The Testis

THE TESTIS  - ANDROGENS & ANABOLIC STEROIDS, ANTIANDROGENS, & MALE CONTRACEPTION

The testis, like the ovary, has both gametogenic and endocrine functions. The onset of gametogenic function of the testes is con-trolled largely by the secretion of FSH by the pituitary. High concentrations of testosterone locally are also required for con-tinuing sperm production in the seminiferous tubules. The Sertoli cells in the seminiferous tubules may be the source of the estradiol produced in the testes via aromatization of locally produced testosterone. With LH stimulation, testosterone is produced by the interstitial or Leydig cells found in the spaces between the seminiferous tubules.

The Sertoli cells in the testis synthesize and secrete a variety of active proteins, including müllerian duct inhibitory factor, inhibin, and activin. As in the ovary, inhibin and activin appear to be the product of three genes that produce a common α subunit and two subunits, A and B. Activin is composed of the two β subunits (βAβB). There are two inhibins (A and B), which contain the α subunit and one of the β subunits. Activin stimulates pituitary FSH release and is structurally similar to transforming growth factor-β, which also increases FSH. The inhibins in conjunction with testosterone and dihydrotestosterone are responsible for the feedback inhibition of pituitary FSH secretion.