Illustrations of structural models for tight (left) and gap intercellular junctions. The plasma membranes of certain neighbouring cells are arranged so that the cells may interact in a specific way. Tight junctions (zonula occludens) serve to seal the space between cells. Intestinal cells are separated by these seam welds", which ensure that the intestinal contents passes through the cells rather than between them. The gap junction or nexus is a form of macula occludens that provides a restricted channel between the cytoplasm of one cell and its neighbour, through which small molecules may pass."