The nature of the epidermal barrier: Structural aspects

The stratum corneum (SC) controls the diffusion and penetration of chemical substances and drugs into and through the skin. The water permeability barrier of the SC seems to be regulated by the bilayers between the corneocytes, which originate largely from polar lipid precursors extruded by the lamellar bodies (LB) of the stratum granulosum cells. The use of a modified ruthenium tetroxide fixation allows detailed studies of the organization of the lipids and the process of post-secretory transformation of the LB-derived lipids into lamellar lipid bilayers in human skin. The transformation is performed in saccular invaginations of the intercellular domains. In this environment LB lipids are first processed into sheets before they become part of the lamellar lipid meshwork of the SC. Moreover, distinct structural relationships between desmosomes and desmosomal remnants (at the stratum disjunctum) on the one hand and lipid layers on the other have been demonstrated, pointing to some important functional interaction of these components in healthy human skin.