Essential fatty acids and epidermal integrity.

The intercellular spaces of the stratum corneum contain multilamellar lipid sheets derived from the extruded contents of lamellar granules. In the absence of linoleic acid, lamellar granules appear empty, and only fragmentary extracellular sheets are found. This defective differentiation is attributable to substitution of oleate for linoleate in O-acylsphingolipids. Normally, linoleate is ester-linked to 30- to 34-carbon omega-hydroxyacids, which, in turn, are amide-linked to sphingosine. Acylglucosylceramides, bearing a beta-D-glucosyl moiety on the sphingosine, may provide the driving force for lamellar granule assembly. The omega-hydroxyacyl chains are long enough to span a lipid bilayer, while the linoleate inserts into an adjacent bilayer. This interaction could promote assembly of lamellar granules. It has also been proposed that acylceramides may stabilize the extracellular sheets by a similar mechanism. In addition, the horny cell has been found to possess a covalently bound lipid envelope consisting principally of omega-hydroxyacylsphingosines derived from O-acylsphingolipids.