Chapter 7: Biology of Hair Follicles

AT-A-GLANCE

EVOLUTION AND FUNCTION OF HAIR

Hair is found only in mammals, in which during the course of evolution, its primary roles were to serve as insulation and protection from the elements. In contemporary humans, however, hair’s main purpose revolves around its profound role in social interactions. Loss of hair (alopecia) and excessive hair growth in unwanted areas (hirsutism and hypertrichosis) can lead to significant psychological and emotional distress that supports a multibillion-dollar effort to reverse these conditions.

Much progress has been made in understanding hair growth, and as a result, new treatments for alopecia are on the horizon.^1,2 These advances resulted from the interest of developmental biologists and other investigators in the hair follicle as a model for a wide range of biological processes. As each hair follicle cyclically regenerates, it recapitulates its initial development. Many growth factors and receptors important during hair follicle development also regulate hair follicle cycling.^3,4 The hair follicle possesses keratinocyte and melanocyte stem cells, nerves, and vasculature that are important in healthy and diseased skin.^5-7 To appreciate this emerging information and to properly assess a patient with hair loss or excess hair (see Chaps. 85, 86, 87, 88, 89, 90), an understanding of the anatomy and development of the hair follicle is essential.

EMBRYOLOGY

Morphologically, hair follicle development has been divided into eight consecutive stages, several of which are illustrated in Fig. 7-1. Each stage is characterized by unique expression patterns for growth factors and their receptors, growth factor antagonists, adhesion molecules, and intracellular signal transduction components.^8-10 Promising advances in understanding the molecular mechanisms behind hair follicle development arose through the discovery that mammalian counterparts (homologs) of genes important for normal Drosophila (fruit fly) development also affect hair follicle development. Decapentaplegic (Dpp/BMP), engrailed (en), Homeobox (hox), hedgehog/patched (hh/ptc), notch, wingless/armadillo (wg/wnt/catenin), and branchless (Fgf) genes are all critical for hair follicle and vertebrate development in general (reviewed in ^8-10). These genes were all first discovered in Drosophila; thus, most of the names assigned to them describe the peculiar appearance (phenotype) of the flies carrying mutations in these genes.¹¹