Molecular control mechanisms of fish spermatogenesis

Fish spermatogenesis, from spermatogonial stem-cell renewal to sperm maturation, is controlled by the sex steroid hormones. Mitotic divisions of spermatogonia can be categorized by spermatogonial stem cell renewal and spermatogonial proliferation. Spermatogonial renewal is regulated by estradiol-17β (E₂; the natural estrogen in vertebrates), and spermatogonial proliferation toward meiosis is promoted by 11-ketotestosterone (11-KT), the main androgen in teleost. The action of E2 and 11-KT is mediated by other factors produced by Sertoli cells; E2 is mediated by spermatogonial stem-cell renewal factor and 11-KT is mediated by spermatogenesis preventing substance and activin B. Although 11-KT also induce meiosis and spermiogenesis, the control mechanisms of these processe are not clear. After spermiogenesis, immature spermatozoa undergo sperm maturation. Sperm maturation is regulated by 17α,20β-dihydroxy-4-pregnen-3-one (DHP), which is progestin in teleost. The DHP acts directly on spermatozoa to active the carbonic anhydrase existed in the spermatozoa. This enzymatic activation causes an increase in the seminal plasma pH, enabling spermatozoa to motile.