CHARACTERIZATION OF LOW-TEMPERATURE (IE, LESS-THAN-65-DEGREES-C) LIPID TRANSITIONS IN HUMAN STRATUM-CORNEUM

This study aims to characterize human stratum corneum (SC), focusing o n those lipid transitions that occur at or below physiologically relev ant temperatures. In the past, a lipid transition near 35 degrees C ha d been thought to be variable and a consequence of superficial sebaceo us lipid contamination. However, analysis here indicates that it is wi dely present, and cannot be attributed to sebum production. We demonst rate that this transition represents a solid-to-fluid phase change for a discrete subset of SC lipids. The reversibility of this transition upon reheating, and its absence in extracted lipid samples imply that these lipids are not uniformly present throughout the SC, but would ap pear to be differentially distributed in response to terminal differen tiation. Further, such an arrangement could involve a close associatio n with other nonlipid (e.g., protein) components. Evidence for a new t ransition at similar to 55 degrees C is presented that suggests the lo ss of crystalline orthorhombic lattice structure. The existence of ort horhombic structure at physiologic temperature is reasoned to involve ceramides and/or free fatty acids. Localization of these lipids at the level of the corneocyte envelope supports a comprehensive picture of water transport across the SC, whereby diffusion occurs primarily via the intercellular lipids. This view, coupled with the hydration-induce d changes in lipid disorder observed here provides additional insight into the mechanism by which skin occlusion increases permeability. Sum marily, these results i) emphasize the inherent danger of over-interpr eting experiments with isolated SC lipids, ii) emphasize the potential advantage(s) of employing several biophysical techniques to study SC structure, and iii) indicate that a full characterization of lipid pha se behavior is requisite to our eventual understanding of SC structure and permeability function, particularly those phase transitions that occur near or at normal skin temperature.