Spermatogenesis and Spermiogenesis, Fish

After introducing the three main phases of spermatogenesis (mitotic, meiotic, and spermiogenic), we describe the Sertoli-germ cell relationship characteristic of cystic spermatogenesis in fish. We then explore how spermatogonial stem cells, along with Sertoli cells, form spermatogenic cysts, the functional units of fish spermatogenesis. The distribution of spermatogonial stem cells determines the two primary testis types in fish: unrestricted and random versus restricted and concentrated in the testis periphery. We then discuss how testis-produced paracrine signals and pituitary gonadotropic hormones, follicle-stimulating hormone, and luteinizing hormone, regulate the testicular growth phase and germ cell differentiation into specialized sperm cells ready for fertilization.