The Hedgehog and the hair follicle: a growing relationship Andrzej Dlugosz Department of Dermatology and Comprehensive Cancer Center, University of Michigan, 3310 CCGC/Box 0932, 1500 E. Medical Center Drive, Ann Arbor, Michigan 48109-0932, USA Phone: (734) 763-0355; Fax: (734) 763-4575; E-mail: dlugosza@umich.edu The various forms of hair loss (alopecia) constitute an extremely common complaint among both men and women. Although sometimes dismissed as merely cosmetic, these conditions can be psychologically and socially devastating. The desire to regrow hair is at the root of a multi-billion dollar industry worldwide; however, there are few truly effective treatments. Given the recent progress in elucidating some of the pictures of alopecia stop hair loss grow hair prevent baldness signals regulating hair follicle growth and development, there is hope that more useful therapies may soon be available. The timely report by Sato et al. (1) presents interesting new data bearing on this issue. The mature hair follicle is constructed of concentric layers of epithelial cells surrounding the hair shaft (Figure 1). The deepest portion of the follicle, the hair bulb, contains a mesenchymal dermal papilla enveloped by rapidly proliferating hair matrix cells. These epithelial cells give rise to 6 terminally differentiated cell types in the hair shaft and inner root sheath. Most hairs contain a medulla, cortex, and cuticle; external to this are balding facts stop hair loss grow hair prevent baldness the inner root sheath cuticle, and Huxley's and Henle's layers. The outer root sheath is continuous with the epidermis and sebaceous gland, and forms the outermost layer, extending down to and surrounding the hair follicle bulb. A specialized region of the outer root sheath called the "bulge," located just beneath the sebaceous gland at the insertion site of the arrector pili muscle, contains cells with stem cell properties (2, 3). Actively growing follicles also contain melanocytes that transfer pigment to matrix keratinocytes, imparting color to the developing hair. The follicle is thus a rich model for studying epithelial-mesenchymal interactions, cell lineage determination, melanogenesis, and dht hair loss stop hair loss grow hair prevent baldness the molecular regulation
