Castration of a male animal results in an immediate rise in FSH and LH secretion. This demonstrates that hormones secreted by the testes exert negative feedback control of gonadotropin secretion. If testosterone is injected into the castrated animal, the secretion of LH can be returned to the previous (precastration) levels. This provides a classical example of negative feedback— LH stimulates testosterone secretion by the Leydig cells, and testosterone inhibits pituitary secretion of LH (fig. 20.13).
The amount of testosterone that is sufficient to suppress LH, however, is not sufficient to suppress the postcastration rise in FSH secretion in most experimental animals. In rams and bulls, a water-soluble (and, therefore, peptide rather than steroid) product of the seminiferous tubules specifically suppresses FSH secretion. This hormone, produced by the Sertoli cells, is called inhibin. The seminiferous tubules of the human testes also have been shown to produce inhibin, which inhibits FSH secretion in men. (There is also evidence that inhibin is produced by the ovaries, where it may function as a hormone and as a paracrine regulator of the ovaries.)
The brain contains testosterone receptors and is a target organ for this hormone. The effects of testosterone on the brain, such as the suppression of LH secretion, are not mediated directly by testosterone, however, but rather by its derivatives that are produced within the brain cells. Testosterone may be converted by the enzyme 5a-reductase to dihydrotestosterone (DHT), as previously described. The DHT, in turn, can be changed by other enzymes into other 5a-reduced androgens— abbreviated 3a-diol and 3P-diol (fig. 20.14). Alternatively,
. Spermatic cord
Rete testis within 'mediastinum testis
Seminiferous tubules Interstitial (Leydig) cells
Germinal epithelial cells
Lumen of seminiferous tubule
■ Figure 20.12 The seminiferous tubules. (a) A sagittal section of a testis and (b) a transverse section of a seminiferous tubule.
testosterone may be converted within the brain to estradiol-17p. Although usually regarded as a female sex steroid, estra-diol is therefore an active compound in normal male physiology! Estradiol is formed from testosterone by the action of an enzyme called aromatase. This reaction is known as aromatization, a term that refers to the presence of an aromatic carbon ring (chapter 2). The estradiol formed from
Interstitial . (Leydig) cells
■ Figure 20.13 The anterior pituitary and testes. The seminiferous tubules are the targets of FSH action; the interstitial (Leydig) cells are targets of LH action. Testosterone secreted by the Leydig cells inhibits LH secretion; inhibin secreted by the tubules may inhibit FSH secretion.
testosterone in the brain is required for the negative feedback effects of testosterone on LH secretion.
The negative feedback effects of testosterone and inhibin help to maintain a relatively constant (that is, noncyclic) secretion of go-nadotropins in males, resulting in relatively constant levels of androgen secretion from the testes. This contrasts with the cyclic secretion of gonadotropins and ovarian steroids in females. Women experience an abrupt cessation in sex steroid secretion during menopause. By contrast, the secretion of androgens declines only gradually and to varying degrees in men over 50 years of age. The causes of this age-related change in testicular function are not currently known. The decline in testosterone secretion cannot be due to decreasing gonadotropin secretion, since gonadotropin levels in the blood are, in fact, elevated (because of less negative feedback) at the time that testosterone levels are declining.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.