Tight/Occluding Junction

Tight junctions prevent some molecules from passing across an epithelium.

The borders of two cells are fused together, often around the whole perimeter of each cell, forming a continuous belt like junction known as a tight junction or zonula occludens (zonula = latin for belt).

Transmembrane proteins from each cell membrane interlock across the intercellular space, all around the cell, in this belt (black lines in the diagram).

The permeability of tight junctions varies from site to site – and are often selectively leaky. For example, these junctions are important in the gut, in acting as a selective diffusion barrier, preventing diffusion of water soluble molecules. They also act to restrict the localisation of membrane bound proteins. (For more, see the section on the gut)