Neighboring epithelial cells, which line the inner intestinal surface (enterocytes), are connected to one another by a complex of substances that is formed directly under the apical (superior) surface of these cells. This surface has numerous long and thin projections called microvilli, they increase the absorption area manyfold. The complex is constituted by a number of specialized intercellular membrane junctions. Occluding or tight junctions (zonula occludens) are found directly under the microvilli. In epithelia with marked polarity of cell surfaces, tight junctions represent a boundary between apical and basilateral plasma membrane portions. These junctions are only found in the epithelia. In the intestinal epithelium, there are adhesive belts (zonula adherens) that are found below the tight junctions. These are adhesive junctions connecting the neighboring cell surfaces mechanically. The connection is provided by special adhesive proteins called cadherins. Near an adhesive belt or strip, one may usually find several desmosomes (macula adherens) #1, #2. These local adhesive junctions act like a rivet when binding the lateral cell surfaces.

The junctional complexes often contain another type of membrane junctions called communicating junctions (gap junctions). They exert a function to form direct transmembrane communications (channels) thereby connecting the cytoplasm in the neighboring cells.

For more details, see fig. «Annular gap junction between membranes of cardiac muscle cells» and «A cluster of connexons in membrane of endothelial cell (cryo-fracture)».