Keratin (ˈkɛrətɪn) is one of a family of structural fibrous proteins also known as scleroproteins. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin among vertebrates. Keratin also 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. Excessive keratinization participate in fortification of certain tissues such as in horns of cattle and rhinos, and armadillos' osteoderm. The only other biological matter known to approximate the toughness of keratinized tissue is chitin.
Keratin comes in two types, the primitive, softer forms found in all vertebrates and harder, derived forms found only among sauropsids (reptiles and birds).
Spider silk is classified as keratin, although production of the protein may have evolved independently of the process in vertebrates.
Alpha-keratins (α-keratins) are found in all vertebrates. They form the hair (including wool), the outer layer of skin, horns, nails, claws and hooves of mammals, and the slime threads of hagfish. The baleen plates of filter-feeding whales are also made of keratin. Keratin filaments are abundant in keratinocytes in the hornified layer of the epidermis; these are proteins which have undergone keratinization. They are also present in epithelial cells in general. For example, mouse thymic epithelial cells react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of mouse thymic epithelial cells in genetic studies of the thymus.
The harder beta-keratins (β-keratins) are found only in the sauropsids, that is all living reptiles and birds.