The changes that take place duringresult in what are known as the secondary sexual characteristics. Before , apart from the obvious differences in the external appearance of the sex organs, the composition of male and female bodies is remarkably similar. However, the sex produced during alter the situation dramatically. Young men, for example, have about one and a half times as much lean (non-fat) body tissue as young women, and by the time women have matured at the end of their teens their bodies carry twice as much fat as those of similarly aged men.
Puberty usually begins about the age of 11 to 13 years in children from most Western countries, although occasionally it may start much earlier. Only a century ago, the average age of onset of puberty was about 16 or 17. The earlier start of puberty is usually attributed to improved diet.
In girls, the earliest signs of puberty are the development of the breasts and the growth of pubic hair around the genitals. The onset of menstrual periods (the menarche) follows soon after. It takes about two years to develop a regular menstrual cycle and the fertility of many teenage girls is low during this time. Nevertheless, this is no reason to assume that teenage girls who have sexual intercourse will not become pregnant.
In boys during puberty the penis and testes enlarge and the seminiferous tubules inside the testes begin to produce sperm.
Boys are capable of producing sperm from about the age of 11 to 15 years and are then quite capable of fathering a child. Other, secondary, sex changes that occur in boys are that the voice breaks (becomes deeper) because of enlargement of the larynx, pubic hair grows around the genitals, and hair also grows on the face and later on the body.
The growth rate of both sexes increases rapidly during adolescence – the so-called growth spurt.
This increase in height is usually greater in boys, although it begins earlier in girls, and is also of course subject to the influences of diet and environment.
All these effects on the body tissues are achieved through the influence of male and female sex hormones. The release of these hormones is governed by two other hormones from the pituitary gland, the so-called ‘master gland’ near the brain. In males the release of luteinising hormone (LH) stimulates the testes to produce testosterone, whereas in females the release of follicle-stimulating hormone (FSH) causes the ovaries to produce oestrogens. There is a feedback system whereby the presence of oestrogen or testosterone in the blood causes the pituitary gland to stop producing its hormones.
Ultimately, the control of all the sex hormones resides in the hypothalamus, the region deep within the brain that co-ordinates the nervous system and most of the hormone-producing glands. Scientists believe that the biological clock responsible for the start of puberty is governed by the hypothalamus. As part of the maturation process, the hypothalamus becomes less sensitive to the minute amounts of sex hormones circulating in the blood of the boy or girl. This provokes an increased secretion of stimulating hormones by the pituitary gland. In turn, this results in an enhanced production of sex hormones by the testes or ovaries, and puberty has started.