Only the follicle is alive; the shaft you see is dead keratin
Hair is a protein filament growing from dermal follicles and a hallmark of mammals. Humans wear thick terminal strands and fine vellus almost everywhere except glabrous skin. What you style is already dead; only the follicle beneath keeps living stem cells and growth machinery.
The follicle is the working part. Tucked into the dermis, it ends in a soft, whitish bulb whose cells manufacture the shaft, and it keeps a supply of stem cells that regrow a hair after it falls out and can be drafted in to rebuild skin after a wound. Each follicle also carries a sebaceous gland that oils the strand and an arrector pili muscle that can pull the hair upright, which in sparsely haired humans shows up as goose bumps.
The shaft itself shows no biochemical activity. Most of its volume is keratin, arranged in three zones. Outside is the cuticle, overlapping flat cells laid like roof tiles and coated with a single molecular layer of lipid that sheds water. Beneath it the cortex supplies most of the strength and soaks up water; it also holds the melanin granules that set colour. At the centre, when present at all, sits the loose, open medulla. Follicle shape decides fibre shape: round fibres give straight hair, oval or flatter ones wave or curl. Human hairs range from about 0.017 to 0.18 millimetres across.
Only two pigments colour natural hair. Eumelanin dominates in brown and black hair, pheomelanin in red, and blond reflects little pigment of either. Grey appears as melanin output falls or stops. Every strand cycles on its own schedule through anagen, catagen and telogen phases. The oft-quoted growth rate of about a centimetre a month is a simplification: scalp hair has been measured at 0.6 to 3.36 centimetres monthly, slowing with age, and thick hairs outpace thin ones, roughly 11.4 against 7.6 millimetres a month.
Hair covers the outer body except mucous membranes and glabrous skin such as palms, soles and lips. The English word traces through Middle English heer to Old English and Old Norse roots, and an obsolete term for head hair, fax, survives in place names such as Fairfax and Halifax.
Source: Hair
