The hair follicle: A paradoxical androgen target organ

Androgens are the main regulator of normal human hair growth. After puberty , they promote transformation of vellus follicles, producing tiny, unpigmen ted hairs, to terminal ones, forming larger pigmented hairs, in many areas, e.g. the axilla. However, they have no apparent effect on the eyelashes, b ut can cause the opposite transformation on the scalp leading to the replac ement of terminal hairs by vellus ones and the gradual onset of androgeneti c alopecia. This paradox appears to be an unique hormonal effect. Hair foll icles are mainly epithelial tissues, continuous with the epidermis, which p roject into the dermis. A mesenchyme-derived dermal papilla enclosed within the hair bulb at the base controls many aspects of follicle function. In t he current hypothesis for androgen regulation, the dermal papilla is also c onsidered the main site of androgen action with androgens from the blood bi nding to receptors in dermal papilla cells of androgen-sensitive follicles and causing an alteration of their production of paracrine factors for targ et cells e.g. keratinocytes. Studies of cultured dermal papilla cells from sites with different responses to androgens in vivo have confirmed the para doxical responses. All dermal papilla cells from androgen-sensitive sites c ontain low capacity, high affinity androgen receptors. However, only some c ells formed 5 alpha -dihydrotestosterone, e.g. beard but not axillary cells , in line with hair growth in 5 alpha -reductase deficiency. Incubation wit h androgens also stimulated the mitogenic capacity of beard cell media, but inhibited that produced by scalp cells. This suggests that the paradoxical differences are due to differential gene expression within hair follicles, presumably caused during embryogenesis. Copyright (C) 2001 S. Karger AG, B asel.