry
Some unpalatable animals closely resemble other equally unpalatable species. Such mimicry is called Müllerian mimicry (in honor of the German zoologist Fritz Müller, who studied it). Presumably each species gains a measure of protection from the occasional, but educational, losses of the other species to predators. Lincoln Brower, who has studied the monarch and viceroy, believes that the viceroy is as unpalatable to potential predators as the monarch, and thus is really an example of Müllerian mimicry.
Aggressive Mimicry
Some carnivores have evolved devices with which they mimic the prey (or potential mate) of other (usually smaller) predators. They use these devices as lures.
Two examples:
• The angler fish (Antennarius) displays a lure resembling a small fish. The lure is a development of the spine of the first dorsal fin. This species of anglerfish, which was found in the Philippines, is 9.5 cm long. Note its use of camouflage: its texture (and color) closely resemble the sponge- and algae-encrusted rocks found in its habitat.
Fireflies use their flashes to attract mates. The pattern differs from species to species. In one species, the females sometimes mimic the pattern used by females of another species. When the males of the second species respond to these "femmes fatales", they are eaten!
Group Behavior
Cooperation between members of a social species often reduces the severity of predation. Grazing ungulates are usually organized so that the stronger are on the outside of the herd, the weaker within.
In this photo (courtesy of Ted Grant, National Film Board of Canada), musk oxen have responded to a threat by forming a circle with the females and young in the center. Both herds of ungulates and flocks of birds often have members who are extra-watchful, ready to give the alarm if danger threatens. When alarmed, smelt (a kind of fish) release a pheromone into the water that warns other members of the school. When a honeybee stings an intruder, she release isoamyl acetate, which excites other bees to join the attack. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.06%3A_Avoiding_Predation.txt |
Pheromones are chemicals released by an organism into its environment enabling it to communicate with other members of its own species.
Pheromones in Insects
Alarm Pheromone: When an ant is disturbed, it releases a pheromone that can be detected by other ants several centimeters away. They are attracted by low concentrations of the pheromone and begin to move toward the region of increasing concentration. As they get nearer to their disturbed nestmate, their response changes to one of alarm. The higher concentration causes them to run about as they work to remedy the disturbance. Unless additional amounts of the alarm pheromone are released, it soon dissipates. This ensures that once the emergency is over, the ants return quietly to their former occupations. Honeybees also have an alarm pheromone (which is a good thing not to elicit around a colony of "Africanized" bees).
Trail Pheromone: Certain ants, as they return to the nest with food, lay down a trail pheromone. This trail attracts and guides other ants to the food. It is continually renewed as long as the food holds out. When the supply begins to dwindle, trailmaking ceases. The trail pheromone evaporates quickly so other ants stop coming to the site and are not confused by old trails when food is found elsewhere. And at least in one species of ant, trails that no longer lead to food are also marked with a repellant pheromone.
A stick treated with the trail pheromone of an ant (left) can be used to make an artificial trail which is followed closely by other ants emerging from their nest (right). The trail will not be maintained by other ants unless food is placed at its end. (Photos courtesy of Sol Mednick and Scientific American).
Queen Pheromone: Honeybee queens spend their lives literally surrounded by a retinue of worker bees. The queen emits a pheromone that is a complex mixture of unsaturated hydrocarbons, fatty acids, and other organic molecules. Among its effects:
• inducing the workers to feed and groom her
• inhibiting the workers from building queen cells and rearing new queens
• inhibiting ovary development in the workers
Sex Attractants
Hundreds of pheromones are known with which one sex (usually the female) of an insect species attracts its mates. Many of these sex attractants or their close chemical relatives are available commercially. They have proved useful weapons against insect pests in two ways:
• Male Confusion: Distributing a sex attractant throughout an area masks the insect's own attractant and thus may prevent the sexes getting together. This "communication disruption" has been used successfully against a wide variety of important pests. For example, the sex attractant of the cotton boll weevil has reduced the need for conventional chemical insecticides by more than half in some cotton-growing areas.
• Insect Monitoring: Insect sex attractants are also valuable in monitoring pest populations. By baiting traps with the appropriate pheromone, a build-up of the pest population can be spotted early. Even if a conventional insecticide is the weapon chosen, its early use reduces
• the amount needed
• damage to the crop
• cost to the grower
• possible damage to the environment.
Early detection of pest build-up is a key ingredient in the system known as integrated pest management (IPM).
The photo (courtesy of USDA) shows the feathery antennae of a male gypsy moth. These detect the pheromone released by the females (who do not fly). In some insects, a single molecule of sex attractant is enough to elicit a response.
Sexual Deception
• By an animal: Many species of spiders prey exclusively on moths of certain species and only on the males. Studies of one species of spider, Mastophora cornigera, show that it releases a mixture of volatile compounds that mimic the sex pheromone of the moth species it preys upon. Male moths flying upwind in search of a female end up eaten instead!
• By a plant: A number of species of orchids - each pollinated by the males of a single species of insect (wasps or bees) — emit the same pheromone that is the sex attractant by which females of the insect species attract the males for mating.
Pheromones in Mammals
Releaser Pheromones: Many mammals (e.g., dogs and cats) deposit chemicals in and/or around their "territory". As these vaporize, they signal to other members of the species of the presence of the occupant of the territory. Domestic rabbit mothers release a mammary pheromone that triggers immediate nursing behavior by their babies (pups). A good thing, too, as mothers devote only 5–7 minutes a day to feeding their pups so they had better be quick about it.
Many animals, including mammals, signal with alarm pheromones. Although neither the source nor the chemical nature of alarm pheromones are known in any mammal, stressed animals release something that triggers quick behavior (e.g., flight or fight) in other members of their species. The pheromone is detected in a special cluster of cells located at the very tip of the nose and thus in a position to detected airborne molecules even before the vomeronasal organ (see below) or nasal epithelium can. The detectors on these cells are primary cilia.
Primer Pheromones: Rats and mice give off pheromones that elicit mating behavior. However, the response is not immediate as it is in the releaser pheromones of mother rabbits and insects. Instead, detection of the pheromone primes the endocrine system of the recipient to make the changes, e.g., ovulation, needed for successful mating. Primer pheromones are detected by the olfactory epithelium with which normal odors are detected and also in most mammals (but not humans) by the vomeronasal organ (VNO). The VNO is a patch of receptor tissue in the nasal cavity distinct from the olfactory epithelium. The receptors are G-protein-coupled transmembrane proteins similar to those that mediate olfaction, but encoded by entirely different genes. The neurons leading from the VNO take a separate path into and through the brain.
Human pheromones: It has long been noticed that women living close together (e.g., college roommates) develop synchronous menstrual cycles. This is thought to be because they release two (as yet uncharacterized) primer pheromones: (1) one prior to ovulation that tends to speed up the onset of ovulation in others and (2) one after ovulation that tends to delay the onset of ovulation in other women. Both pheromones are released from the armpits. The pheromones are not detected consciously as odors, but presumably trigger the hormonal changes that mediate the menstrual cycle. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.07%3A_Pheromones.txt |
All eukaryotes and some microbes (e.g., cyanobacteria) display changes in gene activity, biochemistry, physiology, and behavior that wax and wane through the cycle of days and nights.
Examples:
• The level of the hormone melatonin that rises in your body during the night and falls during the day.
• Fruit flies (Drosophila melanogaster) hatch in greatest numbers just at dawn.
Even when the organism is placed in constant conditions (e.g., continuous darkness), these rhythms persist. However, without environmental cues, they tend to be somewhat longer or somewhat shorter than 24 hours giving rise to the name circadian rhythms (L. circa = about; dies = day).
The genetics and molecular biology of circadian rhythms have been studied in several model organisms including
• some unicellular eukaryotes
• fungi
• plants (Arabidopsis)
• invertebrates (Drosophila)
• mammals (mice, rats, and humans)
What has emerged are some remarkable similarities in mechanisms across these various groups. Let us take a detailed look at the mechanism in Drosophila.
The Circadian Clock in Drosophila
A number of genes in Drosophila are turned on when the animal is exposed to light:
• effector genes whose products mediate the animal's responses (e.g. hatching or molting)
• clock genes whose products regulate the circadian clock. Two key members of this group are:
• period (per)
• timeless (tim)
Activation of all of these genes requires that their promoters are bound by the protein transcription factors
• CLOCK encoded by the gene clock (clk)
• CYCLE encoded by the gene cycle (cyc)
(The names of proteins will be designated with capitalized Roman letters; the genes that encode them indicated in lower case italics.)
The Mechanism
Fig.15.11.8.2 Mechanism
• The PER and TIM proteins (synthesized on ribosomes in the cytoplasm) form dimers.
• When the concentration of these gets high enough (early evening), they dissociate and are transported into the nucleus.
• Here PER
• binds to the CLK/CYC transcription factors, removing them from the promoters of the genes they activate; thus shutting off transcription. Because these genes include per and tim, the result is a negative feedback loop; that is, the product of the per gene inhibit its own synthesis (as well as that of tim). Just as the heat of a furnace turns through the thermostat - its own production off, so a rising level of PER/TIM dimers turns off further synthesis of them. As the level then falls, this inhibition is lifted and PER/TIM activity begins anew.
• turns on clock gene expression.
• The time required for the different effects results in the levels of PER/TIM and CLOCK oscillating in opposite phases with a circadian (~24 hr) rhythm (figure).
Setting the Clock
Even without any external cues (e.g., alternating light and dark), the cycles persist although they tend to drift away from environmental time.
• Under natural conditions, the clocks are precise.
• This is because they are "set" (synchronized) by environmental cues, of which light is one of the most important.
In Drosophila, it works like this.
• Light (blue) is absorbed by the protein cryptochrome (CRY).
• This causes an allosteric change in its conformation enabling it to bind to TIM and PER.
• This causes TIM and PER to break down (in proteasomes) ending their inhibition of gene transcription.
• If this happens when PER/TIM levels are rising (late in the "day"), it sets the clock back.
• If it happens when PER/TIM levels are declining (late in the "night"), it sets the clock ahead.
The Circadian Clock in Mammals
The circadian clock in mammals resembles that in Drosophila in a number of ways with many of the participating genes being homologous. However, there are some differences:
• The transcription factors that turn on the light-induced promoters are dimers of the CLOCK protein and a protein designated BMAL1. These dimers turn on
• three Per genes
• two Cry genes, the genes encoding cryptochrome
• hundreds of effector genes whose products control a wide variety of metabolic functions (e.g., cellular respiration, glycolysis, gluconeogenesis, lipid metabolism)
• The PER and CRY mRNAs are exported to the cytoplasm where they are translated.
• The PER and CRY proteins then enter the nucleus where they inhibit CLOCK-BMAL1 thus turning OFF transcription of Per and Cry, and are then degraded in proteasomes.
In due course these actions allow CLOCK and BMAL1 to once again stimulate transcription of Per and Cry. Thus this negative feedback loop causes the levels of BMAL1 and PER/CRY to oscillate in opposite phases (as CLOCK and PER/TIM do in Drosophila).
Many tissues in mammals, e.g., liver, skeletal muscle, and the beta cells of the pancreas have endogenous clocks. But all of these are under the control of a "master clock", the suprachiasmatic nucleus (SCN) - clusters of neurons in the hypothalamus.
The blood levels of many hormones have strong circadian rhythms. Examples:
• hormones synthesized in the hypothalamus, e.g. vasopressin
• whose secretion is controlled by the hypothalamus such as growth hormone and cortisol
• insulin
Setting the Clock
By light
Mice who are totally blind (lacking both rods and cones) have no trouble keeping their circadian clock on time.
They are able to do this because
• Some 1–2% of the ganglion cells in their retina - instead of depending on signals arriving from rods and/or cones detect light directly.
• These ganglion cells have an extensive network of dendrites that contain the pigment melanopsin.
• When exposed to light (diffuse light is fine), these ganglion cells become depolarized and send their signals back to the suprachiasmatic nucleus (SCN).
By food
In mice, the SCN clock, set by light/dark cycles, is the master clock as long as food is available all the time (the normal situation in the laboratory). However, if food is offered for only a 4-hour period when the mice would normally be asleep, they shift several circadian activities so that, for example, once a day they begin running about just before they expect food to be given to them. This rhythm continues even if the mice are kept in constant darkness.
The clock mechanism is the same as the light/dark-driven clock in the SCN, but the machinery that sets the clock by food is located in a different part of the hypothalamus, the dorsomedial hypothalamic nuclei (DMH).
Mice with both copies of the Bmal1 gene knocked out, are unable to establish circadian rhythms to either light or food. However, injections of an adeno-associated virus vector (AAV) containing the Bmal1 gene
• into the SCN restores the light clock but not the food-set clock
• into the DMH restores the food but not the light-set clock.
Sleep Disorders
Unlike mice, people who are totally blind cannot set the clock in their SCN. As a result, their circadian rhythm drifts out of phase with the actual cycle of day and night. These people often are bothered by feeling sleepy during the day and wide awake when they want to be asleep at night. A report in the 12 October 2000 issue of the New England Journal of Medicine tells of a group of blind people who were able to set their clocks with the help of a dose (10 mg) of melatonin at bedtime. However, this treatment worked only when the subject's circadian rhythm had drifted so that the normal rise in melatonin from the pineal gland was occurring in the early evening; that is, the dose of melatonin had to be given when it could boost the endogenous level of the hormone.
Some people suffer from a disorder called familial advanced sleep-phase syndrome (FASPS). As the name suggests, it is inherited ("familial") and their circadian clocks run fast ("advanced"). Those afflicted tend to wake up several (up to four) hours earlier than normal.
One cause of the disorder turns out to be a point mutation in the human PER2 gene. Exactly how this mutation causes shortening of the circadian cycle is still under investigation.
Photoperiodism
Many plants and animals not only engage in a cycle of daily activities (opening of flowers, waking, feeding, etc.) but also in seasonal activities.
• In plants, such things as production of flowers and making buds dormant in preparation for winter.
• In animals, such things as preparing to migrate and entering and leaving hibernation.
The most reliable clue to the change of season is length of day (temperature is far less reliable!). The farther a plant or animal lives north or south of the equator, the more pronounced the changing ratio of daytime hours to nighttime hours with the changing seasons.
It is not surprising then that both plants and animals mainly depend on photoperiod to prepare for changes in seasonal activities. And what better way to measure the relative length of day and night than by enlisting the machinery by which circadian rhythms are entrained?
As for animals, recent work with Drosophila suggests that this animal uses two circadian clocks to monitor the changing length of day and night.
• An "evening" clock that takes over in the long days of summer.
• A"morning" clock that is inhibited by light but takes over when the nights are getting longer;
The molecular machinery (Cry, Tim, Per, etc.) for each clock is confined to separate neurons in two different parts of the brain.
In these experiments, Drosophila is using the two clocks to adapt daily — not seasonal — cycles of activity to the changing seasons. But this machinery for measuring photoperiod could enable them to prepare for seasonal changes in activity, e.g., to stop forming eggs at the end of the summer. However, other studies examining such seasonal changes in Drosophila find that the photoperiodic response is independent of circadian responses. So we must await more experiments to resolve the question. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/15%3A_The_Anatomy_and_Physiology_of_Animals/15.11%3A_Behavior/15.11.08%3A_Circadian_Rhythms_in_Drosophila_and_Mammals.txt |
Meristematic
The main function of meristematic tissue is mitosis. The cells are small, thin-walled, with no central vacuole and no specialized features.
Meristematic tissue is located in
• the apical meristems at the growing points of roots and stems.
• the secondary meristems (lateral buds) at the nodes of stems (where branching occurs), and in some plants,
• meristematic tissue, called the cambium, that is found within mature stems and roots.
The cells produced in the meristems soon become differentiated into one or another of several types.
Protective
Protective tissue covers the surface of leaves and the living cells of roots and stems. Its cells are flattened with their top and bottom surfaces parallel. The upper and lower epidermis of the leaf are examples of protective tissue.
Parenchyma
The cells of parenchyma are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other and are usually stuffed with plastids. In areas not exposed to light, colorless plastids predominate and food storage is the main function. The cells of the white potato are parenchyma cells. Where light is present, e.g., in leaves, chloroplasts predominate and photosynthesis is the main function.
Sclerenchyma
The walls of these cells are very thick and built up in a uniform layer around the entire margin of the cell. Often, the cell dies after its cell wall is fully formed. Sclerenchyma cells are usually found associated with other cells types and give them mechanical support.
Sclerenchyma is found in stems and also in leaf veins. Sclerenchyma also makes up the hard outer covering of seeds and nuts.
Collenchyma
Collenchyma cells have thick walls that are especially thick at their corners. These cells provide mechanical support for the plant. They are most often found in areas that are growing rapidly and need to be strengthened. The petiole ("stalk") of leaves is usually reinforced with collenchyma.
Xylem
Xylem conducts water and dissolved minerals from the roots to all the other parts of the plant.
In angiosperms, most of the water travels in the xylem vessels. These are thick-walled tubes that can extend vertically through several feet of xylem tissue. Their diameter may be as large as 0.7 mm. Their walls are thickened with secondary deposits of cellulose and are usually further strengthened by impregnation with lignin. The secondary walls of the xylem vessels are deposited in spirals and rings and are usually perforated by pits. Xylem vessels arise from individual cylindrical cells oriented end to end. At maturity the end walls of these cells dissolve away, and the cytoplasmic contents die. The result is the xylem vessel, a continuous nonliving duct.
Xylem also contains tracheids. These are individual cells tapered at each end so the tapered end of one cell overlaps that of the adjacent cell. Like xylem vessels, they have thick, lignified walls and, at maturity, no cytoplasm. Their walls are perforated so that water can flow from one tracheid to the next. The xylem of ferns and conifers contains only tracheids.
In woody plants, the older xylem ceases to participate in water transport and simply serves to give strength to the trunk. Wood is xylem. When counting the annual rings of a tree, one is counting rings of xylem.
Phloem
The main components of phloem are sieve elements and companion cells.
Sieve elements are so named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells. The result is a sieve tube that conducts the products of photosynthesis - sugars and amino acids - from the place where they are manufactured (a "source"), e.g., leaves, to the places ("sinks") where they are consumed or stored; such as
• roots
• growing tips of stems and leaves
• flowers
• fruits, tubers, corms, etc.
Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions. Companion cells move sugars, amino acids and a variety of macromolecules into and out of the sieve elements. In "source" tissue, such as a leaf, the companion cells use transmembrane proteins to take up - by active transport - sugars and other organic molecules from the cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve elements through plasmodesmata. The pressure created by osmosis drives the flow of materials through the sieve tubes.
In "sink" tissue, the sugars and other organic molecules leave the sieve elements through plasmodesmata connecting the sieve elements to their companion cells and then pass on to the cells of their destination. Again, water follows by osmosis where it may leave the plant by transpiration or increase the volume of the cells or move into the xylem for recycling through the plant. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.01%3A_Plant_Anatomy/16.1.01%3A_Plant_Tissues.txt |
The Root Tip
The root tip consists of a
• root cap — a sheath of cells that
• protect the meristem from abrasion and damage as the root tip grows through the soil;
• secrete the growth hormone auxin;
• detect water and nutrients in the soil;
• detect gravity and respond with gravitropism.
• meristem - a region of rapid mitosis, which produces the new cells for root growth.
Because of the frequency of mitosis in the meristem, root tips are often used to demonstrate mitosis in the laboratory The inset is a photo (courtesy of Carolina Biological Supply Co.) of anaphase in the meristem of an onion root tip.
The Region of Elongation
Here the cells produced by mitosis undergo a period of elongation in the direction of the axis of the root.
The Region of Differentiation
Here develop the differentiated tissues of the root.
• Epidermis - A single layer of flattened cells at the surface. When first formed, epidermal cells have extensions — the root hairs — which greatly increase the surface area available for the uptake of nutrients from the soil. The photo below shows the root hairs in the region of differentiation of a germinating radish seed.
• Cortex - A band of parenchyma cells that develops beneath the epidermis. It stores food. Its inner surface is bounded by a single layer of cells, the
• Endodermis
• Stele
• Pericycle - the outer boundary of the stele. Secondary roots branch from it.
• Xylem - arranged in bundles in a spokelike fashion
• Phloem - alternates with xylem
• Cambium - In older parts of the root, another meristem forms between the xylem and phloem. Mitosis in the cambium produces new "secondary xylem" to the inside and secondary phloem to the outside.
Water Uptake
Water enters the root through the epidermis. Once within the epidermis, water passes through the cortex, mainly traveling between the cells. However, in order to enter the stele, it must pass through the cytoplasm of the cells of the endodermis.
Once within the stele, water is free again to move between cells as well as through them. In young roots, water enters directly into the xylem. In older roots, it may have to pass first through a band of phloem and cambium. It does so by traveling through horizontally-elongated cells, the xylem rays.
Mineral Uptake
One might have expected that minerals would enter the root dissolved in water. But, in fact, minerals enter separately:
• Even when no water is being absorbed, minerals enter freely - mostly through the root hairs.
• Minerals can enter against their concentration gradient; that is, by active transport. For example, plants can take up K+ from the soil against a ten-thousand-fold concentration gradient; e.g., from as little as 10 µM in the soil to 100 mM in the cell.
• Anything that interferes with the metabolism of root hairs interferes with mineral absorption.
• The root hairs are also the point of entry of mycorrhizal fungi. These transport minerals - especially phosphorus - to the root hair in exchange for carbohydrates from the plant.
• In legumes, the root hairs are the point of entry of rhizobia that will establish the mutualistic partnership enabling the plant to convert atmospheric nitrogen into protein.
Plants absorb their nutrients in inorganic form
For examples:
• nitrogen enters as nitrate (NO3) or ammonium ions (NH4+)
• phosphorus as PO43−
• potassium as K+
• calcium as Ca2+
When you hear of the virtues of organic fertilizers, remember that such materials meet no nutritional need of the plant until their constituents have been degraded to inorganic forms. Organic matter does play an important role in making good soil texture, but only to the extent that it can yield inorganic ions can it meet the nutritional needs of the plant.
Once within the epidermis, inorganic ions pass inward from cell to cell, probably through plasmodesmata. The final step from the cytoplasm of the pericycle cells to the xylem is probably accomplished once again by active transport.
16.1.03: Stems
javascript:void('Remove Anchor')
The organization of the tissues of the stem differs between dicots and monocots.
The Woody Dicot Stem
The drawing shows a sector of a cross section through a 5-year old twig from a basswood tree (Tilia). The stem has three areas:
• bark
• wood
• pith
Bark
• Cork - The outer part of the bark is protected by layers of dead cork cells impregnated with suberin. Suberin is waxy and cuts down water loss from the stem. But suberin is as impervious to air as it is to water. The gas exchange needs of the living cells beneath the cork are met by openings in the cork called lenticels.
• Cortex - Layers of parenchyma cells. These store food (as they do in the root). In the very young stem (before cork has formed), they may have chloroplasts and carry on photosynthesis.
• Cork cambium - In older stems, a meristem forms between the cork and cortex. Mitosis of its cells produces more cork.
• Expanded pith rays - Regions of parenchyma that store food.
• Phloem - Bundles of sieve tubes surrounded and supported by sclerenchyma. Translocation of food through the stem takes place in the sieve tubes.
Cambium
During the growing season, mitosis in this band of meristematic tissue produces new phloem to the outside and new xylem to the inside.
Xylem
Xylem makes up the wood region. The xylem vessels made in the spring, when water is plentiful, have larger diameters than those made later in the season. No xylem is made during the dormant season. The visual contrast between the late summer xylem of one season and the spring xylem of the next creates the annual ring. Xylem serves two functions:
• transport of water and minerals up the stem
• support
The photograph (courtesy of Turtox) shows the organization of tissues in the corn (maize) stem, a typical monocot. The corn stem consists of:
• an external rind
• an interior filled with pith.
Vascular bundles are scattered through the pith.
Each vascular bundle contains:
• a layer of sclerenchyma that provides support
• a bundle of phloem containing
• sieve tubes used for food transport
• their companion cells.
• four xylem vessels
• a group of xylem tracheids
• both carry water and dissolved minerals up the stem | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.01%3A_Plant_Anatomy/16.1.02%3A_Roots.txt |
Layers in a leaf
Upper epidermis
This is a single layer of cells containing few or no chloroplasts. The cells are quite transparent and permit most of the light that strikes them to pass through to the underlying cells. The upper surface is covered with a waxy, waterproof cuticle, which serves to reduce water loss from the leaf.
Palisade layer
This consists of one or more layers of cylindrical cells oriented with their long axis perpendicular to the plane of the leaf. The cells are filled with chloroplasts (usually several dozen of them) and carry on most of the photosynthesis in the leaf.
Spongy layer
Lying beneath the palisade layer, its cells are irregular in shape and loosely packed. Although they contain a few chloroplasts, their main function seems to be the temporary storage of sugars and amino acids synthesized in the palisade layer. They also aid in the exchange of gases between the leaf and the environment. During the day, these cells give off oxygen and water vapor to the air spaces that surround them. They also pick up carbon dioxide from the air spaces. The air spaces are interconnected and eventually open to the outside through pores called stomata (sing., stoma).
Collectively, the palisade and spongy layers make up the mesophyll.
Lower epidermis
Typically, most of the stomata (thousands per square centimeter) are located in the lower epidermis. Although most of the cells of the lower epidermis resemble those of the upper epidermis, each stoma is flanked by two sausage-shaped cells called guard cells. These differ from the other cells of the lower epidermis not only in their shape but also in having chloroplasts. The guard cells regulate the opening and closing of the stomata. Thus they control the exchange of gases between the leaf and the surrounding atmosphere.
Leaf Veins
Not only must the cells of the mesophyll be close to their air supply to secure CO2 and release O2 and the reverse in the dark but they must be close to a leaf vein with its
• xylem to supply water and minerals
• phloem to remove synthesized food
The photo shows the network of leaf veins in a maple leaf. Probably no cell in the spongy layer is more than two cells away from a vein.
The xylem and phloem of veins is often surrounded by layers of sclerenchyma cells. These impart strength to the vein providing a stiff framework to support the soft tissues of the leaf blade.
16.1.05: Arabidopsis Thaliana
This little plant has become to plant biology what Drosophila melanogaster and Caenorhabditis elegans are to animal biology.
Arabidopsis is an angiosperm, a dicot from the mustard family (Brassicaceae). It is popularly known as thale cress or mouse-ear cress. While it has no commercial value - in fact is considered a weed - it has proved to be an ideal organism for studying plant development.
Some of its advantages as a model organism:
• It has one of the smallest genomes in the plant kingdom: 135 x 106 base pairs of DNA distributed in 5 chromosomes (2n = 10) and almost all of which encodes its 27,407 genes.
• Transgenic plants can be made easily using Agrobacterium tumefaciens as the vector to introduce foreign genes.
• The plant is small - a flat rosette of leaves from which grows a flower stalk 6–12 inches high.
• It can be easily grown in the lab in a relatively small space.
• Development is rapid. It only takes 5– 6 weeks from seed germination to the production of a new crop of seeds.
• It is a prolific producer of seeds (up to 10,000 per plant) making genetics studies easier.
• Mutations can be easily generated (e.g., by irradiating the seeds or treating them with mutagenic chemicals).
• It is normally self-pollinated so recessive mutations quickly become homozygous and thus expressed.
Other members of its family cannot self-pollinate. They have an active system of self-incompatibility. Arabidopsis, however, has inactivating mutations in the genes - SRK and SCR - that prevent self-pollination in other members of the family.
• However, Arabidopsis can easily be cross-pollinated to
• do genetic mapping
• produce strains with multiple mutations.
Many of the findings about how plants work described throughout these pages were learned from studies with Arabidopsis. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.01%3A_Plant_Anatomy/16.1.04%3A_The_Leaf.txt |
Most plants secure the water and minerals they need from their roots. The path taken is:
$\text{soil} \rightarrow \text{roots} \rightarrow \text{stems} \rightarrow \text{leaves}$
The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells), but less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration, which serve two useful functions- it provides the force for lifting the water up the stems and it cools the leaves. Water and minerals enter the root by separate paths which eventually converge in the stele.
The Pathway of Water and Minerals
Soil water enters the root through its epidermis. It appears that water then travels in both the cytoplasm of root cells - called the symplast (i.e., it crosses the plasma membrane and then passes from cell to cell through plasmodesmata) and in the nonliving parts of the root - called the apoplast (i.e., in the spaces between the cells and in the cells walls themselves. This water has not crossed a plasma membrane. However, the inner boundary of the cortex, the endodermis, is impervious to water because of a band of lignified matrix called the casparian strip. Therefore, to enter the stele, apoplastic water must enter the symplasm of the endodermal cells. From here it can pass by plasmodesmata into the cells of the stele. Once inside the stele, water is again free to move between cells as well as through them. In young roots, water enters directly into the xylem vessels and/or tracheids. These are nonliving conduits so are part of the apoplast.
Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids. At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.
Minerals enter the root by active transport into the symplast of epidermal cells and move toward and into the stele through the plasmodesmata connecting the cells. They enter the water in the xylem from the cells of the pericycle (as well as of parenchyma cells surrounding the xylem) through specialized transmembrane channels.
What Forces Water Through the Xylem?
Observations
• The mechanism is based on purely physical forces because the xylem vessels and tracheids are lifeless.
• Roots are not needed. This was demonstrated over a century ago by a German botanist who sawed down a 70-ft (21 meters) oak tree and placed the base of the trunk in a barrel of picric acid solution. The solution was drawn up the trunk, killing nearby tissues as it went.
• However, leaves are needed. When the acid reached the leaves and killed them, the upward movement of water ceased.
• Removing a band of bark from around the trunk - a process called girdling - does not interrupt the upward flow of water. Girdling removes only the phloem, not the xylem, and so the foliage does not wilt. (In due course, however, the roots - and thus the entire plant - will die because the roots cannot receive any of the food manufactured by the leaves.)
Transpiration-Pull
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above. As we have seen, water is continually being lost from leaves by transpiration. Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf. But even the best vacuum pump can pull water up to a height of only 34 ft (10.4 m) or so. This is because a column of water that high exerts a pressure of ~15 lb/in2 (103 kilopascals, kPa) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia (the tallest is 370 feet [113 meters] high)? Taking all factors into account, a pull of at least 270 lb/in2 (~1.9 x 103 kPa) is probably needed.
The answer to the dilemma lies the cohesion of water molecules; that is the property of water molecules to cling to each through the hydrogen bonds they form. When ultrapure water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. It has been reported that tensions as great as 3000 lb/in2 (21 x 103 kPa) are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires. Because of the critical role of cohesion, the transpiration-pull theory is also called the cohesion theory.
support for Cohesion theory
• If sap in the xylem is under tension, we would expect the column to snap apart if air is introduced into the xylem vessel by puncturing it. This is the case.
• If the water in all the xylem ducts is under tension, there should be a resulting inward pull (because of adhesion) on the walls of the ducts. This inward pull in the band of sapwood in an actively transpiring tree should, in turn, cause a decrease in the diameter of the trunk.
• The graph shows the results of obtained by D. T. MacDougall when he made continuous measurements of the diameter of a Monterey pine. The diameter fluctuated on a daily basis reaching its minimum when the rate of transpiration reached its maximum (around noon)
• The rattan vine may climb as high as 150 ft (45.7 m) on the trees of the tropical rain forest in northeastern Australia to get its foliage into the sun. When the base of a vine is severed while immersed in a basin of water, water continues to be taken up. A vine less than 1 inch (2.5 cm) in diameter will "drink" water indefinitely at a rate of up to 12 ml/minute.
If forced to take water from a sealed container, the vine does so without any decrease in rate, even though the resulting vacuum becomes so great that the remaining water begins to boil spontaneously. (The boiling temperature of water decreases as the air pressure over the water decreases, which is why it takes longer to boil an egg in Denver than in New Orleans.)
• Transpiration-pull enables some trees and shrubs to live in seawater. Seawater is markedly hypertonic to the cytoplasm in the roots of the red mangrove (Rhizophora mangle), and we might expect water to leave the cells resulting in a loss in turgor and wilting. In fact, the remarkably high tensions on the order of 500–800 lb/in2 (~3 to 5 thousand kPa) in the xylem can pull water into the plant against this osmotic gradient. So mangroves literally desalt seawater to meet their needs.
Problems with the theory
When water is placed under a high vacuum, any dissolved gases come out of solution as bubbles (as we saw above with the rattan vine) - this is called cavitation. Any impurities in the water enhance the process. So measurements showing the high tensile strength of water in capillaries require water of high purity - not the case for sap in the xylem. So might cavitation break the column of water in the xylem and thus interrupt its flow? Probably not so long as the tension does not greatly exceed 270 lb/in2 (~1.9 x 103 kPa).
By spinning branches in a centrifuge, it has been shown that water in the xylem avoids cavitation at negative pressures exceeding 225 lb/in2 (~1.6 x 103 kPa). And the fact that sequoias can successfully lift water 358 ft (109 m) - which would require a tension of 270 lb/in2 (~1.9 x 103 kPa) - indicates that cavitation is avoided even at that value. However, such heights may be approaching the limit for xylem transport. Measurements close to the top of the tallest living sequoia (370 ft [=113 m] high) show that the high tensions needed to get water up there have resulted in smaller stomatal openings, causing lower concentrations of CO2 in the needles, causing reduced photosynthesis, causing reduced growth (smaller cells and much smaller needles). (Reported by Koch, G. W. et al., in Nature, 22 April 2004.) So the limits on water transport limit the ultimate height which trees can reach. The tallest tree ever measured, a Douglas fir, was 413 ft. (125.9 meters) high.
Root Pressure
When a tomato plant is carefully severed close to the base of the stem, sap oozes from the stump. The fluid comes out under pressure which is called root pressure. Root pressure is created by the osmotic pressure of xylem sap which is, in turn, created by dissolved minerals and sugars that have been actively transported into the apoplast of the stele.
One important example is the sugar maple when, in very early spring, it hydrolyzes the starches stored in its roots into sugar. This causes water to pass by osmosis through the endodermis and into the xylem ducts. The continuous inflow forces the sap up the ducts.
Although root pressure plays a role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport.
• Few plants develop root pressures greater than 30 lb/in2 (207 kPa), and some develop no root pressure at all.
• The volume of fluid transported by root pressure is not enough to account for the measured movement of water in the xylem of most trees and vines.
• Those plants with a reasonably good flow of sap are apt to have the lowest root pressures and vice versa.
• The highest root pressures occur in the spring when the sap is strongly hypertonic to soil water, but the rate of transpiration is low. In summer, when transpiration is high and water is moving rapidly through the xylem, often no root pressure can be detected.
So although root pressure may play a significant role in water transport in certain species (e.g., the coconut palm) or at certain times, most plants meet their needs by transpiration-pull. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.02%3A_Plant_Physiology/16.2A%3A_Xylem.txt |
Food and other organic substances (e.g., some plant hormones and even messenger RNAs) manufactured in the cells of the plant are transported in the phloem. Sugars (usually sucrose), amino acids and other organic molecules enter the sieve elements through plasmodesmata connecting them to adjacent companion cells. Once within the sieve elements, these molecules can be transported either up or down to any region of the plant moving at rates as high as 110 μm per second.
Two demonstrations:
• Girdling. Girdling is removing a band of bark from the circumference of the tree. Girdling removes the phloem, but not the xylem. If a tree is girdled in summer, it continues to live for a time. There is, however, no increase in the weight of the roots, and the bark just above the girdled region accumulates carbohydrates. Unless a special graft is made to bridge the gap, the tree eventually dies as its roots starve.
• The pictures below are autoradiographs showing that the products of photosynthesis are transported in the phloem.
A cucumber leaf was supplied with radioactive water (3HOH) and allowed to carry on photosynthesis for 30 minutes. Then slices were cut from the petiole of the leaf and covered with a photographic emulsion. Radioactive products of photosynthesis darkened the emulsion where it was in contact with the phloem (upper left in both photos), but not where it was in contact with the xylem vessels (center). In the photomicrograph on the left, the microscope is focused on the tissue in order to show the cells clearly; on the right, the microscope has been focused on the photographic emulsion.
Some fruits, such as the pumpkin, receive over 0.5 gram of food each day through the phloem. Because the fluid is fairly dilute, this requires a substantial flow. In fact, the use of radioactive tracers shows that substances can travel through as much as 100 cm of phloem in an hour.
Mechanism that drives translocation of food through the phloem
Translocation through the phloem is dependent on metabolic activity of the phloem cells (in contrast to transport in the xylem).
• Chilling its petiole slows the rate at which food is translocated out of the leaf (above).
• Oxygen lack also depresses it.
• Killing the phloem cells puts an end to it.
The Pressure-Flow Hypothesis
The best-supported theory to explain the movement of food through the phloem is called the pressure-flow hypothesis.
• It proposes that water containing food molecules flows under pressure through the phloem.
• The pressure is created by the difference in water concentration of the solution in the phloem and the relatively pure water in the nearby xylem ducts.
• At their "source" - the leaves - sugars are pumped by active transport into the companion cells and sieve elements of the phloem.
• As sugars (and other products of photosynthesis) accumulate in the phloem, water enters by osmosis.
In the figure, sugar molecules are represented in black, water molecules in red.)
• Turgor pressure builds up in the sieve elements (similar to the creation of root pressure).
• As the fluid is pushed down (and up) the phloem, sugars are removed by the cortex cells of both stem and root (the "sinks") and consumed or converted into starch.
• Starch is insoluble and exerts no osmotic effect.
• Therefore, the osmotic pressure of the contents of the phloem decreases.
• Finally, relatively pure water is left in the phloem, and this leaves by osmosis and/or is drawn back into nearby xylem vessels by the suction of transpiration-pull.
Thus it is the pressure gradient between "source" (leaves) and "sink" (shoot and roots) that drives the contents of the phloem up and down through the sieve elements.
Tests of the theory
1. The contents of the sieve elements must be under pressure.
This is difficult to measure because when a sieve element is punctured with a measuring probe, the holes in its end walls quickly plug up. However, aphids can insert their mouth parts without triggering this response.
Left: when it punctures a sieve element, sap enters the insect's mouth parts under pressure and some soon emerges at the other end (as a drop of honeydew that serves as food for ants and bees).
Right: honeydew will continue to exude from the mouthparts after the aphid has been cut away from them.
2. The osmotic pressure of the fluid in the phloem of the leaves must be greater than that in the phloem of the food-receiving organs such as the roots and fruits. Most measurements have shown this to be true.
Transport of Messenger RNA (mRNA) through the Phloem
Plant scientists at the Davis campus of the University of California (reported in the 13 July 2001 issue of Science) have demonstrated that messenger RNAs can also be transported long distances in the phloem. They grafted normal tomato scions onto mutant tomato stocks and found that mRNAs synthesized in the stock were transported into the scions. These mRNAs converted the phenotype of the scion into that of the stock.
16.2C: Transpiration
Transpiration is the evaporation of water from plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis. But air that is not fully saturated with water vapor (100% relative humidity) will dry the surfaces of cells with which it comes in contact. So the photosynthesizing leaf loses substantial amount of water by evaporation. This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem of the roots and stem.
Transpiration is not simply a hazard of plant life. It is the "engine" that pulls water up from the roots to:
• supply photosynthesis (1%-2% of the total)
• bring minerals from the roots for biosynthesis within the leaf
• cool the leaf
Using a potometer (above), one can study the effect of various environmental factors on the rate of transpiration. As water is transpired or otherwise used by the plant, it is replaced from the reservoir on the right. This pushes the air bubble to the left providing a precise measure of the volume of water used.
Environmental factors that affect the rate of transpiration
1. Light
Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.
3. Humidity
The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases.When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.
The volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn (maize) plants may transpire 400,000 gallons (1.5 million liters) of water. As liquid water, this would cover the field with a lake 15 inches (38 cm) deep. An acre of forest probably does even better. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.02%3A_Plant_Physiology/16.2B%3A_Phloem.txt |
In order to carry on photosynthesis, green plants need a supply of carbon dioxide and a means of disposing of oxygen. In order to carry on cellular respiration, plant cells need oxygen and a means of disposing of carbon dioxide (just as animal cells do). Unlike animals, plants have no specialized organs for gas exchange (with the few inevitable exceptions!). The are several reasons they can get along without them:
• Each part of the plant takes care of its own gas exchange needs. Although plants have an elaborate liquid transport system, it does not participate in gas transport.
• Roots, stems, and leaves respire at rates much lower than are characteristic of animals. Only during photosynthesis are large volumes of gases exchanged, and each leaf is well adapted to take care of its own needs.
• The distance that gases must diffuse in even a large plant is not great. Each living cell in the plant is located close to the surface. While obvious for leaves, it is also true for stems. The only living cells in the stem are organized in thin layers just beneath the bark. The cells in the interior are dead and serve only to provide mechanical support.
• Most of the living cells in a plant have at least part of their surface exposed to air. The loose packing of parenchyma cells in leaves, stems, and roots provides an interconnecting system of air spaces. Gases diffuse through air several thousand times faster than through water. Once oxygen and carbon dioxide reach the network of intercellular air spaces (arrows), they diffuse rapidly through them.
• Oxygen and carbon dioxide also pass through the cell wall and plasma membrane of the cell by diffusion. The diffusion of carbon dioxide may be aided by aquaporin channels inserted in the plasma membrane.
Leaves
The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma).
Normally stomata open when the light strikes the leaf in the morning and close during the night. The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes.
Time Osmotic Pressure lb/in2
7 A.M. 212
11A.M. 456
5 P.M. 272
12 Midnight 191
The table shows the osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at 150 lb/in2 (~1000 kilopascal, kPa). When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.
Opening stomata
The increase in osmotic pressure in the guard cells is caused by an uptake of potassium ions (K+). The concentration of K+ in open guard cells far exceeds that in the surrounding cells. This is how it accumulates:
• Blue light is absorbed by phototropin which activates a proton pump (an H+-ATPase) in the plasma membrane of the guard cell.
• ATP, generated by the light reactions of photosynthesis, drives the pump.
• As protons (H+) are pumped out of the cell, its interior becomes increasingly negative.
• This attracts additional potassium ions into the cell, raising its osmotic pressure.
Closing stomata
Although open stomata are essential for photosynthesis, they also expose the plant to the risk of losing water through transpiration. Some 90% of the water taken up by a plant is lost in transpiration. In angiosperms and gymnosperms (but not in ferns and lycopsids), Abscisic acid (ABA) is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration (which often occurs around mid-day).
The mechanism:
• ABA binds to receptors at the surface of the plasma membrane of the guard cells.
• The receptors activate several interconnecting pathways which converge to produce
• a rise in pH in the cytosol
• transfer of Ca2+ from the vacuole to the cytosol
• These changes stimulate the loss of negatively-charged ions (anions), especially NO3 and Cl, from the cell and also the loss of K+ from the cell.
• The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor.
• The stomata close.
Open stomata also provide an opening through which bacteria can invade the interior of the leaf. However, guard cells have receptors that can detect the presence of molecules associated with bacteria called pathogen-associated molecular patterns (PAMPs). LPS and flagellin are examples. When the guard cells detect these PAMPs, ABA mediates closure of the stoma and thus close the door to bacterial entry.
This system of innate immunity resembles that found in animals.
Density of stomata
The density of stomata produced on growing leaves varies with such factors as the temperature, humidity, and light intensity around the plant. It also depends on the the concentration of carbon dioxide in the air around the leaves. The relationship is inverse; that is, as the concentration of CO2 goes up, the number of stomata produced goes down, and vice versa. Some evidence:
• Plants grown in an artificial atmosphere with a high level of CO2 have fewer stomata than normal.
• Herbarium specimens reveal that the number of stomata in a given species has been declining over the last 200 years — the time of the industrial revolution and rising levels of CO2 in the atmosphere.
These data can be quantified by determining the stomatal index: the ratio of the number of stomata in a given area divided by the total number of stomata and other epidermal cells in that same area.
Q&A
How does the plant determine how many stomata to produce?
It turns out that the mature leaves on the plant detect the conditions around them and send a signal (its nature still unknown - but see below*) that adjusts the number of stomata that will form on the developing leaves.
Two experiments (reported by Lake et al., in Nature, 411:154, 10 May 2001):
• When the mature leaves of the plant (Arabidopsis) are encased in glass tubes filled with high levels (720 ppm) of CO2, the developing leaves have fewer stomata than normal even though they are growing in normal air (360 ppm).
• Conversely, when the mature leaves are given normal air (360 ppm CO2) while the shoot is exposed to high CO2 (720 ppm), the new leaves develop with the normal stomatal index.
*One signal that increases stomatal density in 2-day-old Arabidopsis seedlings (a different experimental setup than the one above) is a 45-amino acid peptide called stomagen that is released by mesophyll cells and induces the formation of stomata in the epidermis above.
Stomata reveal past carbon dioxide levels
Because CO2 levels and stomatal index are inversely related, could fossil leaves tell us about past levels of CO2 in the atmosphere? Yes. As reported by Gregory Retallack (in Nature, 411:287, 17 May 2001), his study of the fossil leaves of the ginkgo and its relatives shows:
• their stomatal indices were high late in the Permian period (275–290 million years ago) and again in the Pleistocene epoch (1–8 million years ago). Both these periods are known from geological evidence to have been times of low levels of atmospheric carbon dioxide and ice ages (with glaciers).
• Conversely, stomatal indices were low during the Cretaceous period, a time of high CO2 levels and warm climate.
These studies also lend support to the importance of carbon dioxide as a greenhouse gas playing an important role in global warming.
Roots and Stems
Woody stems and mature roots are sheathed in layers of dead cork cells impregnated with suberin — a waxy, waterproof (and airproof) substance. So cork is as impervious to oxygen and carbon dioxide as it is to water. However, the cork of both mature roots and woody stems is perforated by nonsuberized pores called lenticels. These enable oxygen to reach the intercellular spaces of the interior tissues and carbon dioxide to be released to the atmosphere.
In many annual plants, the stems are green and almost as important for photosynthesis as the leaves. These stems use stomata rather than lenticels for gas exchange. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.02%3A_Plant_Physiology/16.2D%3A_Gas_Exchange_in_Plants.txt |
All plants carry on photosynthesis by adding carbon dioxide (CO2) to a phosphorylated 5-carbon sugar called ribulose bisphosphate. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RUBISCO). The resulting 6-carbon compound breaks down into two molecules of 3-phosphoglyceric acid (PGA). These 3-carbon molecules serve as the starting material for the synthesis of glucose and other food molecules. The process is called the Calvin cycle and the pathway is called the C3 pathway.
Photorespiration
As its name suggests, RUBISCO catalyzes two different reactions:
• adding CO2 to ribulose bisphosphate — the carboxylase activity
• adding O2 to ribulose bisphosphate — the oxygenase activity
Which one predominates depends on the relative concentrations of O2 and CO2 with
• high CO2, low O2 favoring the carboxylase action
• high O2, low CO2 favoring the oxygenase action
The light reactions of photosynthesis liberate oxygen and more oxygen dissolves in the cytosol of the cell at higher temperatures. Therefore, high light intensities and high temperatures (above ~ 30°C) favor the second reaction.
The uptake of O2 by RUBISCO forms the 3-carbon molecule 3-phosphoglyceric acid, just as in the Calvin cycle, and the 2-carbon molecule glycolate. The glycolate enters peroxisomes where it uses O2 to form intermediates that enter mitochondria where they are broken down to CO2. So this process uses O2 and liberates CO2 as cellular respiration does which is why it is called photorespiration. It undoes the good anabolic work of photosynthesis, reducing the net productivity of the plant. For this reason, much effort so far largely unsuccessful has gone into attempts to alter crop plants so that they carry on less photorespiration. The problem may solve itself. If atmospheric CO2 concentrations continue to rise, perhaps this will enhance the net productivity of the world's crops by reducing losses to photorespiration.
C4 Plants
Over 8,000 species of angiosperms have developed adaptations which minimize the losses to photorespiration. They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C4 plants. (Plants that have only the Calvin cycle are thus C3 plants). Some C4 plants - called CAM plants - separate their C3 and C4 cycles by time, while other C4 plants have structural changes in their leaf anatomy so that their C4 and C3 pathways are separated in different parts of the leaf with RUBISCO sequestered where the CO2 level is high; the O2 level low.
After entering through stomata, CO2 diffuses into a mesophyll cell. Being close to the leaf surface, these cells are exposed to high levels of O2, but they have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle).
Instead the CO2 is inserted into a 3-carbon compound (C3) called phosphoenolpyruvic acid (PEP) forming the 4-carbon compound oxaloacetic acid (C4). Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells are deep in the leaf so atmospheric oxygen cannot diffuse easily to them and often have thylakoids with reduced photosystem II complexes (the one that produces O2). Both of these features keep oxygen levels low in Bundle sheath cells, which is where the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle to form sugars and starch, and pyruvic acid (C3), which is transported back to a mesophyll cell where it is converted back into PEP.
These C4 plants are well adapted to (and likely to be found in) habitats with high daytime temperatures and intense sunlight. Some examples crabgrass, corn (maize), sugarcane, and sorghum. Although only ~3% of the angiosperms, C4 plants are responsible for ~25% of all the photosynthesis on land.
4 cells in C3 plants
The ability to use the C4 pathway has evolved repeatedly in different families of angiosperms - a remarkable example of convergent evolution. Perhaps the potential is in all angiosperms.
A report in the 24 January 2002 issue of Nature (by Julian M. Hibbard and W. Paul Quick) describes the discovery that tobacco, a C3 plant, has cells capable of fixing carbon dioxide by the C4 path. These cells are clustered around the veins (containing xylem and phloem) of the stems and also in the petioles of the leaves. In this location, they are far removed from the stomata that could provide atmospheric CO2. Instead, they get their CO2 and/or the 4-carbon malic acid in the sap that has been brought up in the xylem from the roots.
If this turns out to be true of many C3 plants, it would explain why it has been so easy for C4 plants to evolve from C3 ancestors.
CAM Plants
CAM plants are also C4 plants (CAM stands for crassulacean acid metabolism because it was first studied in members of the plant family Crassulaceae.). However, instead of segregating the C4 and C3 pathways in different parts of the leaf, CAM plants separate them in time instead (Table 1).
Table 1
Night Morning
• CAM plants take in CO2 through their open stomata (they tend to have reduced numbers of them).
• The CO2 joins with PEP to form the 4-carbon oxaloacetic acid.
• This is converted to 4-carbon malic acid that accumulates during the night in the central vacuole of the cells.
• The stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen).
• The accumulated malic acid leaves the vacuole and is broken down to release CO2.
• The CO2 is taken up into the Calvin (C3) cycle.
These adaptations also enable their owners to thrive in conditions of high daytime temperatures, intense sunlight, and low soil moisture. Some examples of CAM plants include cacti, Bryophyllum, the pineapple and all epiphytic bromeliads, sedums, and the "ice plant" that grows in sandy parts of the scrub forest biome.
C4 Diatoms
On 26 October 2000, Nature reported the discovery of both the C3 and C4 pathways in a marine diatom. In this unicellular organism, the two paths are kept separate by having the C4 path in the cytosol, and the C3 path confined to the chloroplast. The presence of a C4 pathway probably reflects the frequent low concentrations of CO2 in ocean waters.
16.2F: Tropisms
A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant. There are two forms: Positive = the plant, or a part of it, grows in the direction from which the stimulus originates. and Negative = growth away from the stimulus.
A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant.
• Positive = the plant, or a part of it, grows in the direction from which the stimulus originates.
• Negative = growth away from the stimulus.
Plants respond to:
• Light = phototropism
• Stems are positively phototropic.
• Roots are negatively phototropic.
• Gravity = gravitropism
• Stems are negatively gravitropic while
• roots are positively gravitropic.
The adaptive value of these tropisms is clear.
• Roots growing down and/or away from light are more likely to find the soil, water, and minerals they need.
• Stems growing up and toward the light will be able to expose their leaves so that photosynthesis can occur.
The Mechanism of Phototropism
The Darwin Experiments
If they placed an opaque cover over the tip, phototropism failed to occur even though the rest of the coleoptile was illuminated from one side. However, when they buried the plant in fine black sand so that only its tip was exposed, there was no interference with the tropism — the buried coleoptile bent in the direction of the light.
From these experiments, it seemed clear that
• the stimulus (light) was detected at one location (the tip)
• the response (bending) was carried out at another (the region of elongation)
The Danish plant physiologist Boysen-Jensen showed (in 1913) that the signal was a chemical passing down from the tip of the coleoptile.
He
• cut off the tip of the coleoptile
• covered the stump with a layer of gelatin
• replaced the tip.
The graph (based on the work of K. V. Thimann) shows the effect of auxin concentration on root and stem growth. The difference between the behavior of roots and stems lies in the difference in the sensitivity of their cells to auxin. Auxin concentrations high enough to stimulate stem growth inhibit root growth.
Possible Mechanism of Gravitropism in Roots
When a root is placed on its side,
• Statoliths (organelles containing starch grains) settle by gravity to the bottom of cells in the root tip.
• This causes PIN proteins to redistribute to the underside of the cell where they pump auxin out of the cell.
• The auxin then accumulates along the under side of the root.
• This INHIBITS root cell elongation (see graph).
• So the cells at the top surface of the root elongate, causing the root to grow down. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.02%3A_Plant_Physiology/16.2E%3A_Photorespiration_and_C4_Plants.txt |
Mosses and liverworts are traditionally classified together in the Division Bryophyta on the basis of their sharing a similar life cycle (alternation of generations), similar reproductive organs (antheridia and archegonia), and a lack of vascular tissue (xylem and phloem).
Some 23,000 species of living mosses and liverworts have been identified. These are small, fairly simple, plants usually found in moist locations. Liverworts have a thin, leathery body that grows flat on moist soil or, in some cases, the surface of still water. The above photo is of a common liverwort, Ricciocarpus natans. Mosses have an erect shoot bearing tiny leaflike structures arranged in spirals. Neither mosses nor liverworts have any woody tissue so they never grow very large. They have neither xylem nor phloem for the transport of water and food through the plant.
The Gametophyte Generation
Fig.16.3.2.2 Moss life cycle
The leafy shoot of mosses is haploid and thus part of the gametophyte generation.
In the common haircap moss, Polytrichum commune (shown here), there are three kinds of shoots:
• Female, which develop archegonia at their tip.
• A single egg forms in each archegonium.
• Male, which develop antheridia at their tip.
• Multiple swimming sperm form in each antheridium.
• Sterile, which do not form sex organs.
In early spring, raindrops splash sperm from male to female plants. These swim down the canal in the archegonium to the chamber containing the egg. The resulting zygote begins the sporophyte generation.
The Sporophyte Generation
Mitosis of the zygote produces an embryo that grows into the mature sporophyte generation. It consists of:
• A foot, which absorbs water, minerals, and food from the parent gametophyte
• A stalk, at the tip of which is formed a sporangium (the brownish objects in the photo).
The sporangium is
• filled with spore mother cells
• sealed by an operculum
• covered with a calyptra. The calyptra develops from the wall of the old archegonium and so is actually a part of the gametophyte generation. It is responsible for the common name ("haircap moss") of this species.
During the summer, each spore mother cell undergoes meiosis, producing four haploid spores - the start of the new gametophyte generation. Late in the summer, the calyptra and operculum become detached from the sporangium allowing the spores to be released. These tiny spores are dispersed so effectively by the wind that many mosses are worldwide in their distribution. If a spore reaches a suitable habitat, it germinates to form a filament of cells called a protonema. Soon buds appear and develop into the mature leafy shoots.
Thus the gametophyte generation is responsible for sexual reproduction. The sporophyte generation is responsible for dispersal.
Evolutionary Position of the Bryophytes
Evidence from the chloroplast genome
The chloroplasts of mosses and liverworts, like those of all photosynthetic eukaryotes, contain multiple copies of a small genome: circular DNA molecules encoding some - but not all of the genes needed for their own replication and photosynthesis. Chloroplast genomes have been sequenced from representatives of most of the plant groups. Although they all contain the same genes, they fall into two distinct groups with respect to the organization of their genes.
• One group is illustrated by the figure, which shows the organization of the genome in Marchantia polymorpha, a liverwort. Between the two arrowheads is a stretch of some 30,000 base pairs. The order of the genes in this region is also found in mosses (supporting the idea that mosses and liverworts are, indeed, close relatives), and in the lycopsids — vascular plants (they have xylem and phloem) that have a fossil record going back over 400 million years.
• In all the other groups of plants, the same genes are present but in inverted order.
Evidence from the mitochondrial genome (mtDNA)
However, the mitochondrial DNA of plants suggests a different evolutionary scenario. The mtDNA of all plants including mosses and lycopsids but NOT liverworts (nor green algae) contain certain shared introns. This suggests that:
• mosses and liverworts are not close relatives
• liverworts and lycopsids are not close relatives
• liverworts may have been the first group of plants to evolve from algal ancestors
16.3C: Fern Life Cycle
Ther are over 10,000 species of ferns. Most are found in the tropics where tree ferns with their above-ground stems may grow as high as 40 feet. In temperate regions, the stems of ferns called rhizome grow underground. The leaves called fronds grow up from the rhizome each spring.
Alternation of Generations
The Sporophyte Generation
The plant we recognize as a fern is the diploid sporophyte generation. Sori form on the fronds. Each contains many sporangia mounted on stalks. Within each sporangium, the spore mother cells undergo meiosis producing four haploid spores each.
When the humidity drops,
• The thin-walled lip cells of each sporangium separate.
• The annulus slowly straightens out.
• Then the annulus snaps forward expelling the spores.
The photo shows the sori on the underside of the leaflets of Polystichum acrostichoides, the Christmas fern.
The Gametophyte Generation
If a spore is blown to a suitable moist location, it germinates into a filament of cells. This may grow into a prothallus with
1. rhizoids, which absorb water and minerals from the soil,
2. archegonia, which produce a single egg (by mitosis) or
3. antheridia, which form swimming sperm (again, by mitosis) or
4. both.
Fertilization
If moisture is plentiful, the sperm swim to archegonia - usually on another prothallus because the two kinds of sex organs generally do not mature at the same time on a single prothallus. Another method for promoting cross-fertilization: The first spores to germinate develop into prothallia with archegonia. These prothallia secrete a gibberellin into their surroundings. This is absorbed by younger prothallia and causes them to produce antheridia exclusively. Fertilization restores the diploid number and begins a new sporophyte generation. The embryo sporophyte develops a foot that penetrates the tissue of the prothallus and enables the sporophyte to secure nourishment until it becomes self-sufficient. Although it is tiny, the haploid fern prothallus is a fully-independent, autotrophic plant. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.03%3A_Reproduction_in_Plants/16.3B%3A_Moss_Life_Cycle.txt |
Angiosperms are the flowering plants (today the most abundant and diverse plants on earth). Most are terrestrial and all lack locomotion. This poses several problems. Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. Moreover, there must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. The functions of the flower solve both of these problems.
The Flower and Its Pollination
In angiosperms, meiosis in the sporophyte generation produces two kinds of spores: (1) microspores which develop in the microsporangium and will germinate and develop into the male gametophyte generation and (2) megaspores that develop in the megasporangium will develop into the female gametophyte generation. Both types of sporangia are formed in flowers. In most angiosperms, the flowers are perfect: each has both microsporangia and megasporangia, although some angiosperms are imperfect, having either microsporangia or megasporangia but not both.
• Monoecious plants have both types of imperfect flower on the same plant.
• Dioecious plants have imperfect flowers on separate plants; that is, some plants are male, some female. Examples include willows, poplars, and the date palm. Most dioecious plants use an X-Y system of of sex determination like that in mammals. However, a few species use an X-to-autosome ratio system like that of Drosophila, and a very few use a ZW system like that of birds and lepidoptera.
Flowers develop from flower buds. Each bud contains 4 concentric whorls of tissue. From the outer to the inner, these develop into
• a whorl of sepals (collectively called the calyx)
• a whorl of petals (collectively called the corolla)
• stamens in which the microsporangia form
• carpels in which the megasporangia form.
Stamens
Each stamen consists of a lobed anther, containing the microsporangia and supported by a thin filament. Meiosis of the diploid microspore mother cells in the anther produces four haploid microspores. Each of these develops into a pollen grain consisting of a larger vegetative cell (also called the tube cell) inside of which is a a smaller germ cell (also called the generative cell). At some point, depending on the species, the germ cell divides by mitosis to produce 2 sperm cells.
Carpels
Carpels consist of a stigma, usually mounted at the tip of a style with an ovary at the base. Often the entire whorl of carpels is fused into a single pistil. The megasporangia, called ovules, develop within the ovary. Meiosis of the megaspore mother cell in each ovule produces 4 haploid cells, a large megaspore and 3 small cells that disintegrate.
Development of the megaspore
The nucleus of the megaspore undergoes three successive mitotic divisions. The 8 nuclei that result are distributed and partitioned off by cell walls to form the embryo sac. This is the mature female gametophyte generation. The egg cell will start the new sporophyte generation if it is fertilized. It is flanked by 2 synergid cells. In several (perhaps all) angiosperms, they secrete an attractant that guides the pollen tube through the micropyle into the embryo sac. The large central cell, which in most angiosperms contains 2 polar nuclei, will after its fertilization develop into the endosperm of the seed. It also contains 3 antipodal cells.
Pollination
When a pollen grain reaches the stigma, it germinates into a pollen tube. If it hasn't done so already, the germ cell divides by mitosis forming 2 sperm cells. These, along with the tube nucleus (also known as the vegetative nucleus), migrate down the pollen tube as it grows through the style, the micropyle, and into the ovule chamber.
In Arabidopsis the pollen tube follows a gradient of increasing concentration of a small defensin-like protein secreted by the synergids. The pollen tube with its contents makes up the mature male gametophyte generation.
Double fertilization
The pollen tube enters the ovule through the micropyle and ruptures. One sperm cell fuses with the egg forming the diploid zygote. The other sperm cell fuses with the polar nuclei forming the endosperm nucleus. Most angiosperms have two polar nuclei so the endosperm is triploid (3n). The tube nucleus disintegrates.
Most angiosperms have mechanisms by which they avoid self-fertilization.
Seeds
After double fertilization, each ovule develops into a seed, which consists of
• a plumule, made up of two embryonic leaves, which will become the first true leaves of the seedling, and a terminal (apical) bud. The terminal bud contains the meristem at which later growth of the stem takes place.
• One or two cotyledons which store food that will be used by the germinating seedling. Angiosperms that produce seeds with two cotyledons are called dicots. Examples include beans, squashes, Arabidopsis. Angiosperms whose seeds contain only a single cotyledon are monocots. Examples include corn and other grasses.
• The hypocotyl and radicle, which will grow into the part of the stem below the first node ("hypocotyl" = below the cotyledons) and primary root respectively. The development of each of the parts of the plant embryo depends on gradients of the plant hormone, auxin.
• In addition to the embryo plant (derived from the zygote), each seed is covered with protective seed coats derived from the walls of the ovule.
The food in the cotyledons is derived from the endosperm which, in turn, received it from the parent sporophyte. In many angiosperms (e.g., beans), when the seeds are mature, the endosperm has been totally consumed and its food transferred to the cotyledons. In others (some dicots and all monocots), the endosperm persists in the mature seed.
The seed is thus a dormant embryo sporophyte with stored food and protective coats. Its two functions are
• dispersal of the species to new locations (aided in angiosperms by the fruit)
• survival of the species during unfavorable climatic periods (e.g., winter). "Annual" plants (e.g., beans, cereal grains, many weeds) can survive freezing only as seeds. When the parents die in the fall, the seeds remain alive — though dormant— over the winter. When conditions are once more favorable, germination occurs and a new generation of plants develops.
Fruits
Fruits are a development of the ovary wall and sometimes other flower parts as well. As seeds mature, they release the hormone auxin, which stimulates the wall of the ovary to develop into the fruit. In fact, commercial fruit growers may stimulate fruit development in unpollinated flowers by applying synthetic auxin to the flower.
Fruits promote the dispersal of their content of seeds in a variety of ways.
• Wind. The maple "key" and dandelion parachute are examples.
• Water. Many aquatic angiosperms and shore dwellers (e.g., the coconut palm) have floating fruits that are carried by water currents to new locations.
• Hitchhikers. The cocklebur and sticktights achieve dispersal of their seeds by sticking to the coat (or clothing) of a passing animal.
• Edible fruits. Nuts and berries entice animals to eat them. Buried and forgotten (nuts) or passing through their g.i. tract unharmed (berries), the seeds may end up some distance away from the parent plant.
• Mechanical. Some fruits, as they dry, open explosively expelling their seeds. The pods of many legumes (e.g., wisteria) do this. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.03%3A_Reproduction_in_Plants/16.3D%3A_Angiosperm_Life_Cycle.txt |
Asexual reproduction is the formation of new individuals from the cell(s) of a single parent. It is very common in plants, less so in animals.
Asexual Reproduction in Plants
All plant organs have been used for asexual reproduction, but stems are the most common.
Stems
In some species, stems arch over and take root at their tips, forming new plants.
The horizontal above-ground stems (called stolons) of the strawberry (shown here) produce new daughter plants at alternate nodes.
Underground stems such as rhizomes, bulbs, corms and tubers are used for asexual reproduction as well as for food storage.
Irises and day lilies, for example, spread rapidly by the growth of their rhizomes.
Leaves
This photo shows the leaves of the common ornamental plant Bryophyllum (also called Kalanchoë) . Mitosis at meristems along the leaf margins produce tiny plantlets that fall off and can take up an independent existence.
Roots
Some plants use their roots for asexual reproduction. The dandelion is a common example. Trees, such as the poplar or aspen, send up new stems from their roots. In time, an entire grove of trees may form - all part of a clone of the original tree.
Plant Propagation
Commercially-important plants are often deliberately propagated by asexual means in order to keep particularly desirable traits (e.g., flower color, flavor, resistance to disease). Cuttings may be taken from the parent and rooted.
Grafting is widely used to propagate a desired variety of shrub or tree. All apple varieties, for example, are propagated this way.
Apple seeds are planted only for the root and stem system that grows from them. After a year's growth, most of the stem is removed and a twig (scion) taken from a mature plant of the desired variety is inserted in a notch in the cut stump (the stock). So long the cambiums of scion and stock are united and precautions are taken to prevent infection and drying out, the scion will grow. It will get all its water and minerals from the root system of the stock. However, the fruit that it will eventually produce with be identical (assuming that it is raised under similar environmental conditions) to the fruit of the tree from which the scion was taken.
Apomixis
Citrus trees and many other species of angiosperms use their seeds as a method of asexual reproduction; a process called apomixis.
• In one form, the egg is formed with 2n chromosomes and develops without ever being fertilized.
• In another version, the cells of the ovule (2n) develop into an embryo instead of - or in addition to - the fertilized egg.
Hybridization between different species often yields infertile offspring. But in plants, this does not necessarily doom the offspring. Many such hybrids use apomixis to propagate themselves.
The many races of Kentucky bluegrass growing in lawns across North America and the many races of blackberries are two examples of sterile hybrids that propagate successfully by apomixis.
Recently, an example of apomixis in gymnosperms was discovered (see Pichot, C., et al, in the 5 July 2001 issue of Nature). In a rare cypress, the pollen grains are diploid, not haploid, and can develop into an embryo when they land on either the female cones of their own species (rare) or those of a much more common species of cypress.
Is this paternal apomixis in a surrogate mother a desperate attempt to avoid extinction?
Breeding apomictic crop plants
Many valuable crop plants (e.g., corn) cannot be propagated by asexual methods like grafting.
Agricultural scientists would dearly love to convert these plants to apomixis: making embryos that are genetic clones of themselves rather than the product of sexual reproduction with its inevitable gene reshuffling. After 20 years of work, an apomictic corn (maize) has been produced, but it does not yet produce enough viable kernels to be useful commercially.
Asexual Reproduction in Animals
Budding
Here, offspring develop as a growth on the body of the parent. In some species, e.g., jellyfishes and many echinoderms, the buds break away and take up an independent existence. In others, e.g., corals, the buds remain attached to the parent and the process results in colonies of animals. Budding is also common among parasitic animals, e.g., tapeworms.
Fragmentation
As certain tiny worms grow to full size, they spontaneously break up into 8 or 9 pieces. Each of these fragments develops into a mature worm, and the process is repeated.
Parthenogenesis
In parthenogenesis ("virgin birth"), the females produce eggs, but these develop into young without ever being fertilized. Parthenogenesis occurs in some fishes, several kinds of insects, and a few species of frogs and lizards. It does not normally occur in mammals because of their imprinted genes. However, using special manipulations to circumvent imprinting, laboratory mice have been produced by parthenogenesis.
In a few nonmammalian species it is the only method of reproduction, but more commonly animals turn to parthenogenesis only under certain circumstances. Examples:
• Aphids use parthenogenesis in the spring when they find themselves with ample food. In this species, reproduction by parthenogenesis is more rapid than sexual reproduction, and the use of this mode of asexual reproduction permits the animals to quickly exploit the available resources.
• Female Komodo dragons (the largest lizard) can produce offspring by parthenogenesis when no male is available for sexual reproduction. Their offspring are homozygous at every locus including having identical sex chromosomes. Thus the females produce all males because, unlike mammals, females are the heterogametic sex (ZW) while males are homogametic (ZZ).
Parthenogenesis is forced on some species of wasps when they become infected with bacteria (in the genus Wolbachia). Wolbachia can pass to a new generation through eggs, but not through sperm, so it is advantageous to the bacterium for females to be made rather that males.
In these wasps (as in honeybees),
• fertilized eggs (diploid) become females
• unfertilized (haploid) eggs become males
However, in Wolbachia-infected females, all their eggs undergo endoreplication producing diploid eggs that develop into females without fertilization; that is, by parthenogenesis.
Treating the wasps with an antibiotic kills off the bacteria and "cures" the parthenogenesis!
Apis mellifera capensis
Occasionally worker honeybees develop ovaries and lay unfertilized eggs. Usually these are haploid, as you would expect, and develop into males. However, workers of the subspecies Apis mellifera capensis (the Cape honeybee) can lay unfertilized diploid eggs that develop into females (who continue the practice). The eggs are produced by meiosis, but then the polar body nucleus fuses with the egg nucleus restoring diploidy (2n). (The phenomenon is called automictic thelytoky.)
Why Choose Asexual Reproduction?
Perhaps the better question is: Why not?
After all, asexual reproduction would seem a more efficient way to reproduce. Sexual reproduction requires males but they themselves do not produce offspring.
Two general explanations for the overwhelming prevalence of sexually-reproducing species over asexual ones are:
• Perhaps sexual reproduction has kept in style because it provides a mechanism to weed out (through the recombination process of meiosis) harmful mutations that arise in the population reducing its fitness. Asexual reproduction leads to these mutations becoming homozygous and thus fully exposed to the pressures of natural selection.
• Perhaps it is the ability to adapt quickly to a changing environment that has caused sex to remain the method of choice for most living things.
Purging Harmful Mutations
Most mutations are harmful - changing a functional allele to a less or nonfunctional one. An asexual population tends to be genetically static. Mutant alleles appear but remain forever associated with the particular alleles present in the rest of that genome. Even a beneficial mutation will be doomed to extinction if trapped along with genes that reduce the fitness of that population.
But with the genetic recombination provided by sex, new alleles can be shuffled into different combinations with all the other alleles available to the genome of that species. A beneficial mutation that first appears alongside harmful alleles can, with recombination, soon find itself in more fit genomes that will enable it to spread through a sexual population.
Evidence (from Paland and Lynch in the 17 February 2006 issue of Science):
Some strains of the water flea Daphnia pulex (a tiny crustacean) reproduce sexually, others asexually. The asexual strains accumulate deleterious mutations in their mitochondrial genes four times as fast as the sexual strains.
Evidence (from Goddard et al. in the 31 March 2005 issue of Nature):
Budding yeast missing two genes essential for meiosis adapt less rapidly to growth under harsh conditions than an otherwise identical strain that can undergo genetic recombination. Under good conditions, both strains grow equally well.
Evidence (from Rice and Chippindale in the 19 October 2001 issue of Science):
Using experimental Drosophila populations, they found that a beneficial mutation introduced into chromosomes that can recombine did - over time increase in frequency more rapidly than the same mutation introduced into chromosomes that could not recombine.
So sex provides a mechanism for testing new combinations of alleles for their possible usefulness to the phenotype:
• deleterious alleles weeded out by natural selection
• useful ones retained by natural selection
Some organisms may still gain the benefits of genetic recombination while avoiding sex. Many mycorrhizal fungi use asexual reproduction only. However, at least two species have been shown to have multiple - similar - copies of the same gene; that is, are polyploid. Perhaps recombination between these (during mitosis?) enables these organisms to avoid the hazards of accumulating deleterious mutations. (See the paper by Pawlowska and Taylor in the 19 Feb 2004 issue of Nature.)
But there are many examples of populations that thrive without sex, at least while they live in a stable environment.
Rapid Adaptation to a Changing Environment
As we have seen (above), populations without sex are genetically static. They may be well-adapted to a given environment, but will be handicapped in evolving in response to changes in the environment. One of the most potent environmental forces acting on a species environment is its parasites.
The speed with which parasites like bacteria and viruses can change their virulence may provide the strongest need for their hosts to have the ability to make new gene combinations. So sex may be virtually universal because of the never-ending need to keep up with changes in parasites.
Evidence:
• Some parasites interfere with sexual reproduction in their host:
• Wolbachia-induced parthenogenesis discussed above is an example.
• Several types of fungi block wind pollination of their grass hosts forcing them to inbreed with its resulting genetic uniformity.
• There is some evidence that genetically uniform populations are at increased risk of devastating epidemics and population crashes.
• Flour beetles (Tribolium castaneum) parasitized by the microsporidium Nosema whitei increase the rate of recombination during meiosis.
• Drosophila females parasitized by bacteria produce more recombinant offspring than non-infected mothers do.
The idea that a constantly-changing environment, especially with respect to parasites, drives evolution is often called the Red Queen hypothesis. It comes from Lewis Carroll's book Through the Looking Glass, where the Red Queen says "Now here, you see, it takes all the running you can do to keep in the same place".
The possibilities outlined above are not mutually exclusive and a recent study [see Morran, L. T., et al., in Nature, 462:350, 19 November 2009] suggests that both forces are at work in favoring sexual reproduction over its alternatives.
The organism for testing these theories was Caenorhabditis elegans. While C. elegans does not reproduce asexually, most worms are hermaphrodites and usually reproduce by self-fertilization with each individual fertilizing its own eggs. This quickly results in its genes becoming homozygous and thus fully-exposed to natural selection just as they are in asexually-reproducing species.
Hermaphrodites have two X chromosomes and self-fertilization ("selfing") usually produces more of the same; that is, hermaphrodites produce more hermaphrodites. However, an occasional nondisjunction generates an embryo with a single X chromosome and this develops into a male. These males can mate with hermaphrodites (their sperm is preferred over the hermaphrodites own) and, in fact, such "outcrossing" produces a larger number of offspring. It also produces 50% hermaphrodites and 50% males.
Testing the role of outcrossing vs. self-fertilization in maintaining fitness in the face of an increased mutation rate.
These workers developed six strains of worms:
• two that could reproduce only by selfing
• two that could reproduce only by crossing a male with an hermaphrodite ("outcrossing")
• "wild-type" worms
All the strains were exposed to a chemical mutagen that increased the spontaneous mutation rate some fourfold.
The results: After 50 generations, the
• the strains of worms that could reproduce only by selfing suffered a serious decline in fitness
• the strains of worms that could reproduce only by outcrossing suffered no decline
• the wild-type worms with intermediate levels of outcrossing (20–30%) suffered only moderate declines in fitness.
Fitness was measured by placing the worms in a petri dish with a barrier that they had to cross to reach their food (E. coli).
The conclusion: the genetic recombination provided by outcrossing protected the worms from loss of fitness even in the face of an increase in mutation rate.
Testing the role of outcrossing vs. self-fertilization in the speed of adaptation to a changed environment.
For these tests, one of each category of mating types was exposed over 40 generations to a pathogenic bacterium (Serratia marcescens) that killed most worms when eaten by them.
The results: After 40 generations,
• the strain of worms that could reproduce only by selfing were just as susceptible to the pathogen as they were at the start while
• the strain of worms that could reproduce only by outcrossing had evolved a high degree of resistance to the pathogen
• the wild-type worms only developed a modest increase in their resistance to the bacteria.
Since these studies were reported, the same team has expanded their experiments to examine the effects of evolution in the pathogen (Serratia marcescens), that is, to look for evidence of coevolution of host and parasite. (Reported by Morran, L. T., et al., in Science, 333: 216, 8 July 2011.)
Over 30 generations of worms, they harvested and tested the bacteria recovered from the bodies of worms that had died within 24 hours of infection. They found that:
• Worms that can maintain genetic variability by outcrossing suffered substantially lower mortality from the coevolved parasite that did worms from the starting population (kept frozen until used).
• Worms that could only reproduce by selfing became so susceptible to the evolving strain of Serratia marcescens that they died out within 20 generations.
• Curiously, the selection pressure of the increasing virulence of Serratia marcescens caused wild-type worms to increase their rate of outcrossing from the normal 20–30% to over 80%. So one response to the pressure of this coevolving parasite was to promote sex in its host.
Reproduction in Rotifers
Rotifers are microscopic invertebrates. They are assigned a phylum of their own (not discussed elsewhere in these pages). The phylum includes:
• a class of ~1,500 species called monogonont rotifers (they have only a single gonad). The monogonont rotifers can choose either asexual or sexual reproduction as circumstances warrant.
• a class of ~350 species called bdelloid rotifers. The bdelloid rotifers are limited to asexual reproduction only. Even after years of study, neither males nor haploid eggs have ever been found in any members of this group. It looks as though they gave up sexual reproduction millions of years ago.
Laboratory studies show that monogonont rotifers favor asexual reproduction when they are living in a stable environment but shift to more sexual reproduction when placed in a varied or unfavorable environment. As they adapt to the new environment, they gradually switch back to asexual reproduction.
But how have the bdelloid rotifers that never engage in sexual reproduction managed to survive? How have they avoided the demands of the Red Queen; that is, avoided extinction at the hands of parasites?
One study (Wilson, C. G. and Sherman, P. W., Science, 327:574, 29 January 2010) reveals a mechanism. These tiny animals can be completely desiccated (dried out) and remain in suspended animation for years. In the desiccated state, they can be blown vast distances (some species are worldwide in their distribution). Once deposited in a moist environment (a few drops of water are sufficient), they resume an active life. Wilson and Sherman have shown that the desiccation that is harmless to the rotifers is lethal to their fungal parasite. So once dried, they are not only cured of their parasite, but can then be blown to a spot where they can resume an active life with no parasites present.
Another way in which these rotifers can avoid the evolutionary dead-end expected of asexually-reproducing organisms has been revealed by DNA sequencing of their genome. It turns out that they can purge their genome of deleterious alleles by gene conversion (during mitosis).
But in any case despite its disadvantages sexual reproduction is here to stay
• reducing the effect of harmful mutations
• increasing the speed with which populations can adapt to changes in their environment | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.03%3A_Reproduction_in_Plants/16.3E%3A_Asexual_Reproduction_in_Plants.txt |
Evolution seems to favor (and be favored by) genetic variability. Genetic variability is promoted by outbreeding - sexual reproduction between genetically dissimilar parents. Just why sexual reproduction is so popular throughout the world of living things is still a hotly-debated question, but the fact remains. Plants, being anchored in position, have a special problem in this regard. Many employ the services of animals (e.g., insects, birds, bats) to transfer pollen from plant to plant. But if the flowers have both sex organs:what is to prevent the pollen from fertilizing its own eggs?
A variety of solutions have been tried in the plant kingdom. These include:
• Having imperfect flowers; that is, flowers that are either male or female.
• Dioecy. The imperfect flowers are present on separate plants. Dioecy is the equivalent of the separate sexes of most animals. But it is rather rare. Some examples include poplars and hollies.
• Monoecy. The imperfect flowers are present on the same plant. But if they mature at different times, self-fertilization is avoided. Corn (maize) is a common example.
But the vast majority of angiosperms have perfect flowers; that is containing both male and female sex organs. So how do they avoid self-fertilization?
• Heteromorphic flowers.
The flowers are perfect but come in two structural types; for example
• long stamens with a short style
• short stamens with a long style
• Homomorphic flowers. All flowers have exactly the same structure. Avoidance of self-fertilization depends on genetic/biochemical mechanisms. There are two quite different types of self-incompatibility.
• Sporophytic self-incompatibility (SSI)
• Gametophytic self-incompatibility (GSI)
Sporophytic Self-Incompatibility (SSI)
This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower.
16.3F: Transgenic Plants
Progress is being made on several fronts to introduce new traits into plants using recombinant DNA technology. The genetic manipulation of plants has been going on since the dawn of agriculture, but until recently this has required the slow and tedious process of cross-breeding varieties. Genetic engineering promises to speed the process and broaden the scope of what can be done.
Making transgenic plants
There are several methods for introducing genes into plants, including infecting plant cells with plasmids as vectors carrying the desired gene or shooting microscopic pellets containing the gene directly into the cell. In contrast to animals, there is no real distinction between somatic cells and germline cells. Somatic tissues of plants, e.g., root cells grown in culture, can be transformed in the laboratory with the desired gene or grown into mature plants with flowers. If all goes well, the transgene will be incorporated into the pollen and eggs and passed on to the next generation. In this respect, it is easier to produce transgenic plants than transgenic animals.
Some Achievements
Improved Nutritional Quality
Milled rice is the staple food for a large fraction of the world's human population. Milling rice removes the husk and any beta-carotene it contained. Beta-carotene is a precursor to vitamin A, so it is not surprising that vitamin A deficiency is widespread, especially in the countries of Southeast Asia. The synthesis of beta-carotene requires a number of enzyme-catalyzed steps. In January 2000, a group of European researchers reported that they had succeeded in incorporating three transgenes into rice that enabled the plants to manufacture beta-carotene in their endosperm.
Insect Resistance
Bacillus thuringiensis is a bacterium that is pathogenic for a number of insect pests. Its lethal effect is mediated by a protein toxin it produces. Through recombinant DNA methods, the toxin gene can be introduced directly into the genome of the plant where it is expressed and provides protection against insect pests of the plant.
Disease Resistance
Genes that provide resistance against plant viruses have been successfully introduced into such crop plants as tobacco, tomatoes, and potatoes.
Tomato plants infected with tobacco mosaic virus (which attacks tomato plants as well as tobacco). The plants in the back row carry an introduced gene conferring resistance to the virus. The resistant plants produced three times as much fruit as the sensitive plants (front row) and the same as control plants.
Herbicide Resistance
Questions have been raised about the safety - both to humans and to the environment - of some of the broad-leaved weed killers like 2,4-D. Alternatives are available, but they may damage the crop as well as the weeds growing in it. However, genes for resistance to some of the newer herbicides have been introduced into some crop plants and enable them to thrive even when exposed to the weed killer.
Effect of the herbicide bromoxynil on tobacco plants transformed with a bacterial gene whose product breaks down bromoxynil (top row) and control plants (bottom row). "Spray blank" plants were treated with the same spray mixture as the others except the bromoxynil was left out.
Salt Tolerance
A large fraction of the world's irrigated crop land is so laden with salt that it cannot be used to grow most important crops. However, researchers at the University of California Davis campus have created transgenic tomatoes that grow well in saline soils. The transgene was a highly-expressed sodium/proton antiport pump that sequestered excess sodium in the vacuole of leaf cells. There was no sodium buildup in the fruit.
"Terminator" Genes
This term is used (by opponents of the practice) for transgenes introduced into crop plants to make them produce sterile seeds (and thus force the farmer to buy fresh seeds for the following season rather than saving seeds from the current crop). The process involves introducing three transgenes into the plant:
• A gene encoding a toxin which is lethal to developing seeds but not to mature seeds or the plant. This gene is normally inactive because of a stretch of DNA inserted between it and its promoter.
• A gene encoding a recombinase — an enzyme that can remove the spacer in the toxin gene thus allowing to be expressed.
• A repressor gene whose protein product binds to the promoter of the recombinase thus keeping it inactive.
How they work
When the seeds are soaked (before their sale) in a solution of tetracycline
• Synthesis of the repressor is blocked.
• The recombinase gene becomes active.
• The spacer is removed from the toxin gene and it can now be turned on.
Because the toxin does not harm the growing plant - only its developing seeds - the crop can be grown normally except that its seeds are sterile.
The use of terminator genes has created much controversy:
• Farmers - especially those in developing countries - want to be able to save some seed from their crop to plant the next season.
• Seed companies want to be able to keep selling seed.
Transgenes Encoding Antisense RNA
These are discussed in a separate page. Link to it
Biopharmaceuticals
The genes for proteins to be javascript:void('Remove anchor')used in human (and animal) medicine can be inserted into plants and expressed by them.
Advantages:
• Glycoproteins can be made (bacteria like E. coli cannot do this).
• Virtually unlimited amounts can be grown in the field rather than in expensive fermentation tanks.
• It avoids the danger from using mammalian cells and tissue culture medium that might be contaminated with infectious agents.
• Purification is often easier.
Corn is the most popular plant for these purposes, but tobacco, tomatoes, potatoes, rice and carrot cells grown in tissue culture are also being used. Some of the proteins that have been produced by transgenic crop plants:
• human growth hormone with the gene inserted into the chloroplast DNA of tobacco plants
• humanized antibodies against such infectious agents as
• HIV
• respiratory syncytial virus (RSV)
• sperm (a possible contraceptive)
• herpes simplex virus, HSV, the cause of "cold sores"
• Ebola virus, the cause of the often-fatal Ebola hemorrhagic fever
• protein antigens to be used in vaccines
• An example: patient-specific antilymphoma (a cancer) vaccines. B-cell lymphomas are clones of malignant B cells expressing on their surface a unique antibody molecule. Making tobacco plants transgenic for the RNA of the variable (unique) regions of this antibody enables them to produce the corresponding protein. This can then be incorporated into a vaccine in the hopes (early trials look promising) of boosting the patient's immune system - especially the cell-mediated branch - to combat the cancer.
• other useful proteins like lysozyme and trypsin
• However, as of April 2012, the only protein to receive approval for human use is glucocerebrosidase, an enzyme lacking in Gaucher's disease. It is synthesized by transgenic carrot cells grown in tissue culture.
Controversies
The introduction of transgenic plants into agriculture has been vigorously opposed by some. There are a number of issues that worry the opponents. One of them is the potential risk of transgenes in commercial crops endangering native or nontarget species.
Examples:
• A gene for herbicide resistance in, e.g. maize (corn), escaping into a weed species could make control of the weed far more difficult.
• The gene for Bt toxin expressed in pollen might endanger pollinators like honeybees.
To date, field studies on Bt cotton and maize show that the numbers of some nontarget insects are reduced somewhat but not as much as in fields treated with insecticides.
Another worry is the inadvertent mixing of transgenic crops with nontransgenic food crops. Although this has occurred periodically, there is absolutely no evidence of a threat to human health.
Despite the controversies, farmers around the world are embracing transgenic crops. Currently in the United States over 80% of the corn, soybeans, and cotton grown are genetically modified (GM) — principally to provide
• resistance to the herbicide glyphosate ("Roundup Ready®") thus making it practical to spray the crop with glyphosate to kill weeds without harming the crop
• resistance to insect attack (by expressing the toxin of Bacillus thuringiensis) | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.03%3A_Reproduction_in_Plants/16.3E%3A_Self-incompatibility_-_How_Plants_Avoid_Inbreeding.txt |
• 16.4A: Plant Growth
Growth in plants occurs chiefly at meristems where rapid mitosis provides new cells. As these cells differentiate, they provide new plant tissue.
• 16.4B: Germination of Seeds
Germination is the resumption of growth of the embryo plant inside the seed.
• 16.4C: Etiolation
The stems of plants raised in the dark elongate much more rapidly than normal, a phenomenon called etiolation. It is a mechanism that increases the probability of the plant reaching the light.
• 16.4D: Flowering
The flowering plants (angiosperms) go through a phase of vegetative growth producing more stems and leaves and a flowering phase where they produce the organs for sexual reproduction. In "annuals", like the snapdragon, the vegetative phase begins with germination of the seed. Flowering follows and ends with the senescence and death of the plant. In biennials, the vegetative phase takes up the first year; flowering followed by death occurs the second year.
• 16.4E: Photoperiodism and Phytochrome
Many angiosperms flower at about the same time every year. This occurs even though they may have started growing at different times. Their flowering is a response to the changing length of day and night as the season progresses. The phenomenon is called photoperiodism. It helps promote cross pollination.
16.04: Plant Development - Fundamentals
Growth in plants occurs chiefly at meristems where rapid mitosis provides new cells. As these cells differentiate, they provide new plant tissue.
Stem Growth
In stems, mitosis in the apical meristem of the shoot apex (also called the terminal bud) produces cells that enable the stem to grow longer and, periodically, cells that will give rise to leaves. The point on the stem where leaves develop is called a node. The region between a pair of adjacent nodes is called the internode. The internodes in the terminal bud are very short so that the developing leaves grow above the apical meristem that produced them and thus protect it. New meristems, the lateral buds, develop at the nodes, each just above the point where a leaf is attached. When the lateral buds develop, they produces new stem tissue, and thus branches are formed.
Under special circumstances (such as changes in photoperiod), the apical meristem is converted into a flower bud. This develops into a flower. The conversion of the apical meristem to a flower bud "uses up" the meristem so that no further growth of the stem can occur at that point. However, lateral buds behind the flower can develop into branches.
The drawing is of a typical woody dicot, the horse chestnut, as seen during the dormant season. The leaves have dropped off, leaving a leaf scar; the dots inside each leaf scar show where the vascular bundles (xylem and phloem) had entered the petiole of the leaf. A flower had been produced the season before, so that during the season just ended two branches had grown out on either side of the flower bud scar. Lenticels are openings that allow oxygen and carbon dioxide to diffuse between the living cells of the stem and the air.
The growth of roots in described in a separate page, | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.04%3A_Plant_Development_-_Fundamentals/16.4A%3A_Plant_Growth.txt |
Germination is the resumption of growth of the embryo plant inside the seed.
Requirements:
• proper temperature
• Water is always needed to allow vigorous metabolism to begin. It is also sometimes needed to leach away a germination inhibitor within the seed. This is especially common among desert annuals. The inhibitor is often abscisic acid (ABA).
• oxygen
• a preceding period of dormancy (often).
The seeds of many temperate-climate angiosperms will germinate only after a prolonged period of cold. An inhibitor within the seed (probably abscisic acid - ABA) is gradually broken down at low temperatures until finally there is not enough to prevent germination when other conditions become favorable. This mechanism is of obvious survival value in preventing seeds from germinating during an unseasonably warm spell in the autumn or winter.
• Correct photoperiod (often).
Germination in a Dicot
• The primary root emerges through the seed coats while the seed is still buried in the soil.
• The hypocotyl ("below the cotyledons") emerges from the seed coats and pushes its way up through the soil. It is bent in a hairpin shape - the hypocotyl arch- as it grows up. The two cotyledons protect the plumule- the epicotyl ("above the cotyledons") and first leaves - from mechanical damage.
• Once the hypocotyl arch emerges from the soil, it straightens out. This response is triggered by light. Both red light, absorbed by phytochrome and blue light, absorbed by cryptochrome can do the job.
• The cotyledons spread apart exposing the epicotyl with the apical meristem at its tip, and two primary leaves
• In many dicots, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
The above image (courtesy of the Pittsburgh Plate Glass Co.) is a time-lapse photograph showing three stages in the germination of a bean seed.
Germination in Monocots
When grass seeds like corn (maize) or oats (shown here) germinate,
• The primary root pierces the seed (and fruit) coverings and grows down.
• The primary leaf of the plant grows up. It is protected as it pushes up through the soil by the coleoptile - a hollow, cylindrical structure.
• Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it.
The coleoptile of grass (e.g., oat) seedlings has been a favorite experimental object for studing phototropism.
16.4C: Etiolation
The stems of plants raised in the dark elongate much more rapidly than normal, a phenomenon called etiolation. It is a mechanism that increases the probability of the plant reaching the light.
Once light shines on it,
• the cotyledons spread apart
• the primary leaves
• grow to full size
• turn green
• the plant begins to produce internodes of normal size.
The image shows seedlings of the common garden bean grown in the light (left) and in darkness (right). The pale color of the dark-grown plant is caused by the lack of chlorophyll. When the food reserves of its seed are used up, the seedling will die (unless placed in the light).
Each seedling has three nodes (the bottom ones where the cotyledons were attached), but the internodes are greatly elongated in the dark-grown seedling. This response is NOT dependent on photosynthesis, because light too dim to be useful in photosynthesis nevertheless halts etiolation. Exposure to dim red light (660 nm) or blue light can halt etiolation. The effect of blue light is mediated by cryptochrome. The red light effect is mediated by phytochrome.
How does phytochrome work?
The prevention of etiolation in Arabidopsis is mediated by phytochrome B.
• When sunlight (660 nm) converts PR into PFR, the PFR moves from the cytoplasm into the nucleus.
• There it stimulates the activity of DELLA proteins.
• These bind to members of a group of helix-loop-helix proteins called PIFs ("phytochrome-interacting factors").
• PIFs, like many helix-loop-helix proteins, are transcription factors. They bind to and turn on promoters of genes that, among other effects, stimulate cell and thus stem elongation.
• However, when DELLA proteins bind PIFs, the PIFs are prevented from binding to the promoters of their target genes.
• The result: reduced cell elongation and so no etiolation.
Gibberellins are plant hormones that promote stem elongation thus mimicking the etiolation response. They do this by triggering the degradation of DELLA proteins thus freeing PIFS to bind to the promoters of genes needed for cell elongation.
The Shade-Avoidance Response
Natural sunlight contains about the same amount of red (660 nm) as far-red (735 nm) light. However, chlorophyll absorbs 660 nm light strongly, so that the light that filters through a canopy of foliage is enriched in far-red light.
• This is absorbed by PFR converting it into inactive PR and relieving the inhibition of PIFs.
• The now-active PIFs turn on the genes needed for the synthesis of auxin.
• Auxin stimulates stem elongation.
The shade-avoidance response helps ensure that the plant reaches enough sunlight to be able to carry on photosynthesis. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.04%3A_Plant_Development_-_Fundamentals/16.4B%3A_Germination_of_Seeds.txt |
The flowering plants (angiosperms) go through a phase of vegetative growth producing more stems and leaves and a flowering phase where they produce the organs for sexual reproduction. In "annuals", like the snapdragon, the vegetative phase begins with germination of the seed. Flowering follows and ends with the senescence and death of the plant. In biennials, the vegetative phase takes up the first year; flowering followed by death occurs the second year. In perennials, flowering typically occurs year after year when conditions are appropriate.
Vegetative growth of the above-ground part of the plant — the shoot — occurs at the apical meristem. This is a mass of undifferentiated cells at the tip of the stem. Mitosis of these cells produces cells that differentiate to form more stem, leaves and secondary meristems. Also called lateral buds, these form in the axils of the leaves and will form branches.
The Signal to Flower
Flowering involves the conversion of the apical meristem into a floral meristem, from which all the parts of the flower will be produced. Signals that change the fate of the apical meristem include:
• maturity of the plant
• temperature
• the arrival of the plant hormone gibberellin
• for many plants, photoperiod - the relative length of day and night.
Temperature
Many annual plants (e.g., winter wheat) and biennial plants have their time of flowering delayed unless they have undergone a preceding period of wintertime cold. The change brought about by this prolonged exposure to the cold is called vernalization.
In the "model" plant Arabidopsis thaliana, vernalization works like this.
• A gene designated Flowering Locus C (FLC) encodes a transcription factor that blocks the expression of the genes needed for flowering.
• The level of FLC mRNA is high in the fall.
• But with the onset of cold temperatures, production of an antisense transcript of FLC (called COOLAIR) increasesas does, later, a sense transcript of part of the FLC gene.
• Both of these RNAs are non-coding; that is, they are not translated into protein.
• But they cooperate in suppressing the production of FLC mRNA and its translation into FLC protein.
• With the arrival of spring, there is no FLC protein remaining to suppress flowering so flowering can begin.
The buds of many species of woody angiosperms found in temperate climates, such as apples and lilacs, also need a preceding period of cold weather before they can develop into flowers. So these plants cannot be grown successfully at lower latitudes because the winters never get cold enough (a few days at 0–10°C). This bud dormancy is localized. Prior chilling of one bud on a lilac stem enables it to flower while the other, nonchilled, buds on the stem remain dormant.
Photoperiod
Photoperiod is detected in the leaves. The cocklebur, drawn here, needs at least 8.5 hours of darkness in order to flower. Even if only a part of one leaf is exposed to the correct photoperiod, the entire plant will bloom (middle figure).
The leaves produce a chemical signal called florigen that is transmitted to the apical meristems to start their conversion into floral meristems. The right-hand drawing shows that grafting a cocklebur (B) that receives the required period of darkness to one (A) that does not causes flowering in both. Evidently the florigen signal passes from B to A through their connected vascular systems.
The chemical nature of florigen has been sought for decades. The most recent evidence suggests that at least one component is the protein encoded by the gene FLOWERING LOCUS T (FT).
Converting the Apical Meristem to a Floral Meristem
In the nucleus of the meristem cells, the FT protein binds to the transcription factor FD and turns on the expression of genes needed for flowering, e.g., APETALA1 and LEAFY.
Structure of the Flower
The floral meristem differentiates into four concentric groups of cells that form the four parts of the flower.
1. The cells in whorl 1 develop into a whorl of sepals. These form at the lowest level. Collectively they make up the calyx.
2. Whorl 2 forms above the calyx, forming the petals. Collectively these make up the corolla of the flower (the part that most ornamentals are grown for).
3. Whorl 3 develops into the stamens, the male reproductive organs.
4. The innermost whorl, 4, forms carpels, the female reproductive organs. Carpels often fuse to form a single structure, which some botanists call the pistil.
What triggers the various parts of the floral meristem to enter one or another of these four developmental pathways?
The ABC Model of Flower Development
Genetic analysis of mutants especially those found in the dicots Arabidopsis thaliana and in the snapdragon (Antirrhinum) support the ABC model of flowering. This model postulates a group of genes that encode the transcription factors needed to turn on the genes for sepal, petal, etc. development. The "master switches" fall into 3 groups: A, B, and C.
These are the rules:
• Cells in which only A genes are expressed develop into sepals. This occurs at the lowest level of the floral meristem.
• Cells in which both A and B genes are expressed develop into petals. This occurs at the next higher level.
• Expression of B and C genes turns on the developmental program to form stamens.
• Expression of C genes alone turns on the development of carpels in the innermost band of cells.
Examples of A, B and C group genes involved in flowering - these have been identified in Arabidopsis thaliana
A group APETALA1 (AP1) and
APETALA2 (AP2)
B group APETALA3 (AP3) and
PISTILLATA (PI)
C group AGAMOUS (AG)
The transcription factor LEAFY plays a major role in turning on the A, B, and C group genes in the appropriate locations.
• The LEAFY protein alone turns on AP1 in whorls 1 and 2.
• LEAFY plus a protein encoded by the gene UFO (for "unusual floral organs") turn on AP3 in whorls 2 and 3.
• LEAFY and a second, still unidentified, protein turn on AG in whorls 3 and 4.
If LEAFY protein alone is sufficient to turn on AP1, why isn't AP1 expressed in all four whorls?
The answer: AGAMOUS blocks the expression of AP1, so any cell expressing AGAMOUS cannot express AP1. In fact, the antagonism is reciprocal: AP2 in whorls 1 and 2 (A group) inhibits AGAMOUS so the gene expression in whorls 3 and 4 remains distinct from that in whorls 1 and 2.
The proteins encoded by APETALA3 and PISTILLATA (Group B) form a heterodimer that binds to
• the APETALA1 protein to form petals
• the AGAMOUS protein to form stamens
Aided by a fourth transcription factor encoded by the gene SEPALLATA3, these quaternary complexes bind to specific sequences of DNA turning on the expression of the various genes needed to form whorls 2 and 3. Further research may reveal similar behavior for the other genes.
SEPALLATA3 (SEP3) is one of four SEP genes in Arabidopsis. If all but SEP4 are inactivated, a flower with only sepals is formed (hence the name). If all four are inactivated, no flowers are formed at all.
So formation of a flower requires a cascade of sequential gene activity that gradually converts a mass of undifferentiated cells (the apical meristem) into the parts of a flower. The genes encode transcription factors that act as master switches, turning on (or off) downstream genes that ultimately make each part of the flower in its appropriate location. This same strategy of genetic control of developmental pathways is seen in animal development. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.04%3A_Plant_Development_-_Fundamentals/16.4D%3A_Flowering.txt |
Many angiosperms flower at about the same time every year. This occurs even though they may have started growing at different times. Their flowering is a response to the changing length of day and night as the season progresses. The phenomenon is called photoperiodism. It helps promote cross pollination.
In 1920 two employees of the U. S. Department of Agriculture, W. W. Garner and H. A. Allard, discovered a mutation in tobacco - a variety called Maryland Mammoth - that prevented the plant from flowering in the summer as normal tobacco plants do. Maryland Mammoth would not bloom until late December. Experimenting with artificial lighting in winter and artificial darkening in summer, they found that Maryland Mammoth was affected by photoperiod. Because it would flower only when exposed to short periods of light, they called it a short-day plant. Some other short-day plants are
• chrysanthemums (bloom in the fall)
• rice (Oryza sativa)
• poinsettias
• morning glory (Pharbitis nil)
• the cocklebur (Xanthium)
Some plants such as spinach, Arabidopsis, sugar beet and the radish flower only after exposure to long days and hence are called long-day plants. Still other plants, e.g. the tomato, are day neutral; that is, flowering is not regulated by photoperiod.
Photoperiodism also explains why some plant species can be grown only in a certain latitude. Spinach, a long-day plant, cannot flower in the tropics because the days never get long enough (14 hours). Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the days become short enough to initiate flowering, a killing frost in apt to occur before reproduction and the formation of seeds is completed.
Photoperiodism in a Short-Day Plant
Experiments with the cocklebur have shown that the term short-day is something of a misnomer; what the cocklebur needs is a sufficiently long night.
• Cockleburs (adapted to the latitude of Michigan) will flower only if they have been kept in the dark for at least 8.5 hours — the critical period. (A and B).
• Interruption of an otherwise long night by light — red (660 nm) rays are particularly effective — prevents flowering. (C) unless
• it is followed by irradiation with far-red (730 nm) light (D).
• An intense exposure to far-red light at the start of the night reduces the dark requirement by 2 hours (E).
These response are mediated by phytochrome.
Phytochrome
• Phytochrome is a homodimer: two identical protein molecules each conjugated to a light-absorbing molecule (compare rhodopsin).
• Plants make 5 phytochromes: PhyA, PhyB, as well as C, D, and E.
• There is some redundancy in function of the different phytochromes, but there also seem to be functions that are unique to one or another. The phytochromes also differ in their absorption spectrum; that is, which wavelengths (e.g., red vs. far-red) they absorb best.
• Phytochromes exist in two interconvertible forms
• PR because it absorbs red (R; 660 nm) light
• PFR because it absorbs far-red (FR; 730 nm) light
• These are the relationships:
• Absorption of red light by PR converts it into PFR.
• Absorption of far-red light by PFR converts it into PR.
• In the dark, PFR spontaneously converts back to PR.
The Hourglass Model
The behavior of phytochrome provided the first model - called the hourglass model - of the mechanism of photoperiodism in short-day plants.
• Sunlight is richer in red (660 nm) than far-red (730 nm) light, so at sundown all the phytochrome is PFR.
• During the night, the PFR converts back to PR.
• The PR form is needed for the release of the flowering signal.
• Therefore, the cocklebur needs 8.5 hours of darkness in which to
• convert all the PFR present at sundown into PR
• carry out the supplementary reactions leading to the release of the flowering signal ("florigen").
• If this process is interrupted by a flash of 660-nm light, the PR is immediately reconverted to PFR and the night's work is undone (C)
• A subsequent exposure to far-red (730 nm) light converts the pigment back to PR and the steps leading to the release of "florigen" can be completed (D)
• Exposure to intense far-red light at the beginning of the night sets the clock ahead about 2 hours or so by eliminating the need for the spontaneous conversion of PFR to PR (E).
The Circadian Rhythm Model
Recent work mostly in the long-day plant, Arabidopsis - supports a different model of photoperiodism. This work suggests that the photoperiodic response is governed by the interaction of daylight with innate circadian rhythms of the plant.
• Virtually all eukaryotes have innate circadian rhythms.
• These are rhythms of biological activities that fluctuate over a period of approximately 24 hours (L. circa = about; dies = day) even under constant environmental conditions (e.g. continuous darkness). Under constant conditions, the cycles may drift out of phase with the environment.
• However, when exposed to the environment (e.g., alternating day and night), the rhythms become entrained; that is, they now cycle in lockstep with the cycle of day and night with a period of exactly 24 hours.
• In Arabidopsis, the entrainment of the rhythms requires that light is detected by the
• phytochromes (absorb red light)
• cryptochromes (absorb blue light)
Long-Day Plants
Arabidopsis is a long-day plant and has provided many clues about the mechanism involved in this photoperiodic response. Any response to photoperiod requires a method of keeping time; that is, a clock. Plants, like so many other organisms, have an innate circadian rhythm that regulates the expression of many genes. Among these in Arabidopsis is CONSTANS (CO), a gene that encodes a zinc-finger transcription factor whose levels of mRNA rise and fall with a circadian rhythm.
Translation of CONSTANS mRNA produces the transcription factor that turns on a number of genes, including FLOWERING LOCUS T (FT), a gene needed to start the conversion of apical buds into flower buds.
CONSTANS messenger RNA (mRNA) is abundant early in the morning, declines during the middle part of the day and then rises to another peak late in the afternoon.
However,
• the CONSTANS protein is quickly degraded (in proteasomes) during the morning and middle part of the day and also during the night.
• The degradation triggered by morning light (rich in 660 nm rays) is mediated by phytochrome B (PhyB).
• By late in the afternoon, if the day has been long enough,
• transcription of the CO gene increases producing a rise in CO mRNA
• translation of the CO mRNA produces more CO protein which is
• no longer degraded.
• These effects are mediated by the absorption of
• red (enriched in far-red) light by phytochrome A (PhyA)
• blue light by cryptochrome.
Now with the CONSTANS protein accumulating, it is available to turn on the gene transcription (e.g., FT) needed for the induction of the flowering. In short days, with darkness falling before the rise in CONSTANS mRNA, there is not enough CONSTANS protein synthesized to induce flowering. So flowering in Arabidopsis seems to require the interaction of daylight perceived by phytochromes and cryptochromes with the intrinsic circadian rhythm of CONSTANS expression.
Short-Day Plants
The roles of circadian rhythms and light in short-day plants are not yet as well understood. Studies with rice, a short-day plant, suggests that the mechanism described for Arabidopsis may work there as well but with CONSTANS acting as a suppressor of FLOWERING LOCUS T and thus as an inhibitor of flowering under long days.
Trees
Photoperiodism not only controls flowering in some trees but also stops vegetative growth and promotes the setting of winter buds as the days grow shorter in the autumn. In aspens (Populus sp.) this control is mediated by CONSTANS and FLOWERING LOCUS T. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.04%3A_Plant_Development_-_Fundamentals/16.4E%3A_Photoperiodism_and_Phytochrome.txt |
Unlike animals, plants cannot flee from potentially harmful conditions like drought, freezing, exposure to salt water or salinated soil. They must adapt or die. The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress.
Here are a few examples.
Closing of stomata
Some 90% of the water taken up by a plant is lost in transpiration. Most of this leaves the plant through the pores called stomata - in the leaf. Each stoma is flanked by a pair of guard cells. When the guard cells are turgid, the stoma is open. When turgor is lost, the stoma closes. In angiosperms and gymnosperms (but not in ferns and lycopsids), ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.
The mechanism:
• ABA binds to receptors at the surface of the plasma membrane of the guard cells.
• The receptors activate several interconnecting pathways which converge to produce
• a rise in pH in the cytosol
• transfer of Ca2+ from the vacuole to the cytosol.
• These changes stimulate the loss of negatively-charged ions (anions), especially NO3 and Cl, from the cell and also the loss of K+ from the cell.
• The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor.
• The stomata close.
Protecting cells from dehydration
ABA signaling turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues - from damage when they become dehydrated.
Root growth
ABA can stimulate root growth in plants that need to increase their ability to extract water from the soil.
Bud dormancy
ABA mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter. ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (photoperiodism) will bud dormancy be lifted.
Seed maturation and dormancy
Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. ABA is essential for seed maturation and also enforces a period of seed dormancy. As we saw for buds, it is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy. Not until the seed has been exposed to a prolonged cold spell and/or sufficient water to support germination is dormancy lifted.
Abscission
ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). It is, in fact, this action that gave rise to the name abscisic acid.
The dropping of leaves in the autumn is a vital response to the onset of winter when ground water is frozen - and thus cannot support transpiration - and snow load would threaten to break any branches still in leaf.
Most nondeciduous species in cold climates (e.g., pines) have "needles" for leaves. These are very narrow and have a heavy waterproof cuticle. The shape aids in shedding snow, and the cuticle cuts down on water loss.
Seedling growth
ABA inhibits stem elongation probably by its inhibitory effect on gibberellic acid.
Apical dominance
ABA - moving up from the roots to the stem - synergizes with auxin - moving down from the apical meristem to the stem - in suppressing the development of lateral buds. The result is inhibition of branching or apical dominance. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.05%3A_Plant_Development_-_Hormones/16.5A%3A_Abscisic_acid_%28ABA%29.txt |
Auxins are plant hormones. The most important auxin produced by plants is indole-3-acetic acid (IAA). It plays important roles in a number of plant activities, including:
• development of the embryo
• leaf formation
• phototropism
• gravitropism
• apical dominance
• fruit development
• abscission
• root initiation and development
• the shade-avoidance effect
Embryonic Development
From the very first mitotic division of the zygote, gradients of auxin guide the patterning of the embryo into the parts that will become the organs of the plant:
• shoot apex,
• primary leaves,
• cotyledon(s),
• stem,
• root.
Leaf Formation
The formation of new leaves in the apical meristem is initiated by the accumulation of auxin. Already-developing leaves deplete the surrounding cells of auxin so that the new leaves do not form too close to them. In this way, the characteristic pattern of leaves in the plant is established. Auxin also controls the precise patterning of the epidermal cells of the developing leaf.
Phototropism
Plant shoots display positive phototropism: when illuminated from one direction, the shoot proceeds to grow in that direction. Proposed Mechanism for phototropism is a multiple process. The direction of light is detected at the tip of the shoot with (blue light is most effective). It is absorbed by a flavoprotein called phototropin. Flavoproteins contain flavin as a prosthetic group. Auxin moves from the tip down. An auxin transporter - one of the PIN proteins - is inserted in the plasma membrane at the lateral face of cells of the shoot. Auxin is pumped out of these efflux transporters and accumulates in the cells on the shady side. This stimulates elongation of the cells on the shady side causing the shoot to bend toward the light.
Gravitropism
Gravitropism is a plant growth response to gravity.
• Plant shoots display negative gravitropism: when placed on its side, a plant shoot will grow up
• Roots display positive gravitropism: they grow down.
Possible Mechanism of Gravitropism in Roots
When a root is placed on its side,
• Statoliths (organelles containing starch grains) settle by gravity to the bottom of cells in the root tip.
• This causes PIN proteins to redistribute to the underside of the cell where they pump auxin out of the cell; that is, they are efflux transporters.
• The auxin then accumulates along the under side of the root.
• This INHIBITS root cell elongation.
• So the cells at the top surface of the root elongate, causing the root to grow down.
Apical Dominance
Growth of the shoot apex (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. This phenomenon is called apical dominance. If the terminal shoot of a plant is removed, the inhibition is lifted, and lateral buds begin growth. Gardeners exploit this principle by pruning the terminal shoot of ornamental shrubs, etc. The release of apical dominance enables lateral branches to develop and the plant becomes bushier. The process usually must be repeated because one or two laterals will eventually outstrip the others and reimpose apical dominance.
Apical dominance seems to result from the downward transport of auxin produced in the apical meristem. In fact, if the apical meristem is removed and IAA applied to the stump, inhibition of the lateral buds is maintained.
The common white potato is really a portion of the underground stem of the potato plant. It has a terminal bud or "eye" and several lateral buds. After a long period of storage, the terminal bud usually sprouts but the other buds do not. However, if the potato is sliced into sections, one bud to a section, the lateral buds develop just as quickly as the terminal bud.
Fruit Development
Pollination of the flowers of angiosperms initiates the formation of seeds. As the seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit that covers the seeds.
Some commercial growers deliberately initiate fruit development by applying auxin to the flowers. Not only does this ensure that all the flowers will "set" fruit, but it also maximizes the likelihood that all the fruits will be ready for harvest at the same time.
Abscission
Auxin also plays a role in the abscission of leaves and fruits. Young leaves and fruits produce auxin and so long as they do so, they remain attached to the stem. When the level of auxin declines, a special layer of cells — the abscission layer — forms at the base of the petiole or fruit stalk. Soon the petiole or fruit stalk breaks free at this point and the leaf or fruit falls to the ground.
The figure on the right shows a nice demonstration of the role of auxin in abscission. If the blade of the leaf is removed, as shown in the figure, the petiole remains attached to the stem for a few more days. The removal of the blade seems to be the trigger as an undamaged leaf at the same node of the stem remains on the plant much longer, in fact, the normal length of time. If, however, auxin is applied to the cut end of the petiole, abscission of the petiole is greatly delayed.
Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping.
Root Initiation and Development
The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots. Auxin also stimulates the formation of adventitious roots in many species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant.
Horticulturists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting. The process can often be hastened by treating the cuttings with a solution or powder containing a synthetic auxin.
Once a root is formed, a gradient of auxin concentration develops highest at the tip promoting the production of new cells at the meristem, and lowest in the region of differentiation, thus promoting the elongation and differentiation of root cells. The drop in auxin activity in the regions of elongation and differentiation is mediated by cytokinin — an auxin antagonist.
Translocation of Auxin
Auxin moves through the plant by two mechanisms: It passes in the sap moving through the phloem from where it is synthesized (its "source", usually the shoot) to a "sink" (e.g., the root). It also passes from cell to cell by the following mechanism.
Auxin can enter the cell by diffusion and also through influx transporters in the plasma membrane. It moves out through efflux transporters - called PIN proteins. Eight different types of PIN proteins have been identified so far. These are transmembrane proteins inserted in localized portions of the plasma membrane, e.g.,
• at the top of the cell where they move auxin toward the top of the plant;
• at the basal surface of the cell where they move auxin down the plant;
• at the lateral surface of the cell where they move auxin laterally (e.g., to mediate phototropism and gravitropism).
Identifying the signals that direct the appropriate placement of the PIN proteins is an active area of research.
How does auxin achieve its many different effects in the plant? Auxin effects are mediated by two different pathways: immediate, direct effects on the cell and turning on of new patterns of gene expression
Direct effects of auxin
The arrival of auxin in the cytosol initiates such immediate responses as
• changes in the concentration of and movement of ions in and out of the cell
• reduction in the redistribution of PIN proteins
Some of the direct effects of auxin may be mediated by its binding to a cell-surface receptor designated ABP1 ("Auxin-binding protein 1").
Effects of auxin on gene expression
Many auxin effects are mediated by changes in the transcription of genes. Auxin enters the nucleus and binds to its receptor, a protein called TIR1 ("transport inhibitor response protein 1") which now can bind to proteins responsible for attaching ubiquitin to one or another of several Aux/IAA proteins. This triggers the destruction of the Aux/IAA proteins by proteasomes. Aux/IAA proteins normally bind transcription factors called auxin response factors (ARF) preventing them from activating the promoters and other control sequences of genes that are turned on (or off) by auxin. Destruction of the Aux/IAA proteins relieves this inhibition, and gene transcription begins.
This mechanism is another of the many cases in biology where a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). For example, the gibberellins, another group of plant hormones, exert their effects using a similar strategy. The presence in the cell of many different Aux/IAA proteins (29 in Arabidopsis), many different ARFs (23 in Arabidopsis) and several (~4) TIR1-like proteins provides a logical basis for mediating the different auxin effects that I have described. But how this is done remains to be discovered.
Synthetic auxins as weed killers
Some of the most widely-used weed killers are synthetic auxins. These include 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T). As the formulas show, 2,4,5-T is 2,4-D with a third chlorine atom, instead of a hydrogen atom, at the #5 position in the benzene ring (blue circles).
2,4-D and its many variants are popular because they are selective herbicides, killing broad-leaved plants but not grasses (no one knows the basis of this selectivity).
Why should a synthetic auxin kill the plant? It turns out that the auxin influx transporter works fine for 2,4-D, but that 2,4-D cannot leave the cell through the efflux transporters. Perhaps it is the resulting accumulation of 2,4-D within the cell that kills it.
A mixture of 2,4,-D and 2,4,5-T was the "agent orange" used by the U.S. military to defoliate the forest in parts of South Vietnam. Because of health concerns, 2,4,5-T is no longer used in the U.S. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.05%3A_Plant_Development_-_Hormones/16.5B%3A_Auxin.txt |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.