Puberty
The brain's hypothalamus begins to release pulses of gonadotropin-releasing hormone, or GnRH. These signals travel to the pituitary gland in the center of the head. Cells there respond by secreting luteinizing hormone and follicle-stimulating hormone into the blood. The ovaries or testes then grow and begin producing estradiol and testosterone. Rising levels of these sex steroids produce the physical changes of puberty. This neurohormonal process may precede the first visible body changes by one to two years. Ernst Knobil found that GnRH signals from the hypothalamus induce pulsed secretion of LH at roughly one to two hour intervals. Robert M. Boyar discovered that gonadotropin pulses occur only during sleep before puberty progresses. By the end of puberty, there is little day-night difference in the amplitude and frequency of these pulses. Neurokinin B and kisspeptin are critical parts of the control system that switches on the release of GnRH at the start of puberty.
Females typically begin the process of puberty around age ten and a half. Males start at ages eleven and a half to twelve. Puberty generally ends between fifteen and seventeen for females and sixteen to seventeen for males. Females attain reproductive maturity about four years after the first physical changes appear. In contrast, males accelerate more slowly but continue to grow for about six years after the first visible pubertal changes. Testicular enlargement is the first physical manifestation of puberty in boys. The size of the testicles reaches maximal adult size about six years after the onset of puberty. While eighteen to twenty cubic centimeters is an average adult size, there is wide variation in normal populations. Maximal adult height is achieved at an average age of fifteen years for an average female and eighteen years for an average male. Potential fertility usually precedes completion of growth by one to two years in females and three to four years in males.
The average age at which the onset of puberty occurs has dropped significantly since the 1840s. A 2006 study in Denmark found that breast development started at an average age of nine years and ten months. This was a year earlier than when a similar study was done in 1991. Scientists believe the phenomenon could be linked to obesity or exposure to chemicals in the food chain. In the 19th century, it was age 15 for females and 17 for males. Modern archaeological research suggests that the rate of puberty as it occurs now is comparable to other time periods. Growth spurts began at around 10 to 12, but markers of later stages had delays correlated with severe environmental conditions such as poverty. The median age of menarche for a population may be an index of the proportion of undernourished females in the population. The earliest average onset of puberty is for African-American females and the latest average onset for high altitude subsistence populations in Asia.
Scientists are concerned about Bisphenol A or BPA's behavioral effects on fetuses and children at current exposure levels. The Centers for Disease Control and Prevention found measurable amounts of BPA in the bodies of more than 90 percent of the U.S. population studied. Many plastic baby bottles contain BPA, and it is more likely to leach out when temperature is increased. Obesity influence and exercise link early obesity with an earlier onset of puberty in females. They have cited obesity as a cause of breast development before nine years and menarche before twelve years. Mental illnesses occur in puberty, including major depressive disorder and bipolar disorder. Females aged between 15 and 19 make up 40% of anorexia nervosa cases. Various studies have found direct genetic effects to account for at least 46% of the variation of timing of puberty in well-nourished populations. Researchers have hypothesized that early puberty onset may be caused by certain hair care products containing estrogen or placenta.
Common questions
What hormones trigger the start of puberty in humans?
The brain's hypothalamus releases pulses of gonadotropin-releasing hormone to initiate puberty. These signals travel to the pituitary gland which secretes luteinizing hormone and follicle-stimulating hormone into the blood.
At what age do females typically begin puberty compared to males?
Females typically begin the process of puberty around age ten and a half while males start at ages eleven and a half to twelve. Puberty generally ends between fifteen and seventeen for females and sixteen to seventeen for males.
How has the average onset age of puberty changed since the 1840s?
The average age at which the onset of puberty occurs has dropped significantly since the 1840s when it was age 15 for females and 17 for males. A 2006 study in Denmark found that breast development started at an average age of nine years and ten months.
Why does early onset of puberty cause psychological damage to females?
Early-maturing females usually look larger than females who have not yet entered puberty and may face teasing about their visible breasts. These experiences lead to lower self-esteem, more depression, and poorer body image among affected individuals.
What percentage of the U.S. population contains measurable amounts of Bisphenol A or BPA?
The Centers for Disease Control and Prevention found measurable amounts of BPA in the bodies of more than 90 percent of the U.S. population studied. Many plastic baby bottles contain BPA and it is more likely to leach out when temperature is increased.