This chapter is concerned with the function of the female reproductive system from birth, through puberty and adulthood, and finally to menopause.
After birth, the gonads are quiescent until they are activated by gonadotropins from the pituitary to bring about the final maturation of the reproductive system. This period of final maturation is known as adolescence. It is often called puberty, although strictly defined, puberty is the period when the endocrine and gametogenic functions of the gonads first develop to the point where reproduction is possible.
After sexual maturity, there are regular periodic changes of the adult female reproductive system, each in preparation for pregnancy. The cyclic changes are primarily divided into the ovarian and uterine cycle, though changes can also be seen in the uterine cervix, vagina, and breasts. Control of the cycle is exerted through the regulation of hypothalamic, pituitary, and ovarian hormones.
With advancing age, these cycles become irregular and eventually cease in the period known as menopause. The ovarian follicles are less responsive to central regulation, and there is an acute decrease in estrogen levels, which may lead to vasomotor symptoms, labile mood, and many changes in the female reproductive tract.
The age at the time of puberty is variable. In Europe and the United States, it has been declining at the rate of 1–3 months per decade for more than 175 years. In the United States in recent years, puberty has generally been occurring between the ages of 8 and 13 in girls and 9 and 14 in boys depending on ethnic background.
Another event that occurs in humans at the time of puberty is an increase in the secretion of adrenal androgens (Fig. 4–1). The onset of this increase is called adrenarche.
Change in serum dehydroepiandrosterone sulfate (DHEAS) with age. The middle line is the mean, and the dashed lines identify ±1.96 standard deviations. (Reproduced with permission from Smith MR, Rudd BT, Shirley A, et al. A radioimmunoassay for the estimation of serum dehydroepiandrosterone sulfate in normal and pathological sera. Clin Chim Acta 1975;65:5.)
It typically happens in males and females before the onset of puberty occurring at age 8–10 years in girls and 10–12 years in boys. Dehydroepiandrosterone (DHEA) values peak at about 25 years of age and are slightly higher in boys. They then decline slowly to low values after the age of 60.
The increase in adrenal androgen secretion at adrenarche occurs without any changes in the secretion of cortisol or adrenocorticotropic hormone (ACTH). Adrenarche is probably due to a rise in the lyase activity of a 17α-hydroxylase. Thereafter, there is a gradual decline in this activity as plasma adrenal androgen secretion declines to low levels in old age.