Sertoli cell structure and function in anamniote vertebrates

Anamniote vertebrates (fishes and amphibians) show cystic spermatogenesis, in which Sertoli cells present important characteristics that provide new insights into Sertoli cell physiology. Unlike amniotes (in reptiles, birds, and mammals), anamniote Sertoli cells are mitotically active, and this activity can be differentiated into two modes: (i) Sertoli cell progenitors proliferate to form new niche space that then can be occupied by single Aund spermatogonia to form new spermatogenic cysts that are regulated by estrogens, thyroid hormone, follicle-stimulating hormone (FSH), and insulin-like growth factor 3; and (ii) Sertoli cells already associated with existing cysts proliferate to accommodate the growing germ cell clone, which is modulated by FSH, androgens, and progestins. Sertoli cells’ terminal differentiation occurs when meiosis is completed by the accompanied germ cells and, simultaneously, tight junctions among Sertoli cells are formed. Because anamniote Sertoli cells show very high support capacity for germ cells, comprehensive studies of Sertoli cells may provide important clues to the regulatory mechanisms of these cells in vertebrates.