Introduction

Hair growth plays significant roles in human social and sexual communication. People all over the world classify a person's state of health, sex, sexual maturity and age, often subconsciously, by assessing their scalp and body hair. The importance of hair is seen in many social customs in different cultures, such as shaving the head of Buddhist monks or no cutting of scalp hair by Sikhs. Body hair is also involved; for example, the widespread customs of daily shaving men's beards and women's axillary hair in Northern Europe and the USA. When this is considered, it is not surprising that abnormalities of hair growth, either greater or less than “normal”, even including common male pattern baldness, cause widespread psychological distress.

Androgens are the most obvious regulators of human hair growth. Although hair with a major protective role, such as the eyelashes, eyebrows and scalp hair, is produced by children in the absence of androgens, the formation of long pigmented hair on the axillae, pubis, face etc. needs androgens in both sexes. In contrast, androgens may also inhibit hair growth on the scalp, causing baldness. How one type of hormone can simultaneously cause these contradictory effects in the same tissue in different body sites within one person is an endocrinological paradox. The hair follicle has another exciting characteristic. It is the only tissue in the adult body which can regenerate, often producing a new hair with different features. This is how androgens can stimulate such major changes.