Stratum Corneum
Definition
The stratum corneum is the outermost sublayer of the epidermis and the body's first physical barrier against the external environment. It is composed of 15-30 layers of dead, flattened skin cells (corneocytes) embedded in a lipid matrix of ceramides, cholesterol, and fatty acids — often described using the 'bricks and mortar' model. Despite being made of dead cells, the stratum corneum is metabolically active and performs critical functions: it prevents transepidermal water loss (keeping moisture in), blocks the entry of pathogens, irritants, and allergens (keeping threats out), maintains the skin's acidic pH (acid mantle), and regulates the desquamation process. The stratum corneum is constantly renewing — new cells arrive from below as old cells shed from the surface. When this layer is damaged by harsh products, over-exfoliation, or environmental stress, the entire skin barrier becomes compromised, leading to dryness, sensitivity, and increased vulnerability to irritation.
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