Hair is a protein filament that grows from follicles found in the dermis, or skin. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein, notably alpha-keratin. Attitudes towards different hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person’s personal beliefs or social position, such as their age, sex, or religion. The study of hair is part of trichology, a branch of dermatology. The cross-section of a hair can be divided into three zones: the medulla (innermost zone), the cortex (middle zone), and the cuticle (outermost zone). The strength and flexibility of a hair are determined by its cross-sectional shape. The quality and quantity of the cross-linkages between the alpha-keratin molecules determine whether the hair will be strong or weak. Many animals have developed specialized hairs that serve particular functions. These


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