In this video, we're going to talk about the structure of connective tissue. And so recall from our previous lesson videos that all tissues including connective tissue are made of groups of cells and their extracellular matrix or their ECM. But recall that the extracellular matrix can be more prominent in some types of tissues versus other types of tissues. And so for example, recall from our previous lesson videos that epithelial tissue consists of cells that are really tightly packed together and have relatively little ECM between the cells. However, once again, when it comes to connective tissues, they actually have the most prominent ECM or extracellular matrix that can take up most of the volume of the tissue and actually separate the cells because of the relatively large amounts of ECM. And so because connective tissue has the most prominent ECM, this is why we focus on its structure in this video. Now recall that the extracellular matrix or the ECM is really just the material that is immediately outside of the cells and the extracellular matrix is actually made of ground substance and protein fibers. Now the ground substance is really just unstructured material that's between the tissue cells and between the protein fibers and the ECM and the ground substance, depending on the type of connective tissue can actually range in its viscosity or range in its thickness from being a rock hard solid like bone, for example, to being a liquid like blood, for example. And so the different types of protein fibers that are found in the extracellular matrix can actually contribute to the viscosity of the ground substance. Now, moving forward, we'll learn that there are three different types of protein fibers. And we'll talk about the differences in those three different types of protein fibers later in our course. But for now, you should note that the different types of fibers and the different amounts of each of those fibers can actually contribute to the tissue's physical properties. For example, contributing to the strength of the tissue, the flexibility of the tissue and the ability for the tissue to recoil or spring back to its original shape. Now, once again, all tissues including connective tissues are made of groups of cells. And so the cells of connective tissue can secrete and maintain the components of the extracellular matrix, including the ground substance and the protein fibers. And also the cells will support the specialized functions of the connective tissue. Now, in most types of connective tissue, there are two main types of cells which are the blast cells and the site cells. Now the blast cells are going to end with the root blast, which is why they're called blast cells. And these blast cells are going to be immature cells that have the ability to divide and they are more active and they are going to be actively building or they actively build and actively secrete the components of the ECM again, including the ground substance and the protein fibers. And so as we'll learn, moving forward in our course, the blast cells of connective tissue proper are fibroblasts which notice ends with the root blast again, which is why they are blast cells. Now, other examples of blast cells include osteoblasts which are found in the bone and chondroblast which are found in cartilage. Now, the site cells are going to end in the root site and these cells are going to be more mature cells that are a little bit less active. And so they don't really divide as much and they are more about maintaining the extracellular matrix rather than actively building and secreting the components of the extracellular matrix. And so in terms of maintenance, it's only going to do minor repairs and just routine maintenance. And so as we'll learn, moving forward, an example of a cell in connective tissue proper is going to be the fibrocyte which notice ends in the root sites, which is why they are site cells and other types of site cells. Uh Examples of site cells are going to be osteocyte which are found in bone and chondrocytes which are found in cartilage. Now, if major or significant repair or major or significant growth of the tissue is needed, then sometimes these site cells can actually revert back to blast cells so that they can actively build and secrete the ECM for that significant repair. And again, that significant growth. But if it's really just maintaining the ECM, then it's just going to be minor repairs. And again, those would be the site cells. So notice down below, we have an example. And it's asking us how is connective tissue like ice cream with different toppings mixed in? And so notice that we've got these two really delicious looking ice cream cones down below. And what you'll notice is that we have the major components of the tissues here in the middle. And again, recall that all tissues are going to be composed of, again, groups of cells, but also their extracellular matrix. And we've learned that the extracellular matrix is going to be composed of brown substance and protein fibers or just fibers for short. And so what you can see here is that we've got these um basically this key here that is showing us how ice cream with different toppings mixed in is similar to connective tissue. And so what you'll notice is that the ground substance, the unstructured material between the the cells and the protein fibers is like the base of the ice cream, either a chocolate base, for example, or a vanilla base for example. And so just changing the ground substance can change the tissue. And again, the ground substance can change in viscosity or its thickness. It could be a rock hard solid uh or a liquid. And so just like ice cream, when you just take it out of the freezer, it's kind of more rock hard and more solid. Uh or if you have ice cream that's been left out for a little bit, it's kind of more liquidy and the protein fibers are going to be things such as chocolate swirls or caramel swirls or little candy cane swirls. And again, there are three different types of fibers and different amounts and different types of those fibers can contribute to the viscosity of the ground substance and also contribute to the physical properties of the connective tissue. And then of course, every tissue is going to be composed of groups of cells and those cells can uh be of different types and the different types of cells can lead to different connective tissues. And so notice we're using things like marshmallows and little chocolate chips and gummy bears and sprinkles and other candies to represent the different types of cells. And so notice that this ice cream over here, which um you know, has a chocolate ground substance and marshmallows and chocolate chips and chocolate swirls as protein fibers is going to be a different connective tissue or represent a different connective tissue. Then this other vanilla based ice cream here uh representing the ground substance with little gummy bears and sprinkles and candies as the cells. And again, little cotton candy swirls as the protein fibers. And so this is how connective tissue can resemble ice cream with different toppings mixed in. And so as we move forward in our course, we'll be able to talk more details about connective tissue. But for now, this year concludes our brief lesson on the structure of the connective tissue. And we'll be able to get some practice applying these concepts moving forward. So I'll see you all in our next video.