Tight Junctions

Epithelial cells cover the body (e.g. skin), cavities (e.g. stomach, uterus, bladder) and ducts (e.g. renal tubules, intestine) of multicellular organisms, and thus constitute the frontier between the individual and the external milieu. In areas that withstand strong mechanical or chemical stress (e.g. skin, esophagus, cornea, vagina) epithelia are stratified, whereas in the rest of the body, the epithelia independently of their morphology (e.g. columnar of the intestine, tubular of renal tubules, squamous of the lung), are organized in monolayers (Fig 1A). Epithelia protect the tissues that lie beneath, from microorganisms, toxins, trauma and water evaporation, and regulate the exchange of substances between the content of body cavities and ducts and the underlying tissues. Transport across epithelia occurs through the transcellular and paracellular pathways and requires the presence in epithelial cells of two basic features: a polarized plasma membrane and tight junctions (Fig. 1B).