Describe the process of spermatogenesis, including the roles of sustentocytes and the blood-testis barrier.

Spermatogenesis in the seminiferous tubules begins when spermatogonia divide by mitosis; one daughter cell stays at the basal lamina as a stem cell, while the other becomes a primary spermatocyte. After moving into the adluminal compartment, the primary spermatocyte undergoes meiosis I and II to form haploid spermatids, which then differentiate into sperm through spermiogenesis. Sustentocytes (Sertoli cells) support and nourish the developing cells, and their tight junctions form the blood-testis barrier that prevents sperm antigens from entering the bloodstream and triggering an immune response.

Spermatogenesis occurs in the epithelial walls of the seminiferous tubules. The outermost stem cells, spermatogonia, divide continuously by mitosis. After puberty, each mitotic division produces a type A daughter cell that remains at the basal lamina to maintain the germ cell pool, and a type B daughter cell that is pushed toward the lumen and becomes a primary spermatocyte. The primary spermatocyte then undergoes meiosis I to produce two haploid secondary spermatocytes, which quickly complete meiosis II to form four haploid spermatids. Each spermatid then undergoes spermiogenesis, elongating, shedding excess cytoplasm, and forming a tail to become a mature spermatozoon with a head, midpiece, and tail. Sustentocytes, also called Sertoli cells, extend from the basal lamina to the lumen and are connected laterally by tight junctions. These junctions divide the tubule into a basal compartment containing spermatogonia and a few primary spermatocytes, and an adluminal compartment containing meiotically active cells and the lumen. The tight junctions create the blood-testis barrier, which prevents membrane antigens of differentiating sperm from escaping into the bloodstream, where they would activate the immune system. The spermatogonia, recognized as self, remain outside the barrier and can receive bloodborne signals. When type B daughter cells move into the adluminal compartment, the tight junctions temporarily open to allow passage. Sustentocytes also nourish spermatogenic cells, move them toward the lumen, secrete testicular fluid that transports sperm, phagocytize faulty cells and excess cytoplasm, and produce inhibin and androgen-binding protein, which help regulate spermatogenesis.