Hair cycle

During the anagen phase (the period when a hair follicle is actively growing), which can vary from a few weeks to several years, epithelial cells in the hair bulb undergo vigorous mitotic activity and differentiate as they move distally to form the hair fiber and its surrounding inner root sheath and inner layer of the outer root sheath. Eventually, epithelial cell division ceases, and the follicle enters the catagen phase, in which the proximal end of the hair shaft keratinizes to form a club-shaped structure, which eventually sheds, and the lower part of the follicle involutes by apoptosis. The telogen phase marks the period between follicular regression and the onset of the next anagen phase (solid arrows). The development of the anagen follicle, which closely replicates embryonic development of the hair follicle, is conventionally divided into six stages (I to VI, with anagen VI representing the fully formed anagen follicle). In most mammals, hair cycles are coordinated in a wave-like fashion across the skin (moult waves), but in humans, follicles cycle independently of their neighbors. In alopecia areata (dashed arrow), anagen VI follicles are precipitated prematurely into telogen phase. Although they are probably able to re-enter anagen phase, the development of the follicle is halted at the anagen III/IV stage, when they prematurely return to the telogen phase. Truncated cycles may continue if and until disease activity declines, and follicles are able to progress further into the anagen phase.