ABSTRACT
INTRODUCTION Stratum corneum, the permeability barrier between the internal milieu and the environment, is composed of fibrous protein-enriched corneocytes and a lipid-enriched intercellular matrix (1). The corneocytes protect against desiccation and environmental challenge by regulating efflux and retention of water vapor and other gaseous substances. Maintenance of an optimal level of hydration by the stratum corneum is largely dependent on several factors. First, intercellular lamellar lipids, organized predominantly in an orthorhombic gel phase, provide an effective barrier to the passage of water through the tissue. Secondly, the diffusion path length also retards water loss, since water must traverse the tortuous path created by the stratum corneum layers and corneocyte envelopes. Thirdly, and equally important, the amount of natural moisturizing factor , a complexmixture of low-molecular-weight, water-soluble compounds, first formed within the corneocytes by degradation of the histidine-rich protein known as filaggrin. Each maturation step leading to the optimal moisture balance-including corneocyte strengthening, lipid processing, and natural moisturizing factor generation-is influenced by the level of stratum corneum hydration. These processes, as well as the final step of corneodesmolysis that mediates exfoliation, are often disturbed upon environmental challenge, resulting in dry, flaky skin conditions (2).
