Hi. In this video, we're going to be talking about cell to cell junctions. So what is a cell to cell junction? Well, it's a junction that connects cells together. Now there are many different types based on what kind of molecules are being used, so let's go through them.
The first type is called the adherens. You may see this as the anchoring junctions, and these anchor cells together. Cadherins are the important protein here, but actin is also another important component. The cadherins and anchoring junctions are anchored to actin. Here's like a zoomed out version of two cells with an adherent junction with actin in them, and then here's the zoomed-in. You have the cadherin, and if you follow the cadherin through to the inside of the cell, it's going to be connected to actin filaments. That's the adherens junction.
Then we have the desmosomes, which act to bind epithelial cells to each other. Cadherins are also important here, but they instead link to intermediate filaments instead of actin filaments. There is a similar structure called a hemidesmosome, which links epithelial cells to the basal lamina instead of to each other. Here's what a desmosome looks like: you have these cadherins, and if you follow them through, they're going to be linked to different types of internal filaments, including intermediate filaments. That's desmosomes.
Next, we have what's known as the tight junctions or the occluding junctions, and these form seals between cells that prevent leakage of water or ions between the cells. The proteins that make up tight junctions are called claudins or occludins, and these are really important in creating polarized regions in cells, so the apical or basal surface. Here is an example. This surface here is the apical, and this surface here is the basal surface. They look different. They have different proteins on them. The reason is because there are tight junctions that sit here that prevent the proteins on the lower side from going to any other part of the membrane. If you zoom in on those tight junctions, what you see is you get proteins that form these really tight interactions between the two cells. What it prevents is proteins from going from the apical side to the basal side, and also prevents water or other ions from flowing through these areas. This describes the tight junctions.
One of the final types we're going to talk about is the gap junction. These are regions of the plasma membrane that align in parallel to connect. The proteins here are called connexins. These cylindrical proteins adjoin adjacent membranes, and these types of junctions allow water and inorganic ions to pass between the cytosols. Here we have one plasma membrane and here we have another; there's some intracellular space here. You can see these gap junctions forming with connexins that allow water and inorganic molecules to pass; this is super important.
Finally, we have the last type, which we have talked about before. This is plasmodesmata; they connect plant cells together. You can see them here, which you've seen in these images before, so I'm not going to spend a ton of time, but you can also see them here where you have different plasmodesmata connecting the cytosol of one cell to the cytosol of the other. So those are the different types of cell junctions. Let's now move on.
