ABSTRACT
INTRODUCTION Over a decade ago, Rawlings et al. (1) summarized the knowledge on the state of the art of stratum corneum (SC) biology and dry skin, and this was recently elaborated by Rawlings and Matts (2). This chapter builds on these publications and Chapter 7 to discuss the latest understanding of dry skin (Fig. 1) and the effect of moisturizers on this most common dermatological problem. First, however, we need to consider the role of water loss through the SC. We are all losing water through our skin unless we live in 100% relative humidity (RH) climates. Under normal circumstances, the SC must be as impermeable as possible, except for a small amount of water loss, to (1) hydrate the outer layers of the SC to maintain flexibility and (2) provide enough water to allow enzyme reactions that facilitate SC maturation events, together with corneodesmolysis and, ultimately, desquamation (Fig. 2) (3-6). This inbuilt mechanism of transepidermal water loss (TEWL) is vital for the normal functioning of the SC. Naturally, in this process, water gradients are generated within the tissue. However, a key to precipitating the condition we call “dry skin” is a perturbation of these water gradients within the SC, and Warner and his team at Procter & Gamble were the first to demonstrate the changes in SC water gradients in dry skin (7) where about one-third of the outer layers of the SC are reported to contain less than 10% water content (Fig. 3). At this level of water, the SC will be dysfunctional and brittle (8).
