Body Story

Teen dreams: Hormones

Hormones are the driving forces behind the body's changes at puberty. Though your brain decides when puberty is to begin, it is hormones that carry out the work. They are the body's chemical messengers, sent out in the blood to ring the changes required by the brain.

In puberty, the two most important hormones are testosterone, the male sex hormone, and oestrogen, the female sex hormone. In fact, both sexes need both hormones, though in different quantities. Men have approximately 10 times more testosterone than women do, and women have around 10 times more oestrogen. These hormones control the development of secondary sexual characteristics, such as the growth of body hair in both sexes, the development of breasts and the menstrual cycle in women, and sperm production and the growth of facial hair in men.

• How hormones work
Hormones circulating in the blood come into contact with all cells but only trigger changes in their 'target' cells, which have special hormone receptors. For example, cells in Darren's testicles have testosterone receptors, and the testosterone triggers these cells to organise themselves into tubes, building factories that will one day produce sperm.

• Testosterone
In men, 95% of testosterone is produced in the testicles; the rest is produced by the adrenal glands at the top of the kidneys. Testosterone is a powerful hormone that stimulates the body's development of muscle, bone, skin and sex organs, along with 'masculine' physical features, such as the growth of body hair. At about the age of 13 there is sufficient testosterone in a boy's body to begin triggering the changes that transform his body into the body of a man. See boy's story.

In women, testosterone is produced in small amounts by the ovaries and the adrenal glands. It controls a woman's sex drive and energy levels and it also strengthens and maintains bones.

• Oestrogen
Women produce oestrogen in their ovaries. It stimulates breast growth by causing cells in the chest to absorb fat from the blood, making them swell. It also regulates the menstrual cycle. When levels of oestrogen in the blood reach their peak, they trigger the release of the matured egg from its follicle, in ovulation. See girl's story.

Oestrogen is also necessary for the health and growth of bones.

Men do not have cells that produce their own oestrogen. Instead, they make oestrogen by converting excess testosterone. As well as for growth, men need the female sex hormone for the healthy function of the brain and libido.

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