The function of the ovaries changes dramatically with age, yet, the reasons for these changes are still not well understood. The beginnings of estrogen secretion, LH secretion, ovulation, and menstruation in girls typically starts around the age of twelve, but can begin as late as age fifteen.
Why do these changes, called puberty , begin?
The causes of puberty are difficult to study, for several reasons. Study of experimental animals such as rats has been helpful, but reproduction in these animals is not precisely like that in humans, so it is difficult to apply these findings to humans. Studies of puberty in rhesus monkeys, which are more like humans, have produced a variety of interesting results.
Work done has shown that changes in the activity of nerve cells in the hypothalamus appear to initiate puberty by increasing the secretion oh LH from the pituitary gland. What causes this maturation of hypothalamic nerve cells is unclear. Curiously, this monkey: if a monkey is given a restricted diet, puberty will be delayed.
These data in monkeys show a certain “logic” in the endocrine system, if not enough food is available to fatten up a monkey, then it certainly doesn’t “make sense” to initiate puberty and the risk of a pregnancy. These monkey data are also consistent with the known delay of puberty that has been found in human populations exposed to periods of famine.
However, how does the hypothalamus “know” that the body is too undernourished for reproduction? Perhaps insulin, affecting cell function in the hypothalamus, regulates the hypothalamic control of puberty as well as food intake.
At the other end of life, at ages forty five to fifty, the ovarian cycles of a woman become more irregular and finally cease, a process known as menopause. Once again, the reasons for menopause are not at all well understood. Work in rats suggest that the ovaries are not at fault: if ovaries from older rats are transplanted into younger rats, they resume a normal function. Many people suspect that changes in the hypothalamus are again to blame, but what exactly happens is still unclear.
Regardless of the cause of menopause, the loss of ovarian hormones in older women has a disruptive, and even perhaps unhealthy, influence upon the remainder of life. Physicians have increasingly supported the administration of small doses of estrogen to replace the missing estrogen in older women, and have found that this combats the loss of minerals in bones with age and reduces the risk of heart disease.
Thus, the use of sex hormones can improve both the quality and quantity of life in older women.