THIS IS HAIR: Hair is a filamentous biomaterial, that grows from follicles found in the dermis. The human body, apart from its glabrous skin, 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. Hair-like structures, called cilia, but which are not classified as hair, are visible in organs such as the nose and ear and these structures also occur in many other organs of mammals, other animals, and plants. Although non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" either, in the scientific sense. These so-called "hairs" (trichomes) also are found on plants. The projections on arthropods, such as insects and spiders, are classified as bristles, which are composed of a polysaccharide called chitin. There are varieties of cats, dogs, and mice bred to have little or no visible hair (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 skin, 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;and 2) the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones. Starting from the outside: 1) the cuticle which consists of several layers of flat, thin cells out overlapping one another as 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.