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تسجيل

Biology of hair , Hair growth

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  154

2026-10-04

133

Biology of hair

Hair growth

A hair shaft begins in cells located in a germination centre, called the matrix, located in the base of the follicle (Fig. 1). Hair does not grow continually, but in cycles, alternating between periods of growth and quiescence. A follicle that is actively producing hair is said to be in the anagen phase. Hair is produced during 4–8 years for head hair (<12 months for non-head hair) at a rate of approximately 0.22–0.52 mm/day or 0.6–1.42 cm/month  for head hair (growth rate depends on hair type and anatomical location). After this period, the follicle enters a relatively short transition period of about 2 weeks, known as the catagen phase, during which cell division stops and the follicle begins to degenerate. Following the transition phase, the hair follicle enters a resting or quiescent period, known as the telogen phase (10 weeks), during which the hair shaft stops growing completely and hair growth begins to shut down. Factors such as race, disease states, nutritional deficiencies and age are known to influence both the rate of growth and the length of the quiescent period. On the scalp of an adult, approximately 85% of the hair is in the growing phase and the remaining 15% is in a resting stage.

Figure 1 Cross-section through hair follicle.