part of part of occurs in epidermal cell keratinocyte differentiation keratinization The process in which the cytoplasm of the outermost cells of the vertebrate epidermis is replaced by keratin. Keratinization occurs in the stratum corneum, feathers, hair, claws, nails, hooves, and horns. Reactome:R-HSA-6805567 GO:0031424 biological_process multicellular organismal process