spermatozoa often occurs in association with increased DNA damage. DNA fragmentation and increased in situ DNA susceptibility to denaturation, the features similar to these seen during apoptosis of somatic cells, characterize abnormal spermatozoa in cases of male infertility. Avoidance of immune system response Glycoprotein molecules on the surface of ejaculated sperm cells are recognized by all human female immune systems, and interpreted as a signal that the cell should not be rejected. The female immune system might otherwise attack sperm in the reproductive tract. The specific glycoproteins coating sperm cells are also utilized by some cancerous and bacterial cells, some parasitic worms, and HIV-infected white blood cells, thereby avoiding an immune response from the host organism. The blood-testis barrier, maintained by the tight junctions between the Sertoli cells of the seminiferous tubules, prevents communication between the forming spermatozoa in the testis and the blood vessels (and immune cells circulating within them) within the interstitial space. This prevents them from eliciting an immune response. The blood-testis barrier is also important in preventing toxic substances from disrupting spermatogenesis. Spermatozoa in other organisms Animals Fertilization relies on spermatozoa for most sexually reproductive animals. Some species of fruit fly produce the largest known spermatozoon found in nature. Drosophila melanogaster produces sperm that can be up to 1.8 mm, while its relative Drosophila bifurca produces the largest known spermatozoon, measuring over 58 mm in length. In Drosophila melanogaster, the entire sperm, tail included, gets incorporated into the oocyte cytoplasm, however, for Drosophila bifurca only a small portion of the tail enters the oocyte. The wood mouse Apodemus sylvaticus possesses spermatozoa with falciform morphology. Another characteristics which makes these gametocytes unique is the presence of an apical hook on the sperm head. This hook is used to attach to the hooks or to the flagella of other spermatozoa. Aggregation is caused by these attachments and mobile trains result. These trains provide improved motility in the female reproductive tract and are a means by which fertilization is promoted. The postmeiotic phase of mouse spermatogenesis is very sensitive to environmental genotoxic agents, because as male germ cells form mature spermatozoa they progressively lose the ability to repair DNA damage. Irradiation of male mice during late spermatogenesis can induce damage that persists for at least 7 days in the fertilizing spermatozoa, and disruption of maternal DNA double-strand break repair pathways increases spermatozoa-derived chromosomal aberrations. Treatment of male mice with