How does a hair on a human leg "know" it has reached the right length, and stop growing?. What mechanism differentiates it from a scalp hair which continues to grow unless cut?
Interesting question; the same might be asked for eyebrow hairs, for instance. I am not sure about leg hairs, but eyebrow hairs begin to grow to abnormal length once we age, supposedly due to our DNA breaking down. I would assume that it is the DNA which dictates how each kind of hair should grow and shape.
I guess if we had better knowledge and ability to control hair growth on legs, waxing or shaving would become obsolete :). Meh im a guy, what do I care ..
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mike hello, i realy dont fammiliar with the exact molecular mechanism but maybe this short explanation that is taken from the internet site http://dermatology.about.com/cs/hairanatomy/a/hairbiology_2.htm
will help.
There are three stages of hair growth: catagen, telogen, and anagen. Catagen - The catagen phase is a transitional stage and 3% of all hairs are in this phase at any time. This phase lasts for about 2-3 weeks. During this time growth stops and the outer root sheath shrinks and attaches to the root of the hair. This is the formation of what is known as a club hair.
Telogen - Telogen is the resting phase and accounts for 10-15% of all hairs. This phase lasts for about 100 days for hairs on the scalp and much longer for hairs on the eyebrow, eyelash, arm and leg. During this phase the hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. About 25-100 telogen hairs are shed normally each day.
Anagen - Anagen is the active phase of the hair. The cells in the root of the hair are dividing rapidly. A new hair is formed and pushes the club hair up the follicle and eventually out. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2-6 years. Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. On the other hand, people with very long hair have a long active phase of growth. The hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase of about 30-45 days explaining why they are so much shorter than scalp hair. best regards. galc.
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