Epithelial cells by the nature of their function as linings or as glands must always “hold on to each other”. Cells lining our intestines for example, must hold on to each other to prevent any material from passing between them. This ensures that anything that goes through our bodies passes through them (the intestinal cells) for proper processing, and not between them. So how is this possible? Well, intestinal cells as well as most other epithelial cells are attached to each other by junctional complexes, a primary component of which is the tight junction.
Tight junctions function as seals between and around epithelial cells. These consist of closely apposed plasma membranes of adjacent cells with no space at all between them. Thus, nothing escapes between cells.
For example, urine remains inside our urinary bladder and does not leak through it because of tight junctions. The contents of our intestines and stomach are kept inside our bodies because of tight junction. Food is absorbed by the cells but once absorbed cannot leave the cells. Contents of our body do not leak through our skin because of tight junction.
These are just some examples of the importance of tight junctions between cells.
By the way, gap junctions mentioned in my Nov 17 (intercellular communication) post, also are components of junctional complexes. Another component is the adhesive junction and this will be the subject of a future post.