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Jul 23, 2010, 9:43:10 PM7/23/10

Hair
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For other uses, see Hair (disambiguation).
Cross section of a hair

Hair is a filamentous biomaterial, that grows from follicles found in
the dermis. The human body, apart from the palms of the hands and
soles of the feet, 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 keratin.

Found exclusively in mammals, hair is one of the defining
characteristics of the mammalian class.[1] Although other non-mammals,
especially insects, show filamentous outgrowths, these are not
considered "hair" in the scientific sense. So-called
"hairs" (trichomes) are also found on plants. The projections on
arthropods such as insects and spiders are actually insect bristles,
composed of a polysaccharide called chitin. There are varieties of
dogs, cats, and mice bred to have little or no visible fur. In some
species, hair is absent at certain stages of life.

Hair often refers to two distinct structures: 1) the part beneath the
skin, called the hair follicle or when pulled from the head, called
the bulb. This organ is located in the dermis and maintains stem cells
which not only re-grow the hair after it falls out but also are
recruited to regrow skin after a wound;[2] and 2) the shaft, which is
the hard filamentous part that extends above the skin surface. The
hair shaft, in cross-section, can be divided roughly into three zones.
Starting from the outside: 1) the cuticle which consists of several
layers of flat, thin cells laid out like roof shingles, 2) the cortex
which contain the keratin bundles in cell structures that remain
roughly rod like and in some cases, 3) the medulla, a disorganized and
open area at the fiber's center.[3]

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