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.
Give an example of circumstances where close contact between adjacent cells is important.
One example is in the epithelial tissue that lines the gut wall, where it is important that there are no large spaces between the cells, where harmful microbes in ingested food could enter the body. Another example is the skin, which also has a protective role.
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.
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.
Activity 1 A tour of the cell
'A tour of the cell' is an interactive activity in which you can look in more detail at subcellular structures in animal and plant cells. After completing the activity you will be able to identify major organelles and structures in transmission electron micrograph (TEM) images of eukaryotic cells, and outline how these structures relate to cell function.