The epithelial layer of the skin is continuously replaced. As the replacement cells move closer to the surface of the epidermis, they produce keratin (from the Greek keras, meaning “horn”), a tough protein. The transformation of cells into keratin breaks down the cells’ nuclei and organelles until they can no longer be distinguished. When the cells’ nuclei have broken down, the cells cannot carry out their metabolic functions. By the time the cells reach the superficial layer of the skin, they are dead and composed mostly of keratin.