ABSTRACT
The hair follicle, a highly unusual, dynamic organ found only in mammalian skin, has contributed greatly to the evolutionary success of mammals. Each follicle synthesizes and supports a hair and, most excitingly, possesses the ability to partially recapitulate embryogenesis to replace this hair during the hair growth cycle [1, 2]. The new hair can resemble the previous one or differ from it in size and/or color. This is seen very dramatically in Scottish mountain hares, whose thick white winter coat markedly contrasts with the shorter brown summer version [3]. Major changes also occur in human hair growth, mainly under the influence of androgens; around and after puberty, follicles that produced tiny, almost transparent vellus hairs in childhood form larger, thicker and darker terminal hairs in many areas such as the axilla in both sexes and the face of boys [4-6]. To achieve this, hair follicles pass through regular three-phase growth cycles: (1) development and growth (anagen), when the lower follicle is regenerated, forming a new hair that generally replaces the original hair; (2) regression (catagen), when growth stops, the lower follicle is resorbed, and the whole hair becomes keratinized; and (3) rest (telogen), when the hair normally remains in the skin until replaced by the next hair. An additional process of exogen, involving the active release of the old hair, has also recently been proposed [7].
