SC / Horny Layer / Outermost Skin Layer
The outermost layer of the epidermis — approximately 10–20 cells thick, composed of flattened, protein-filled dead cells (corneocytes) embedded in a lipid matrix of ceramides, cholesterol and free fatty acids. This "brick and mortar" architecture is the primary barrier between the body and the external environment. The stratum corneum prevents water loss, blocks pathogens, and regulates the penetration of topical actives. It is continuously shed (desquamation) and replaced from below — the complete cycle takes approximately 2–4 weeks.
Understanding the SC is essential to understanding why application order, pH, and ingredient selection matter in skincare. Actives must either stay in the SC or penetrate through it to reach deeper skin layers where fibroblasts and other target cells reside.
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