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Hair build-up and hair cycle
Hairs are long threads made of keratin, whose hard cortex encloses the core which is of softer build-up. The hair follicle (the hair root and all the surrounding cells that are responsible for hair growth) are supplied with nourishment by thin vascular veins. The hair length is determined by the various developement phases of the hair cycle.
The growth phase lasts 2 to 6 years, which is followed by a two to three week transitional phase. Subsequently the hair goes into a three month rest period. After the rest period the hair falls out and the hair cycle starts over. Fortunatly this cycle does not occure for all hairs at the same time. About 50 to 150 hairs enter this cycle daily, thus - assuming about 100,000 hairs on a head - the hair density is not affected. Hairs usually grows about 1 cm per month and have a thickness of about 0.07 mm. The hair colour depends on the amount, type and distribution of the melanosomes ( hair pigments ) in thehair cortex.
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