maximum plasma concentrations at about day 14 of the cycle. The release in LH causes the follicle to break open, releasing a mature ovum. Under the stimulation of lH, the follicle changes into the corpus luteum, which begins secreting progesterone as well as estrogen.
The increased concentrations of estrogens and progesterone regulate the hypothalamus and the anterior pituitary by a feedback inhibition process that decreases GnRH, LH, and FSH production. The result is that further ovulation is inhibited. As described later in this chapter, this is the primary mechanism by which steroid birth control products inhibit ovulation.
If fertilization does not occur by about day 25, the corpus luteum begins to degenerate, slowing down its production of hormones. The concentrations of estrogens and progesterone become too low to maintain the vascularization of the en-dometrium, and menstruation results.
The pharmacological actions of hCG are essentially the same as those of LH. In females during pregnancy, the hCG secreted by the placenta maintains the corpus luteum to continue secretion of estrogen and progesterone, thus inhibiting ovulation and menstruation.
In males (Fig. 25.10), LH stimulates testosterone synthesis by the testes, and together, testosterone and LH promote spermatogenesis (sperm production) and development of the testes. Testosterone is also essential for the development of secondary sex characteristics in males. FSH stimulates production of proteins and nutrients required for sperm maturation.
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