Early days
Whether a baby develops into a boy or a girl depends on the pair of sex chromosomes they get from their parents. If there are two X chromosomes a girl develops, while a boy develops from a combination of an X and a Y chromosome.
Very early on in pregnancy, at about six to eight weeks, the Y chromosome kicks into action and triggers testes to develop - without a Y chromosome this embryonic tissue would have become ovaries. The foetal testis produces the male hormone testosterone and this promotes the development of the penis and scrotum. In doing this, testosterone prevents the formation of fallopian tubes, a womb (uterus) and vagina.
Puberty
In boys the process of sexual development, called puberty, starts between the ages of 12 and 15 and lasts for about four years. During this time hormones secreted by the pituitary gland in the brain, called follicle stimulating hormone (FSH) and luteinising hormone (LH), prompt the testes to produce more testosterone, causing a sharp increase in the levels of this male sex hormone.
This testosterone surge stimulates the physical changes that turn a boy into a man
This testosterone surge stimulates the physical changes that turn a boy into a man. These changes are known as the secondary sexual characteristics. They include enlargement of the penis, testicles and scrotum; the appearance of pubic hair, body hair and facial hair; voice breaking; and physical growth and muscle development.
Once puberty is reached sperm are continuously manufactured by the testes. Arousal in response to sexual stimulation develops and sexual attraction to other people occurs.
Adulthood
During the adult years testosterone continues to promote sperm formation in the testes. It also continues to cause hair to appear on the face and body, and ironically is also believed to be involved in hair loss from the head or male pattern baldness.
Ageing
With increasing years the level of testosterone in the body gradually declines. In the middle years many men experience tiredness, loss of libido, poor erections, loss of muscle mass, mood swings and irritability.
Some believe these symptoms are a result of lower testosterone levels and that giving men replacement testosterone therapy helps to overcome these symptoms. Others believe the symptoms are more likely to be because of psychological issues than physical ones.
Whether the male menopause - more accurately called the andropause (male hormones are known as androgens) - exists or not continues to be hotly debated and the jury is still out.
This article was last medically reviewed by Dr Rob Hicks in May 2005.
First published in October 1997.
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