Occluding junction definition
Occluding junction refers to the cell junction that seals the space between cells of an epithelium in such a way that it creates a barrier, which prevents the passage of molecules.
Cell junctions
Tissue cells are in constant communication due to existence of cellular junctions. There are various types of cell junctions and each one perform distinct functions in cells:
- anchoring junctions are responsible for maintaining cell shape and location in tissues;
- gap (or communicating) junctions are the true responsible for chemical communication between cells, making possible the exchange of ions and molecules between neighbour cells; and
- occluding junctions provide a barrier through the sealing of one (or several) cell layer(s), which prevents the passage of water and some solutes.
Occluding junctions’ characteristics
Occluding junctions are characterized by intimate contact between adjacent cells plasma membranes. This contact is so close that it was initially thought that the outer layers of neighbour cells plasma membranes were fused. Many proteins in cells associate to form occluding junctions. Some of these proteins regulate the cell polarity, determining its apical and basolateral domain.
These cell junctions do not totally seal the space between cells and the resulting permeability varies greatly, depending on the tissue. On the other hand, by forming barriers, occluding joints make it possible to maintain the different chemical composition of adjacent fluids.
Tight junction
A specific type of occluding junctions, tight junctions, are characteristic of vertebrate epithelia (figure 1). In this type of tissue, tight junctions exert a variety of functions: they separate the basolateral and apical domain of cells by controlling proteins and lipids diffusion between cell compartments; and seal the plasma membranes of adjacent cells, forming a selective permeable barrier of epithelial tissue. Tight junctions’ permeability is regulated by epithelial cells themselves, which make these cell junctions more permeable if there is a greater demand for water or solutes.
Tight junctions are formed by chains of transmembrane adhesion proteins – the claudins and occludins. These proteins are arranged in a network conformation that is included in the plasma membrane, from end to end of the cell through the inside. Extracellular domain of these proteins interacts with extracellular domain of claudins and occludins of neighbour cells. In this way, this cell junction makes adjacent cell membranes so intimate that they seal the space between them.
Claudins are the most important proteins in these cellular junctions, differing among the various types of existent tight junction in vertebrate epithelia. Claudins and occludins interact with intracellular peripheral membrane proteins, called ‘ZO proteins’. Protein chains associate with the actin of cell’s cytoskeleton through ZO proteins.
Figure 1 – Tight junction illustrative schematic image.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
