[noun] Keratinocytes (which produce keratin) are the main cells composing the epidermis, arranged in layers stacked on top of one another. These cells are formed from basal keratinocytes located in the deepest part of the epidermis and then rise to the surface, where they die and turn into dead cells. In this way, the epidermis is naturally renewed over a period of 21 to 28 days, the time required for these cells to migrate to the surface.