The biology of hair

Hair is a product of small pits in the skin known as hair follicles. The most important feature of hair follicles is that their activity is intermittent; each active phase or anagen is succeeded by a transitional phase (catagen) and a resting phase (telogen), during which the fully formed "club hair" is retained for a period and then shed. The growth of facial, body, axillary, and pubic hair depends on androgens. Facial hair and body hair require high levels of testosterone and its conversion to 5-alpha-dihydrotestosterone. Pubic and axillary hair follicles require much lower levels of hormone, and 5-alpha-reduction appears to be unnecessary. Paradoxically, male pattern alopecia and its female equivalent also require androgen for their manifestation. The differing lengths of hair in the various regions of the body result largely from differences in the duration of anagen and only to a small extent from differences in the rates of growth. Some hair loss from the scalp can be characterized in terms of the hair growth cycle, and some involves long-term changes in the follicular architecture. Thus postfebrile and postpartum alopecias are telogen effluvia that involve shedding of club hairs, whereas drug-induced alopecia and alopecia areata involve shearing and loss of growing hairs. Male pattern baldness and female diffuse alopecia involve gradual shortening of the periods of anagen and shrinkage of the hair follicles over a long term.