ABSTRACT
Dry skin is a complex phenomenon in which the skin can feel rough, tight, and itchy and visibly look “dry” due to the appearance of macroscopic flakes or scale on the skin surface [1]. To understand dry skin, we must first understand the processes taking place both in the epidermis and the stratum corneum, but particularly within the superficial layers of the stratum corneum because this is ultimately where dry skin is manifest. Dry skin results from a perturbation in the process of desquamation, the progressive degradation of the cohesive forces binding the corneocytes of the stratum corneum [2]. In healthy skin, desquamation is a carefully regulated process in which the surface corneocytes are shed in careful balance with the underlying formation of new corneocytes at the stratum granulosum/corneum boundary, without compromising the overall integrity of this critical tissue. Desquamation is not only responsible for maintaining stratum
corneum thickness, but by ensuring a continual turnover of corneocytes it also protects against the ever-present damaging effects of the environment. The major event occurring in desquamation is the controlled degradation of the corneodesmosomes, rivet-like protein complexes that, by linking neighboring corneocytes, represent the principal cohesive element of the tissue. Less well-understood changes in lipid organization and phase behavior also play a role in facilitating this process. Under normal conditions, the corneodesmosomes are degraded by proteases, located in the stratum corneum intercellular space, which hydrolyze the binding regions of these structures. (For reviews see Refs. 3 and 4.)
Fundamentally, dry skin occurs when the desquamation process is perturbed and the peripheral corneodesmosomes are not degraded, resulting in an accumulation of cohesive rafts of corneocytes on the skin’s surface [5]. These accumulations of surface corneocytes are manifest as dry flaky scale. Probably the major extrinsic factor involved in perturbation of the desquamation process is reduced humidity, although low temperature and UV damage can also precipitate this condition [6]. Low environmental humidity increases the desiccation stress on the outermost layers of the stratum corneum, leading to a reduction in water content. Since the desquamatory enzymes require water for functionality, the reduced water content then leads to a decrease in the activity of these hydrolytic enzymes, perturbed corneodesmosomal hydrolysis resulting in skin scale [7].
