Excess hair growth hirsutism from minoxidil

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The cuticle is responsible for much of the mechanical strength of the hair fibre. It consists of scale-shaped layers. Human hair typically has 6-8 layers of cuticle. Wool has only one, and other animal hair may have many more layers. Hair responds to its environment, and to its mechanical and chemical history. For example, hair which is wetted, styled and then dried, acquires a temporary 'set', which can hold it in style. This style is lost when the hair gets wet again. For more permanent styling, chemical treatments (perms) break and re-form the disulphide links within the hair structure.

Hair is a biological polymer; over 90% of its dry weight is made up of proteins called keratins. Under normal conditions, human hair contains around 10% water, which modifies its mechanical properties considerably. Hair proteins are held together by disulfide bonds, from the amino acid cysteine. These links are very robust; for example, virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material.

Among humans, nature selected for little body hair as part of a set of adaptations including bipedal locomotion and an upright posture. Bipedal locomotion is extremely inefficient, and many animals can outrun human beings for short periods of time; such animals, however, are inefficient radiators of heat, and cannot run for long periods of time. Thus, human hunters must be able to chase animals for long periods of time, and must therefore have an efficient mechanism for radiating body heat. Upright posture, which exposes less surface area of the body to direct solar radiation, and subcutaneous sweat glands, which operate best with an absence of hair.