ABSTRACT
The normal stratum corneum (SC) is composed of orderly interdigitating stacks of corneocytes coated by layers of intercellular lipids. The protein-enriched corneocytes are filled with a dense array of disulfide cross-linked keratin filaments bound to filaggrin. Permeating through this matrix are low molecular weight water-soluble molecules forming the natural moisturizing factor (NMF), which largely derives from the enzymatic degradation of filaggrin. The NMF avidly and effectively binds water. The resulting osmotic pressure inside the cells does not lead to their disintegration because corneocytes are made of so strong a crosslinked protein matrix surrounded by a thick cornified envelope, formed itself from highly cross-linked isopeptide bonded proteins. The lipid-enriched intercellular matrix provides a rate-limiting barrier to water evaporation from the skin surface and to the transport of other chemicals across the skin. With failure of the barrier, xerosis develops and may evolve to flaky and scaly presentations (Fig. 1). This represents an abnormal process of desquamation. In fact, the SC is not sim-
ply a collection of dead cells, but rather represents a dynamic and metabolically active tissue interacting with subjacent cell layers that reacts to various environmental threats [1,2].
