Hormonal regulation of the menstrual cycle
Last reviewed: 23.04.2024
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The menstrual cycle reflects the activity of the hypothalamus-pituitary-ovary system, which is manifested by structural and functional changes in the reproductive tract: uterus, fallopian tubes, endometrium, and vagina. Each cycle ends with menstrual bleeding, the first day of which is considered the beginning of the cycle.
During the first part of the menstrual cycle (folliculin phase), FSH, secreted by the anterior pituitary gland, stimulates the production of estradiol with granular cells of the ovary. FSH and estradiol cause the proliferation of these cells, and the secretion of estradiol is increased. These hormones stimulate LH receptors. Estradiol acts on the endometrium of the uterus, causing it to thicken and vascularize, thereby preparing it for implantation of the oocyte. As the follicles mature, the level of inhibin increases in them and in the blood, which has a selectively inhibitory effect on the secretion of FSH.
The peak concentration of estradiol in the blood, which falls on the middle of the menstrual cycle (day 14), triggers a surge of LH release from the pituitary gland. LH stimulates ovulation (the exit of a mature egg from the follicle). The remaining cells in the postovulatory follicle form a yellow body, which begins to secrete progesterone and estradiol. Progesterone has a inhibitory effect on the secretion of inhibin.
During the second, luteal phase, progesterone together with estradiol cause an even greater thickening of the endometrium. There is an intensive vascularization of endometrial cells and their differentiation, the cells become secretory.
Approximately 1 week after the formation of the yellow body, it begins to reverse development and secrete less estradiol and progesterone. By the 28th day of the menstrual cycle, the level of ovarian steroids becomes inadequate to sustain the life of the thickened endometrium and it undergoes destruction, which leads to menstruation. Bleeding lasts 3-5 days. Low levels of estradiol and progesterone at the end of the cycle are removed (by the negative feedback principle) inhibition of GHG hypothalamic secretion. The level of GnRH in the hypothalamus rises, which stimulates the secretion of FSH and LH by the pituitary gland, and the menstrual cycle begins again.
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