Juctional Complex

There are specialized regions in which there is firmer intercellular attachment. One type of contact found is some epithelial cell is called the junctional complex. In epithelial cells four types of junctional differentiation of ceil membranes are present.

1. The zonula occludens or occlusion zone (tight junction). Sometimes also called the terminal bar, it is found near the free surface of cells. The ‘terminal bar’ of light microsopists includes both the tight junction and the zonula adherens. In the region of the tight junction the plasma membranes of two adjacent cells are fused at a series of points containing sealing strands which form lines of attachments.




2. Cells adhere to each other means of junctions called desmosomes. These are to two main types, belt desomosomes and spot desomosomes.
The belt desomosomes is found just below the tight junction. It is in the form of a band on the inner side of the cell membrane of epithelial cells. The intercellular space in the junctional zone is filled with fine filaments.

3. Spot desmosomes (maculae adherens) are disc-shaped points of contact between the plasma membranes of adjacent cells. The spot desmosome consists of a disc-shaped plaque on the cytoplasmic surface of each cell membrane. Non-contractile 100A filaments called tonofilaments, which form the structural framework of the cytoplasm, are attached to the plaques.



4. Gap junctions are patch-like areas of close intercellular contract.  The intercellular space is narrow in the region of the gap junction. The gap junction consists of a disc-shaped area on each cell membrane containing several hollow, cylindrical particles. Calcium ions regulate the permeability of gap junction channels. The sensitivity of gap junctions to calcium ions appears to be an essential factor in tissue repair after injury.






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