Direct observation of domains in model stratum corneum lipid mixtures by Raman microspectroscopy

Several studies on intact and model stratum corneum (SC), the top layer of the epidermis, have suggested the presence of crystalline domains. In the p resent work, we used micro-Raman mapping to detect lipid domains in model l ipid mixtures formed by an equimolar mixture of ceramides, cholesterol, and palmitic, acid, the three main lipid species of SC. We were able to determ ine the spatial distribution of the three compounds individually based on t he systematic analysis of band areas. As a control, we studied freeze-dried lipid mixtures, and the Raman microspectroscopy reported faithfully the ho mogeneous distribution of the three compounds. Spectral mapping was then pe rformed on hydrated equimolar mixtures carefully annealed. In this case, cl ear phase separations were observed. Domains enriched in cholesterol, ceram ides, or palmitic acid with a size of a few tens of square microns were det ected. These findings constitute the first direct evidence of the formation of heterogeneous domains in the SC lipid models in a bulk phase. Raman mic rospectroscopy is an innovative approach to characterize the conditions lea ding to the formation of domains and provides new insights into the underst anding of the skin barrier.