Chapter 22: The Lipid Phase of the Stratum Corneum Studied by Solid-state NMR: A Not So Rigid Barrier

The lipid phase of the stratum corneum (SC), the outermost layer of mammalian skin, represents a unique assembly of lipids organized in stacked layers representing the most important permeation barrier to protect the body from excessive water loss. What makes this lipid assembly so unique is the fact that its main molecular species are represented by ceramides of varying chain lengths, free fatty acids, and cholesterol. Also, the packing and lateral organization of these lipids is quite special, forming short and long periodicity phases of densely packed lipids in an orthorhombic phase at very low hydration. Most lipids are considered to be highly rigid in this assembly. Recent biophysical investigations have revealed that some lipid segments are more dynamic than originally assumed, giving rise to a reconsideration of the current models of the SC lipid phase. This chapter summarizes the NMR view of the SC lipid phase. We also discuss modifications of the rigid lipid phase model considering more mobile lipid segments within the rigid SC lipid phase and possibly dynamic domain formation.