Three Types of Cell Junctions

00:00 There are three major types of connections between cells. We break them into three classes.

00:06 We'll see the cadherin based junctions again but more complicated situations are tight junctions.

00:14 Tight junctions are where cells are really closely held together. Recall the example that I gave of cells lining the gut in the digestive system, say the small intestine. We don't want food passing between the cells.

00:27 So they need to be held very tightly together so that things cannot sneak between the cells.

00:33 Let's say you had some proteins that you eat and they have been broken down in the gut and they were trying to sneak between the cells. Then they get into the internal environment or the extracellular matrix on the other side, then you're going to have a huge immune response to that.

00:49 And that's one of the things that actually happens with gluten intolerance.

00:52 Gluten, one of the moieties of the gluten proteins is able to sneak between the tight junctions wreak havoc with the immune system. So people that have coeliac disease or gluten intolerance may experience some leakage because that protein moiety can make its way through the digestive lining sometimes.

01:14 So another type of junction are adhesive junctions. They are desmosomes.

01:19 They are very much like rivets. They snap cells together.

01:22 Here's an example where you see the cadherin based linkages in between the two membranes.

01:28 You've got a snap on this side, and a snap on this side. And between the two cell membranes, you've got this cadherin fingers.

01:36 And then again you can see that we could be anchored to some cytoskeletal elements in the middle.

01:43 So things could pass between cells in the case of desmosomes or adhesive junctions.

01:50 On occasion, we see half of a desmosome or a hemidesmosome which will anchor cells to a basement membrane.

02:00 So for example, when we consider epithelial tissues which are our skin and linings of our digestive tract and such, those cells are anchored to a basement membrane by hemidesmosomes. So that keeps everything in place.

02:18 The third type of cell connection we see are gap junctions. And gap junctions physically allow substances even fluid itself to move back and forth between each of those cells.

02:31 So not only do we have a transmembrane protein that's a channel but we have that channel passing through both cell membranes.

02:40 Much like we see in the nuclear pores. These are called gap junctions. And examples of where we see gap junctions are in the heart muscles where electrolytes are allowed to pass directly through these channels into the neighbouring cells.

02:58 So overall, we have these three types. One the tight junctions, do just what they sound like.

03:04 They held cells tightly together.

03:06 We've got the snaps that hold cells tightly together but allow passage of materials between the cells on occasion.

03:14 And then we have gap junctions that allow passage of materials from one cell to another cell.

03:24 So with that, we can bring this lecture to a close.

03:28 And at this time, you should be able to identify where many of the macromolecules that we've already discovered are involved in cell membrane structures. So we've got carbohydrates, we've got proteins and we've got lipids involved in the cell membrane structure.

03:45 In addition to that, you should have an understanding of the fluidity of the membrane and how that is maintained based on the phospholipids and cholesterol etc that are involved.

03:57 And you should also be able to describe the roles of the three types of microtubules and actin or filaments inside of the cell, and how the highways work through the cell, as well as how cellular cytoskeletal elements hold the structure of the cell.

04:18 In addition to that, you should be able to explain the mechanism cells use to stick together.

04:24 So, gap junctions, tight junctions and desmosomes.

04:30 Thank you so much for your attention. I look forward to seeing you in a future lecture.