Anagen phase of the hair indicates (AIPG 2009)
|A||The phase of activity and growth|
|B||The phase of transition|
|C||The phase of resting|
|D||The phase of degeneration|
The phase of activity and growth
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.
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 of 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 hairs have a fixed length, while hairs on the head seem to have no length limit.
Growth cycles are controlled by a chemical signal like epidermal growth factor.
Hair growth cycle times
Scalp: The time these phases last varies from person to person. Different hair colour and follicle shape effects the timings of these phases.
a. Anagen phase, 2–3 years (occasionally much longer)
b. Catagen phase, 2–3 weeks
c. Telogen phase, around 3 months
o Anagen phase, 4–7 months
o Catagen phase, 3–4 weeks
o Telogen phase, about 9 months