Stratum Corneum

What Is the Stratum Corneum?

The stratum corneum (SC) is the outermost layer of the epidermis — the layer that interfaces directly with the environment. Despite being only 10–20 micrometres thick (about the diameter of a human hair), it performs the majority of the skin's barrier function. The SC consists of 15–20 layers of flattened, anucleate (nucleus-free), keratin-filled dead skin cells called corneocytes, embedded in an organised lipid matrix of ceramides, cholesterol, and fatty acids. This "brick and mortar" structure is the foundation of skin barrier science.

Functions of the Stratum Corneum

The SC performs multiple critical roles simultaneously: it prevents transepidermal water loss (TEWL), keeping the body hydrated; it blocks penetration of pathogens, allergens, and environmental irritants; it provides mechanical protection against physical trauma; it contains natural moisturising factors (NMFs) — hygroscopic molecules including amino acids, urocanic acid, lactic acid, and PCA that maintain corneocyte hydration from within; and it creates the acidic surface pH environment that supports the microbiome and barrier enzymes.

The Stratum Corneum and Skincare

Every topical skincare product must interact with or penetrate the SC to have any effect. Small molecules (retinol, certain peptides, low-molecular-weight HA) can penetrate through the lipid lamellae or via follicular routes. Larger molecules largely remain at the surface. The SC is also the primary target of moisturisers, exfoliants, and barrier-repair products — making it central to essentially all of skincare science.

See Skin Layers Science and Lipid Barrier.