<P> Keratin (/ ˈkɛrətɪn /) is one of a family of fibrous structural proteins . It is the key structural material making up hair, horns, claws, hooves, and the outer layer of human skin . Keratin is also the protein that protects epithelial cells from damage or stress . Keratin is extremely insoluble in water and organic solvents . Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized epidermal appendages found in reptiles, birds, amphibians, and mammals . The only other biological matter known to approximate the toughness of keratinized tissue is chitin . </P> <P> Keratin derives from Greek κερατίνη keratíni from κέρας keras (genitive κέρατος keratos) meaning "horn" originating from the Proto - Indo - European * ḱer - of the same meaning . It is composed of "horn like", i.e., kerato, to which the chemical suffix - in is appended . The Greek keras is used in many animal names, e.g. Rhinoceros, meaning "nose with a horn". </P> <P> Keratin filaments are abundant in keratinocytes in the cornified layer of the epidermis; these are proteins which have undergone keratinization . In addition, keratin filaments are present in epithelial cells in general . For example, mouse thymic epithelial cells (TECs) are known to react with antibodies for keratin 5, keratin 8, and keratin 14 . These antibodies are used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus . </P> <Ul> <Li> the α - keratins are found in all vertebrates . They form the hair (including wool), stratum corneum, horns, nails, claws and hooves of mammals and the hagfish slime threads . </Li> <Li> the harder β - keratins are found only in the sauropsids, that is all living reptiles and birds . They are found in the nails, scales, and claws of reptiles, some reptile shells (Testudines, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds . (These keratins are formed primarily in beta sheets . However, beta sheets are also found in α - keratins .) </Li> </Ul>

Keratin is not present in the epidermis of