The Menstrual Cycle

Secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. FSH and LH regulate the production of estrogen and progesterone by the ovary. Ovarian estrogen and progesterone secretion proceed in a cyclical manner. It is this cyclical release of estrogen and progesterone that determines the regular hormonal changes in the uterus, vagina, and cervix associated with the menstrual cycle. Cyclical changes in blood levels of estrogen and progesterone, together with FSH and LH, modulate the development of ova, ovulation, and the corpus luteum in the ovary.

During the first, or follicular, phase of the menstrual cycle, estradiol blood levels rise slowly and then fall quite rapidly. Estradiol blood levels peak around mid-cycle (days 12-14). The midcycle estrogen peak is thought to be important in triggering a midcycle surge of LH and FSH secretion. Estrogens have a biphasic effect on LH and FSH release, with high levels of estrogen at midcycle triggering LH and FSH release; subsequently they suppress LH and FSH secretion. This suppression is mediated by inhibition of GnRH release from the hypothalamus.

The luteal phase of the menstrual cycle follows the LH and FSH surge (days 14-28). The brief elevation of the LH level stimulates production of the ovarian corpus luteum. The high levels of estradiol and the FSH surge at midcycle inhibit estradiol biosynthesis by the ovarian granulosa cells. As a consequence, during the luteal phase, estrogen production is reduced and andro-gens produced by the ovarian thecal cells accumulate. Androgens, together with low levels of FSH, stimulate the production of progesterone by the granulosa cells in the corpus luteum. The menstrual cycle ends about 14 days later with the regression of the corpus luteum and a concomitant fall in estrogen and progesterone production. The triggering mechanism for this regression may involve both estrogens and prostaglandins. In the event that pregnancy occurs, human chorionic go-nadotropin secretion by the embryo maintains the corpus luteum through stimulation of progesterone and estrogen synthesis.

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