(A) The mammalian testis is composed of seminiferous tubules intertwined so that the “start” and “end” of these tubules are both connected to the rete testis. Immotile sperm flow from the lumen of the seminiferous tubules into the epididymis via the rete testis. During their passage through the epididymis to the vas deferens, sperm acquire their motility. Adapted with permission from Nature Reviews Genetics (59). (B) Histological cross section through an adult mouse testis depicting seminiferous tubules, the peritubular myoid cells, and the interstitium (space between tubules). (C) Expansion of both the undifferentiated (A spermatogonia) and differentiated (A1 spermatogonia) spermatogonial populations occurs by mitosis of these cell types, regulated in part by FSH. Undifferentiated spermatogonia enter the differentiation pathway at the time of the A-to-A1 spermatogonia transition. The red arrow denotes the required action of vitamin A (in the form of RA) in this transition. The subsequent conversion of differentiated spermatogonia to spermatocytes, representing the initiation of meiosis, also requires RA activity. The differentiation of secondary spermatocytes (m2°m) and the process of spermiogenesis (round spermatids to elongated spermatids, then spermatozoa) requires testosterone.