Epithelium: How Are Epithelial Cells Joined Together?

00:01 Let's now look and see how epithelial cells are joined together. Well there are three different types of junctional complexes and they have three different sorts of functions. The first juntional complex is at the very apex of the cells, and they are called occluding or tight junctions. They are barrier. They prevent material passing between the cells. It's important that the body can control what passes across or what it is absorbed across the epithelial surface. And so these occluding or tight junction prevent material passing through between cells. As sometimes these occluding junctions can become a little bit permeable and do allow things to pass through, but that's controlled by the epithelial cells in certain locations. But essentially they present themselves as a barrier. They are also a barrier to the migration of components to the cell membrane.

01:11 They seperate the apical border of the cell from the lateral border of the cell. And that's very important because these molecules in the apex of the cell and on the lateral surface of the cell that have very important functions. So it's important to seperate these two domains.

01:32 And one function of the tight junction or the occluding junctions is to act as maintain the integrity of these two domains. Lower down in the diagram, you can see anchoring junctions and there are two types of anchoring junctions. Some are called the zonula adherens, meaning they form a belt like structure around the cell, the occluding junction do the same. These zonula adherens are actually attached to the actin filaments of the cytoskeleton, and they are linked to the cytoskeleton from one cell to its neigbouring cell. The other top of anchoring junction is the desmosome or macula adherens. These are little spot junctional complexes, spot weld, if you like and they are attached to the intermediate filaments of the cytoskeleton, of one cell and also to another cell. So these anchoring junctions link the cells together so that the epithelium acts as a cohesive unit. And the remaining type of junction is the communicating junction or gap junction. These consists of proteins, connexons which enable things to pass between the cells, things like anions, nutrients and various other chemical signals. So cells can communicate with each other via these gap junctions. And that's very important for smooth muscle cells to be able to contract in a sequential manner, and also cardiac muscle to contract in a similar sequential manner.