Epidermis as a shield from radiation and oxidative stress

The skin consists of two main layers, the epidermis and the dermis. The dermis is superficial to the subcutaneous fat tissue and is reachable by blood capillaries. Dermal fibroblasts synthesize a complex extracellular matrix containing collagenous and elastic fibers. The epidermis contains mostly keratinocytes that rise to the skin surface as they differentiate progressively to form the non-nucleated corneocytes that consists of the superficial part of the epidermis, the stratum corneum (SC). The SC comprises a unique two-compartment system of structural, nonnuclated cells (corneocytes) embedded in a lipid-enriched intercellular matrix, forming stacks of bilayers that are rich in ceramides, cholesterol and free fatty acids. SC functions as a physiochemical barrier to protect and prevent water loss from the epidermis, maintaining its integrity, and to provide protection from the environment by producing antioxidant molecules which interact with reactive oxygen species (ROS) or their by-products to either eliminate or to minimize their deleterious effects. The SC supports the absorption of liposoluble compounds and promotes the penetration of lipophilic molecules. There are several ways how molecules can penetrate in the SC such as: intercellular (penetration between the corneocytes), transcellular (penetration through the keratinized corneocytes), intrafollicular (penetration through hair follicles), and/or polar (penetration between polar pores) (Pouillot, et al., 2008). Physiochemical factors which regulate penetration include molecular mass, concentration, solubility, partition coefficient, pH variations, co-solvents, temperature, and enhancers. Because of its critical location, the SC is a major interface between the body and the environment and provides a biological barrier against an array of chemical and physical environmental pollutants. Due to the constant exposure to oxidants including ultraviolet (UV) radiation and other environmental pollutants such as diesel fuel exhaust, cigarette smoke (CS), halogenated hydrocarbons, heavy metals and O3 (one of the most toxic of these compounds, the SC can be defined as our first defense against the outdoor environment.