Apart from some protective roles, the primary function of human hair is to communicate information about a person's age and state of sexual maturity. Therefore, major differences from the norm, such as hirsutism and balding, often cause psychological distress.
Androgens are the main regulator of changes in human hair growth. They stimulate the production of pigmented terminal hair in many areas after puberty, including pubic and axillary hair in both sexes and beard growth in men. Androgens also cause the reverse transformation of large, terminal, scalp follicles to small vellus ones producing tiny, pale hairs and causing balding. Both beard growth and balding can occur on the same person demonstrating an endocrine paradox. How does the same hormone stimulate hair growth on the face while taking it away on the scalp?
The various androgen-insensitivity syndromes reveal another paradox. Androgen receptors are required within hair follicles for all androgen-dependent hair growth to occur.
However, the enzyme 5alpha-reductase type 2 which metabolises testosterone to the more potent androgen, 5alpha-dihydrotestosterone, is only necessary for androgen action in male specific follicles such as the beard and balding scalp. In this second paradox axillary and female pubic hair will grow well in 5alpha-reductase deficient individuals!
Both these paradoxes suggest that androgen action within individual follicles is specific to the individual follicle i.e. relates to its gene expression. Androgens are believed to act on the mainly epithelial hair follicle via the mesenchyme-derived dermal papilla situated at its base. Cultured dermal papilla cells derived from follicles with differing responses to androgens (i)in vivo(/i) retain their characteristics (i)in vitro(/i)offering a useful model system with which to study these paradoxical responses to hormones. Understanding these should facilitate the development of better treatments for andogen-dependent hair disorders.