Sertoli cells are critical to germ cell development, as indicated by their close contact. As many as 6 to 12 spermatids may be attached to a Sertoli cell. Sertoli cells phagocytose residual bodies (excess cytoplasm resulting from the transformation of spermatids to spermatozoa) and damaged germ cells, provide structural support and nutrition for germ cells, secrete fluids, and assist in spermiation, the final detachment of mature spermatozoa from the Sertoli cell into the lumen. Spermiation may involve plasminogen activator, which converts plasminogen to plasmin, a prote-olytic enzyme that assists in the release of the mature sperm into the lumen. Sertoli cells also synthesize large amounts of transferrin, an iron-transport protein important for sperm development.
During the fetal period, Sertoli cells and gonocytes form the seminiferous tubules as Sertoli cells undergo numerous rounds of cell divisions. Shortly after birth, Sertoli cells cease proliferating, and throughout life, the number of sperm produced is directly related to the number of Sertoli cells. At puberty, the capacity of Sertoli cells to bind FSH
and testosterone increases. Receptors for FSH, present only on the plasma membranes of Sertoli cells, are glycoproteins linked to adenylyl cyclase via G proteins. FSH exerts multiple effects on the Sertoli cell, most of which are mediated by cAMP and protein kinase A (Fig. 37.6). FSH stimulates the production of androgen-binding protein and plasmino-gen activator, increases secretion of inhibin, and induces aromatase activity for the conversion of androgens to estrogens. The testosterone receptor is within the nucleus of the Sertoli cell.
Androgen-binding protein (ABP) is a 90-kDa protein, made of a heavy and a light chain, that has a high binding affinity for dihydrotestosterone and testosterone. It is similar in function, with some homology in structure, to another binding protein, sex hormone-binding globulin (SHBG), synthesized in the liver. ABP is found at high concentrations in the human testes and epididymis. It serves as a carrier of testosterone in Sertoli cells, as a storage protein for androgens in the seminiferous tubules, and as a carrier of testosterone from the testes to the epididymis.
Other products of the Sertoli cell are inhibin, follistatin, and activin. Inhibin suppresses FSH release from the pituitary gonadotrophs. The pituitary gonadotrophs and testicular Sertoli cells form a classical negative-feedback loop in which FSH stimulates inhibin secretion and inhibin suppresses FSH release. Inhibin also functions as a paracrine
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