Localization of Lipid Microdomains and Thermal Phenomena in Murine Stratum Corneum and Isolated Membrane Complexes: An Electron Spin Resonance Study

The intercellular region of the stratum corneum can be isolated in the form of membrane complexes (intercellular lipids and adjacent cornified envelopes) which are devoid of the cytoplasmic components. In this study the temperature induced phase transitions and corresponding lipid domain reorganization in isolated stratum corneum (SC) sheets and SC membrane complexes (SCM) were determined using the electron spin resonance (ESR) spin probe technique. The spin probe perdeuterated di-tert-butyl nitroxide (pdDTBN) in SC and SCM revealed less well-defined physiologic phase transitions in the SCM and a more polar lipid domain in SC. However, the ESR results show the coexistence of highly ordered domains (immobilized spin probe) and less viscous domains in intact SC, which persist in SCM. Above ≍ 20°C the spin probe is dissolved in a highly disordered (isotropic) lipid domain in both SC and SCM. In both solvent extracted SC and SCM, the spin probe is dissolved in a highly ordered lipophilic domain associated with the lipids complexed to the corneocyte envelope and exhibiting no thermal transitions between -23° to 60° C. Further, the amount of mobile spin probe is related to the amount of residual lipid. An unexpected finding was the apparent reduction of the spin probe in solvent extracted SCM, suggesting the presence of a previously unrecognized free radical reducing mechanism in these sites. The mobilities of the spin probe when dissolved in model lipids, non-hydroxy, and hydroxy containing ceramides and cholesteryl oleate, differed significantly from those observed in sc or SCM. These studies demonstrate the usefulness of ESR for the localization and characterization of lipid microenvironments in the stratum corneum.