15.5 Sexual Development
Sexual Development Occurs at Puberty
Puberty is the stage of development at which individuals become sexually mature. Though the outcomes of puberty for boys and girls are very different, the hormonal control of the process is very similar. In addition, though the timing of these events varies between individuals, the sequence of changes that occur is predictable for male and female adolescents. As shown in Figure 15.6, a concerted release of hormones from the hypothalamus (GnRH), the anterior pituitary (LH and FSH), and the gonads (either testosterone or estrogen) is responsible for the maturation of the reproductive systems and the development of secondary sex characteristics, which are physical changes that serve auxiliary roles in reproduction as shown in Figure 15.6.
The first changes begin around the age of eight or nine when the production of LH becomes detectable.
In addition to age, multiple factors can affect the age of onset of puberty, including genetics, environment, and psychological stress. One of the more important influences may be nutrition; historical data demonstrate the effect of better and more consistent nutrition on the age of menarche in girls in the United States, which decreased from an average age of approximately 17 years of age in 1860 to approximately 12.75 years in 1960. Some studies indicate a link between puberty onset and the amount of stored fat in an individual. This effect is more pronounced in girls, but has been documented in both sexes. Body fat, corresponding with secretion of the hormone leptin by adipose cells, appears to have a strong role in determining menarche. This may reflect to some extent the high metabolic costs of gestation and lactation. In girls who are lean and highly active, such as gymnasts, there is often a delay in the onset of puberty.
Signs of Puberty
Different sex steroid hormone concentrations between the sexes also contribute to the development and function of secondary sexual characteristics. Examples of secondary sexual characteristics are listed in Table 15.1.
| Male | Female |
|---|---|
| Increased larynx size and deepening of the voice | Deposition of fat, predominantly in breasts and hips |
| Increased muscular development | Breast development |
| Growth of facial, axillary, and pubic hair, and increased growth of body hair | Broadening of the pelvis and growth of axillary and pubic hair |
In boys, the growth of the testes is typically the first physical sign of the beginning of puberty, which is followed by growth and pigmentation of the scrotum and growth of the penis. The next step is the growth of hair, including armpit, pubic, chest, and facial hair. Testosterone stimulates the growth of the larynx and thickening and lengthening of the vocal folds, which causes the voice to drop in pitch. The first fertile ejaculations typically appear at approximately 15 years of age, but this age can vary widely across individual boys. Unlike the early growth spurt observed in females, the male growth spurt occurs toward the end of puberty, at approximately age 11 to 13, and a boy’s height can increase as much as 4 inches a year. In some males, pubertal development can continue through the early 20s.
Tanner Scale of secondary sex organs by age for males and females
The Tanner staging scale, also known as the Sexual Maturity Rating (SMR) chart, is a classification system used by pediatricians and other health care providers. It helps track and document the development of a patient’s secondary sex characteristics as they change from those of a child through puberty to those of an adult (see Figure 15.7).
Figure: 15.7: A five point scale of physical development of secondary sexual characteristics such as breast size, genitals, testicular volume, and pubic hair growth. Tanner scale-male.svg and Tanner scale-female.svg by M.Komorniczak are licensed under CC BY-SA 3.0.
