So recall from our previous lesson videos that connective tissue proper is also sometimes referred to as fibrous connective tissue because these tissues are very rich in terms of their protein fibers found in the extracellular matrix. And often, those protein fibers are distinct and pretty easily seen under a light microscope. And so in this video, we're going to talk more about what these protein fibers actually are. And so protein fibers can be defined as relatively large and thread or rope-like structures that are found in the extracellular matrix and are made up of smaller proteins. Now, really, there are 2 main smaller proteins that you should be aware of that make up the protein fibers.
And so the first smaller protein that you should be aware of are collagen proteins, which are actually nonelastic proteins, meaning that they do not allow for elasticity or the ability to stretch and then return back to their original shape. However, although collagen does not really allow for elasticity, it does allow for strength. And so that is going to be a feature of the protein fibers that collagen proteins make up. Now it is worth noting that collagen is actually the most abundant protein in the human body, where it can actually make up anywhere between a quarter to a third of the total protein content in the human body. So that's a lot.
Now the second main smaller protein that you should be aware of is elastin. And elastin, as its name implies, is an elastic protein, which means that it allows for elasticity or the ability to stretch and then return back to its original shape after being stretched. Now, again, the collagen proteins and elastin proteins can form larger protein fibers. And so notice that down below this entire rope-like structure that you can see here, we are indicating is representing our entire protein fiber. And as we mentioned, the protein fiber is a relatively large thread or rope-like structure in the ECM that's made up of smaller proteins.
And so notice here on the right, we're indicating, these smaller proteins and showing them separated here. And so you can see that each of these highlighted regions would represent smaller proteins. And so these smaller proteins could either be collagen or elastin. And again, they come together to form the protein fibers. And so now that we have a better understanding of what protein fibers are, in our next lesson video, we're going to talk about the 3 different types of protein fibers.
So I'll see you all in that video.