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It has been estimated that the average scalp has about 100.000 hairs  and that most females have about 5 square meters of hair, which grow at a rate of about 1 cm a month. Hair density varies from individual to individual and generally depends on hereditary factors. Under normal health conditions, the hair falls out at a rate of 50 to 100 daily.
Each hair consists of two parts: an external one, called the shaft, and an internal one, the root, which is located in the skin's innermost layer.
The final part of the root expands into a bulb, shallow at its bottom. This bulb holds the papilla, which is the "vital core" of the hair, rich in nourishment-conveying blood vessels. The bulb and the papilla as a whole form the hair follicle. A sebaceous gland is annexed to each hair follicle. Sebaceous glands produce the sebum necessary for hair lubrication and protection: sebum hyperproduction results in oily hair, while sebum hypoproduction in dry hair.
The hair bulb is the core of the hair, since it carries out an important, biochemical function leading to hair formation and growth.
Hair growth is not a continuous process, but includes three, different stages. During the first stage, known as anagen, the hair grows 3 to 4 tenths of a millimetre daily on average. During the second stage, known as catagen, cell proliferation begins to slow down to a complete stop. Finally, during the third stage, known as telogen, the hair follicle practically "hibernates". During this stage, which lasts three months on average, the hair is only retained in the scalp by some intercellular bonds. Then, the hair follicle having resumed its activity (anagen stage), the hair falls out when a new hair pushes it out. As a result, each hair can live two to four years and grows depending on the multiplying power of the cells which generate the root. Interestingly, the mean diameter of a hair and the length of the anagen stage are interrelated: the greater the diameter, the longer the cycle. In fact, in alopecia or excess hair loss, the anagen stage is considerably shorter and the hair is always very thin.
Depending on age, site, race, hormones and many other factors, the duration of each phase varies:
5-6 years for the anagen period, about 5 weeks for the catagen  and about 5 months for the telogen.

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