Intercellular junctions and cell-extracellular matrix junctions

Epithelial cells attach to their neighbors and to the underlying basement membrane using specialized types of intercellular junctions and cell-extracellular matrix junctions, respectively. These junctional complexes also link the cytoskeletons of neighboring epithelial cells, allowing them to coordinate their functions.

Take a look at the three main types of intercellular (cell-to-cell) adhesion junctions in this TEM image of intestinal epithelium at the right:

1. Tight or occluding junctions- composed of occludins and claudin proteins, these junctions are located in the lateral plasma membrane near the apical surface, forming a tight seal between the cells to separate the luminal environment from the underlying connective tissue
2. Adherent junctions or zonula adherens- composed of E-cadherin and catenin proteins, these junctions are located below the tight junctions, where they connect the actin cytoskeletons of neighboring cells and support the tight junctions
3. Desmosomes or macula adherens- composed of cadherin family proteins, these junctions form spot-like welds, connecting the intermediate filament networks of neighboring cells to provide strong structural support to the epithelium.

Epithelial cells also directly communicate with their neighbors through gap junctions (Nexus). These are small channels composed of connexin proteins located in the plasma membranes of neighboring cells. Gap junctions allow small molecules such as ions and metabolites (nucleotides, sugars, and amino acids) to move between cells. In this TEM image of a gap junction, notice how the plasma membranes of the neighboring cells are in direct contact, allowing for intercellular communication.
 

Next are cell-to-extracellular anchoring junctional complexes.