Spermatogenesis and Spermiogenesis, Fish

Spermatogenesis in fish resembles that of other vertebrates: the germ cells develop in close contact with a somatic cell type, the Sertoli cells that provide the required structural, nutritional and regulatory support. In fish, initially one or two Sertoli cells envelop a single type A spermatogonium, thus forming the earliest stage of a spermatogenic cyst. Consecutive rounds of mitotic divisions result in a geometrically (i.e., 2, 4, 8, 16, etc.) growing number of differentiating spermatogonia, such that also the cyst-forming Sertoli cells need to proliferate, resulting overall in a growing spermatogenic cyst. After completion of the mitotic expansion, spermatogonia continue to develop, as in other vertebrates, via meiosis and eventually spermiogenesis to terminally differentiate into spermatozoa. They are released by Sertoli cells (spermiation) via opening of the cyst. Two important differences to higher vertebrates are that in fish, a given Sertoli cell contact only one or two germ cell stages, and that Sertoli cell proliferation is typically observed in the adult testis. Mitotic proliferation of spermatogonia and meiosis are mostly regulated by FSH and steroids, in turn modulating the production of paracrine factors often produced by Sertoli cells. During spermiogenesis, LH regulates the final steps to form the flagellated spermatozoa, which does not show an acrosome in fish.