A thread-like structure composed of dead cells containing keratin, a fibrous protein. The root of each hair is embedded in a tiny pit in the dermis layer of the skin called a hair follicle. Each shaft of hair consists of a spongy semihollow core (the medulla), a surrounding layer of long, thin fibres (the cortex), and, on the outside, several layers of overlapping cells (the cuticle). While a hair is growing, the root is enclosed by tissue called a bulb, which supplies the hair with keratin. Once the hair has stopped growing, the bulb retracts from the root and the hair eventually falls out. Hair is involved in the regulation of body temperature (known as thermoregulation). If the body is too cold, arrector pili muscles in the skin contract, pulling the hairs upright to form goose pimples. Erect hairs trap an insulating layer of air next to the skin. Brittle hair may be due to excessive styling, hypothyroidism, or severe vitamin or mineral deficiency. Very dry hair can be caused by malnutrition. Ingrown hairs occur when the free-growing end of the hair penetrates the skin near the follicle, which may cause inflammation. (See also hirsutism; hypertrichosis.)


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