Final answer:
Epidermal cells become keratinized as they migrate up through the epidermal layers, accumulating keratin which provides protection and gives the skin its characteristic toughness.
Step-by-step explanation:
Epidermal cells that move up through the strata become keratinized. This process occurs as the cells are produced in the basal layer, or stratum basale, primarily by the basal cells and also mix with melanocytes and Merkel cells. As these keratinocytes are pushed outward through the layers of the epidermis, they accumulate more of the protein keratin, which provides skin with its protective properties. The outermost layer consists of dead, keratinized squamous cells, which slough off and are continuously replaced by new cells from the lower layers. The epidermis is therefore referred to as keratinized stratified squamous epithelium.