Hair

Hair is a filamentous outgrowth of dead cells from skin, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other life forms, especially insects, show filamentous outgrowths, these are not considered "hair" in the accepted meaning of the term. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles. The hair of non-human mammal species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life.
The primary component of hair fiber is keratin. Keratins are proteins, long chains (polymers) of amino acids. Keratin proteins form the cytoskeleton (miniature skeleton within a cell) of all epidermal cells. Keratin filaments run within a cell from the inside of the outer membrane to weave a "basket" around the nucleus of the cell. Keratins are a principal part of the cells in the epidermis, hair, nails, and feathers.