So first let's discuss how the cells in these tissues are held together as this is a very
important point and some of these vocabulary words will be useful in upcoming slides. So first
of all, cells can be held together in a number of different ways and these points of contacts
are referred to as cell junctions. There are 4 different types of cell junctions found in the
human body. You have gap junctions, adherens junctions, tight junctions, and desmosomes.
The first type of junction that we'll discuss is the tight junction. The tight junction is a type
of junction between 2 cells that prevents movement of substances between those 2 cells.
Think about an apartment where you have your apartment and the apartment next door and
there is a wall in between and no one or nothing should be getting in between those 2
apartments. The way this works is there are transmembrane proteins that are between the
2 cells that link the cells together and hold them together tightly. The 2nd type of junction
that we find in our body are adherens junctions. In an adherens junction, you have a specialized
structure called a plaque and what this plaque is is a dense layer or proteins that's going to
connect transmembrane proteins from each of the cells to the cytoskeleton inside of those
cells. The purpose of these junctions is to actually resist separation during contractile
movement and it's found in the stomach. In these plaques, you have 2 types of proteins. You
have the cadherins which are going to be the transmembrane proteins in the membranes of
each of the cells and the cytoskeletal element or the protein associated with the cytoskeleton
is going to be actin microfilaments. This is going to become important because when we talk
about other junctions, the different types of transmembrane proteins are going to be different
depending on the type of junction that we're talking about. So, in desmosomes, we have
plaques as well and in this case the transmembrane protein is still cadherin but the cytoskeletal
element in this case is now keratin and intermediate filament. Desmosomes have the same type
of function as the adherens junctions and that we're going to be resisting separation during
contractile movements except for in this case we're going to find desmosomes in places like
cardiac muscle cells. Here is another type of junction protein, but I want to be clear that
this is not a cell-to-cell junction. Instead, what we find in cells that are anchored to a
basement membrane or some type of basal membrane are these junctions called hemidesmosomes.
Hemidesmosomes are going to anchor cells to a basement membrane using a plaque, but instead
of this plaque being made of cadherin as its transmembrane protein, the transmembrane
protein is actually integrin. Finally we have our gap junctions and gap junctions are my favorite
because it allows a group of cells to almost act as one singular cell and the reason why is
because it allows communication between the cells without the substance that's being
communicated having to leave the cell. So it creates tunnels between neighboring cells and in
this case the protein that's involved in this is connexins.