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HORMONAL CONTROL OF SPERM PRODUCTION: TESTOSTERONE
Testosterone usually is thought of as the male hormone, but it alone does not regulate the production of sperm. Two other hormones, produced by the pituitary gland located at the base of the brain, are necessary for sperm production and maturation. These hormones, called gonadotropins because of their effect on the gonads, are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones initially were identified in the female, and their names reflect their effect in the female reproductive system. Later they were found to play comparable roles in the male, but the original names have largely been retained (although LH is sometimes called interstitial cell-stimulating hormone [ISCH] in men).
The blood carries these two gonadotropins from their site of synthesis in the pituitary to their site of action in the testes. In the testes, FSH and, to a lesser extent, testosterone are both needed to stimulate sperm production in the large number of seminiferous tubules present. LH is also required for sperm production insofar as it stimulates the secretion of testosterone from the.surrounding cells and plays a role in the maturation of the sperm. Finally, gonadotropin-releasing hormone (GnRH), which is produced by the hypothalamus (a gland located at the base of the brain and attached to the pituitary gland by means of a stalk), is required for sperm production because it regulates the production of both FSH and LH.
With this in mind, we can compare the regulation of the hormones testosterone, FSH, LH, and GnRH with the negative feedback mechanism described earlier. When the concentration of testosterone in the blood becomes elevated, it not only inhibits the release of LH from the pituitary gland; it also inhibits the release of GnRH from the hypothalamus. In the absence of GnRH, no LH or FSH is released; and in the absence of these hormones, no more testosterone is released. Consequently, sperm production is decreased. In sum, when too much testosterone is produced, its further production is inhibited until its level in the blood drops. No sooner does the concentration of testosterone in the blood begin to drop, how ever, then its inhibiting effect on GnRH is lost. As GnRH levels rise, LH is again secreted which, in turn, stimulates the production of more testosterone, leading to the production of more sperm.
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