Puberty and Menarche
Puberty means the onset of adult sexual life, and menarche means the beginning of the cycle of menstruation.
The period of puberty is caused by a gradual increase in gonadotropic hormone secretion by the pituitary, beginning in about the eighth year of life, as shown in Figure 81–9, and usually culminating in the onset of puberty and menstruation between ages 11 and 16 years in girls (average, 13 years).
In the female, as in the male, the infantile pituitary gland and ovaries are capable of full function if appro-priately stimulated. However, as is also true in the male, and for reasons not understood, the hypothala-mus does not secrete significant quantities of GnRH during childhood. Experiments have shown that the hypothalamus itself is capable of secreting this hormone, but the appropriate signal from some other area of brain to cause the secretion is lacking. There-fore, it is now believed that the onset of puberty is initiated by some maturation process that occurs else-where in the brain, perhaps somewhere in the limbic system.
Figure 81–10 shows (1) the increasing levels of estro-gen secretion at puberty, (2) the cyclical variation during the monthly sexual cycle, (3) the further increase in estrogen secretion during the first few years of reproductive life, (4) the progressive decrease in estrogen secretion toward the end of reproductive life, and, finally, (5) almost no estrogen or progesterone secretion beyond menopause.
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