ABSTRACT
The flexibility of the stratum corneum (SC) plays an important role in keeping the skin supple and in giving it a radiant appearance. Water present within the SC is essential for maintaining the flexibility of the SC, and is constitutively regulated by the water-holding capacity of the SC. Much evidence suggests that water-soluble materials, such as free amino acids, organic acids, urea, and inorganic ions determine the water-holding properties of the SC; these materials have been termed natural moisturizing factors [1]. Based upon this theory, many moisturizers have been designed and developed in the cosmetic field. Well-known removers of lipids, such as organic solvents, despite their poor ability to remove water-soluble materials, induce dry skin, which is characterized by a reduction in the water-holding function of the SC. Thus, we hypothesized that structural lipids, mainly comprised of ceramides, play a significant role in the water-holding potential of the SC. In this chapter, we introduce a new mechanism underlying the water-holding properties of the SC and elucidate the role of ceramides as natural moisturizing factors and their efficacy in the clinical treatment of dry skin.
