Keratinocytes are the major cellular component of the epidermis, which is the stratified squamous epithelia forming the outer-most layer of skin. The keratinocytes lie on a basement membrane and are organized into distinct cell layers which differ morphologically and biochemically These regions from the basement membrane outward are the basal, spinous, granular, and cornified layers. Cellular proliferation takes place mainly in the basal layer. On division, keratinocytes give rise to either replacement progenitor cells and/or cells that are committed to undergo the process of terminal differentiation These latter cells leave the basal layer and gradually migrate upward, simultaneously progressing along the differentiation pathway as they go. Finally they reach the outer surface of the epidermis in the form of fully mature functional cells, the corneocytes. The function of these mature cells is the protection of the underlying viable tissues from the external milieu.