Puberty | Physical maturity

Puberty is the period of sexual maturity when sexual organs mature and secondary sexual characteristics develop. Puberty is also the second major growth period of life—the first being infancy. A number of hormones under the control of the hypothalamus, pituitary, ovaries, and testes regulate this period of sexual growth, which begins for most boys and girls between the ages of nine and 15 years. The initial obvious sign of female puberty is the beginning of breast development, whereas the initial obvious sign in males is testicular enlargement. Since early signs of female puberty are more noticeable, it is sometimes assumed that female puberty precedes male puberty by quite a bit. However, males usually start puberty just a few months after females, on average. In males, puberty is marked by testicle and penile enlargement, larynx enlargement, pubic hair growth, and considerable growth in body height and weight. In females, puberty is marked by hip and breast development, uterine development, pubic hair growth, menstruation, and increases in body height and weight. Because of the extensive growth that occurs at this time, a balanced, nutritious diet with sufficient calories is important for optimal growth. Although puberty was originally used to classify the initial phase of early fertility, the term is also used to include the development and growth which culminates in fertility. In this sense, puberty usually lasts two to five years and is accompanied by the psychological and emotional characteristics called adolescence.

Physical maturity

Puberty marks the physical transition from childhood to adulthood. While the changes that accompany this time are significant, their onset, rate, and duration vary from person to person. In general, these changes are either sexual or growth related. The pubertal growth spurt is characteristic of primates. Although other mammals may have increased reproductive organ growth, their overall size does not increase as dramatically. The major control center for human pubertal development is the hypothalamus for both sexes, but puberty is accompanied by additional growth of the adrenal glands, as well. The added adrenal tissue secretes the sex hormones, androgens or estrogens, at low levels. The adrenal sex hormones are thought to initiate the growth of pubic and axillary (under-arm) hair. This adrenal maturation is called adrenarche.

It is not known exactly what triggers puberty to begin. However, the hypothalamus sends out gonadotropin hormones responsible for sperm and egg maturation. One theory holds that normal brain growth towards the end of childhood includes significant hypothalamic changes. Hypothalamic receptors are thought to become more sensitive to low levels of circulating sex steroids. These changes enable the neuroendocrine system to initiate spermarche (sperm maturation) and menstruation in puberty. However, these early hormonal fluctuations begin at night and remain a nocturnal pulse for some time before they are detectable while awake. Some behavioral changes are related to pubertal hormonal changes, as well. The increase in testosterone is associated with more aggressive behavior in males. And libido (sex drive) increases occur for some teenagers in association with estrogen and testosterone increases. These effects are also carried out through sex hormone receptors on the hypothalamus.

Male puberty

Major pubertal hormones secreted by the hypothalamus include gonadotropin releasing hormone (GRH) and growth hormone releasing hormone (GHRH). Both target the anterior pituitary gland, which in turn releases gonadotropins and growth hormone (also known as somatotropin). GRH is released in a pulsative fashion. This pulsation triggers release of the gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). LH stimulates testosterone release by the testes, and FSH is required for early stages of sperm maturation. GHRH is released on a daily basis throughout life, but growth hormones have an enhanced effect during puberty when they are combined with sex hormones.

The age of onset of puberty varies but can be between the ages of nine and 14 years in boys. However, individuals can mature as late as 20 years of age. When all of a male’s organs and endocrine functions are normal but testicular development never occurs, he is said to display eunuchoidism. This name originates from China where servile classes of eunuchs were created by removing their testicles. Because of their lack of testosterone, they were less aggressive. Puberty that begins before the age of eight years is called precocious. Precocious puberty can result from neurological disorders of the posterior hypothalamus or pituitary disorders such as tumors or infections. Precocious puberty requires measurement of the several hormones involved to determine which are lacking or which are in excess. There are blood tests for each one. If a tumor is suspected, imaging of the suspect organ needs to be done with x rays, computed tomography scans (CT scans), or magnetic resonance imaging (MRI)

The initial sign of male puberty is testicular enlargement. The testes secrete testosterone, which stimulates many primary and secondary sexual characteristics. Testosterone causes the prostate gland and seminal vesicles to mature. The seminal vesicles begin to secrete fructose which is the primary nutrient sperm require. During puberty, primitive male germ cells begin to mature into primary spermatocytes. This early step in sperm maturation is testosterone-independent. However, the final stage of sperm maturation into spermatozoa is testosterone-dependent. Testicular size may double or quadruple at the start of puberty, but the rate of testicular growth is greatest in the middle of puberty. By the end, they will have doubled in size again. There is great variability in the final testicular size from different men, but this difference has no affect on sexual ability.

The general progression of male genital area development is the onset of testicular enlargement, onset of penile enlargement, and the appearance of pubic hair (pubarche). The scrotal skin also becomes darker and more wrinkled. Penile enlargement usually begins about a year after testicular growth begins. The penis first becomes longer, and then becomes broader. Initial ejaculations usually occur later during sleep. Sperm count is low, at first.

Facial hair growth and a deepening voice are two secondary sexual characteristics that develop about two years after pubic hair appears in males. Facial hair begins on the upper lip, becomes more confluent, extends to side-burns, and then grows on the chin. Hair also begins to appear on a pubertal boy’s chest and abdomen. The voice deepens by dropping in pitch due to enlargement of the vocal cords in the larynx, voice box. In addition, other body hair grows, and the areola (pigmented ring around the nipple) enlarges.

Boys grow considerably in both height and mass during puberty. On average, boys will grow about 3.7 in/year (9.5 cm/year) at the peak year of their growth spurt. Boys average 4 ft 7 in (1.4 m) in height prior to the onset of puberty and grow an additional 15 in (38 cm) taller during their pubertal growth spurt. At the end of puberty, the average male height is 5 ft 10 in (1.8 m). The initial growth occurs in the leg bones increasing leg length. Then the torso lengthens causing an increase in sitting height. Between leg growth and torso growth, the arms, shoulders, and hips of boys grow considerably, as well. Muscle mass also increases-particularly in the shoulders. A temporary drop in subcutaneous fat occurs in the arms during this time with fat levels returning to normal at the end of puberty.

Female puberty

At the beginning of puberty, a girl’s face rounds out, her hips widen, and her breasts begin to develop. Breast development can occur as early as eight but starts between 10 and 14 years for most girls. Full breast development may take two to five years. Pubic hair begins to grow shortly afterwards, followed by the first menstrual period, or menarche. Like male puberty, female puberty is initiated by hypothalamic hormones. GRH secreted from the hypothalamus triggers LH and FSH release from the anterior pituitary. The LH and FSH, in turn, stimulate ova maturation. GHRH is also released from the hypothalamus and stimulates growth hormone secretion from the pituitary.

Breast development is called thelarche and can be measured in stages. The initial accumulation of tissue pads the underside of the areola around the nipple. Before puberty, the areola is usually about 0.5 in (1.2 cm) in diameter. By the end of puberty, it can be about 1.5 in (3.8 cm) in diameter. The breast enlarges developing a smooth curve. Then a secondary mound of tissue grows under the areola. Usually by age 18 years, a girl’s breasts have reabsorbed the secondary mound giving a rounded contour to the now adult shape.

Breast budding is followed by menarche between 12 and 14 years for most girls. However, normal menarche may occur between 10 and 16 years. Menstruation occurs as part of the menstrual cycle which lasts about 28 days. The initial hormonal cycles associated with the menstrual period usually begin months before menarche, so for a while a girl usually has hormonal cycles without menstruation. The menstrual cycle is divided into two halves, the follicular and the luteal phases. During the follicular phase, an immature egg follicle ripens and estrogen levels rise. On around day 14, LH and FSH trigger the egg to travel into the adjacent fallopian tube. During the luteal phase, high progesterone and estrogen levels prevent another egg from beginning another cycle. After about eight days, if the egg is not fertilized, then the uterine lining is shed as menstrual blood. Menstruation can last one to eight days but usually lasts three to five days. The amount of blood lost varies from slight to 2.7 oz (80 ml) with the average being 1 oz (30 ml) lost for the whole period.

A number of factors affect when menstruation begins. Normal menarche is associated with good nutrition and health. Girls who are malnourished or ill may have later menarche. In addition, girls who are particularly athletic or involved in strenuous physical activities such as ballet often start menstruating later. Once menarche occurs, cycles are usually irregular for up to two years. Because of this irregularity, girls may be less likely to conceive during this time. However, it is possible to conceive and therefore they should use contraception if they are sexually active and wish to prevent pregnancy.

The pubertal growth spurt, of height and weight, in girls usually occurs a year or two before boys, on average. Increases in height and weight are followed by the increases in hip size, breast size, and body fat percentage. The peak growth rate during this time is 3.2 in (8 cm) per year, on average. The average female is 4 ft 3 in (1.3 m) tall at the beginning of puberty and gains 13.5 in (34 cm) total during her pubertal growth spurt. At the end of puberty, the average female height is 5 ft 4.5 in (1.6 m) tall. Girls also increase body fat at the hips, stomach, and thighs.

Related topics

Around the world, entry into adulthood is often marked ceremoniously in males and females. A rite of passage ceremony is held to honor this transition. This type of ceremony is usually held in less-industrialized countries where boys and girls are expected to assume adult roles at the end of puberty. The Arapesh of New Guinea build the young woman a menstrual hut at the home of her husband-to-be. Her girlish ornaments are removed, and the girl acquires womanly markings and jewelry. The ceremony marks the beginning of her fertility. Young Mano men of Liberia go through a ceremonial death at puberty. These young men used to be stabbed with a spear and thrown over a cliff to symbolize death and rebirth into adulthood. Actually, a protective padding kept the spear from penetrating them, and a sack of chicken blood was tied over the spot to appear as though the boy had been stuck. He was not tossed over the cliff, but a heavy object was thrown over instead to sound like he had been thrown. Pubertal Apache girls are sometimes showered with golden cattail pollen (considered holy) as part of a four-day ritual. Boys and girls in Bali, Indonesia, formally come of age when a priest files their six top teeth even so they will not appear fanged.

By comparison, industrialized countries seldom have pubertal rites of passage. In fact, puberty may not be discussed often. Instead, these teenagers are usually expected to continue their education for some time before they can settle down and have a family. The changes that accompany puberty often bring on new feelings, however. Adolescents begin to contemplate independence from their parents and assume more adult roles in their family. In addition, puberty is a time when some boys and girls begin to think about their sexuality and sexual activity. Because the human body undergoes such significant and seemingly rapid changes in puberty, it can be a frightening time if a boy or girl does not understand what they are experiencing. Studies have shown that boys and girls who have been told about pubertal changes are less frightened and have fewer emotional problems related to puberty than children who have not been informed about what to expect.

With sexual maturation comes fertility. Many people do not become sexually active during puberty, but those who do have the additional adult responsibility to respect the possibility of pregnancy. For teenagers who begin having intercourse, contraceptive options exist to prevent pregnancy. Another serious consideration is the possibility of contracting a sexually transmittable disease (STD). Not all STDs are curable. Some are debilitating, and others are fatal. The key is protection. Most contraceptives do not protect against both pregnancy and STDs.

Adolescence is not a good time to play Russian roulette with a poor diet either. A diet of potato chips and ice cream or celery and water will not optimize healthy growth. They will both hinder it. Loading up on junk food or slimming down by fasting are both dangerous. During puberty, a lot of body mass is constructed, and the right nutritional building blocks are essential. Calcium, protein, carbohydrates, minerals, and vitamins are all important. And enough calories to fuel development is also needed. During puberty, adolescents need about 2,000 to 2,500 calories a day. Some girls become self-conscious of their developing bodies and try to minimize fatty tissue growth by fasting or making themselves throw up food they have eaten. Both of these mechanisms to stay thin are extremely dangerous, can have long-term detrimental effects on health, and should be avoided.

KEY TERMS

Adolescence— The psychological and emotional changes that accompany puberty.

Adrenarche— Maturation of the adrenal glands to secrete low levels of sex hormones.

Androgens— Male sex hormones including testosterone and androstenedione.

Contraceptive— Any substance or device used to prevent the fertilization of an egg by a sperm during sexual intercourse.

Fertility— The ability to reproduce.

Menarche— The beginning of menstruation.

Menstruation— The cyclic shedding of the endometrial lining of the uterus in fertile women who do not become pregnant.

Neuroendocrine— The interaction between the endocrine system (hormones) and the nervous system (brain) to modulate physiological events.

Sex hormones— Estrogen and testosterone.

Adolescents who can turn to a trustworthy adult with their questions or concerns about puberty may find this transition easier.

See also Adrenals; Endocrine system; Reproductive system.

Resources

BOOKS

Emde, Robert N., and John K. Hewitt, eds. Infancy to Early Childhood: Genetic and Environmental Influences on Developmental Change. New York: Oxford University Press, 2001.

Fisher, Celia B., and Richard M. Lerner. Encyclopedia of Applied Developmental Science. Thousand Oaks, CA: Sage Publications, 2005.

Hayward, Chris, ed. Gender Differences at Puberty. Cambridge, UK: Cambridge University Press, 2003.

Kaplowitz, Paul. Early Puberty in Girls: The Essential Guide to Coping with this Common Problem. New York: Ballantine Books, 2004.

Rice, F. Philip. The Adolescent: Development, Relationships, and Culture. Boston, MA: Allyn & Bacon, 2005.

Santrock, John W. Adolescence. Boston, MA: McGraw-Hill, 2007.

Louise Dickerson