The stratum corneum is the outermost layer of the epidermis. Historically viewed as simply a layer of "dead cells," it is now recognized as a highly bioactive and structurally complex tissue.
The Brick and Mortar Model
Proposed by Elias in 1983, this is the most accurate conceptual model for the stratum corneum:
The Bricks (Corneocytes)
Anucleated, flattened cells packed with keratin filaments and Natural Moisturizing Factors (NMF) like urea, lactic acid, and amino acids. They act as water-holders.
The Mortar (Lipid Matrix)
Highly organized lamellar bilayers composed of Ceramides (50%), Cholesterol (25%), and Free Fatty Acids (15%). This matrix is what actively prevents water from escaping (TEWL).
Cosmetic Penetration (The 500 Dalton Rule)
To cross the intact stratum corneum, a molecule generally must weigh less than 500 Daltons, have sufficient lipid solubility (log P ~ 1-3), and be uncharged. This is why high molecular weight Hyaluronic Acid (>1000 kDa) remains entirely on the surface of the skin, acting as a film-former rather than deeply hydrating the epidermis.