What is Histology -2-

The plasma membranes of adjacent epithelial cells frequently contain transmembrane proteins called junctional complexes (also called intercellular junctions) that link cells together. Several types of junctional complexes exist:

Anchoring junctions (desmosomes, adherens junctions, hemidesmosomes) - Proteins embedded in, and spanning, the plasma membranes of adjacent cells allowing epithelia to adhere to one another forming structural units of cells - i.e., allowing individual cells to function as a solid sheet rather than as isolated cells. Fibers attached to anchoring junctions within cells help them to resist stretching forces. This is particularly important in those areas of the body that are subject to mechanical stresses (e.g., epidermis of the skin).
Tight junctions - These are junctional complexes that serve as barriers to the free diffusion of molecules across epithelial surfaces. By fusing the plasma membranes on their lateral surfaces, tight junctions prevent molecules from moving between the cells. This means that any substance absorbed across a layer of epithelial cells has to pass through the cells - not around them. This provides a mechanism by which epithelial tissue can control absorption (selective permeability), and keep tissue compartments with different chemical compositions separate from one another.
Gap junctions - These junctional complexes contain channels which permit ion flow and the passage of small molecules between contiguous cells. Cells with gap junctions are able to communicate and coordinate their activity.