Stratum Corneum

The stratum corneum is the outermost layer of the epidermis, made up of flattened, non-living skin cells embedded in a lipid matrix. It is the physical part of the skin barrier and the layer most cosmetic products interact with first.

Its cells, called corneocytes, are filled with keratin and surrounded by a mortar of ceramides, cholesterol and fatty acids. Within the corneocytes sits the natural moisturizing factor, a mix of water-binding compounds that helps the layer stay flexible and hydrated. This structure regulates water content, limits transepidermal water loss, and provides the first line of defence against friction, chemicals and microbes.

The stratum corneum is constantly renewed: new cells push up from below while the oldest cells shed from the surface in a controlled process called desquamation. When shedding slows or lipids run low, the surface can look dull, feel rough, or flake. Skincare relevance is direct here, since exfoliants act on this layer to encourage even turnover, while humectants and barrier lipids support its hydration and cohesion. Over-treating it, however, can thin and weaken the barrier, so a balance between renewal and protection is generally the goal.