Keratinocytes, the major cell type of the epidermis, are formed at the basal layer just above the dermis. They are metabolically active cells with normal constituents such as a nucleus and cytoplasm.
Keratinocytes serve many important functions including the production of the structural protein keratin. As a keratinocyte progresses up through the epidermis, it is transformed at the stratum granulosum into a nonviable corneocyte before being shed in the normal epidermal turnover process. The transformation of the keratinocyte includes the loss of the cell nucleus and cytoplasm, formation of a tough outer structure called the cell envelope, aggregation of keratins and expellation of lipids into extracellular spaces. The resultant corneocyte is comprised of about 80% keratin by dry weight.
Corneocytes are also referred to as squames. This name is derived from a Latin word squama meaning scales or armor as corneoctyes form the protective outer layer of the skin. Corneocytes are roughly 30 µm in diameter and 0.3 µm thick. Less than the diameter of a human hair, individual corneocytes that are shed from the stratum corneum are not visible to the human eye.
Many skin care products target either keratinocytes or corneocytes; therefore, understanding their differences and the basic structure of human skin is important for formulators.