9 Pilosebaceous physiology in relation to hirsutism and acne

PSAs, with few exceptions, consist of a piliary and a sebaceous component. In androgen-sensitive areas, each has the capacity to develop into either a terminal hair follicle or a sebaceous follicle depending upon its location. Without androgen, there is no development of the sexual hair follicle or sebaceous gland. Androgens appear to promote sexual hair growth by recruiting a population of PSAs that have preset genetic sensitivity to initiate the production of terminal hairs. The site of action of androgens within the PSA is unclear. There are indications that androgens may act at more than one site in a system that requires two-way reciprocal interaction between dermal and epithelial cells for the generation of hair growth. Growth hormone appears to exert an important synergism with androgen in affecting the PSA, seemingly through the mediation of insulin-like growth factors.

Hirsutism is due to an increased density of growing terminal hairs. The majority of cases of moderately severe hirsutism in women are due to hyperandrogenaemia, as are half the cases of mild hirsutism and about one-quarter of the cases of mild acne vulgaris. We advocate reserving the term idiopathic hirsutism or idiopathic acne for those patients in whom excessive growth of terminal hair or acne is not explained by androgen excess. We believe that highly variable sensitivity to androgen within the population explains both idiopathic hirsutism and cryptic hyperandrogenaemia; that is, these disorders lie at opposite ends of the normal spectrum of sensitivity to androgen. The biological basis for the variations in responsiveness of PSAs to androgens is unknown. The regression of hirsutism induced by antiandrogen treatment is characterized by the growth of hairs that are more vellus in character, i.e. smaller and less medullated.