[Biochemical analysis of epidermal lipids].

According to the knowledge acquired some 15 years ago, the cutaneous lipids may be classified into 2 families: the "neutral" lipids, represented by cholesterol, cholesterol esters, cholesterol sulphate, triglycerides, free fatty acids, squalen and alcanes, and the "polar" lipids including phospholipids (phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingomyeline) and sphingolipids (ceramides I-VII, monohexosylceramides). From the functional point of view, free fatty acids, cholesterol, and ceramides organised in layers are the most important components of intercellular spaces of the stratum corneum. Analytic methods have been recently developed to help understand the structural organisation of these various molecules within the horny layer and their influence on the epidermal barrier function. Raman microspectroscopy or X-ray diffraction are most frequently used. Differential calorimetry and fluorescence or infrared spectroscopy provide complementary information. The principal findings are: lamellar structure depends on the presence of ceramides supplemented by adequate quantities of free fatty acids and cholesterol; ceramide chains interact to provide the ordered structure and ceramide-1 is necessary for stabilisation of lipid layers; cholesterol may regulate the molecular mobility of hydrocarbon chains within the bi-layers. Knowledge of the molecular structure of the barrier lipids finds several applications, e.g.: in pharmacology--conception of new formulations adapted for percutaneous and topical application of drugs; in dermatology--comprehension of physiopathologic mechanisms of various dermatoses; in biotechnology--development of skin substitutes with valid stratum corneum barrier; in cosmetics--choice of best formulations suited for reconstruction of the intercellular lipid substance.