keratinizing

Keratinizing, in a biological context, refers to keratinization—the process by which certain stratified squamous epithelia produce and accumulate keratin to form a tough, protective outer layer. In keratinizing epithelia, basal cells proliferate and migrate toward the surface, differentiating as they go. During this maturation, cells synthesize keratin proteins and other components. The cytoplasm becomes filled with keratin filaments, nuclei and many organelles are lost, and the cells become corneocytes. Lipids are released into the cell membranes, contributing to a waterproof barrier and the formation of a cornified envelope. The fully matured superficial layer, the stratum corneum, consists of dead, flattened keratinocytes that are periodically shed.

Where keratinization occurs and its degree vary by tissue. The epidermis of the skin is the most prominent example, producing a thick, highly keratinized barrier. Some mucous membranes are also keratinized, such as the hard palate and gingiva. Many other mucosal surfaces, including portions of the oral cavity and the esophagus, are non-keratinized or only partially keratinized.

Clinical relevance includes hyperkeratosis, a thickening of the stratum corneum often in response to chronic irritation, and parakeratosis, where superficial keratinocytes retain nuclei. Keratinization is essential for barrier function, protecting against abrasion, dehydration, and pathogen entry, while the degree and pattern of keratinization help distinguish different epithelial tissues.