tight junction A region of the cell surface, of variable size, and situated just below the apical border where the membranes of adjacent cells are fused to form a pentalaminar structure. In simple epithelial layers tight junctions often form a continuous layer (the zonula occludens). Their function appears to be the provision of areas of contact between cells of enhanced permeability and low electrical resistance. They occur in tissues such as the blood-brain barrier and cardiac muscle, where such features are advantageous.

tight junction (zonula occludens) The region between the plasma membranes of two adjacent cells that are so closely positioned that there is no intercellular space between them. This type of junction fuses cells together and provides a selective barrier to the diffusion of substances between cells. Tight junctions are common between cells of the intestinal epithelium, where they act to seal off the intestinal lumen from the intercellular fluid. See also cell junction.