The Hair Cycle

Hair follicles undergo cycles of degeneration and regeneration. These are divided into different stages:

Anagen Phase

The anagen phase of hair growth lasts about three or more years. During the onset of this stage, the cycling portion of the follicle regenerates, a process that necessitates a reservoir of follicle stem cells. Stem cells are able to self-renew, as well as give rise to differentiating cells. About 84% of hair remain in this phase.

Catagen Phase

catagen phase, or transition phase, which lasts around three weeks or less, is marked by the end of proliferation and apoptosis of epithelial cells below the bulge. During this stage, there is major cell death, and only a small portion of hair follicles remain in a state of full growth. At the end of the catagen phase, a shift occurs that sets the stage for new hair growth. About 1% of hair stay in this phase.

Telogen Phase

In the telogen phase, most bulge cells are in a dormant state. The hair is dying and falling out. This is then followed, once again, by the anagen phase. Once follicle stem cells become activated, they migrate to the base of the follicle, where they produce a new hair shaft.

The duration of each phase varies, depending on the type, site, and genetic programming of the follicle.