An epithelial layer covers most external (for example skin) and internal (for example the lung lining) surfaces. These epithelial layers are made up of sheets of epithelia. Epithelial cells are attached directly to each other, however, there are gaps/junctions between these cells that offer various roles. For example, these gaps contribute to the impermeable nature of the epithelia by sealing any spaces between the cells; linking the cytoplasm of neighbouring cells where it is necessary to transfer substances between cells; they contribute to communication by passing signals from one cell to another across the plasma membrane. As well as this, the epithelia are attached to cytoskeletal filaments in the cell to improve stability and keep them in position. This can be seen most clearly in specific tissues such as the heart muscle where anchoring is of key importance.
Depending on which of these functions is required from the epithelial cells, different types of junction can be seen between them. They are called anchoring junctions, occluding junctions, channel-forming junctions and signal-relaying junctions.
- ↑ B. Alberts, A. Johnson, J. Lewis at al. (2008) Molecular Biology of The Cell, 5th Edition, New York: Garland Science. Chapter 19, pages 1132-1133.