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The very thin stratum corneum (SC), 10–30 µm or 1/10 the thickness of a piece of paper, is the skin’s outer and most exposed layer, and the permeability barrier between the internal milieu and hostile environmental assault. Because it is exposed externally, the skin provides protection and acts as a barrier to the outside environment in order to maintain internal homeostasis. Although the SC has many functions, its main ability is to serve as a protective barrier that prevents excess fluid and electrolyte loss, allowing life to exist in a fragile terrestrial environment.
The SC accounts for up to 75% of the epidermis, making it a virtual wall of protein protection for the cellular and inner-cellular inhabitants. “Stratum Corneum Defense Functions: An Integrated View,” written by Peter M. Elias, MD, states, “The permeability barrier is mediated by the organization of the extra cellular lipids of the SC into a series of parallel membrane structures, and its distinctive composition.”1
This fascinating upper layer of the epidermis is composed of fibrous protein-enriched corneocytes and an essential lipid-enriched intercellular matrix. During corneocyte formation, the plasma membrane of viable cells is replaced by a ceramide monolayer, covalently bound to the underlying cornified envelope. This lipid envelope functions as a template for accumulating extra cellular lipid layers.
According to the article “Adhesion and Debonding of Biological Soft Tissues—Stratum Corneum,” by Kenneth Wu of the Reinhold H. Dauskardt Research Group at Stanford University in Stanford, California, “The layered construction of the skin represents a composite material in which the components possess specialized functionalities to accommodate a variety of conditions from mechanical stresses to variable ambient moisture, and to resist the presence of toxic chemicals, pathogens and radiation.” E Proksch, et al, in the study “Barrier Function Regulates Epidermal Lipid and DNA Synthesis,” explain, “The lipids are a mixture of sphingolipids, cholesterol and free fatty acids, which form intercellular membrane bi-layers.”2
These bi-layers house essential intercellular residents that are affected by exposure to the environment, and other elements, including professional spa treatments and daily cleansing that remove these inner-cellular lipid substances. “The hygroscopic water-soluble substances within the stratum corneum are responsible for much of the water-binding ability of the horny layer,” states JD Middleton in the article “The Mechanism of Water Binding in Stratum Corneum.”3 The ability of corneum to bind water is reduced when it is extracted with solvents, which remove lipids, and then with water, which removes the water-soluble substances. This fact is important to note because it affects the outcome of both professional skin care services and home care treatments, and should be front and center when organizing skin care remedies.