Human hair growth cycle
Under normal circumstances hair growth in each hair follicle occurs in a cycle. There are three main phases of the hair growth cycle; anagen, catagen and telogen with anagen further subdivided into proanagen, mesanagen and metanagen. Anagen is the active growth phase when hair fiber is produced. Proanagen marks initiation of growth with RNA and DNA synthesis in a follicle which then quickly progresses through mesanagen to metanagen and maximum follicle length and girth.
In this mature state of proliferation and differentiation the hair follicle consists of a total of eight concentric layers of different cell types and melanogenesis occurs within pigmented hair follicles. Anagen is followed by catagen, a period of controlled regression of the hair follicle. Ultimately the hair follicle enters telogen, when the follicle is in a so-called resting state.
Anagen is the longest phase with up to 90% of follicles on a normal human scalp in this active hair growth state at any given time and correspondingly telogen hair follicles comprise up to 10% on the scalp. The average rate of hair fiber growth is around 0.35mm a day but this rate varies depending on the site of the hair follicle and the age and sex of the individual.
The length of the anagen growth phase for scalp hair is usually 3-6 years while telogen lasts just 30-90 days and catagen is best estimated at 14-21 days. In many young mammals the anagen growth phase occurs in a wave like pattern across the skin surface, but the hair follicles of humans can run through the normal cycles of growth apparently independently of neighboring follicles.
Normally this cycle of hair production and inactivity will continue for the duration of the individual’s life but other factors can influence and inhibit hair production and in some cases lead to physical destruction of the hair follicle. Factors may include adverse reactions to drugs and cosmetics, or as a result of scarring, tumors, radiation, the genetics of the individual, hormones and/or their immune system.