Menopause:How Does It Happen?
The ovaries contain structures called follicles that hold the egg cells. You are born with about 2 million egg cells and by puberty there are about 300,000 left. Only about 400 to 500 ever mature fully to be released during the menstrual cycle.The rest degenerate over the years. During the reproductive years, the pituitary gland in the brain generates hormones that cause a new egg to be released from its follicle each month. The follicle also increases production of the sex hormones estrogen and progesterone, which thicken the lining of the uterus. This enriched lining is prepared to receive and nourish a fertilized egg following conception. If fertilization does not occur, estrogen and progesterone levels drop, the lining of the uterus breaks down, and menstruation occurs.For unknown reasons, the ovaries begin to decline in hormone production during the mid-thirties. In the late forties, the process accelerates and hormones fluctuate more, causing irregular menstrual cycles and unpredictable episodes of heavy bleeding. By the early to mid-fifties, periods finally end altogether. However, estrogen production does not completely stop. The ovaries decrease their output significantly, but still may produce a small amount. Also, another form of estrogen is produced in fat tissue with help from the adrenal glands (near the kidney). Although this form of estrogen is weaker than that produced by the ovaries, it increases with age and with the amount of fat tissue.
Progesterone, the other female hormone, works during the second half of the menstrual cycle to create a lining in the uterus as a viable home for an egg, and to shed the lining if the egg is not fertilized. If you skip a period, your body may not be making enough progesterone to break down the uterine lining. However, your estrogen levels may remain high even though you are not menstruating.
At menopause, hormone levels don't always decline uniformly. They alternately rise and fall again. Changing ovarian hormone levels affect the other glands in the body, which together make up the endocrine system. The endocrine system controls growth, metabolism and reproduction. This system must constantly readjust itself to work effectively. Ovarian hormones also affect all other tissues, including the breasts, vagina, bones, blood vessels, gastrointestinal tract, urinary tract, and skin.
The Endocrine Glands
Endocrine glands secrete hormones into the blood and interact with one another to help regulate total body metabolism, growth, and reproduction.
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Reproduced from the National Institute on Aging, National Institutes of Health.
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