Biology and genetics of hair.

The mammalian hair follicle (HF) is a complex structure composed of several distinct cell layers. The HF is an ectodermal appendage that resides in the skin, and unlike other tissues and organs, it possesses the remarkable ability to self-renew and undergoes a hair cycle that persists in adult life. Stem cells in the bulge region of the HF, as well as dermal papilla cells, play key roles in the regulation of successive hair cycles. Recent advances in molecular genetics have enabled the identification of many genes and pathways that are involved in HF morphogenesis and cycling. Furthermore, mutations in some of these genes are associated with hereditary hair diseases in humans. Identification of causative genes for hair diseases has provided a better understanding of the crucial roles of these genes in HF morphogenesis, development, and hair growth in humans.