The skin has its own efficient mechanisms and locations for storing water in the stratum corneum (SC). A well-hydrated SC is essential for shedding cells and preventing thickened, dry, rough, scaly skin. But what if the hydration system breaks down? How can the cosmetic chemist help?
One way is to encourage the storage of water in the SC. Can you find four places to store water in Figure 1? This figure was taken from a report1 written by Christina Marino, MD, with the Safety & Health Assessment & Research for Prevention (SHARP) program at the Washington State Department of Labor and Industries. The figure illustrates the cells of the SC and the lipid bilayers that surround them. The SC is the outermost layer of the epidermis. Its function is to provide a physical barrier between the interior body and the exterior environment.
The cells in this barrier are called corneocytes. They were living cells, with a nucleus and DNA, when they were formed lower in the epidermis, but they mature and lose their vitality and experience other changes as they move up from the epidermis and through the SC on their way to being shed at the skin surface. In the SC they are flat, hexagonal-shaped cells filled with water-retaining keratin proteins surrounded by a protein envelope and lipids. The corneocytes are stacked 10–30 layers high and connected to each other by protein bridges called desmosomes. Stacked bilayers of lipids surround the cells in the extracellular space. The resulting structure is the natural physical and water-retaining barrier of the skin. One writer called it “a wall of hydrophilic protein bricks (the corneal cells) sealed by a hydrophobic ceramide lipidic cement.”