Avian Reproduction

Atsushi Asano, Atsushi Tajima

Terminally differentiated avian sperm consist of a head which male genetic material locates and flagellum that provides the motive force to propel them towards the fertilization site. The apical end of the sperm head accommodates a secretory vesicle, called an acrosome, that undergoes acrosome reaction releasing proteolytic content to penetrate the peri-vitelline membrane of an egg. Transcriptionally and translationally inactive, sperm need to rely on these distinct compartments in which different functions are preassembled, in order to achieve the goal of "fertilization". How are these complex structures with high functionality formed? Spermatogenesis is divided into an early stage in which diploid spermatogonia is proliferated into round spermatids thorough mitotic and meiotic divisions, and a late stage in which round spermatids are transformed into sperm though nuclear condensation and elongation of the sperm head, and formation of accessory structures. Recently, it was reported in aves that morphologically differentiated sperm undergo post-testicular maturation during passage through the male genital tract, suggesting that a similar system to mammals might be involved in the acquisition of fertilizing ability in avian sperm. Investigation for mechanisms underlying how sperm regulate their functions which are necessary to achieve fertilization is important for developing reproductive biotechnology in aves, because cryopreservation of poultry sperm is still not reliable for use in commercial production or for the preservation of genetic resources. In this review, we firstly provide an update on avian spermatogenesis, and then discuss the uniqueness of structure and functions of avian sperm, highlighting differences from mammalian sperm. Lastly, we discuss the molecular mechanism and current techniques of cryopreservation for avian sperm.