Hair grows in cycles of various phases:
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 85% 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.
Anagen phase 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. A hair pulled out in this phase will typically have the
root sheath attached to it which appears as a clear gel coating the first few mm of the hair from its base; this may be misidentified as the follicle, the root or the sebaceous gland by non-health care professionals. 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. 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, which 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. A bulb of keratin attaches to the bottom tip of the hair and keeps it in place while a new hair begins to grow below it. A hair pulled out in this phase will have the bulb of keratin attached to it which appears as a small white ball on the end of the hair. 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, about three months. 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. The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair. Fifty to one-hundred club hairs are shed daily from a normal scalp.
Timeline • 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: • Anagen phase, 4–7 months • Catagen phase, 3–4 weeks • Telogen phase, about 9 months ==Clinical significance==