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The biology of hair shape

Few studies have been devoted to the biology of African hair, although its curly appearance is a characteristic trait. Published reports have mainly focused on hair density and hair cycle, 1,2 growth rate, 2 chemical analysis 3–5 and scanning electron microscopy. 6 Many different definitions have been used in the literature to describe the curly nature of hair. Hrdy 5,7,8 developed a nomenclature which has been widely accepted. The terminology defines a kink as “a sudden constriction and twisting of the hair shaft, producing an obvious discontinuity in the curvature”. 7 Many theories were put forward to explain why hair crimped and curled. One such theory was based on the geometric shape of the hair shaft. It was indeed generally considered that hair shape was related to the cross-section of the hair shaft. A circular cross-section was assumed to give rise to straight hair, while a pronounced elliptical cross-section was supposed to produce curly hair. It was well known that African hair had a large maximum diameter and a small minimum diameter, resulting in the most elliptical shaft found in any racial group. However, Hrdy had observed that the curliest hair often was not the most elliptical, the relationship between hair shaft cross-sectional shape and hair form being thus open to debate. Other theories were based on the morphology of the hair bulb, and an explanation of crimping as being a result of keratinization within a retroverted hair bulb was proposed. 9