Figure 1.

Epidermal structure and keratinocyte differentiation. The epidermis is a self-renewing tissue composed mainly of keratinocytes in various stages of terminal differentiation. Keratinocytes are produced in the stratum basale (basal layer), and move outward through the epidermis, undergoing a programmed series of differentiation involving enucleation and accumulation of cytokeratins and tight junctions with each other. Keratinocytes also receive melanin from melanocytes in the form of pre-packaged organelles termed melanosomes. The basic layers from the basement membrane outward are the stratum basale, stratum spinosum, stratum granulosum, and the stratum corneum, each identified by the morphology and differentiation state of the keratinocyte as indicated by expression of cytokeratins and other proteins.