Keratin, highly fibrous and resistant protein that makes up most of the material in the cells forming the epidermis, hair, nails, scales, feathers, beaks, horns, and hooves of animals. These cells originate from permanent populations of germinal cells, and as they migrate outward they undergo specific patterns of differentiation in a process called keratinization. That is, the cells become increasingly filled with microfibrils of keratin, and the nuclei and organelles of the cells are reabsorbed. Little is understood, however, about how the cells differentiate to form such diverse and efficient structures as the elastic outer layer of the skin of mammals or the stiff scales of fish. An important quality of keratin is its ability to extend and contract reversibly.