In my textbook, it says about adhering junctions: \"Typically, adhering junction

Question

In my textbook, it says about adhering junctions:

"Typically, adhering junctions are located deep to the tight junctions; the anchoring of the microfilament proteins provides the only means of junctional support for the apical surface of the cell. The ultra-strong tight junctions are needed only near the apical surface and not along the entire length of the cell. Once neighboring cells are fused together by tight junctions near the apical surface, adhering junctions support the apical surface..."

I don't understand:
-How do adhering junctions support the apical surface if tight junctions work near the apical surface to seal it off?
-I thought tight junctions and adhering junctions both surrounded the entire perimeter of the cell?

Thanks!

Explanation / Answer

Both tight and adhering junctions are found around the perimeter of the cell horizontally (imagine a single ribbon wrapped around a box, or the equator wrapped around the Earth), but the junctions do not extend from the top of the cell to the base of the cell. The passage is saying that tight junctions are found around the perimeter of the cell at the top end only. Since the upper sides of the cells are fused tightly with these junctions, the middle and lower portions of the cell's sides will be forced close together too even though they are not tightly fused like the upper portion. There is a massive amount of strain on tight junctions, so to alleviate some of this, the cell uses adherent junctions just below the tight junctions. Adherent junctions help to connect the actin filaments of joined cells, allowing the tension present throughout the connected cells to diffuse more easily. Think of it this way: You have two boxes side-by-side on a table. Where the two sides touch, imagine that the upper sections are stapled together. If you then try to pull the boxes apart, all of the force is place on those few staples holding them together. Eventually, they will rip and the boxes will come apart. If you instead staple the boxes a bit further down the sides too, then when you pull, the force is spread between the top and middle portion of the boxes. This makes them much less likely to rip apart. The middle staples (adherent junctions) are helping to support the top staples (tight junctions) by diffusing the force. Hope this helps some.

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