Sources and Role of Stratum Corneum Hydration
Water is essential for life. All living cells require water to function but equally some ‘‘dead’’ cells also require water to be metabolically active. For humans to survive, the loss of water from the skin must be carefully regulated. This function of the epidermis is highly dependent on its sophisticated outer layer: the stratum corneum SC (1). However, this tissue must also retain sufﬁcient water to allow it to function in arid and desiccating environments. Under normal circumstances this complex tissue must be as impermeable as possible except for a small amount of water loss to hydrate the outer layers of the stratum corneum to maintain its ﬂexibility and to provide enough water to allow enzyme reactions that facilitate stratum corneum maturation events together with corneodesmolysis and ultimately desquamation (2-4). The waterretaining capacity of the stratum corneum is highly dependent upon the thickness of the SC, the precise phenotypes of the corneocytes and their organization, the precise composition and physical packing state of barrier lipids, and ﬁnally the presence of highly hygroscopic compounds largely found within the corneocytes (Fig. 1).