In this video, we're going to talk broadly about the cells and connective tissue proper, including loose and dense connective tissues, which are heavily differentiated in terms of their amounts and arrangements of protein fibers in the extracellular matrix, but are not differentiated as much in terms of their cell types. And so the cells and connective tissue proper, again, loose and dense connective tissues, can actually be broken up into 2 groups. The first group are the fixed or resident cells and the second group are the migratory cells. Now, the fixed or resident cells, as their name implies, are going to be cells that permanently reside in the tissue in a fixed or stable fashion. And so these fixed or resident cells don't actually move in and out of the tissue, they reside in the tissue.

So they don't move out of the tissue into other areas of the body. These cells are pretty much always going to be in the tissue. Now, the fixed or resident cells of connective tissue proper are going to include fibroblasts, which recall is a blast cell because it ends with the root "blasts." And recall that these fibroblasts are immature cells that are active and they tend to divide more and they are actively building and secreting components of the extracellular matrix, such as the ground substance and the protein fibers. Now, the fixed or resident cells of connective tissue proper are also going to include fibrocytes, which are actually site cells because they end in the root "site."

And so recall that these are mature cells that are less active, they tend to divide less, and they are more about maintaining the extracellular matrix by performing minor routine maintenance rather than active building and secreting of the ECM. Now it is worth noting that sometimes these fibrocytes can actually revert back to being fibroblasts under the right conditions when lots of new extracellular matrix needs to be made. Such as, for example, if there is significant tissue injury or damage and there needs to be significant repair or significant growth of the extracellular matrix. Now another fixed or resident cell of connective tissue proper that's found in some types of connective tissue proper are adipocytes. And adipocytes are actually fat cells that store lots of fat or lipid molecules.

And so these adipocytes or fat cells, once again, are found in some types of connective tissue proper. And we'll be able to talk more about them as we move forward in our course. Now, the second major group of cells in connective tissue proper are the migratory cells, which as their name implies are actually going to migrate around into different tissue. And so these migratory cells are usually going to be immune cells that again, are able to migrate or move in and out of the tissue. And so when they move out of the tissues, they can actually migrate to different areas of the body and move into other tissues.

And they do so in order to protect us against infection. Now, the migratory cells of connective tissue proper include macrophages and mast cells. The macrophages are large phagocytic cells that perform phagocytosis or the engulfment of an invader in order to eliminate it. And then mast cells are going to be immune cells that release chemicals called histamines, which are important for inflammation, which is going to help protect against invaders as well, protecting us from infection. And so, if we take a look at our image down below, notice we're focusing in on the cells of connective tissue proper.

And here what we have is a sketch of some connective tissue proper. And once again, connective tissue proper is also sometimes referred to as fibrous connective tissue. And so you can see the fibers in the extracellular matrix being labeled here. We know that it's going to have a gelatinous or semi-fluid type of ground substance. And, in many cases or some cases, we'll find that connective tissue proper can actually contain blood vessels.

Not all the time, but sometimes it can have blood vessels. Now, notice again, we're focusing in on the cells and we know that there are 2 major groups of cells, the fixed or resident cells, which we are labeling on the left side of the image. And then we also have the migratory cells, which we have on the right side of the image. Now, the fixed or resident cells that are important for building and secreting the components of the extracellular matrix are the fibroblasts.

And so you can see the fibroblasts being indicated here in this sketch. Now, the cells that maintain the extracellular matrix with minor routine repairs and maintenance, those are going to be the fibrocytes, which you can see are also labeled and indicated here in our diagram. And then the third type of resident or fixed cell are going to be the fat cells, which are adipocytes. And again, these adipocytes or fat cells are found in some types of connective tissue proper. And then again, when it comes to the migratory cells, these cells tend to be immune cells that can move into and out of the tissues, and those are going to include mast cells, which release histamine for inflammation, and macrophages, which are relatively large phagocytic cells that perform phagocytosis or engulfing of invaders for elimination.

And so, it's important to note that although fixed or resident cells are permanently residing in the tissue, that doesn't necessarily mean that they are not moving around within the tissue. They can still move around within the tissue, they just reside in the tissue, they don't move out of the tissue and into other areas of the body. And so this here concludes our brief lesson on the cells of connective tissue proper. And as we move forward in our course, we'll be able to apply these concepts and learn more. So I'll see you all in our next video.