[Abby]

Abby asked the Naked Scientists:
   
Hi!  I've got a question I've wondered for years, and even asked
friends who are doctors, but never quite got an answer.  

Why is it that the hair on our arms and legs and eyebrows and all other bits grow back when plucked, trimmed or shaved, and then stop growing at a certain point, but the hair on our heads keeps growing and growing and growing?

What do you think?

[Chemistry4me]

This is bound to get you through until your next haircut []

Hair-follicle cycling
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 much shorter length limit compared to hairs on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor.


Anagen Phase
Anagen is the active growth phase of hair follicles. The cells in the root of the hair are 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. The amount of time the hair follicle stays in the anagen phase 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 a club hair is formed.


Telogen Phase
The telogen phase is the resting phase of the hair follicle. 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.


Hair growth cycle times

Scalp: The time these phases last varies from person to person. Different hair colour and follicle shape affects the timings of these phases.
•anagen phase, 2–3 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

http://en.wikipedia.org/wiki/Hair_follicle#Hair-follicle_cycling