Hormonal Changes In The Course Of The Menstrual Cycle

The menstrual cycle is controlled by two ovarian hormones. Under the stimulating and determinant influence of the gonadotrophins follicle-stimulating hormone (FSH) and luteinizing hormone (LH), cyclic morphological changes take place in the ovaries of the sexually mature woman. At the beginning of the cycle, after completion of menstruation, the buildup of the endometrium and the synthesis of the endometrial progesterone and estrogen receptors are triggered as a result of an increasing secretion of estradiol. The ovarian estrogen synthesis takes place via the intermediate products androstenedione and testosterone, which are aromatized to estrone and estradiol subsequently.

The processes are stimulated by LH in synergy with FSH, which is presumably responsible for the development of a large number of primary follicles in the early follicular phase. Via negative feedback, estrogen production—which increases in the preovulatory phase and reaches the first peak at the time of ovulation—causes FSH to decrease, which leads to a regression of most of the stimulated follicles. Only a dominant follicle becomes independent of the stimulation by pituitary FSH and reaches ovulation maturity. The ovulation in the middle of the cycle is associated with a peak in the LH production and a peak in the FSH production, although the latter is less pronounced. The LH peak lasts approximately 36 hours and is controlled by the pulsatile release of gonadotrophin-releasing hormone (GnRH) from the hypothalamus. After ovulation, the corpus luteum develops, accompanied by an increasing secretion of gestagen, which is responsible for the increase in the thickness of the endome-trium and which, via negative feedback, inhibits the release of FSH from the pituitary gland and, thus, the further maturation of the follicles in the corpus luteum phase. As the luteal phase progresses, estradiol reaches the second peak. The new premenstrual increase in FSH, which is the result of a decrease of the progesterone formation in the corpus luteum, causes the stimulation of a new generation of follicles in the ovaries. The secretory phase is controlled jointly by estradiol and progesterone. The endometrial breakdown that causes menstrual bleeding is caused by decreases in the levels of these two sex hormones. Cyclic hormonal changes, however, have an influence not only on the endometrium but also on the vaginal epithelium and the skin (2). Figure 1 shows the pattern of the pituitary hormones LH/FSH and the two sex hormones estradiol/progesterone during a normal menstrual cycle.

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