Browse the corpus

Human hair serves a variety of purposes. These include physical protection, thermal insulation, dispersion of sweat and sebum, as well as sensory and tactile functions. Human hair also plays a role in social interactions. The hair follicle is considered a mini-organ and forms from ectodermal hair follicle stem cells as a result of neuroectodermal-mesodermal interactions. This mini-organ is composed of the hair follicle, sebaceous gland, apocrine gland, and arrector pili muscle.[1] A fully formed hair is composed of a hair bulb, hair follicle, and hair shaft and is derived from ectodermal placodes (thickened areas of epidermal cells) which also give rise to teeth and mammary glands. The development of these structures occurs within the first 4 months of embryologic life when a hair bud is formed and gives rise to the hair follicle and associated hair structures.[2]

Because of the heavy role that molecular signaling plays in the formation of hair follicles, genetic defects affecting transcription factors may cause a congenital absence of hair, also known as atrichia. This is the result of defective morphogenesis during embryologic development. Due to their common lineage from ectodermal placodes, disorders of hair development may be associated with nail and teeth abnormalities. Less several mutations may result in hypotrichosis or decreased hair growth. These are commonly unrelated to morphogenesis issues, and many affected people have normal hair density at birth. In these patients, the sparse hair is more commonly related to issues with the regenerative processes of cycling and anchoring the hair shaft. However, morphogenesis-related defects in WNT signaling have been associated with hypotrichosis. Research has also shown that DKK-1 is significantly up-regulated in dermal papilla cells found in balding areas. [8]