ABSTRACT
INTRODUCTION The skin has to form an effective barrier to protect the body from external assaults (e.g., penetration of harmful substances, invasion of pathogens, and UV irradiation). In addition, it has to prevent the uncontrolled loss of water and solutes. Therefore, the skin comprises several interacting barriers, which are important for outside-in and/or inside-out barrier function. Examples are physical barriers [e.g., stratum corneum (SC), nucleated epidermis, and basal lamina], as well as immunological barriers and UV-protective barriers. In recent years, tight junctions (TJs) have been discussed to be an important part of the epidermal barrier system.
