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Abstract
Hair is a specialized skin appendage. It has many useful biologic function, including protection from the elements and dispersion of sweat-gland products i.e. pheromones. It also has psychosocial importance in our society. Patients with hair loss or excessive hair growth often suffer tremendously.
After uterine life, human exhibit two main types of hair: Terminal hairs and vellus hairs. The former are conspicuous and exceed 0.03 mm in diameter and 1 cm in length, and pigmented and medullated. The hair fiber diameter remains uniform during a single growth phase under normal conditions. Also terminal hairs grow to a specific length, which varies with tha individual. Hair length is determined by the rate and duration of the growth phase. Terminal hairs are rooted in subcutaneous tissue or deep dermis. However the later are inconspicuous and are 0.03 mm or less in diameter and less than 1 cm in length and lack melanin and medulla. Vellus hairs are rooted in upper dermis and do not penetrate deep in to the dermis. Terminal hairs miniaturized to vellus hair proportions are termed vellus-like hairs. Therefore, hair follicle is a complicated highly dynamic structure. It remodels itself repeatedly throughout the life of a mammal. The cycle have three phases; growing phase (anagen), involutional phase (catagen) and dormant phase (telogen). At any one time, approximately 90 to 95 percent of hairs are in the catagen phase. The anagen phase of scalp hair lasts approximately 2 to 3 years (1000 days) during which the average hair grows 0.3 to 0.4 mm/day. In contrast, the telogen phase lasts an average of 2 to 3 months, whereas the catagen phase lasts only 2 to 3 weeks. Up to 100 telogen hairs are lost each day from the head, and approximately the same numbers of follicles enter anagen. Therefore, the amount of hair depends on the ratio of follicles in anagen versus telogen.