An androgen is a substance that stimulates the growth of the male reproductive tract and the development of secondary sex characteristics. Androgens have effects on al most every tissue, including alteration of the primary sex structures (i.e., the testes and genital tract) and stimulation of the secondary sex structures (i.e., accessory glands) and development of secondary sex characteristics responsible for masculine phenotypic expression. Androgens also affect both sexual and nonsexual behavior. The relative potency ranking of androgens is DHT > testosterone > an-drostenedione > DHEA. The action of sex steroid hormones on somatic tissue, such as muscle, is referred to as "anabolic" because the end result is increased muscle size. This action is mediated by the same molecular mechanisms that result in virilization.
Between 8 and 18 weeks of fetal life, androgens mediate differentiation of the male genitalia. The organogenesis of the wolffian (mesonephric) ducts into the epididymis, vas deferens, and seminal vesicles is directly influenced by testosterone, which reaches these target tissues by diffusion rather than by a systemic route. The differentiation of the urogenital sinus and the genital tubercle into the penis, scrotum, and prostate gland depends on testosterone being converted to DHT. Toward the end of fetal life, the descent of the testes into the scrotum is promoted by testosterone and insulin-like hormones from Leydig cells (see Chapter 39).
The onset of puberty is marked by enhanced androgenic activity. Androgens promote the growth of the penis and scrotum, stimulate the growth and secretory activity of the epididymis and accessory glands, and increase the pigmentation of the genitalia. Enlargement of the testes occurs under the influence of the gonadotropins (LH and FSH). Spermatogenesis, which is initiated during puberty, depends on adequate amounts of testosterone. Throughout adulthood, androgens are responsible for maintaining the structural and functional integrity of all reproductive tissues. Castration of adult men results in regression of the reproductive tract and involution of the accessory glands.
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