Introduction to Connective Tissue Proper

In this video, we're going to begin our introduction to connective tissue proper, which recall from our previous lesson videos is one of two major classes of connective tissue. In our lesson, the other being specialized, connective tissue and so connective tissue proper is a collective term to refer to both loose connective tissue and dense connective tissues. And so connective tissue proper, including both loose and dense connective tissues are actually found all throughout the entire human body in various locations as we'll get to learn more about moving forward in our course. And connective tissue proper is going to be very rich in terms of the protein fibers found in the extracellular matrix. And so these protein fibers are often pretty distinct and easily seen under a light microscope, which is why sometimes connective tissue proper is also referred to as fibrous connective tissue. Now, in terms of the functions connected tissue proper has many different functions. And these functions tend to be more generalized functions that apply to multiple body systems, multiple areas of the body and multiple body processes. And so these generalized functions is why sometimes connective tissue proper is also referred to as general connective tissue. Now loose and dense connective tissues are actually made from relatively the same types of cells and made from the same exact protein fibers. However, loose and dense connective tissues are actually going to differ from each other in terms of the amounts and the arrangements of those protein fibers in the extracellular matrix. And so if we take a look at our image down below, notice we're showing you a map of our lesson and how we can organize connective tissue into two major classes. Again, they can be organized into connective tissue proper or general or fibrous connective tissue. And they can also be organized into specialized connective tissues. And again, we'll talk about the specialized connective tissues later in our course for now, we're focusing in on connective tissue proper, which is always going to include loose connective tissue and dense connective tissue. Now, as their name implies, loose connective tissue is going to have loosely spaced protein fibers in the extracellular matrix. And these loosely spaced protein fibers are going to be separated by a relatively large amount of ground substance. And so notice down below, we're showing you a micrograph of some example, loose connective tissue forming the layer under the epithelia. And what you'll notice about this micrograph of loose connective tissue is that these darker spots that you see throughout represent the nuclei of cells. And notice that all of these lines going through the image represent the protein fibers which again, are pretty distinct and clearly seen here in this micrograph. And what you'll notice is that these protein fibers are pretty loosely arranged in such a manner that you can actually see these small spaces or holes in between these protein fibers. And so that is showing that loose arrangement of loose connective tissue. Now, on the other hand, dense connective tissue as its name implies, is going to have more densely packed protein fibers in the extracellular matrix. And the extracellular matrix is going to have less ground substance in terms uh in comparison to the ground substance of loose connective tissue. And so notice here, we're showing you a micrograph of some example, dense connective tissue forming tendons and ligaments. And immediately what you'll notice is that these pink fibers that you can see in this image are really densely packed together so much so that you don't really see those open spaces like you did over here. And this micrograph with loose connective tissue and their loose arrangement of those fibers. Uh again, you can see how densely packed the fibers are in this particular tissue. And so this here concludes our brief introduction to connective tissue proper. And again, as we move forward in our course, we'll be able to learn a lot more about connective tissue proper and apply these concepts in problems. So I'll see you all in our next video.

True or False: Connective Tissue Proper is categorized into two types based on their cell types.

• A. True—the presence of -blast and -cyte cells determines if it is loose or dense connective tissue proper.
• B. False—the arrangement of fibers determines if it is loose or dense connective tissue proper. 

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 red or rope like structures that are found in the extracellular matrix and are made up of smaller proteins. Now, really, there are two 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 non elastic 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 pro 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 red or rope like structure in the ECM that's made up of smaller proteins. And so notice here on the right, we're indicating uh these smaller proteins and showing them separated here. And so you can see that each of these highlighted regions would represent a uh 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 three different types of protein fibers. So I'll see you all in that video.

So recall from our last lesson video, we said that there are two main types of smaller proteins that form the protein fibers of the extracellular matrix. And those two smaller proteins are collagen proteins and elastin proteins. And so in this video, we're going to introduce the three types of protein fibers that those smaller proteins can form. And so once again, there are three different types of protein fibers that can be found in the ground substance of the extracellular matrix. And so those three different types of protein fibers are collagen fibers, reticular fibers and elastic fibers. Now, the collagen fibers as their name implies are going to be made up of the smaller protein collagen and the collagen fibers are going to be longer fibers that are actually going to have an unb branched or a straight structure. And so this unb branched or straight structure of collagen fibers is partially what gives collagen fibers its strength that it's most commonly associated with. And so the strength of collagen fibers is going to allow it to resist forces such as for example, pulling and pushing forces. Now, although collagen fibers are greatly associated with strength, it is important to note that collagen fibers are not stiff structures. And in fact, they actually do have a little bit of flexibility to them. And so it's important to note that collagen fibers are going to provide strength, but they are also still a little bit flexible. Now, it's worth noting that collagen fibers are also going to form relatively thicker fibers in comparison to the other two types of fibers. And these thicker collagen fibers can also form bundles of collagen fibers. And those bundles of collagen fibers make it thicker and uh able to provide even more strength. Now, the second type of protein fiber is reticular fibers and the reticular fibers like collagen fibers are also made of the smaller protein collagen. But those smaller collagen proteins are going to take on a different arrangement in reticular fibers. And so what's important to note is that the word reticular found in reticular fibers is actually a word that means netlike. And so this tells us that the arrangement of the collagen proteins is going to be in a net like or a branched structure. And so the reticular fibers actually do form a branched structure. And so because the collagen proteins of reticular fibers are in a net like or a branch structure, this is really what allows them to resist forces in multiple directions or in many directions. Now, the third and final type of protein fiber here in our lesson is elastic fibers and as its name implies with the term elastic, these fibers are going to be made of the smaller protein elastin, which recall from our previous lesson videos is an elastic protein which means that it allows for stretching and then it allows for the return to the original shape after stretching. Now, these elastic fibers are going to be branched and wavy, which also helps to contribute to its stretching ability. And again, uh because it is so stretchy, that is what gives it its elastic ability, again, its ability to stretch. But then it of course, return back to its original shape after stretching. Now, what you'll notice is that over here on the right, we have some analogies for you to help you remember these three different types of fibers. Now, when it comes to collagen fibers, again, these are going to be long and unb branched or straight fibers. And so they are going to provide strength with a little bit of flexibility. And so a rope is going to be the analogy that we're using for collagen fibers because ropes notice are going to be unb branched straight structures and we know that they can provide strength but also a little bit of flexibility as well. And ropes tend to be also thicker fibers, especially in comparison to nets. And in some cases, ropes can be also thicker than bungee cords as well. And so that helps to remind us that collagen fibers are going to be thick in their nature. Now for reticular fibers, the analogy that we use is a net because again, recall the word reticular means net like. And so this reminds us that this these reticular fibers are going to have a net like or a branched structure just like a net has a branch structure. And this net like or branch structure allows it to resist forces in multiple directions. And then for elastic fibers, the analogy that we use is a bungee cord because bungee cords are quite elastic where they can actually stretch. But then after stretching, they can return back to their original shape. And so it's worth noting that elastic fibers in the extracellular matrix can actually stretch more than double their length in many cases. And so that's a pretty fascinate. And so this here concludes our lesson on the three different types of fibers. And uh as we move forward in our course, we'll be able to apply these concepts. So I'll see you all in our next video.

So here we have an example problem that says fill out the interactive blanks in the flow chart. And so notice down below we have a flow chart that is flowing from the left to the right as it branches. And notice that the two interactive blanks over here on the left, both end with the word protein. And so in each of these interactive blinks, we're going to fill in one of the two smaller proteins that make up protein fibers, either collagen or elastin. And then notice that the three interactive blanks on the far right, all end with the word fibers. And so in each of these three interactive blanks, we're going to fill in just one of the three types of protein fibers. And so notice that in the far left of our flow chart, there's a question that's asking, do you want the rope to stretch and notice the rope here is going to represent the protein fiber. And so if we can say yes to this question that we do want the rope or the protein fiber to stretch, then of course, that means that the smaller protein that must be used is going to be the protein elastin, which recall is going to have the elastic property, which is the ability to stretch, but then return back to its original shape after being stretched. And of course, if the elastin protein is being used, then that is going to form the elastic fiber and the elastic fiber again, is going to allow for that elasticity, the ability to stretch. But then again, return back to its original shape after being stretched. Now, if we say no to this initial question that we don't want our rope or our protein fiber to be able to stretch, then instead of using the elastin protein, we are going to use the protein collagen. And if the collagen protein is being used, then we know that that collagen protein can actually be assembled in different ways, it can be assembled in an unb branched or straight structure or it could be assemble assembled in a branched or net like structure. And so notice that the next question uh being asked here is, does the rope or protein fiber need to act as a net? And so, of course, if we can say yes to this question, then we know it's going to be reticular fibers because we're called the term reticular means netlike. And so reticular fibers are going to be made of collagen proteins. But those collagen proteins are going to be arranged in a thinner netlike and branched structure. And that is what allows reticular fibers to resist forces in multiple directions. Now, if we say no to this question that the rope does not need to act as a net, then of course, we are going to have collagen fibers. And in collagen fibers, the collagen proteins are going to be arranged in an unb branched and straight structure. And this is what gives collagen fibers its strength. And also don't forget that collagen fibers are not stiff structures. And so they do have a little bit of flexibility as well. And so this here concludes our example problem and as we move forward, we'll be able to apply these concepts in problems. So I'll see you all in our next video.

A student knows a particular structure is made of connective tissue and they want to know what type of fiber exists in its ground substance. The structure must withstand force from muscles pulling in different directions. What type of fiber is likely used in this connective tissue?

• A. Collagen.
• B. Reticular Fibers.
• C. Elastic.

Both collagen fibers & reticular fibers are made of the same protein collagen. How are the two different?

• A. Collagen fibers are wavy while reticular fibers are straight.
• B. Collagen fibers are branched while reticular fibers are long and straight.
• C. Collagen fibers are thin while reticular fibers are thick.
• D. Collagen fibers are long and straight while reticular fibers are branched.

Ehlers-Danlos syndromes are a group of genetic conditions that affect the production of collagen. One symptom of Ehlers-Danlos syndrome can be hyper-elasticity of the skin (skin that stretches much more than normal). This symptom is due to changes in the connective tissue supporting the skin. Relate this symptom to the roles of different fibers in connective tissue:

• A. Collagen is not elastic; the condition likely results in less collagen, allowing skin to stretch more than usual.
• B. Both elastic and reticular fibers are elastic; the condition likely results in higher concentrations of these.
• C. Collagen has highly elastic properties; the condition must result in an overproduction of collagen.
• D. Both A and B are correct.

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 two 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 fibro sites 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 fibrocyte 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 adipocyte and adipocyte are actually fat cells that store lots of fat or lipid molecules. And so these adipocyte 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 and 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 mass cells. The macrophages are large phagocyte cells that perform phagocytosis or the engulfment of an invader in order to eliminate it. And then mass 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 that 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 semifluid type of ground substance. And uh in many cases or uh some cases, we find that connective tissue proper can actually contain blood vessels, not all of 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 two major groups of cells, the fixed or resident cells, which we are putting on the left side of the image. Over here, we're 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. 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 fibrocyte, which you can see are also labeled and indicated here in our diagram. And then the third type of resonant or fixed cell are going to be the fat cells which are adipocyte. And again, these adipocyte 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 mass cells which release histamine for inflammation and macrophages, which are relatively large phagocyte cells that perform phagocytosis or engulfing of invaders for elimination. And so uh 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 year 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.

So here we have an example problem that asks how does the structure of connective tissue proper benefit the function of macrophages and mass cells. And we've got these four potential answer options down below. Now, we need to recall from our previous lesson videos that connective tissue proper or fibrous or general connective tissue is going to have a ground substance that is gelatinous or somewhat of a semifluid in between a solid and a liquid. And we also need to recall from our previous lesson videos that macrophages and mass cells are examples of migratory cells in connective tissue proper. And so these are immune cells that can actually move into and move out of the tissue into other areas of the body to protect against infection. And so if we are looking for a feature of connective tissue proper, that benefits the function of macrophages and mast cells, then it must benefit their migratory nature, their ability to move in and out of the tissue. And so, because connective tissue proper has a gelatinous ground substance that is not a rock hard solid, that actually benefits these migratory macrophages and mast cells because a rock hard ground substance would make it more difficult for these immune cells to migrate through the tissue and also to migrate out of the tissue. And so what you'll notice is that answer option A which suggests that the gelatinous matrix allows movement of migratory cells. That is actually going to be the correct answer for this example problem. So we can go ahead and indicate option A here is correct. Now, option B says that the gelatinous matrix restricts the movement of fixed cells. Now these fixed cells are not going to be macrophages and mass cells. We know macrophages and mast cells are migratory cells and also uh the macrophages and mass cells because they are migratory. Their movement is not going to be restricted and also restricting their movement would not be a benefit of these migratory immune cells. And so for those reasons, we can eliminate interaction. B Now, option C says that the protein fibers allow movement of migratory cells but this is not really the main function of these protein fibers is uh not to allow movement of the migratory cells. So for that reason, we can eliminate option C and then option D says that the protein fibers restrict movement of fixed cells. But really this is not going to be the case. Again, the fixed or resident cells are going to reside in the tissue in a fixed or stable fashion. But that doesn't mean that these fixed cells are not able to move within the tissue. Uh And so their movement is not going to be restricted as option D suggests here. So again, option A is correct for this example and I'll see you all in our next video.

What is the difference in function between fixed and migratory cells in connective tissue proper?

• A. Migratory cells maintain the matrix while fixed cells defend against infection.
• B. Migratory cells defend against infection while fixed cells maintain the matrix.

Fibrocytes are mature fibroblasts. But in some cases, fibrocytes can revert to fibroblasts. Based on their function, when would you expect a fibrocyte may revert to a fibroblast?

• A. A fibrocyte may revert to a fibroblast as a normal aging process as the cells become less active over time.
• B. A fibrocyte may revert to a fibroblast in the case of significant tissue injury/damage to produce new ECM.
• C. A fibrocyte may revert to a fibroblast in order to store energy in the form of lipids.
• D. A fibrocyte may revert to a fibroblast during an acute infection as fibroblasts provide direct immune protection against pathogens.