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the accumulation of keratins—a type of protein—in cells of the epithelial tissue in vertebrates and humans. Keratinization mainly occurs within the surface layers of the skin—the epidermis and all its derivatives (scales, bill, plumage, wool, hooves, horns, hair, and nails).
In humans the keratinized cells in the surface layers of the skin are converted into squamules, which form the cornified layer of the epidermis. This layer is highly elastic and conducts heat poorly, thus protecting the underlying layers of the skin from such harmful factors as physical injury, desiccation, and bacterial infection. Keratinization is most evident on the palms and soles, less so at flexures. (For disorders in the keratinization process, seeHYPERKERATOSIS.)