Source of image: http://www.nature.com/nrm/journal/v2/n4/images/nrm0401_285a_f1.gif
A.Diagrammatic representation of junctional complex in intestinal cells
B.Electron micrograph of of actual intestinal cells
Let’s put together all the components of our junctional complex.
In epithelial cells lining the small intestine for example, the components of the junctional complex are usually arranged in the following sequence, starting from the free surface or exposed surface:
• Tight junction http://acellstoryaday.blogspot.com/2009/02/tight-junction-holding-on-tight.html
• Adhesive junction http://acellstoryaday.blogspot.com/2009/02/adhesive-junction-lets-stick-together.html
• Gap junction http://acellstoryaday.blogspot.com/2008/11/intercellular-communication.html
The adhesive or adherens junction that is labelled in the image above is what we mentioned last time as the belt desmosome while the one labelled as desmosome is what we called as spot desmosome.
There is logic to this kind of arrangement based on the nature and function of the components of the junctional complex.
The tight junction is always at the topmost or most exposed part of the cells because it is supposed to prevent any entrance or exit of materials between cells. The scientific term for tight junction is actually zonula occludens, meaning ring-like occlusion.
The adhesive junctions are usually located below the tight junction because they glue cells together and provide mechanical support to the tight junction.
Finally, gap junctions are at the lowermost part or the least exposed part of the cells. There is actually some space between cells here such that there is rapid exchange of information between cells.
By the way, there may be a 3rd kind of desmosome, the hemidesmosome. As the name implies, it is half of a desmosome. This kind of adhesive junction usually glues epithelial cells to the basal lamina which is in contact with the connective tissues underneath epithelial tissues.
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