Cell to cell junctions. Cells of epithelia are closely connected and are not separated by intracellular material. There are three basic types of connections: tight junctions, anchoring junctions, and gap junctions. The tight junction separates cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction there is no extracellular space between them and the movement of substances through the extracellular space between the cells becomes blocked. This enables the epithelia to act as selective barriers. An anchoring junction includes several types of cell junctions that function to stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of these types of cells. Anchoring junctions provide strong and flexible connections between cells. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membrane of cells. The patches are structural proteins on the inner surface of the cell's membrane. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecule of adjacent cells. These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix. While similar in appearance to desmosomes, they include the adhesion proteins called integrins, rather than cadherens. Adherens junctions use either cadherens or integrins, depending on whether they are linked to other cells or the matrix. The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches, or can form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue. A gap junction forms an intracellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates their function.
