Question about the order of intercellular junctions

Q. As we study the different intercellular junctions is it important to know/label the order they are found in?

A. I think you’re probably referring to this slide:

This is the order that these junctions tend to show up in: the zonula occludens at the very top, the zonula adherens right underneath, and then the macula densa (or spot desmosome) somewhere underneath the zonula adherens.

You don’t need to memorize all three of these in order, because really, there’s really only one junction that has a meaningful position, and that’s the zonula occludens. The other two – meh, it’s not super relevant to the cell’s function that they are located where they are.

So all I’d like you to know from this slide is that the zonula occludens is at the very top of the cell. The reason it is located there is because it forms a super tight seal between the two adjacent cells – and you want that seal to be at the very top, for a couple reasons:

  1. So the cells are connected at their uppermost regions (rather than at the bottom, which would let the cells flop around!). Think of a six pack of pop or beer – the plastic connecting rings are at the very top.
  2. So that nothing (ions, gluten molecules, bacteria) can slip in between the cells at all.

Number 2 is really important if you’re trying to transport stuff through a cell! Let’s look at an example. Here’s slide 70. It shows an ion-transporting cell, obviously – but take a look at the zonula occludens:

This cell wants to transport sodium into itself, and then when it’s ready, push the sodium out into the blood. It’s really important that all the sodium goes through the cell (like you see with the black arrow below). That way the cell can decide when it’s a good idea to push the sodium into the blood – and you can regulate your blood sodium level. So good thing that the zonula occludens is at the very top of the cell, preventing sodium from getting into the intercellular space:

If the zonula occludens wasn’t there, some of the sodium would just slip right between the cells and get into the blood whenever it wanted, and you’d lose your ability to regulate your blood sodium! It would look something like this:

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