Hypothesis Volume 2, Issue 5 pp 265—273

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.