What are called cell junctions?
Cell junctions are specialized regions affirm intercellular connections between the plasma membranes of adjacent cells. These are observed only in some animal tissues and are produced by interaction, attachment and modification of some areas of the adjoining plasma membranes. Cell junctions have three major functions: i) Cell junctions provide mechanical support to the tissues by holding the cells together. ii) Cell junctions help in communication between the cells. iii) Cell junctions may form an impermeable barrier between the interstitial space and an epithelial surface (lumen).
Types of cell junctions
There are four major types of cell junctions. Those are: i) Tight cell junctions ii) Desmosome cell junctions iii) Gap cell junctions iv) Interdigitated cell junctions. Let us have a very easy discussion about the above said four types of cell junctions.
Tight cell junctions
These are also called Zonula occludens. Tight junctions are specially differentiated regions where the lateral plasma membranes of two neighboring cells fuse together at series of points containing sealing strands, which form lines of attachment. The sealing strands are made up of rows of integral membrane proteins contributed by each plasma membrane. This type of junction is found in between the brush bordered epithelial cells of intestine and renal tubules. Just below the microvilliIt is located at the apical region of the cells. Tight junctions occlude the intercellular space and prevent the passage of substances across the epithelium. Thus, such impermeable junctions form a barrier between the two sides of the epithelial lining, so that the materials absorbed across the epithelial cells at the expense of energy may not leak back into the lumen of the intestine or renal tubule. Within the membrane such junctions also help to prevent random movement of the integral proteins.
Desmosome cell junctions
Desmosomes cell junctions are for mechanical union between the cells. Desmosomes are of two types:— i) Belt desmosome ii) Spot desmosome.
Belt desmosome is also called zonula adherens or terminal bar or intermediary junction. It is found at the boundary between the columnar epithelial cells, just below the tight junctions. It is a band like zone that runs parallel to the free surface of the cells. It consists of two actin-made microfilaments each of which is located on the cytoplasmic surface of each membrane. The belt desmosomes remain connected with the terminal web present on each side of the joint.
Spot desmosome is a disc-shaped spot or area of contact between the adjacent cell membranes. It consists of a disc-shaped, dense, protein-made cytoplasmic plate under each membrane. These plates are joined by fine filaments, the trans-membrane linkers, that pass through the intercellular space. Spot desmosomes are found in the epithelial cells of uterus, vagina and epidermis of skin to support the cells against severe mechanical stress.
Gap cell junctions
These are also called the Nexus. A gap junction is a disc-shaped area of close intercellular contract having a narrow intercellular space between the two adjacent plasma membranes. At this junction, each membrane contains several protein molecules, which are arranged in such a way that they form an intercellular channel through which various small molecules and ions can pass from one cell to the next. As a result, the gap junctions provide a corridor for communication between the contiguous cells. Gap junctions are found in cardiac and smooth muscles, liver cells, embroyonic cells etc. In electrically excitable tissues such as cardiac and smooth muscles, this type of junction facilitates scattering of impulse from one cell to another. In other tissues like liver, embroynic tissues etc., the gap junctions provide a system by virtue of which the adjacent cells can share a common pool of metabolites and ions that pass liberally from one cell to another.
Interdigitated cell junctions
These cell junctions are found in cardiac muscle cells. These are the areas of extensive cell contact running crosswise between the two adjacent fibers arranged in longitudinal series. In such junctions, the adjacent plasma membranes are thrown into folds and the evaginations of one membrane fit into the invaginations of the other. Such junctions’ keep the cells tightly adhered and increase the surface area for exchange of important materials.