The primary purpose of hair in humans is to influence social interactions.
Hair follicle development depends on interactions between epithelial and mesenchymal cells. The genes important for this interaction are slowly being elucidated.
Genes important for hair follicle development also play a role in hair follicle cycling.
The hair follicle bulge possesses stem cells important for the continual regeneration of the follicle during cycling.
Hair pigmentation depends on melanocyte stem cells and differentiated cells in the follicle. Many genes important for melanocyte behavior and hair pigmentation have been defined.
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.
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
Molecular regulation of hair follicle morphogenesis. The schematic shows the expression of different growth factors, their ...