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Synopsis Concepts regarding the structure and function of the stratum corneum (SC) have changed from that of a tough film of loosely adherent cells to that of a two-compartment system of protein-enriched cells embedded in intercellular lipids. The two-compartment arrangement enlarged the role of epidermal lipids from that of the plasticizing component of the SC to that of the epidermal barrier governing water-holding properties as well as take-up of water, the differences in permeability of topically applied lipophilic and hydrophilic agents as well as cohesion and desquamation of the SC. Barrier properties of SC are dependent largely on the intactness of the lipid lamellae that surround the corneocytes. The pliability of the SC depends on a correct balance of lipids, hygroscopic water-soluble substances and water. Mammalian differentiation involves characteristic changes in lipid composition consistent with the requirements for waterproofing. There is a progressive depletion of phospholipids and glycosphingolipids with enrichment in ceramides, cholesterol, free fatty acids, and small amounts of other polar (e.g. cholesterol sulphate) and non-polar species (e.g. hydrocarbons, cholesterol esters, triglycerides). The sphingolipids account for the most lipid by weight and are presumed to be of major importance for the water-retaining properties of the epidermal barrier. Decrease of sphingolipid content occurs in aged skin. Deficiency of essential fatty acids leads to enhanced transdermal water transport in addition to dryness and scaliness of the skin. A mixture of both saturated and unsaturated fatty acids produces the optimal barrier to water loss from the SC. The balance between solid crystal and liquid crystalline phases in epidermal lipids is determined by the degree of fatty acid unsaturation and the amount of water.

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