5.2 Cell junctions

Cell-cell contacts also play extremely important roles in tissue function. For example in some tissues it is essential that there is very close contact between cells so that an effective barrier is set up and pathogenic microbes, and even harmful or unwanted molecules, cannot readily pass between the cells.

Cells may have a number of different types of specialised cell-cell contacts known as cell junctions. Epithelial tissue, which is composed of closely packed sheets of cells has several types of cell junction each with specific functions and formed by different specialised proteins. Examples of the arrangement of the main types of junction between epithelial cells are shown in Figure 26.

Figure 26 Schematic diagram showing cell junctions and their functions in epithelial tissue. Epithelial cells are characterised, in part, by their close packing. They are held together by specialised membrane proteins that form specialised junctions between adjacent cells.

Show description|Hide description

This diagram shows a small section of epithelial tissue and the location and schematic structure of the different junction types. The tight junction holds the cells very close together via their lateral surfaces, and is situated very near the apical end of the cells. The adherens junction is present on the lateral surface further from the apical end, and links the actin filaments of adjacent cells together via membrane-spanning proteins and intracellular proteins that link these to the cytoskeleton. The gap junction, which is also on the lateral surface, holds the cells tightly together: adjacent cells are separated by a narrow gap which is spanned by channel-forming protein. Also shown are the two types of anchoring junction; both of these comprise membrane-spanning adhesion proteins, which link the cells together across the intercellular space, and are linked on their cytosolic side to intermediate filaments of the cytoskeleton. The desmosome links adjacent epithelial cells together via their lateral surfaces; and the hemidesmosome links an epithelial cell to the extracellular matrix via its basal surface.

Figure 26 Schematic diagram showing cell junctions and their functions in epithelial tissue. Epithelial cells are characterised, in part,...

Gap junctions are a type of specialised cell junction that allows intercellular communication. In the smooth muscle of the intestine, which must contract in a coordinated manner for peristalsis to occur, the smooth muscle cells have gap junctions, which allow transmission of molecules and electrical signals between the adjacent smooth muscle cells.

Tight junctions, as their name suggests, link cells very closely forming a barrier that prevents the passage of molecules and ions between cells. They also prevent the movement of integral membrane proteins within the cell membrane. Hence they are important in maintaining, for example, the polarity of the epithelial cells of the intestine (Figure 20).

Adherens junctions link the actin cytoskeleton of adjacent cells together, often forming a belt-like arrangement around each of the cells in an epithelial sheet. The link occurs via transmembrane proteins known as cadherins, and intracellular proteins that link cadherins to the cytoskeleton.

Anchoring junctions (called desmosomes) link adjacent cells together, while hemidesmosomes link cells to the extracellular matrix. Again, these links occur via cadherins. In this case, however, the link is to intermediate filaments of the cytoskeleton rather than actin filaments.

There are other specialised types of connections between animal cells, such as the contacts between neurons and their targets, known as synapses. In plants, there are the plasmodesmata. These channels, lined by a cell membrane, make a connection between the cytoplasm of plant cells (through a gap in the cell walls), enabling transport and communication between the cells.