The epidermis contains 2 major and 2 minor cell populations specialized for specific functions. Major populations include the keratinocytes and melanocytes. Minor populations include Langerhans' and Merkel's cells.
A. Keratinizing System: The keratinocytes make up most of the epidermis. They participate in the continuous turnover (renewal) of the skin surface by passing through 4 overlapping processes: cell renewal, or mitosis; differentiation, or keratinization; cell death; and exfoliation (the sloughing of dead cells from the skin surface). The entire process takes 15-30 days and occurs in waves. A cell layer produced by a mitotic wave in the basal layer undergoes keratinization in synchrony. Each wave pushes the cell layers produced in earlier waves toward the surface. The layers from several waves, each at a different depth and step in the process, give a stratified appearance to vertical sections of the epidermis. The 5 layers of the epidermis are distinguished by the shape, staining properties, contents, and orientation of the keratinocytes they contain.
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