A model membrane approach to the epidermal permeability barrier.

The permeability barrier of mammalian skin is found in unusual intercellular domains in the upper layers of the epidermis, and is composed mainly of three lipid classes: ceramide, cholesterol, and free fatty acid. These are organized as lamellae, but the details of lipid organization are not precisely known. To examine the relationship between lipid composition and phase behavior, aqueous dispersions of bovine brain ceramide, cholesterol, and perdeuterated palmitic acid were examined by 2H NMR and compared to analogous systems in which sphingomyelin replaced ceramide. The sphingomyelin systems give rise as expected to a stable fluid lamellar signal over the temperature range 20-75 degrees C and pH 5.2-7.4, whereas the ceramide dispersions show complex polymorphism as a function of both temperature and pH. Prominent features of the ceramide dispersions containing cholesterol are phase coexistence and the presence of a "solid" phase in which molecular motion is more inhibited than in a classical phospholipid gel phase: T1z measurements indicate that lateral diffusion of the palmitic acid probe effectively does not occur. In the absence of cholesterol, a fluid lamellar signal is not observed, but the appearance of a "solid" signal is also influenced by the pH. In the presence of cholesterol, a fluid lamellar signal is present at 50 degrees C, and the 2H NMR order parameter profile is very similar to that derived from the analogous sphingomyelin dispersions.(ABSTRACT TRUNCATED AT 250 WORDS)