In the fetus, the testis is responsible for differentiation of the male genitalia. At puberty, the testis promotes the development of secondary sex characteristics and sexual behavior. In the adult male, the central function of the testis is spermatogenesis and maintenance of coital capacity.
Older men show some evidence of a physiologic decline in gonadal function.
Female development does not require the positive influence of either the fetal gonad or maternal estrogens. Male differentiation, however, requires a functioning fetal testis, which differentiates from the bipotential gonad of the early embryo in response to a gene known as SRY , the sex-determining region of the Y chromosome. Two hormones secreted by the fetal testis are required for male genital differentiation. Testosterone, produced by the fetal testis in response to stimulation by chorionic gonadotropin, determines the remaining steps of masculine development by stimulating the wolffian ducts to develop into the vas deferens, seminal vesicle, and epididymis and by fostering external virilization of the embryo. Each receptor has three domains.