A hair follicle is a mammalian skin organ that produces hair. Hair production occurs in phases, including growth (anagen), cessation (catagen), and rest (telogen) phases. Stem cells are responsible for hair production.
Anagen is the active growth phase of hair follicles during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase, the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2–7 years and is genetically determined. At the end of the anagen phase, an unknown signal causes the follicle to go into the catagen phase.