The finely tuned interaction of female sex hormones has a key role in the development and maintenance of the female genital organs and is an essential requirement for fertility. During the fertile years, the 3 x 5 cm large paired ovaries are the main organ of sex hormone production. In newborn girls, ovaries comprise more than 1 million follicles (small cellular structures that each contain an immature egg) which remain dormant until the onset of puberty when during each menstrual cycle up to a 1000 follicles start to grow and to mature. Depending on the menstrual cycle phase, considerable amounts of estradiol (E2, the classical classical female hormone) and progesterone are produced.
The Menstrual Cycle
During the first cycle phase, which starts immediately after the of menstual bleeding and lasts until the 14th cycle day, the release of hypophyseal gland-derived Follicle-stimulating hormone (also called »FSH«) results in the maturation of numerous estradiol-producing follicles. It is under the influence of luteizing hormone (»LH«) that the most developed follicle then »bursts« around the 14th cycle day and consequently releases the egg cell that is now ready for fecundation. The empty follicle converts into a tissue that is rich in fat - and thus yellow – and is now called corpus luteum (»luteum« means yellow). The corpus luteum is responsible for the production of progesterone during the second cycle phase which prepares the uterus for a possible pregnancy. Although ovaries initially harbour abundant follicles, each menstrual cycle depletes the ovarian reserve over the years. Other factors such as irradiation, certain drugs, but also specific anti-cancer therapies enhance this process and can lead to a profound hormonal deficit considerably earlier.
Hormonal Changes Begin at 35
Women aged 18 to 40 can usually pretty much predict their next menstrual bleeding. But already at the age of 35 the regular production of sex hormones gradually begins to change. And somewhere between 40 and 50 menstrual cycles becomes more and more unpredictable in terms of timing and extent of bleeding episodes. In most cases, the first sign of the imminent menopause is a shorter bleeding interval. It is probably caused by changes in the production of FSH and LH. Approximately two years prior to definitive menopause the monthly rupture of the lead follicle becomes less and less regular. The follicle degenerates rather than being converted into the corpus luteoum. This, in turn, leads to a lack of progesterone and is typically associated with long and irregular menstrual cycles. Menstrual bleeding can now cease for several months. And if bleeding finally occurs, it tends to be prolonged and can be so heavy that serious health problems can develop.
Estrogen Equilibrium and Heat Flashes
Around the age of 45, ovaries become somewhat less responsive to pituitary-derived FSH. Since they are the by far most important source of female sex hormones in younger women, this consequently leads to a lack of estrogens and progesterone. The pituitary attempts to counterbalance the lack of hormones by producing more FSH. The remaining follicles can sometimes react to increased FSH level by a sudden and immediate release of large amounts of estrogens. The quickly alternating states of extremely high and low estrogen levels give rise to classical complaints such as heat flushes and night sweats. Over time remaining follicles become less and less responsive even to high levels of FSH and at one point in time no functional follicles are left. Menopause has begun.
Ovaries remain Important during Menopause
Although the ovaries cease to produce estrogens and progesterone during menopause, this does not necessarily mean that they have become superfluous. They can still produce certain hormones that do have an effect on the female body. Ovaries remain, for example, an important producer of the male sex hormone testosterone. In women, testosterone is responsible for muscle mass and libido. Other ovary-derived hormones include DHEA and androstendione, which can both be converted to estrogens in the liver and in fatty tissue.