Fig. 1.

A: simplified schematic view of the location of the cellular junctions in juxtaposed epithelial cells (EC). Tight junctions (TJs) form the most apical junction and interconnect laterally neighboring cells in the epithelium. TJs allow selective diffusion of fluids, electrolytes, and small molecules through the paracellular space while providing a highly selective barrier for larger molecules, thereby regulating paracellular permeation of ions and other molecules. Adherens junctions are involved in cell-cell adhesion and intracellular signaling. Other basolateral epithelial junctions include desmosomes and gap junctions, which are involved in cell-cell adhesion and intracellular communication, respectively. B: TJs are composed of several types of occludins, junctional adhesion molecule (JAM) proteins, and members of the claudin protein family that influence the charge selectivity of the TJ. These are all transmembrane proteins that form intermolecular and intercellular connections within the paracellular space. All transmembrane junctional proteins interact with intracellular scaffold proteins (such as ZO-1, -2, and -3) that interact with other proteins, including actin in the cytoskeleton.