Tanya Climo, of Westland High School, Hokitika, asks :-

Why does pubic hair grow where it does in human beings?

Richard Donald, an endocrinologist (a medical scientist who studies glands) at the School of Medicine, Christchurch Hospital, responded.

The presence of pubic hair depends on a number of factors, especially hormones (chemical messengers), hormone receptors (protein molecules in the cell which "recognise" the hormones), and the hair follicles themselves which produce the hair.

There are different types of hair, but pubic hair is "sex hair" and responds to sex hormones, especially male-type hormones such as testosterone which is made in the testicles in boys, and the ovaries in girls. Androgens from the adrenal glands also affect pubic hair growth, and are probably responsible for starting pubic hair growth when puberty first begins. If for some reason the testicles or ovaries and the adrenal glands stop working, then pubic hair drops out.

Sex hormones have no effect on pubic hair unless androgen receptors are present in the hair follicles. Occasionally boys are born without these receptors, and then fail to "recognise" male hormone. Others are unable to convert male hormone to a more active form inside the follicle cells. Boys with these problems develop as girls, but do not have any pubic hair or hair under the arms.

Normally the hormone binds to the receptor which is then able to influence the genes in the nucleus of the cells of the hair follicle. The genes then send messages to the parts of the cell which make the hair. This is a complicated process which you will learn more about if you study molecular biology.

What stops pubic hair from growing all over the body? Normally males, because of their higher testosterone levels, have more pubic hair than females, extending up to the umbilicus. However other hair follicles are lacking in androgen receptors and do not respond to sex hor- mones. The genes which promote hair follicle growth may also not be expressed.

Unfortunately for males, scalp hair is different from sex hair, and tends to fall out when exposed to prolonged high concentrations of testosterone.