Body Story

Teen dreams: Boy's story

• The origins of change
For both boys and girls, puberty starts years before there are any outward signs — usually between the ages of 8 and 12. For reasons that no one understands, it always starts at night.

Cells in the brain release a chemical signal in the form of a hormone — gonadotrophins — into the blood. These carry signals from the brain around the body. In a boy, their work begins inside the testicles, where cells begin to divide, organising themselves into tube-like factories which will one day produce sperm. The testicles begin to grow. Between the ages of 9 and 15, they will double in size.

Over the next few years, gonadotrophins will seep out of the tubes, soaking into specialist cells and triggering them to produce testosterone, the male sex hormone. The levels of testosterone gradually build up until they are high enough to begin the physical transformation of a boy into a man.

When testosterone levels reach a certain level, men convert the excess into the female hormone, oestrogen. Oestrogen is necessary for, among other things, the health and growth of bones. See also hormones.

• Physical changes
So it is not until years after the first gonadotrophins are released into the blood that the evidence of their work starts to appear in the body. The changes are designed to make boys sexually attractive to girls, but they initially seem to have precisely the opposite effect.

Testosterone sets off a reaction inside the sebaceous glands, situated deep in the pores of the skin. The glands produce an oily substance called sebum, which clogs the pores and produces spots.

Testosterone also stimulates hair growth. This begins with body hair — most boys don't need to start shaving until their late teens. With hair comes body odour: bacteria living on skin feast on the sweat that is trapped by hair. These bacteria excrete waste in the form of a gas, which has a powerful smell. This applies to girls too.

Between the ages of 11 and 16, a boy's body grows in bursts of up to 1cm a month. Over the whole period, it adds up to a quarter of its final height. The problem with this fast rate of growth is that the brain finds it difficult to keep up. So, for a while, teenagers can be clumsy and badly co-ordinated.

The body does not simply become taller, it fills out. Boys' shoulders become broader, and they develop muscles as testosterone makes the fibres in their muscles multiply and thicken.

Boys' genitals also grow quickly during adolescence. The testicles grow to up to seven times their original size, enabling them to produce the 100 million sperm that are released on orgasm. The penis also becomes longer and thicker and, along with the rest of the genitalia, becomes highly sensitive to touch. The slightest thing can trigger an erection, which can be extremely inconvenient.

Pubescent boys also have to deal with changes in their vocal chords, which cause their voices to 'break'. The vocal chords, like other parts of the body, grow in a burst, making it difficult for the brain to control them at first. The result is a rather embarrassing and unpredictable squeakiness.

• Mind-altering substances
But preparing for sexual activity is not just about physical changes. Testosterone seeps from within blood vessels into the fluid around the brain cells, and from there into dormant cells, triggering a chemical reaction. A group of interconnected cells — part of the reproductive circuit — are brought to life. The chemical change is mind-altering: it creates the desire for sex.

Not only does testosterone create a sex drive, it transforms a young male's behaviour to give him a better chance of actually having sex. He becomes more aggressive and competitive. Although humans may have evolved socially, physically evolution has lagged behind. So although men don't usually have to fight, physically, for a mate, the circuits in their brains for sex and aggression overlap.

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