Coexistence of domains composed of intercellular lipids in stratum corneum as studied by X-ray diffraction and differential scanning calorimetry.

The intercellular lipid matrix of stratum corneum is composed of many lipid components, including ceramides, free fatty acids, and cholesterol, which nonetheless form regularly arranged structures; short- and long-period lamellar structures, and hexagonal and orthorhombic hydrocarbon-chain packing structures. From the viewpoint of compatibility among the structures, there should be a correlation between a lamellar structure arranged periodically along the long axis of the lipid molecules and a packing structure of the hydrocarbon chains in the plane orthogonal to the long axis. To address this issue, differential scanning calorimetry and temperature-dependent X-ray diffraction experiment were performed on human stratum corneum. A detailed comparative analysis of the results on phase transitions obtained by the two methods revealed that the short-period lamellar structure has the orthorhombic hydrocarbon-chain packing structure that is normally observed, while the long-period lamellar structure has another orthorhombic hydrocarbon-chain packing structure hidden in the normal structure, and furthermore, the hexagonal hydrocarbon-chain packing structure does not appear to form a corresponding multilamellar structure and start to undergo a phase transition to the liquid state approximately at 33 °C. The domain constructed with the short-period lamellar structure and the orthorhombic hydrocarbon-chain packing structure is important in considering the water regulation mechanism in stratum corneum, since it provides evidence not only of the water layer of the former but also of changes in the head group of the latter.