The Onset of Puberty

Secretion of FSH and LH is high in the newborn, but falls to very low levels a few weeks after birth. Gonadotropin secretion remains low until the beginning of puberty, which is marked by rising levels of FSH followed by LH secretion. Experimental

642 Chapter Twenty

Table 20.2 Development of Secondary Sex Characteristics and Other Changes That Occur During Puberty in Girls

Characteristic

Age of First Appearance

Hormonal Stimulation

Appearance of breast buds

8-13

Estrogen, progesterone, growth hormone, thyroxine, insulin, cortisol

Pubic hair

8-14

Adrenal androgens

Menarche (first menstrual flow) Axillary (underarm) hair Eccrine sweat glands and sebaceous glands; acne (from blocked sebaceous glands)

10-16

About 2 years after the appearance of pubic hair About the same time as axillary hair growth

Estrogen and progesterone Adrenal androgens Adrenal androgens

Table 20.3 Development of Secondary Sex Characteristics and Other Changes That Occur During Puberty in Boys

Table 20.3 Development of Secondary Sex Characteristics and Other Changes That Occur During Puberty in Boys

Age of First Appearance

Characteristic

Hormonal Stimulation

Growth of testes Pubic hair Body growth Growth of penis Growth of larynx (voice lowers) Facial and axillary (underarm) hair Eccrine sweat glands and sebaceous glands; acne (from blocked sebaceous glands)

10-14

10-15

11-16 11-15

Same time as growth of penis About 2 years after the appearance of pubic hair About the same time as facial and axillary hair growth

Testosterone, FSH, growth hormone Testosterone

Testosterone, growth hormone

Testosterone

Testosterone

Testosterone

Testosterone evidence suggests that this rise in gonadotropin secretion is a result of two processes: (1) maturational changes in the brain that result in increased GnRH secretion by the hypothalamus and (2) decreased sensitivity of gonadotropin secretion to the negative feedback effects of sex steroid hormones.

The maturation of the brain that leads to increased GnRH secretion at the time of puberty appears to be programmed— children without gonads show increased FSH secretion at the normal time. Also during this period of time, a given amount of sex steroids has less of a suppressive effect on gonadotropin secretion than the same dose would have if administered prior to puberty. This suggests that the sensitivity of the hypothalamus and the pituitary to negative feedback effects decreases at puberty, which would also help to account for rising gonadotropin secretion at this time.

During late puberty there is a pulsatile secretion of gonadotropins—FSH and LH secretion increase during periods of sleep and decrease during periods of wakefulness. These pulses of increased gonadotropin secretion during puberty stimulate a rise in sex steroid secretion from the gonads. Increased secretion of testosterone from the testes and of estradiol-170 (estradiol is the major estrogen, or female sex steroid) from the ovaries during puberty, in turn, produces changes in body characteristic of the two sexes. Such secondary sex characteristics (tables 20.2 and 20.3) are the physical manifestations of the hormonal changes occurring during puberty. These changes are accompanied by a growth spurt, which begins at an earlier age in girls than in boys (fig. 20.10).

The age at which puberty begins is related to the amount of body fat and level of physical activity of the child. The aver-

Physiology Puberty

10 12 14

Age (years)

■ Figure 20.10 Growth as a function of sex and age. Notice that the growth spurt during puberty occurs at an earlier age in females than in males.

age age of menarche—the first menstrual flow—is later (age 15) in girls who are very active physically than in the general population (age 12.6). This appears to be due to a requirement for a minimum percentage of body fat for menstruation to begin; this may represent a mechanism favored by natural selection to ensure that a woman can successfully complete a pregnancy and nurse the baby. Recent evidence suggests that the secretion of leptin from adipocytes (chapter 19) is required for puberty. Later in life, women who are very lean and physically active may

Reproduction have irregular cycles and amenorrhea (cessation of menstruation). This may also be related to the percentage of body fat. However, there is also evidence that physical exercise may act to inhibit GnRH and gonadotropin secretion.

Clinical Investigation Clue

Remember that Gloria has normal secondary sexual development and used to have regular periods. ■ Could her low body weight and strenuous exercise be responsible for her symptoms? How?

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