hematopoiesis is defined as the formation of all types of blood cells from hematopoietic stem cells ( hscs ) ( orkin and zon , 2008 ) . this complex process involves the coordination of intrinsic master blood transcriptional regulators and signaling molecules from the surrounding tissues ( huang and zon , 2008 ) . understanding the molecular and cellular mechanisms that drive normal hematopoiesis and hematopoietic malignancies is crucial for developing new treatments for blood diseases . despite the identification of many essential genes involved in hematopoiesis this genetically tractable model has many appealing features , such as the ease of manipulation of transparent embryos and the capacity to carry out large - scale genetic and chemical screens ( lieschke and currie , 2007 ) . 1a ) ( davidson and zon , 2004 ; de jong and zon , 2005 ) ; however , hematopoiesis in zebrafish occurs in sites that are distinct from other vertebrates . for example , zebrafish hscs reside in the kidney marrow , which is functionally equivalent to the bone marrow niche of mammals . nonetheless , the developmental processes and genetic programs of hematopoiesis are highly conserved in zebrafish ( fig . 1b ) ( chen and zon , 2009 ; huang and zon , 2008 ; paik and zon , 2010 ) . in the past two decades , a wealth of studies has been published that has supplied many novel insights and discoveries into the field of hematology . additionally , the parallels between the hematopoietic system in zebrafish and humans prompt the increasing use of zebrafish to model hematological disorders . excellent recent reviews cover the different aspects of these studies in great detail ( bertrand and traver , 2009 ; carradice and lieschke , 2008 ; chen and zon , 2009 ; ellett and lieschke , 2010 ; huang and zon , 2008 ; paik and zon , 2010 ; payne and look , 2009 ) . in this primer article , we survey various methods for research in zebrafish , illustrate the advantages of using zebrafish to study normal and pathogenic hematopoiesis , highlighting key recent advances , and discuss the contribution and potential of zebrafish to identify new disease treatments . the zebrafish is amenable to forward genetic screens on a scale that is infeasible in any other vertebrate models that have the same low cost and space requirements ( patton and zon , 2001 ) . the embryos can survive without red blood cells by passive diffusion of oxygen for the first week , allowing the identification of mutations that would be embryonic lethal in mice . screens using the chemical mutagen enu ( n - ethyl - n - nitrosourea ) or retroviral mutagenesis have successfully yielded a plethora of blood mutants ( amsterdam et al . , 2004 ; ransom et al . , 1996 ; weinstein et al . , these mutants have helped elucidate the genetic pathways that regulate hscs as well as different hematopoietic lineages , and many of them are also excellent models of human blood diseases ( carradice and lieschke , 2008 ; north and zon , 2003 ; shafizadeh and paw , 2004 ) . in some instances , zebrafish mutants have revealed genes that have later been shown to be involved in human disease . for example , cloning of the hypochromic anemic mutant weissherbst ( weh ) identified a previously unknown iron transporter , ferroportin1 ( donovan et al . , 2000 ) . subsequent to the zebrafish findings , mutations in this gene were found in patients with type iv hemochromatosis ( iron overload ) ( gordeuk et al . , 2003 ; montosi et al . , genetic modifier screens that are routinely used in invertebrates such as flies and worms are effective for identifying genes that suppress or enhance the function of the gene of interest , and thus have the potential to reveal new therapeutic targets . the first zebrafish suppressor screen was recently carried out in moonshine mutants , which are deficient for transcriptional intermediary factor 1- ( tif1 ; also known as trim33 ) ( bai et al . , 2010 ) . these embryos have severely impaired erythropoiesis because they fail to appropriately express gata1 , a crucial factor for erythropoiesis ( ransom et al . , 2004 ) . tif1 also promotes erythropoiesis in mammals ( he et al . , 2006 ) , but its molecular mechanism remained elusive . created a viable homozygous tif1 line by rescuing zebrafish by using a green fluorescent protein ( gfp)-labeled wild - type tif1 transgene , and subsequently performed a haploid genetic suppressor screen ( see fig . the screen identified that mutation of the cdc73 gene [ which encodes a component of the paf ( pol - ii - associated factor ) complex ] , restores erythropoiesis in tif1 embryos . knocking down the expression of other paf subunits or components of the 5,6-dichloro-1--d - ribofuranosylbenzimidazole ( drb ) sensitivity - inducing factor ( dsif ) complex restored red blood cells to wild - type levels in tif1 embryos . mechanistically , paf and dsif stall rna polymerase ii ( pol ii ) at target gene promoters . tif1 recruits transcription elongation factors such as positive transcription elongation factor b ( p - tefb ) and facilitates chromatin transcription ( fact ) complex to erythroid genes to release the paused pol ii , thereby promoting transcription . the suppressor screen thus uncovered an unexpected role of tif1 in transcriptional elongation during erythroid cell differentiation . embryonic primitive hematopoiesis ( yellow ) starts at around 11 hours post fertilization ( hpf ) when hemangioblasts ( which have the potential to become either endothelial vascular cells or hematopoietic cells , shown in white ) appear in the anterior lateral mesoderm ( alm ) and posterior lateral mesoderm ( plm ) , which collectively are analogous to the blood islands in the mammalian yolk sac . hemangioblasts in the plm later converge medially to form the intermediate cell mass ( icm ; not shown ) , where primitive erythrocytes ( red ) predominantly arise . the alm , which later becomes the rostral blood island ( rbi ; not shown ) , is the major site for primitive myeloid cells ( blue ) . at around 24 hpf , a transient definitive wave ( pink ) initiates shortly after multi - lineage erythromyeloid progenitors appear in the posterior blood island ( pbi ) . starting from 26 hpf , definitive hscs ( gray ) emerge from hemogenic endothelial cells of the dorsal aorta in the aorta - gonad - mesonephros ( agm ) region . shortly thereafter , hscs migrate to and seed the caudal hematopoietic tissue at 48 hpf , which is an expansion of the pbi and acts as a transient hematopoietic site that gives rise to erythroid , myeloid and thromboid ( yellow ) cells . kidney marrow , which is functionally similar to mammalian bone marrow , gives rise to all blood lineages , including erythroid , myeloid , thromboid and lymphoid ( green ) cells for the larval and adult zebrafish . at around 54 hpf , common lymphoid progenitor ( clp ) cells from the agm region seed the thymus , which is the site for maturation of lymphoid t cells . the names of corresponding available zebrafish mutants are shown in parenthesis ; the mutated genes are unknown for the mutants preceded by  ? . adapted from chen and zon ( chen and zon , 2009 ) and paik and zon ( paik and zon , 2010 ) , with permission . the function of candidate genes during hematopoietic development and the genetic interactions that affect disease phenotypes can be quickly examined in zebrafish embryos and larvae through microinjections of mrna ( overexpression ) and morpholino antisense oligomers ( knockdown ) ( nasevicius and ekker , 2000 ) , without the need to make stable animal strains . for instance , ribosomal proteins are frequently mutated in patients with diamond - blackfan anemia ( dba ) , but it is not clear how the ribosomal defects lead to the disease . two reports have found that knockdown of ribosomal protein 19 ( rps19 ) in zebrafish results in dramatic loss of red blood cells as well as morphological abnormalities that resemble dba ( danilova et al . , 2008 ; uechi et al . , 2008 ) . in addition , the maturation and proliferation of erythroid lineages are severely blocked , as found in humans with the disease . the dose effect of rps19 on disease phenotype was demonstrated by the injection of different amounts of morpholinos . loss of rps19 induces broad activation of the p53 pathway and apoptosis in embryos , including of erythroid cells , which is consistent with observations from murine models . in addition , mutations in other ribosomal proteins all led to similar activation of p53 family proteins . these studies pointed out that the p53 protein family could be a target for dba therapies . lethal mutations in a gene of interest ( gi ) can be rescued by co - expressing a transgene in which a wild - type copy of the gene ( gi ) marked with gfp is driven by an appropriate promoter [ note that a bacterial artificial chromosome ( bac ) transgene that contains the entire locus of the gi gene could be used in the absence of suitable promoter , as in the case for moonshine ( bai et al . , 2010 ) ] , enabling survival to adulthood ( f0 ) . then , male f0 fish are mutagenized with the chemical enu and are crossed to untreated female fish to generate f1 progeny . the f1 female fish that carry one copy of the transgene eggs are squeezed out from the mother and fertilized with uv - treated sperm ( which still activates the eggs but does not contribute chromosomes ) . all of the eggs develop into haploid embryos ( f2 ) , which stay alive until 5 days post - fertilization . 50% of the f2 embryos are gfp - positive ( i.e. they express the rescuing transgene ) and develop normally . the other 50% of haploid embryos are gfp - negative ( i.e. they do not express the rescuing transgene ) and develop recessive mutant phenotypes . however , if the mother carries a suppressor mutation in its genome ( contributed by the enu - mutagenized f0 male ) , 50% of the gfp - negative haploid embryos will be rescued ( yellow ) . although gene targeting by homologous recombination is widely used in mice , this method is still lacking in zebrafish . however , recent studies have developed the techniques tilling ( targeting induced local lesions in genome ) ( stemple , 2004 ) and zinc - finger nuclease ( zfn)-mediated mutation ( doyon et al . , 2008 ; meng et al . , 2008 ; sander et al . , 2011 ) to engineer stable strains carrying defects at specific gene loci . recently , the von hippel - lindau tumor suppressor ( vhl ) mutant was identified through tilling ( van rooijen et al . , 2009 ) the mutants develop polycythemia , resembling the clinical manifestation of vhl - associated chuvash polycythemia , thereby offering the first in vivo system to study the progression of this disease . furthermore , injection of mrna and cdna into zebrafish embryos can be used to quickly dissect the functions of specific mutations identified in patients . in the aforementioned human disorder hemochromatosis , different mutations in ferroportin1 ( slc40a1 ) lead to iron overload in either macrophages or hepatocytes . determining the effect of specific mutations often requires monitoring the localization of the protein in cultured cells and the level of cellular iron accumulation . de domenico and colleagues injected the mutant ferroportin1 cdna into zebrafish embryos and found that only mutations affecting macrophages , not hepatocytes , severely limit erythropoiesis , demonstrating a rapid method by which to diagnose the effect of specific ferroportin1 mutations found in humans ( de domenico et al . , 2007 ) . zebrafish cancer models are predominantly generated through transgenic expression of oncogenes . generating stable transgenic zebrafish is efficiently done with tol2 transposons ( kawakami , 2007 ) . several approaches , such as the cre - loxp and gal4-uas systems , the first zebrafish model of leukemia was of t - cell acute lymphoblastic leukemia ( t - all ) , and involved transgenic zebrafish expressing the mouse myc gene tagged with enhanced gfp ( egfp ) under the control of the rag2 promoter ( rag2:egfp - myc ) ( langenau et al . , 2003 ) . the leukemia induced in this model mirrors many aspects of the human disease , with the added benefit that the egfp - labeled leukemic cells can be directly visualized . the induction of leukemia in this model is so efficient that the majority of the injected animals die before reaching sexual maturity . a conditional cre - loxp system was subsequently introduced : in double transgenics expressing rag2:loxp - dsred - loxp - gfp - myc and heat - shock - inducible cre recombinase ( hsp70:cre ) , t - cell lymphoblastic lymphoma ( t - lbl ) was efficiently induced after heat - shock induction , and t - lbl rapidly progressed to t - all ( feng et al . , 2007 ) . the authors of this study then used the transgenic zebrafish to dissect the molecular pathways involved in the disease progression by testing candidate genes ( feng et al . , 2010 ) because , in humans , the molecular cause for the progression from the highly localized thymic t - lbl to disseminated t - all is not clear . the zebrafish experiments revealed that apoptosis regulator bcl2 , sphingosine 1-phosphate receptor 1 ( s1p1 ) and intercellular adhesion molecule 1 ( icam1 ) are highly expressed in t - lbl ; this inhibits apoptosis and prevents the tumor cells from invading the vasculature owing to increased cell - cell adhesion . akt activation can overcome the block in intravasation and promote the dissemination of t - lbl cells . these studies indicate that treating t - lbl patients with a combination of bcl2 and akt inhibitors could promote lymphoblast death and also prevent the dissemination of tumor cells . the identified markers can further be used to determine t - lbl- and t - all - specific therapies . the aml1-eto ( ae ) translocation ( also known as runx1-runx1t1 ) is the most common chromosomal translocation in acute myeloid leukemia ( aml ) . ubiquitous overexpression of the oncogenic ae protein in zebrafish embryos leads to early lethality and thus requires inducible expression by the heat - shock promoter in transgenic fish ( hsp70 : aml1-eto ) ( yeh et al . , 2008 ) . the induced ae protein in embryos redirects erythroid progenitor cells to develop into granulocytic cells , strongly resembling the cell fate redirections observed in ae - associated aml patients . this study further found that downregulation of the hematopoietic master regulator gene scl is required for the cell fate switch . zebrafish embryos are permeable to water - soluble chemicals and can be housed in 96-well plates , making them ideal for high - throughput chemical screening for novel bioactive compounds ( zon and peterson , 2005 ) . compared with cell - based drug screening , a whole - animal model substantially reduces the likelihood of encountering common issues that arise during late stages of drug development , such as drug toxicity and off - target side effects ( bowman and zon , 2008 ) . chemical screens based on zebrafish disease models have the potential to identify lead compounds for novel therapeutics . for example , in a chemical screen that was conducted to identify new regulators of hscs , north et al . found that prostaglandin e2 ( pge2 ) increased hsc number ( north et al . , 2007 ) . pge2 interacts with the wnt signaling pathway to regulate hscs through camp - pka - mediated -catenin phosphorylation ( goessling et al . , 2009 ) . a stable derivative of pge2 ( dmpge2 ) increases the frequency of long - term repopulating hscs in irradiated murine bone marrow , strongly suggesting a therapeutic potential ( north et al . , 2007 ) . dmpge2 is currently in clinical trials for treating patients receiving cord blood stem cell transplants . as another example , yeh and colleagues utilized the transgenic hsp70:aml1-eto line discussed above and screened for compounds that suppressed the inducible erythropoiesis - to - granulopoiesis conversion ( yeh et al . , 2009 ) . they found that chemicals that inhibit pge2 or -catenin pathways block the capacity of ae to redirect hematopoiesis , a strategy that could potentially provide therapeutic benefits in aml patients . these two independent screens collectively demonstrate the crucial roles of pge2 and -catenin interaction in normal and leukemic hematopoiesis . in addition to identifying therapeutic compounds , chemical screens can also reveal genetic pathways that are involved in hematopoiesis . in the same chemical screen that unveiled pge2 as a regulator of hscs , compounds that regulate blood flow were also found to affect hsc formation during embryogenesis . further analysis demonstrated that the initiation of circulation seems to trigger the formation of hscs and that circulation regulates hscs via nitric oxide ( no ) signaling ( north et al . , 2009 ) . zebrafish embryos , which are transparent , allow direct visualization of hematopoietic cells in live animals . various fluorescent transgenic lines have been generated that facilitate the imaging of hematopoietic cells ( bertrand and traver , 2009 ) . three studies in 2010 reported imaging of the birth of hscs during development ( bertrand et al . , 2010 ; boisset et al . , 2010 ; kissa and herbomel , 2010 ) . the precise origin of hscs  whether they arise from the aortic floor or the underlying mesenchyme within the aorta - gonad - mesonephros ( agm ) region has been a longstanding question in the field . two studies directly imaged hsc creation in zebrafish embryos using fluorescent reporter transgenes to mark nascent hscs ( cmyb : egfp or cd41:gfp ) and vascular precursors ( flk1:memcherry , flk1:gfp or lmo2:dsred ) . the authors found that hscs directly emerge from aortic endothelial cells through unique cell movement in which single endothelial cells bend out of the aortic ventral wall to the sub - aortic space ( bertrand et al . these results were corroborated in a third study , in which the mouse embryos were cut into transverse sections and the deeply located aorta was imaged in the cultured live sections ( boisset et al . , 2010 ) . these studies provide a better understanding of hsc emergence during development , which will aid efforts to eventually instruct hsc formation from embryonic stem cells or induced pluripotent stem cells ( ipscs ) for clinical applications . photo - activation of caged fluorescent compounds has been employed in zebrafish embryos to follow the migration and cell fate of individual hematopoietic precursor cells or hscs ( bertrand et al . , 2008 ; , 2008 ; vogeli et al . , 2006 ; warga et al . , 2009 ) . in addition , an inducible cre transgene ( encoding cre protein fused to estrogen receptor ) and lox - switch transgene ( loxp - stop - loxp - gfp ) have been used to permanently label hematopoietic cells ( bertrand and traver , 2009 ) . in principle , by carefully choosing cre and loxp transgenes , a small set of hematopoietic cells can be labeled during a specific temporal window . then , the migration , proliferation and fate potentials of these cells can be studied in their physiological environments . furthermore , kidney marrow transplantation into sub - lethally irradiated animals has been developed in zebrafish ( pugach et al . , 2009 ; traver et al . , 2004 ) , and each major adult blood lineage can be isolated by flow cytometry ( traver et al . , 2003 ) . these assays allow the functional studies of adult hscs , and are analogous to bone marrow transplantation assays in mice . furthermore , a transparent adult zebrafish casper line was recently created , which enables the visualization of transplanted hscs during homing and engraftment process in adult animals ( white et al . , 2008 ) . zebrafish embryos are permeable to water - soluble chemicals and can be housed in 96-well plates , making them ideal for high - throughput chemical screening for novel bioactive compounds ( zon and peterson , 2005 ) . compared with cell - based drug screening , a whole - animal model substantially reduces the likelihood of encountering common issues that arise during late stages of drug development , such as drug toxicity and off - target side effects ( bowman and zon , 2008 ) . chemical screens based on zebrafish disease models have the potential to identify lead compounds for novel therapeutics . for example , in a chemical screen that was conducted to identify new regulators of hscs , north et al . found that prostaglandin e2 ( pge2 ) increased hsc number ( north et al . , 2007 ) . pge2 interacts with the wnt signaling pathway to regulate hscs through camp - pka - mediated -catenin phosphorylation ( goessling et al . , 2009 ) . a stable derivative of pge2 ( dmpge2 ) increases the frequency of long - term repopulating hscs in irradiated murine bone marrow , strongly suggesting a therapeutic potential ( north et al . , 2007 ) . dmpge2 is currently in clinical trials for treating patients receiving cord blood stem cell transplants . as another example , yeh and colleagues utilized the transgenic hsp70:aml1-eto line discussed above and screened for compounds that suppressed the inducible erythropoiesis - to - granulopoiesis conversion ( yeh et al . , 2009 ) . they found that chemicals that inhibit pge2 or -catenin pathways block the capacity of ae to redirect hematopoiesis , a strategy that could potentially provide therapeutic benefits in aml patients . these two independent screens collectively demonstrate the crucial roles of pge2 and -catenin interaction in normal and leukemic hematopoiesis . in addition to identifying therapeutic compounds , chemical screens can also reveal genetic pathways that are involved in hematopoiesis . in the same chemical screen that unveiled pge2 as a regulator of hscs , compounds that regulate blood flow were also found to affect hsc formation during embryogenesis . further analysis demonstrated that the initiation of circulation seems to trigger the formation of hscs and that circulation regulates hscs via nitric oxide ( no ) signaling ( north et al . , 2009 ) . zebrafish embryos , which are transparent , allow direct visualization of hematopoietic cells in live animals . various fluorescent transgenic lines have been generated that facilitate the imaging of hematopoietic cells ( bertrand and traver , 2009 ) . three studies in 2010 reported imaging of the birth of hscs during development ( bertrand et al . , 2010 ; boisset et al . , 2010 ; kissa and herbomel , 2010 ) . the precise origin of hscs  whether they arise from the aortic floor or the underlying mesenchyme within the aorta - gonad - mesonephros ( agm ) region has been a longstanding question in the field . two studies directly imaged hsc creation in zebrafish embryos using fluorescent reporter transgenes to mark nascent hscs ( cmyb : egfp or cd41:gfp ) and vascular precursors ( flk1:memcherry , flk1:gfp or lmo2:dsred ) . the authors found that hscs directly emerge from aortic endothelial cells through unique cell movement in which single endothelial cells bend out of the aortic ventral wall to the sub - aortic space ( bertrand et al . these results were corroborated in a third study , in which the mouse embryos were cut into transverse sections and the deeply located aorta was imaged in the cultured live sections ( boisset et al . , 2010 ) . these studies provide a better understanding of hsc emergence during development , which will aid efforts to eventually instruct hsc formation from embryonic stem cells or induced pluripotent stem cells ( ipscs ) for clinical applications . photo - activation of caged fluorescent compounds has been employed in zebrafish embryos to follow the migration and cell fate of individual hematopoietic precursor cells or hscs ( bertrand et al . , 2008 ; jin et al . , 2007 , 2008 ; vogeli et al . , 2006 ; warga et al . , 2009 ) . in addition , an inducible cre transgene ( encoding cre protein fused to estrogen receptor ) and lox - switch transgene ( loxp - stop - loxp - gfp ) have been used to permanently label hematopoietic cells ( bertrand and traver , 2009 ) . in principle , by carefully choosing cre and loxp transgenes , a small set of hematopoietic cells can be labeled during a specific temporal window . then , the migration , proliferation and fate potentials of these cells can be studied in their physiological environments . furthermore , kidney marrow transplantation into sub - lethally irradiated animals has been developed in zebrafish ( pugach et al . , 2009 ; traver et al . , 2004 ) , and each major adult blood lineage can be isolated by flow cytometry ( traver et al . , 2003 ) . these assays allow the functional studies of adult hscs , and are analogous to bone marrow transplantation assays in mice . furthermore , a transparent adult zebrafish casper line was recently created , which enables the visualization of transplanted hscs during homing and engraftment process in adult animals ( white et al . , 2008 ) . studies using zebrafish have greatly infused our understanding of hematopoiesis and the perturbation of hematopoietic programs in disease . future chemical and genetic screens will continue to identify additional genes and new compounds that modulate the generation of hscs and other downstream lineages . the speed and scale of the screens will be enhanced by incorporating fluorescent transgenic lines , particularly with the new automated high - throughput manipulation and imaging platforms ( pardo - martin et al . , 2010 ) . functional assays of long - term hsc self - renewal in zebrafish ( i.e. competitive and serial kidney marrow transplantation ) are under development , and screens for hsc function are highly anticipated . the use of zebrafish models for studying blood diseases such as leukemia is frequently limited by the low penetrance and long latency of the disease phenotype . therefore , it is necessary to develop stable transgenic lines using different approaches to express pathogenic genes at sufficiently high levels to enhance the use of zebrafish models . the low penetrating phenotype might also indicate that multiple genetic mutations are required to provide a predisposing background for disease induction . zebrafish are particularly amenable to multigenetic analysis through a combination of convenient microinjections of morpholinos and mrna or cdna . however , a significant challenge in using zebrafish is the current lack of target - specific conditional mutagenesis technology , although the new zfn method has the potential to enable the development of a knock - in approach ( woods and schier , 2008 ) . furthermore , modeling a disease in embryonic zebrafish provides a convenient platform in which to carry out genetic or chemical screens for suppressors or enhancers of the disease phenotype . however , it is important to understand how the disease and zebrafish models differ at a mechanistic level in order to select the optimal assays for high - throughput screens . finally , these recently developed genetic tools allow modeling of human chronic blood disease in zebrafish adults . however , a better understanding of anatomy and physiology of adult hematopoiesis will be required to make optimal use of them . high fecundity , small size and fast embryonic development make zebrafish suitable for large - scale forward and modifier genetic screens with low cost and space requirements . transient genetic manipulations mediated by morpholino , mrna or cdna injections offer convenient methods for functional analysis of candidate genes without the need to make transgenic animals . human blood diseases have been modeled in zebrafish , and small - molecule screens in such models have the potential to reveal new drugs for human disease treatment . the transparency of zebrafish embryos and of the adult zebrafish casper line allows live imaging of hematopoietic cells and of the pathologies of the diseases that affect them . in addition to the lack of knock - in technology , several other major limitations need to be overcome in order to fully exploit the potential of zebrafish in hematological research . for example , few antibody reagents are available for biochemical and immunohistochemistry studies in zebrafish systems . production of target - protein - specific antibodies , including those that recognize cell - surface markers , for zebrafish is ongoing . in addition , studies in zebrafish have been hindered by the lack of methods that are available to culture and differentiate hematopoietic cells in vitro . two recent papers reported in vitro culture and clonal assays of zebrafish hematopoietic progenitor cells ( stachura et al . , 2009 ; stachura et al . , 2011 ) . the identification of additional growth factors and the optimization of in vitro techniques will eventually enable the culture , proliferation , differentiation and therefore more in - depth study of all zebrafish blood - cell lineages .
zebrafish studies in the past two decades have made major contributions to our understanding of hematopoiesis and its associated disorders . the zebrafish has proven to be a powerful organism for studies in this area owing to its amenability to large - scale genetic and chemical screening . 
 in addition , the externally fertilized and transparent embryos allow convenient genetic manipulation and in vivo imaging of normal and aberrant hematopoiesis . 
 this review discusses available methods for studying hematopoiesis in zebrafish , summarizes key recent advances in this area , and highlights the current and potential contributions of zebrafish to the discovery and development of drugs to treat human blood disorders .