Could crystalline arrangements of fats help explain how skin forms a protective barrier?

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The permeability of mammalian skin is determined in large part by layered lipid domains packed between cells in the stratum corneum, the upper layer of the epidermis. These layers share some features with typical biological membranes, but their lipids consist mainly of ceramides, cholesterol, and free fatty acids.

To investigate how lipid composition relates to phase behavior in this unusual system, the researchers had previously used deuterium NMR [Kitson et al. (1994), Biochemistry 33, 6707–6715]. They examined aqueous dispersions of nonhydroxylated bovine brain ceramide, cholesterol, and perdeuterated palmitic acid. These showed complex phase behavior that varied with temperature and pH. In comparable dispersions in which sphingomyelin replaced ceramide, the spectra were consistent with a fluid lamellar phase under the same conditions.

The present laboratory study extended those observations by examining the same dispersions at pH 5.2 using X-ray diffraction. The ceramide dispersions formed complex crystalline phases from room temperature to about 40 °C. Most of the crystalline cholesterol was not in a separate phase. In contrast, the comparable sphingomyelin dispersions formed a fluid lamellar phase under the same conditions.

The researchers concluded that ceramides can form crystalline lamellar structures even in the presence of considerable mole fractions of cholesterol. They suggested that the existence of such structures in the stratum corneum may be important for the epidermal permeability barrier. They also suggested that interactions between ceramide and cholesterol in other biological membranes may produce regions with unique physical properties.