Menstrual cycle and menopause. Part 2.

What happens every month with regular bleeding?

Every month, at the beginning of bleeding, the hypothalamus (cells in a certain area of ​​the brain) detects a reduced level of estrogen hormones (the lowest concentration occurs just before and during menstruation). Using the gonadoliberin GnRH (i.e. gonadotropin-releasing hormone; another name luliberin; it is a decapeptide) it sends information to the anterior pituitary gland - also an area in the brain. This anterior pituitary gland releases the follicle-stimulating hormone FSH and the luteinizing hormone LH. If GnRH is secreted at a rate of 1 pulse per hour, LH secretion increases, if the rate is 1 pulse per 3 hours, non-FSH secretion increases preferentially. Continuous secretion of GnRH leads to a decrease in the sensitivity of receptors to the action of this hormone (down-regulation), which causes a decrease in the secretion of FSH and LH (these hormones are glycoproteins).

In women, small amounts of estrogen released by the ovaries inhibit the hypothalamic-pituitary-ovarian axis by negative feedback, and large doses of estrogen for about 3 days act by positive feedback and increase primarily LH and, to a lesser extent, FSH.

In the first phase of the cycle, called the follicular phase, because the released FSH stimulates about 20 ovarian follicles (Graff's) to grow. Only 1 (less often 2-3) of them mature to ovulation. The remaining ones degenerate and become fibrous, producing the so-called corpus fibrosus, The maturing follicle releases estrogen hormones (granular layer of the ovaries), which in turn act on the uterine mucosa, which thickens. Is this how the "substrate" for the egg is prepared? The first phase of the cycle is called the follicular phase. In the follicular phase, estrogens exert a negative feedback effect on the release of GnRH, LH and FSH.