Figure 3.From programmed puberty to quasi-programmed menopause. For
simplicity, only the FSH-estrogen feedback loop is shown. FSH stimulates follicles
and production of estrogens (Est). Estrogens inhibit FSH production (negative
feedback). (A) In girls, the hypothalamus is extremely sensitive to
estrogens and even low levels of estrogens inhibit FSH. (B) The onset of
menstrual cycle. While the hypothalamus is becoming resistant to estrogens, FSH
stimulates the ovaries and estrogen production. Progressive activation of follicles
from the dormant pool serves as the source of fertilizable ova. (C) Pre-menopause.
While the hypothalamus is becoming progressively resistant to estrogens, FSH
progressively over-stimulates the ovaries. (D) The ovaries fail. Menopause
occurs when the primordial follicle pool is exhausted. Estrogen levels drop. The
feedback between hypothalamus and the ovaries is disrupted.
