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Hair grows in cycles of various phases:[7] anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase (names derived using the Greek prefixes ana-, kata-, and telos- meaning up, down, and end respectively). Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one or several hairs that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase, while 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hair have a much shorter length limit compared to hair on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor. DLX3 is a crucial regulator of hair follicle differentiation and cycling. Specifically, colocalization of phosphorylated Smad1/5/8 complex and DLX3 regulate role for BMP signaling to Dlx3 during hair morphogenesis in animal models.[8][9]
Anagen phase
Anagen is the active growth phase of hair follicles[10] 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; this period is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
Catagen phase
The catagen phase is a short transition stage that occurs at the end of the anagen phase.[11] It signals the end of the active growth of a hair. This phase lasts for about 2–3 weeks while the hair converts to a club hair. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase.
Telogen phase
The telogen phase is the resting phase of the hair follicle.[12] When the body is subjected to extreme stress, as much as 70 percent of hair can prematurely enter the telogen phase and begin to fall, causing a noticeable loss of hair. This condition is called telogen effluvium.[13] The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair.[6] Fifty to one-hundred club hair are shed daily from a normal scalp.[6]
Hair growth cycle times
Scalp: The time these phases last varies from person to person. Different hair color and follicle shape affects the timings of these phases.
Anagen phase, 2–8 years (occasionally much longer)
Catagen phase, 2–3 weeks
Telogen phase, around 3 months
Eyebrows etc.:
Anagen phase, 4–7 months
Catagen phase, 3–4 weeks
Telogen phase, about 9 months
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