The total amount of body hair is difficult to estimate. The head alone has approximately 100,000 hairs. There are three phases in the life cycle of the hair that need to be distinguished:
1. the growing stage, called anagen;
2. the involution stage, called catagen;
3. the resting stage, called telogen.
Many factors, including general health, climate, drugs, hormones, and genetic programming, affect the growth of the hair. The rate of hair growth varies in different body regions (Fig. 11.5). On average, human hair grows 0.37 mm daily. In males, the scalp hair grows slower and the body hair grows faster than in females. Androgen increases the hair growth on the trunk and plays a role in male-pattern balding, however the exact mechanism is not fully understood. Seventy to 100 scalp hairs are shed daily in the normal telogen phase. In the ageing process, the hair also loses color because melanin fails to be synthesized in the hair matrix.
Fig. 11.5 Daily hair growth in different body areas
Hair growth (mm)
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