Lecture 23 - Bone Tissue PDF

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 23 - Bone Tissue Video 1 Introduction Welcome to the next online lecture. So we finally finished all of the components of the nervous system that we are going to be covering in th...

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 23 - Bone Tissue Video 1 Introduction Welcome to the next online lecture. So we finally finished all of the components of the nervous system that we are going to be covering in this term, and we're moving on to the next tissue, which is bone. So if you remember, bone is one of our supporting connective tissues. So in today's online lecture, we're going to begin by looking at a general overview of the functions of bone, and then we're going to look more closely at the histology of bone by looking at what makes up bone tissue. We're also going to compare that to cartilage, another of the supporting connective tissues. So let's get started. Slide 1 The material for this online lecture can be found in chapter 6 of your textbook called the skeletal system, bone in bone tissue. Today we're going to focus primarily on Section 6.1: the functions of the skeletal system, section 6.2: cartilage, and section 6.3: bone histology. I also highly recommend revisiting chapter 4 of your textbook and looking at section 4.2 in connective tissue. In there, you can look at the supporting connective tissues, cartilage, and bone. I also recommend revisiting lecture 8. There was an animation at the end of the connective tissue section where I told you to watch the beginning of it, but if you actually watch the end of it now, you'll get to see the animation that relates to cartilage and bone, and now you'll have the knowledge to go with that as well. So again, that was in lecture module 8, it was the animation near the end of the lecture. If you want to watch that animation again, it has bone and cartilage included in there. Slide 2 We're going to begin our look at bone by looking at the general functions of the skeletal system. Now when we think of the human skeleton, of course we think of the bones of the skeleton, but the skeletal system is really comprised of many different tissues working together. We have bone, we have cartilage, we have dense connective tissue, we have epithelium, we have adipose tissue, and we have nervous tissue moving blood vessels. So all of those things and all of those different tissues work together to create the functions of the skeletal system. Now of course, the first function we think of for bones is supporting the body, and in fact, if you recall they're supporting connective tissues. So bone itself is a hard and rigid structure, however, it's actually quite complex and it's dynamic. Now I mean that it's dynamic in the fact that it's not actually a static structure. It's not like you build bone and it stays that way for the rest of your life. In fact, we're constantly breaking down and rebuilding bone over the course of your lifespan, and we will talk about that a little bit more in the next lecture. But because it is hard and rigid, it has a very important role in supporting the soft tissues of the body, along with the other supporting connective tissue, cartilage. Now cartilage and bone have many similarities as we'll see today. Cartilage is basically the flexible version and a little less strong then bone, so it works together with bone to provide this supporting role. Now when we think about holding bones together, remember that dense regular collagenous connective tissue ligaments are also really important in the supporting roles because the bones wouldn't stay together without the ligaments holding them together. So the ligaments are also very important. So again, things like cartilage are going to provide support for structures like your nose. We saw a little bit of that when we talked about olfaction and I showed you the nasal cavity. We also have cartilage in our external ear, so in that auricle that we talked about with our special sense of hearing. We see it in our trachea, it basically opens or keeps open our windpipe. And we're also going to see it in joins where we have two bones coming together. If we didn't have cartilage than those bones would constantly rub up against each other. So instead we have a more flexible supporting tissue in the joint regions and that's the cartilage. It's the second strongest tissue in the body. The next role of the skeletal system is protection, and again, there's lots of different places where the skeletal system can provide protection to the soft tissues of the body. Our skull, for example, protects our brain because it completely surrounds it. We also have the ribs and the sternum and the vertebrae protecting the organs of the thoracic cavity. So essentially they're creating a bony fortress around our lungs and our heart. Will also see that in the vertebrae itself we have the spinal cord. So the vertebrae stacked on one another are going to provide protection for the spinal cord. There's lots of different ways that the bony features of the body can protect the soft structures of the body. The next function of the skeletal system is movement. Now muscles when they contract need to pull on something, and if it wasn't for bones, the muscles would have nothing to pull on, and in fact, almost all skeletal muscles in the body attach to bones, at least on one end. There are a couple of examples where we have the muscle attached to bone on one end and something else on the other end, like the eyeball that we saw in the visual lecture, but it's very rare for muscles to only attach to soft tissue, they need the bones in order for movement to occur. So they sort of tether themselves to the bone end and then pull on something else to make it move. In most cases we're actually moving across a joint, so we're moving two bones or more, relative to each other. And when we start to talk about the muscles in the body, remember this because all muscles have to cross joints and pull on bones in order for the movements to happen. If we didn't have bones, the muscles wouldn't pull on anything and movement wouldn't occur. Now there are a couple of instances where our muscles attached, just a soft tissues and some examples of that might be in your face, so they're attaching to skin. So we can make movements like the movement for pursing your lips or going to kiss someone that is simply moving soft tissues and it's not attached to bone at all. So there are some instances where the skeletal system isn't necessary for movement to occur, but for almost all other movements in the body, we need the skeletal system and those bones. Now recall that muscles attach to bones via tendons, and we're going to in the next lecture, start to look at how the outer surface of bone helped to turn into the dense, regular collagenous connective tissue that we have for ligaments and tendons as well. Now, those are kind of the common things that you think about for the skeletal system. One of the less common things that you might think about is a storage function. Now you might think, oh, our bone stores things?? But in fact it stores many minerals for release in the blood when the levels drop, especially calcium and phosphorus. So calcium is needed for many different processes in the body. For example, we've already seen it in the presynaptic terminals for all of our nervous system, we also need it for muscle contraction. Phosphorus, on the other hand, is needed for things like the production of ATP, some metabolism steps, the growth and maintenance and repair of our cells and other tissues. So these are important minerals that are largely stored in the bone and then released when we have low levels of them in the body. We can also store fat or adipose tissue in our bone. Most of this is going to be found in the center of our more hollow bones, which we'll talk a little bit more next day, but in this center region, we can store something called bone marrow. So when we have yellow bone marrow that's highly made up of adipose tissue, but we can also have another type of bone marrow, and that's red bone marrow. So when we think about the insides of our bones, we're going to have these cavities, and when we're a newborn, all of those cavities are actually filled with red bone marrow. Now the job of red bone marrow is to actually produce all the different blood cells in the body. So it can make red blood cells, white blood cells, and platelets. As we age, much of that red bone marrow will turn into yellow bone marrow or basically turn into storing adipose tissue on the inside of our bones. There are some regions where we still have the red bone marrow, so places like the ends of many of our long bones, like our arms and our legs, and many of the irregularly shaped bones in the body also contain red bone marrow. But we're going to be talking more about the different shapes of bones in the next lecture. So let's take our first pause here and we'll do some practice questions and then we'll come back and started to talk about cartilage. Video 2 Slide 3 So since most bones start as cartilage, this is a good place to start. The information that we're going to talk about on this slide can also be found in chapter 4 of your textbook, and specifically the different types of cartilage are found in Table 4.10. So when we think of cartilage as a supporting connective tissue, it's firm, it's smooth, it's resilient, it's non-vascular, and of course it's a connective tissue. Now, I'm sure you can understand what firm and smooth mean. Resilient means that if you can press this tissue, it's able to bounce back to its original shape, and we'll talk about what allows for that in a second. It's also non- vascular, so that means that it doesn't have a direct blood supply. And in fact, we're going to talk about where the blood supply is located, but it's going to act very similar to what we saw in epithelial tissue, where the blood supply surrounds the tissue and then the nutrients and gases have to diffuse through the tissue. Now cartilage itself is made up of cartilage cells and matrix. Now what makes cartilage unique is not only the cell types found within the cartilage, but what the matrix is made up of, it's actually 70 to 85% water, and because it's got so much water in the matrix, that's actually what gives cartilage its resiliency. So imagine if you had a water balloon and you stuck your fist into the water balloon, it bounces back to its original shape. The fluid within the substance allows it to be resilient. In addition to this, we're also going to have some protein fibers, of course all of our matrix has protein fibers, as well as some ground substance. Now, when we think about the ground substance, it's going to be made up of a large portion of proteoglycans. Now if you remember, proteoglycans are going to be those molecules that trap water. So in order to have this high concentration of water in the matrix, we need lots of those proteoglycans. For the protein component, we're using collagen fibers and sometimes elastic fibers for one of the types of cartilage, and this is what's going to give cartilage its strength. There are three types of cartilage, hyaline cartilage, elastic cartilage, and fibrocartilage, and it's showing each of these on the slides here. So in terms of the structure of these different cartilages, they're all going to have cartilage cells, and we'll talk about the cells on the next slide, they're all going to have matrix, what's really gonna differ between these is the amount of protein that they have and how it's organized, as well as the types of proteins in the matrix. So in the case of something like fibrocartilage, you can see that there's lots of protein fibers and they're more sort of organized compared to the other types of tissues. So what's the difference between these? So the first one, hyaline cartilage, this is the most common of the cartilages in the body, and it's often associated with bone. In fact, it's the type that we find in any of the joints of the body. So when we're talking about two bones coming together to form a joint, a movable joint, that's where we're going to have hyaline cartilage on the surface of that, and in fact, this specific type of hyaline cartilage that we find in a joint is known as articular cartilage. So again, what makes it different is the amount and orientation of the collagen fibers and ground substance. This type of cartilage is the type that we form an embryological development and will eventually be turned into our skeletons. Again in adults, it's found in joints, this is the articular cartilage. We also find it in our nose, so if you grab your nose right now, the bridge of your nose, that's hyaline cartilage and those rings of your trachea, so if you feel the front of your throat right now, you'll feel some rings there holding your windpipe open or your trachea open. Those are also made of hyaline cartilage. So again, we convert that hyaline cartilage into bone when you are developing as a fetus, but also into childhood, into adolescence until you've reached your full maturity. So we'll talk about some of the bones that actually calcify later in life, in fact, some bones don't fully calcify until you're about 40 years of age, so cartilage still plays an important role even in childhood growth up into adulthood. The next type of cartilage is the elastic cartilage, and that's the type that you can see down here in the bottom. So this is the kind that we're going to find in areas where we need a little bit more flexibility. So our auricle of our ear or our external structures of hearing. So in this case, if you take your ear and you sort of bend it, you'll notice that there's a firm structure in there, but it's much more flexible, and what's allowing for this is the fact that of the protein fibers of elastic cartilage, we're going to have a lot of elastic fibers. So that's what's going to allow for much more flexibility. Finally, the last of the cartilages is the fibrocartilage. So fibrocartilage is the strongest of all the cartilages, and that's because of the orientation and the density of the collagen fibers within it. So fibrocartilage is going to be found in areas of the body where we need to have a little bit more support. So for example, in between the bodies of our vertebrae, so this is known as our intervertebral discs. We're also going to see it as sort of a structure in some of the joints of the body like the knee. So you can't really see it too much in this picture up here, but the cartilage that lines the bone itself, that's the hyaline cartilage, but in between that, there's also a pat of cartilage and that's known as a meniscus. And that is made of fibrocartilage. So this is going to be the strongest of the cartilages and we use it in areas where we need a little more support, a little more structure, and it needs to be able to support the body's weight. So things like in between the vertebrae supporting the body's trunk, the knee joint is supporting all of the body above the knee. So that can be a place where we need a little more weight support. Now as I've already mentioned, when we are an embryo we build a cartilage skeleton. So that's what we're using in the development of our skeletal system in the first place. But we also use cartilage in repair. So when our bone is broken, it will first fill in with cartilage and then we turn that cartilage into bone. So the bone basically is developing from cartilage. Not all bone develops from cartilage, as we'll see in the next lecture, but a large proportion of bone does develop from cartilage. Slide 4 So if we look a little bit more closely at the histology of cartilage, cartilage has basically one main cell type and these are known as chondrocytes. Remember -cyte cells are the type of cell that are the mature version of the cell. So these are some examples of some chondrocytes you can see this portion of the cartilage. All of our chondrocyte cells are located in a cavity, and that cavity is known as a lacuna. So we've seen those terms before. So you can see in some of these images, you can see the little cells on the inside here, And then surrounding those cells is a space that's the lacuna. So there's a little space around each of these individual chondrocyte cells. Now the immature version of our chondrocyte cells are the chondroblast cells, and remember, -blast cells build matrix. So we can see some of those chondroblast cells up here near the surface of our cartilage. So here's an example of a chondroblast cell. Basically what the chondroblast cells do is they produce matrix, secrete that matrix around themselves, and then eventually they trap themselves within the matrix. And then once they've trap themselves within the matrix, they turn into chondrocyte cells. So all chondrocyte cells once were chondroblast cells. That's sort of the immature version. And in the case of the chondroblast cells, they're located along this edge or surface of our cartilage, so we'll talk about what this is in a second. Now again, our chondroblast cells are making matrix and a large portion of the ground substance will have proteoglycans and that's what's going to allow to have so much water in the matrix, and then the fiber component is going to be mostly collagen, but in elastic cartilage we will also see elastin as well. So this is just depicting the matrix around the different cells here. Now on the surface of all of our cartilage or most of our cartilage, we have a double layer of connective tissue. So this is going to be where the blood supply and the nerve supply are located. This is known as the perichondrium, and so this is actually a double layer. On the outside layer we have dense irregular connective tissue. So the cells that are going to be located out here actually fibroblasts, because their job is to produce those protein fibers that make up that layer. Now, the second layer that's deep towards the cartilage, that's actually a layer of cells, and that layer of cells are going to be chondroblast cells. And the other type of cell that actually precedes the condroblast cells, which are called osteochondral progenitor cells, and we'll see that in an upcoming slides. So don't worry about spelling it right now. But basically the cells that are going to produce more cartilage are located on the outside surface, just beneath this dense irregular connective tissue layer, but both of those together are what forms the perichondrium. And so in fact, our blood supply and our nervous tissue would actually be found in the outer layer of the pericardium, and the inner layer of the perichondrium is basically where we have this layer of cells that are going to help produce or make our cartilage. There are some exceptions to the perichondrium rule. Articular cartilage, for example, doesn't have a perichondrium. Now if we think about what articular cartilage is again, this is hyaline cartilage that covers over the ends of our bones that form a joint with another bone. So if you had nerve supply in blood supply in there, then you would actually be constantly rubbing nerves and blood vessels up against each other in the joint. So it doesn't make sense to actually put a perichondrium layer there, now because of this, of course then we don't have a loss of blood every time we damage our joint, but it also makes it really, really hard to repair this type of cartilage because it doesn't have a blood supply on the outside of it. In fact, the blood supply for articular cartilage is going to come from the bone side. So imagine that this is the surface side, well the perichondrium and would be missing, surface side would be here and then the bone side would be down here. So the blood vessels from the bone side are what are going to feed nutrients and gases to these tissues. Now when we think about the blood vessels on the outside in order to drop off gases and nutrients, it's going to have to diffuse through this tissue to get to all of these chondrocyte cells which need nutrients and oxygen to stay alive. Because we have a lot of water in the matrix, it's easy enough for us to move the nutrients and oxygen through, but if you damaged cartilage because it doesn't have its own direct blood supply, it does take a little bit longer to heal, and that articular cartilage is going to be even worse. The other exception to the perichondrium rule is fibrocartilage. Which again, when you think about where fibrocartilage is located in between the bodies of our vertebrae, in our knee joints, if we damage those, then they're really, really hard to fix. If anyone has ever had a meniscal tear, so that's the little pad in between your knee, in the middle of your knee, of fibrocartilage, if you've ever had a problem or torn that, essentially it doesn't get fixed, you can just fix it or heal it yourself because there's no direct blood supply. So it's really, really hard to fix those types of cartilage. Now when we think about cartilage, there's two different ways that we can grow cartilage. Now, the first one is known as appositional growth, and this is the type that I've really kind of been describing so far. So we're taking the chondroblast cells that are already found on the surface of the cartilage. Now remember chondroblast cells are going to secrete matrix around themselves and eventually trap themselves within that matrix. These chondroblast cells, after they sort of multiply so that we don't run out of them, they will secrete matrix, trap themselves and then there'll become part of the mature cartilage. So appositional growth is basically growth on the outside of the cartilage. Now again, it doesn't look like we're on the outside here, but it's growth just underneath the outside layer of the perichondrium. So imagine the perichondrium keeps sort of moving up here and we're gonna get more and more layers of cartilage in between the mature cartilage and the perichondrium. and so it's sort of just growing beneath this perichondrium with those chondroblast layers that make up the inner layer of the perichondrium. So that's appositional growth. The other type of growth is known as interstitial growth. Now interstitial growth is happening in the region where we have the mature cartilage. What happens with interstitial growth is the mature chondrocytes can divide. Now, this is unique to cartilage because the matrix has got so much water in it, it's easier for the cells to move around, and again, they also had those little lacuna that they're in, but the chondrocytes themselves can divide, and when they divide they can secrete a little bit more matrix around themselves, that matrix will push the cells away from each other, and then eventually they become independent cells. So this is known as interstitial growth or growth from within the tissue. So cartilage is quite unique because it can grow from the outside and it can grow from the inside. So when we're really young, we tend to grow our cartilage via interstitial growth, and this will continue into early adolescence or when you reach puberty. From then, we tend to grow more via appositional growth. So there are times in your life where you're going to use one form of growth over the other form, and that's really the sort of the pattern that you'll see. So interstitial growth when you're a child into adolescence, and then appositional growth as you continue into adulthood. So those are the two types of growth of cartilage, qnd we'll see that there are a lot of similarities with this to bone as well, but bone is more rigid so it's not able to do both types of growth. So let's take another pause here and try some practice questions and then we'll come back and start to talk about the histology of bone. Video 3 Slide 5 So now we're going to start to look at the histology of bone. Now you'll notice that there are some similarities between cartilage and bone. One of course they're both supporting connective tissues, so they're going to be comprised of both cells and matrix. But of course, the cell types that we have in bone are slightly different then what we have in cartilage, and the matrix is going to be different as well. So for the cell side of things, it's very similar to what we see in cartilage, our cells produce matrix around themselves, and eventually they trap themselves within that matrix. Once they're trapped within that matrix, the cell becomes the mature version of the cell, and we're wanting to looking at the different cell types in a moment. The difference between bone and cartilage is that bone is constantly being broken down and replaced over your lifespan. So in cartilage, we don't break cartilage down. We can build more or we can repair it if it's been damage, although it does take a long time, whereas in bone, we're actually breaking it down all the time and rebuilding it. So these bone cells aren't trapped forever within the matrix. So of course, the other component is the matrix. So the matrix is made up of both inorganic and organic components. Sixty-five percent of the bone matrix is inorganic and it's and basically made up of crystallized minerals salts. There are different types of mineral salts that you can find within the matrix, but the largest type of mineral salt that we find is something called hydroxyapatite. Hydroxyapatite is a form of a calcium phosphate crystals, and essentially when we talk about calcifying the bone, or mineralizing the bone, or ossifying the bone, then we're talking about adding these mineral salts into the matrix. So when we start to talk about how we make matrix and a moment, think about hydroxyapatite being added into that matrix to make it more solid or more calcified. The other component is going to be our organic component, and this is 35% of our bone matrix. This is going to be made up of proteins that we have, like collagen fibers, proteoglycans, a lot fewer then you see in cartilage because it's not as resilient, so we're only going to have a small amount of proteoglycans in bone compared to cartilage, and there is a tiny bit of water as well. So really what's different between bone and cartilage is that we've sort of taken away some of the proteoglycans and water and replaced those with the calcium phosphate crystals, hydroxyapatite and other mineral salts. Now why do we need both components? While collagen fibers and proteoglycans give bone flexible strength, whereas our mineral salts give it compressive or weight-bearing strength. Slide 6 Now to give you a nice visual of what this looks like, here's a demonstration of what you would have in a bone if you are missing either these components. So here's a bone that we have at the top, and in the first image, what we've done is we've added the bone to an acid. When you add a bone to acid, it breaks down the mineral component so that all that you're left with is the organic component, and that organic component again is going to be the proteoglycans as well as the proteins. So what happens now is our bone isn't strong anymore in terms of it can stay in one solid piece. It's quite flexible now, it will be hard to tear this apart, but it has that flexible strength so you can bend it. So that's the organic component. Now if we took this exact same bone and instead of putting it into an acid, we put it into a bunch of enzymes that break down the protein component. Then what would happen is we'd be only left with the mineral component. Now, you can see here that makes our bones very brittle. When our bones are very brittle, they can shatter quite easily. So this is why we need to have both the flexible strength as well as the compressive strength, because without the minerals we we'd have bendy bones, without the proteins, we would have shattered bones. So we actually need both components in the bone in order to have the strength that are bones need, and this is actually a good example of what's going to happen with one of our cell types that we're going to look at in a second. Slide 7 So now that we know about the matrix, we're going to talk about the different cell types that are found in bone. The first cell type is called the osteoblasts, and we talked about these before when we talked about -blast cells. Again, blast cells build matrix. Now what these osteoblast cells do is they're going to produce both the organic component and the inorganic component of the matrix. So they produce collagen fibers as well as the proteoglycans, and they package these into vesicles. These vesicles then can move towards the membrane of the osteoblast cell and via exocytosis can dump the contents out into the extracellular space. So essentially what they do is they push the proteins out and those proteins start to make sort of a framework around the cell. Now if you think about building the house, the first thing that you want to do is build the frame and then you fill in in between that frame with things like insulation and wiring, and then you cover everything over with your drywall. So that's kind of what the osteoblast cells are doing. They're building the proteins and the proteoglycans inside the solid a package them into vesicles and then they dump them out via exocytosis with the side of the cell membrane. So that sort of makes the framework around the cell. They also are going to produce the mineral component. So they form something known as matrix vesicles. So these form a little bit differently. They form via pinching off a portion of the osteoblast membrane. So in the osteoblast cell you have lots of calcium and you have lots of phosphate. So what happens is when you wrap some of the osteoblast membrane around a portion of the inside of the osteoblast is going to concentrate these calcium and phosphate molecules together to form the hydroxyapatite. So essentially you make a little vesicle, it makes the hydroxyapatite inside that vesicle when you're forcing the minerals to come together, and then those are going to sort of migrate out to our college and framework and calcify it or ossify it. So they basically fill in this space and harden by mineralizing that area. So that's kind of how the two vesicles are going to work. So once we release these vesicles, bone matrix is formed. Now as I said, osteoblast cells are building matrix around themselves, but eventually they're going to get trapped in that matrix, and that's when they're going to turn into the next cell type osteocytes. So these are the ones that are going to be responsible for ossification or calcification or mineralization of our bone. So they're forming the bone matrix. Now as I mentioned, in order for us to get osteoblast cells, they have to come from a form of stem cells. And the stem cells are called osteochondral progenitor cells. So this is actually the precursor to both osteoblasts and chondroblasts. And that's why osteoblasts and chondroblasts actually have quite a lot in common because they originate as the same stem cell. So in the example here, we have an osteochondral progenitor cell. It's going to in this example, develop into an osteoblast, but it could also differentiate into a chondroblast if we were in cartilage, and in fact in that inner layer of the perichondrium, we're going to have a bunch of these osteochondral progenitor cells, which will turn into chondroblast cells. Because remember once the condroblast cell, or osteoblast cell for that matter surround matrix around themselves, they become chondrocytes. So eventually we would run out of the blast cells. So this cell is going to allow us to continue to make blast cells throughout our lifespan. So from there, it will form an osteoblast which will make the bone matrix and that will then turn into the osteocyte once we have the matrix completely formed and now the cells trapped within that matrix. It does still maintain its ability to maintain the matrix, as we'll see on the next slide. So in the image here you can see the osteoblast cell is in a lacuna, so it's got that space around it, and you can also see these little projections coming off of the cell. We'll see those again in a second. So osteochondral progenitor cells is where we're going to get osteoblasts and chondroblasts. Slide 8 So the next cell type is the osteocytes, which I've already talked about. Really the osteocyte is the mature version of the osteoblast that's been surrounded by bone matrix. So here's our developing bone. After several weeks or months, we have newly formed bone. So you can see the cells are exactly the same, we're just changing the matrix around those cells. So there's an osteoblast cell that's the same version of the cell as it becomes an osteocyte cell. This is the bone surface and this is the matrix once it's been complete. So we're basically building the protein structures and then we're ossifying those protein structures or calcifying then. So just like we saw on the last slide, these cells become trapped within a little cavity known as a lacuna. In addition to that, remember those little extensions that were coming off of the cells. We have these connecting cell extensions. So our osteoblast cells actually connect to each other physically through these little connections. Now that's really important because if they didn't do that and they secrete matrix around themselves, they would actually separate themselves from all the neighbouring cells. So this is going to allow them to still maintain a connection to each other while they're making matrix. And in fact, what happens is when you get to the mature version, where you have the osteocyte cells, these connecting cell extensions turn into little canals that connect the cells to each other. These are known as canaliculi or a singular canaliculus. Now these are going to be very important because things like nutrients and oxygen are going to need to get to these cells, and if they are all completely embedded in matrix, then they would have no chance of surviving. So these canaliculi are going to be able to be a pathway for our nutrients and gases to get to these osteocytes cells once they're trapped in the matrix. So osteocytes cells are considered to be inactive, however, they still can help to maintain the matrix, surrounds them. So they're basic role is just maintenance of the matrix. Now, as I said, they're not trapped forever, like we see with chondrocyte cells, we have another cell type that's unique to bone that breaks bone down and that's known as the osteoclast cells. Slide 9 So osteoclast cells are large multi-nuclear cells. So these are much larger than what we see for the osteoblast cells or the osteocytes cells. These are found on the surface of bones. So the large cells are formed from the fusion of monocyte cells. So monocyte cells are a type of white blood cell, and if we take a bunch of these monocyte cells and we fuse them together, that's going to create not only a large cell, but a multi-nuclear cells, so there's many nuclei within the cell. So this is our osteoclast, and this is the nuclei within it because we've had a fusion of several monocyte cells. Now again, they're very large cells and they're found on the surface of our bone. There job is resorption of bone. Now resorption is a word for breaking the bone down. In your textbook, you'll see that it says re-absorption of bone. That's because they're talking about taking the minerals out of the bone and sending it back into the blood, but the actual term for breaking the bone down is called resorption. So how does it really work? Well, much like what we saw in the demonstration of what happens if you take the minerals out or the proteins out. That's basically what osteoclasts do. They secrete acids and those acids will dissolve the calcium and phosphorus components or the mineral component. But they also secrete enzymes that will break down the protein component. So essentially they're breaking down both the organic and the inorganic components of the bone, and they do this via this attachment to the bone itself. So in this example here, our cell has attached itself onto the bone. So in most of the time our osteoclast cells are sort of sitting dormant, but once they get activated, then what happens is they attach onto the bone via some protein attachments known as podosomes, and essentially this creates a little sealed compartment. Then a portion of the membrane of our osteoclasts becomes extended and it makes something known as a ruffled border, essentially this is just to increase the surface area of the membrane. Then it's going to start to release the acids into this sealed compartment along with enzymes that will dissolve away the protein components. All of the fragments from the bone that are broken down are then taken up into the osteoclast cell and released on the other side of the cell into the extracellular space and ultimately into the blood. Now why would we want to do this? We'll remember bone is a storage place for many minerals in the body. If our body is not getting the calcium it needs from our diet and we need more calcium than these osteoclast cells become activated and they break the bone down so that they can release the calcium into our blood supply, and that's why it's so important to make sure that you get enough nutrients and minerals in your diet so that you're not breaking down your body's tissues in order to supply them instead. So that's one reason why we break down our bone, but in addition to that, we're always breaking down our bone and remodelling it. So as our bodies change, as the activities that we do change, the osteoclasts cells will become activated, break down the bone and remodel it. And we'll really talk about how this works in the next lecture when we look at bone remodelling. Conclusion So that's what we're going to end this online lecture. And today's lecture, we looked at the functions of bone, and then we looked more closely at cartilage. We looked at the different types of cartilage, we looked at what makes up cartilage tissue, and then we looked at how cartilage grows. Then we started to look at bone tissue, and specifically we looked at the main components of bone and the cell types in bone. In the next online lecture we're going to look at the different types of bone and how bone grows. So until then, take care.