keratinize

Keratinize is a biological term referring to the process by which certain epithelial cells acquire keratin and become part of a protective, keratin-rich layer. In humans, keratinization occurs mainly in the epidermis of the skin, as well as in nails and hair follicles.

During keratinization, basal keratinocytes proliferate and migrate toward the skin surface through layers of stratified squamous epithelium. As they move upward, they synthesize keratin proteins, accumulate keratin-associated proteins, and gradually lose their nuclei and organelles. In the stratum granulosum, keratohyalin granules promote the formation of a dense keratin cytoskeleton, while lamellar bodies release lipids that contribute to the skin’s permeability barrier. The final stage yields corneocytes—flattened, anucleate cells rich in keratin—that form the outermost stratum corneum, providing mechanical protection and waterproofing.

Keratinization also occurs in some specialized skin regions and accessory structures, giving nails and hair their hard, resilient properties. The process can vary in degree and pattern across different body sites and species.

Clinical significance includes disorders of keratinization, such as hyperkeratosis, which involves thickening of the keratin layer, and ichthyosis, where abnormal keratinization leads to dry, scaly skin. Conversely, impaired keratinization can compromise the skin barrier. Keratinization is closely related to cornification in the epidermis, terms often used interchangeably to describe terminal differentiation of keratinocytes.