ABSTRACT
The cutaneous permeability barrier, which allows life in a terrestrial environment, resides in the outer layer of the epidermis, the stratum corneum (SC). The lipiddepleted cells (corneocytes) of the SC are embedded in a continuous, lipid-enriched, extracellular matrix organized into characteristic lamellar membrane unit structures. This lipid-enriched matrix mediates permeability barrier function by a variety of mechanisms (Table 1). SC lipids consist predominantly of ceramides (Cer) (approximately 50% by weight) with lesser amounts of cholesterol (Chol) and free fatty acids (FA) (108). Importantly, these three species comprise about 10% of the dry weight of the SC, and they are present in about a 1:1:1 molar ratio. Although the SC traditionally is viewed as inert, recent studies have shown that it is both metabolically active, and interactive with the underlying nucleated cell layers of the epidermis (23).
