CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

27. Human Reproduction, Sex, and Sexuality

27.5. The Sexual Maturation of Young Adults

Following birth, sexuality plays only a small part in physical development for several years. However culture and environment shape the responses that the individual will come to recognize as normal behavior for his or her age. Puberty is the developmental period when the body changes to the adult form and becomes able to reproduce. During puberty, which begins at about 10 years of age in females and about 12 years of age in males, an increased production of sex hormones causes major changes as the individual reaches sexual maturity. These changes are generally completed by 15-18 years of age.

The Maturation of Females

Girls typically begin to produce sex hormones from several glands, marking the onset of puberty. The hypothalamus, pituitary gland, ovaries, and adrenal glands begin to produce sex hormones at 9 to 12 years of age (figure 27.8). Table 27.1 lists the principal hormones involved in human reproduction.

FIGURE 27.8. Hormones and Sexual Function

The hypothalamus, pituitary gland, adrenal gland, and ovaries and testes are the primary hormone-producing glands that affect sexual development and behavior.

TABLE 27.1. Human Reproductive Hormones

The hypothalamus is a portion of the brain that controls the functioning of many glands throughout the body, including the pituitary gland. At puberty, the hypothalamus begins to release gonadotropin-releasing hormone (GnRH), which stimulates the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Increased levels of FSH stimulate the development of follicles, saclike structures that produce eggs in the ovaries. The increased luteinizing hormone stimulates the ovary to produce larger quantities of estrogens. The increasing supply of estrogens is responsible for the many changes in sexual development. These changes include breast growth, changes in the walls of the uterus and vagina, changes in the pelvic bone structure, and increased blood supply to the clitoris, a small, elongated erectile structure located at the head of the labia; it develops from the same embryonic tissue as the male penis. Estrogens also stimulate the female adrenal gland to produce androgens, male sex hormones. The androgens are responsible for the production of pubic hair, and they seem to have an influence on the female sex drive.

The features that are not primarily involved in sexual reproduction but are characteristic of a sex are called secondary sexual characteristics. In women, breast development, the distribution of body hair, the patterns of fat deposits, wider hips, and a higher voice are secondary sexual characteristics.

Major developments during this time are ovulation, the release of eggs from the ovary, and the establishment of the menstrual cycle, the periodic growth and shedding of the lining of the uterus. These changes are under the control of a number of hormones produced by the pituitary gland and ovaries.

Initially, as girls go through puberty, menstruation and ovulation may be irregular; however, in most women hormone production eventually becomes regulated, so that ovulation and menstruation take place on a monthly basis, although normal cycles vary from 21 to 45 days.

As girls progress through puberty, curiosity about their changing body form and new feelings lead to self-investigation. Studies have shown that sexual activity, such as stimulation of the clitoris, vagina, or anus, which causes a pleasurable sensation, orgasm, is performed by a large percentage of young women. This stimulation is termed masturbation, and it is a normal part of sexual development.

The Maturation of Males

Males typically reach puberty about 2 years later than females, but puberty in males also begins with a change in hormone levels. At puberty, the hypothalamus releases increased amounts of gonadotropin-releasing hormone (GnRH), resulting in increased levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These are the same changes that occur in female development. Luteinizing hormone is often called interstitial cell-stimulating hormone (ICSH) in males. LH stimulates the testes to produce testosterone, the primary sex hormone in males. The testosterone produced by the embryonic testes causes the differentiation of internal and external genital anatomy in the male embryo. At puberty, the increase in testosterone is responsible for the development of male secondary sexual characteristics and is important in the maturation and production of sperm.

The major changes during puberty include growth of the testes and scrotum, pubic hair development, and increased penis size. Secondary sex characteristics also begin to become apparent; facial hair, underarm hair, and chest hair are some of the most obvious. The male voice changes as the larynx (voice box) begins to change shape. Body contours also change, and a growth spurt increases height. In addition, the proportion of the body that is muscle increases and the proportion of body fat decreases. At this time, a boy’s body begins to take on the characteristic adult male shape, with broader shoulders and heavier muscles.

In addition to these external changes, increased testosterone causes the production of semen, also known as seminal fluid, a mixture of sperm and secretions from three accessory glands—the seminal vesicles, prostate, and bulbourethral glands. They produce secretions that nourish and activate the sperm. Semen also lubricates the reproductive tract and acts as the vehicle to help carry the sperm.

Seminal vesicles secrete an alkaline fluid, containing fructose, and hormones. Its alkaline nature helps neutralize the acidic environment in the female reproductive tract, improving the sperm’s chances of reaching the egg. The fructose provides energy for the sperm. Seminal vesicle secretions make up about 60% of the seminal fluid. The prostate gland produces a thin, milky fluid, which makes up about 25% of semen. It contains sperm-activating enzymes. Bulbourethral gland secretions make up the remaining 15%; they, too, are alkaline.

FSH stimulates the production of sperm cells. The release of sperm cells and seminal fluid, ejaculation, begins during puberty. This release is generally accompanied by a pleasurable sensation, orgasm. During sleep, males frequently have erections, sometimes resulting in ejaculation of seminal fluid. This is termed a wet dream. It is normal and related to the amount of seminal fluid produced. Wet dreams occur less often in men who engage in frequent sexual intercourse or masturbation. Masturbation is a common and normal activity as a boy goes through puberty. Studies of adult sexual behavior have shown that nearly all men masturbate at some time during their lives.

27.5. CONCEPT REVIEW

11. What are the effects of the secretions of the pituitary, the gonads, and the adrenal glands at puberty in females?

12. What role does testosterone play in male sexual maturation?