The stratum corneum (10-20µm thick) has several roles: it reduces water loss, protects against abrasion, keeps out micro-organisms, and acts as a permeability barrier. It is made of corneocyte cells laid 10-20 cells deep. Corneocytes are flat, polyhedral cells lacking a nucleus and consisting mainly of bundles of insoluble protein keratins. Cross-linking proteins and lipids are also present.10

Around half of the lipids in the space between corneocytes are ceramides that play a significant role in epidermal water retention. Ceramides are covalently bound to the outer surfaces of the corneocytes. In addition, the intercellular space contains cholesterol, fatty acids, free sterols and sterol esters.12,13,14

Helping the stratum corneum to stick together and linking the corneocytes are the corneodesmosome membrane junctions. Also in the epidermis are melanocytes providing skin pigmentation, Langerhan’s cells involved in the immune response, and touch-receptor Merkel cells.10

Vasculature in the dermis supports the epidermis with nutrients. But the epidermis is dynamic and its renewal has a complex regulatory system. Alterations in the stratum corneum, caused by abrasion, detergents that remove lipids, or other skin irritants, leads to loss of its barrier function and transdermal water loss. This can, in turn, stimulate more keratinocyte proliferation and differentiation, or maturing, to restore the barrier effect.10