Stages of hair growth

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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.

Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. The difference between body hair and scalp hair (and, in males, chin and moustache hair) is that head hair for practical purposes grows continuously, whereas body hair alternates regular periods of growth and dormancy. During the growth portion of the cycle, body hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After a few weeks, body hair growth stops. The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows to great length, whereas body hair does not. Anthropologists speculate that the functional significance of long head hair is almost certainly adornment, a by-product of natural selection.

Structurally, hair consists of an inner cortex, comprising spindle-shaped cells, and an outer sheath, called the cuticle. Within each cortical cell are many fibrils, running parallel to the fibre axis, and between the fibrils is a softer material called the matrix. It grows from a hair follicle.