The stratum corneum is the outermost layer of the epidermis. Though only a fraction of a millimetre thick, it forms the skin's first line of defence — it is essentially the physical embodiment of what we call the skin barrier.
It follows the "brick and mortar" model: corneocytes (flattened, keratin-filled cells) are the bricks and the lipid layers between them are the mortar, made of ceramides, cholesterol and free fatty acids arranged in an orderly, lamellar structure.
Its condition can be assessed through trans-epidermal water loss (TEWL) and surface hydration measurements. Barrier-friendly care aims to support this layer without disrupting its lipid arrangement — the basis of Zeitschild's emulsifier-free, skin-identical lipids approach.