Physiological Relationships

A. Puberty and Sexual Development

Puberty, or the developmental stage at which anatomical, physiological, and endocrinological changes occur to develop a female competent for sexual reproduction, occurs over the time of 9-14 years of age in the human. Puberty in the female is initiated endocri-nologically at age 9-10 by the gradually increasing CNS-mediated secretion of gonadotrophins; the men-arche or first menstrual period normally occurs between ages 12 and 16. Previously the juvenile years

Hormones, Second Edition before puberty were postulated to be endocrinologi-cally relatively inert, but now due to increasing sensitivity of radioimmunoassay techniques for quantitat-ing very low levels of various hormones it is clear that significant changes in many hormones do occur. However, as yet there are no unifying hypotheses to explain these changes.

Puberty, then, is initiated by increased pituitary output of the gonadotrophins, FSH and LH. It is not precisely clear what factors initiate this change. Possibly, throughout childhood the low levels of estrogen secreted by the ovaries of the prepubescent female are sufficient at the CNS-hypothalamus level to block the release of GnRH (see Figure 13-6). Then, as a consequence of a changing CNS-hypothalamus sensitivity to the prevailing blood estrogen levels, GnRH release is gradually increased, which results in an increased release of FSH and LH. In both females and males during the interval of puberty, there are bursts of both FSH and LH secretion that occur during sleep; their etiology is not known. Associated with these changes is an increased sensitivity of the ovaries to FSH and LH, which results in a gradually increasing metabolic production of estrogens and androgens. This in turn accelerates the growth of the uterus, vagina, accessory sex glands, genitalia, pelvis, breasts, and axillary and public hair. These changes then culminate after the menarche in the monthly cyclical processes of ovulation and, in the absence of fertilization, menstruation.

B. The Female Reproductive Cycle

1. Background

Figure 13-7 details the cyclical changes in FSH, LH, progesterone, estradiol, and inhibin, while Figure 138 depicts the morphological changes in the uterine endometrium that occur during a normal adult female menstrual cycle. The cycle can be conveniently divided into two phases: pre- and postovulatory. The key events of the preovulatory phase are growth and maturation of the ovarian follicle and initiation of the maturation of the uterine endometrium. The key events of the postovulatory phase are the growth, development, and involution of the corpus luteum and, in the absence of initiation of pregnancy, the shedding of the uterine endometrium. These changes are all orchestrated by CNS-hypothalamus regulation of pituitary release of FSH and LH (see Figure 13-6).

A 28-day cycle, as shown in Figure 13-8, is generally regarded as the mean length of normal cycles, but the range for different women extends from 25 to 35 days. The preovulatory phase is usually much more variable

LH mlU/ml

FSH mlU/ml e2


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