ABSTRACT
The intercellular lipids of the human stratum corneum (SC) are unique in composition and quite different from the lipids found in most biological membranes (1).
History
Early evidence indicating a significant role for lipids in the barrier function of the skin came from experiments in which removal of lipids from the skin in vitro by extraction with organic solvents resulted in increased permeability (2). A more detailed understanding of the roles of membrane lipids in the formation and function of the epidermal barrier was based on electron microscopic observations made in the early 1970s (3,4). In transmission electron micrographs, lamellar granules were shown to extrude their membranous contents into the intercellular space at the boundary between the granular layer and the SC, and the freeze-fracture technique revealed, for the first time, that the intercellular spaces of the SC contain broad, multiple membranous structures (5). In addition, the use of water-soluble tracers to monitor the movement of water in the skin by electron microscopy demonstrated that the entire SC was essentially water impermeable (3,4). A major finding that emerged from this body of work was that molecules traversing the SC do so by passive diffusion through the intercellular spaces. This was most clearly indicated by an experiment in which n-butanol was allowed to diffuse across the tissue prior to exposure to osmium vapor, which reacts with the butanol and precipitates in situ (6).
