MAMMALIAN EPIDERMAL BARRIER LAYER LIPIDS - COMPOSITION AND INFLUENCE ON STRUCTURE

[摘要] The outer layers of the mammalian epidermis protect the organism from H2O loss and external injury. The barrier as visualized with tracers was shown to develop initially in the stratum granulosum concurrent with the intercellular deposition of extruded lamellar-body contents. This lamellar material greatly expands the intercellular compartment, and following cornification appears to form broad sheets which freeze fracture like lipids, deviating the fracture-plane from the hydrophobic interior of the membrane to the apparently more hydrophobic material in the intracellular space. Isolated upper epidermal sheets were first obtained from neonatal mice with staphylococcal exfoliatin, and then were treated with lipid solvents in order both to identify membrane structures formerly obscured by intercellular lipid and to characterize lipids contributing to barrier function. Histochemical stains were also used to localize phospholipid and neutral lipid deposits in barrier layers. After solvent treatment, the fracture plane reverted from the intercellular space to the plasma membrane, revealing sparse membrane particles low in the stratum corneum and particle-free fracture faces in mid to upper regions. Extracted upper epidermal sheets contained 3-4% lipid, almost equally distributed between polar and nonpolar substances, with unexpected large quantities of glycolipid, hydrocarbons and free fatty acids. Gas liquid chromatography demonstrated unusually large quantities of long-chain fatty acids (C[carbon]24:0 and C26:0) in all polar lipid fractions and in free fatty acids. Histochemical stains demonstrated neutral lipids in stratum corneum intercellular spaces, phospholipids within granular cells, and PAS[periodic acid-Schiff]-positive material both within and between granular cells. These combined histochemical and biochemical studies confirm earlier reports demonstrating a shift from polar to neutral lipid during cornification. They also indicate that within the stratum corneum lipids are segregated in the intercellular spaces, a pattern consistent with EM images. They suggest that granular layer polar lipid, through as yet undefined mechanisms, may be transformed into the neutral lipids of the cornified layer. The observed admixture of lipids with long-chain, highly saturated fatty acid chains, as well as the sterols are ideally suited (and situated) to function as the epidermal permeability barrier.