The lipoidal permeability barriers of the skin and alimentary tract.

The major routes of administration of drugs to humans involve transport either through the intestinal wall or through the skin. Both these barriers are nonpolar in nature and are subserved by membrane lipids. The lipid composition of the brush border of the intestinal wall is unusual, possessing unusually large quantities of glycosylceramide. The lipid composition of the stratum corneum of the skin is also unusual, possessing large quantities of ceramides and free fatty acids. These atypical membrane components are generally more ordered than the other common membrane lipids at body temperature and, thus, are suited for involvement in formation of barriers between the organism and its environment.