Lecture 24 - Bone Tissue PDF

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 24 - Bone Tissue Video 1 Introduction Welcome to the next online lecture. In this lecture we're continuing our look at bone tissue. So we'll start by looking at the different type...

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 24 - Bone Tissue Video 1 Introduction Welcome to the next online lecture. In this lecture we're continuing our look at bone tissue. So we'll start by looking at the different types of bones found in the body. And then we're going to start to look at how we develop those types of bone. Then we'll look at how bone grows and how we remodel bone throughout our lifespan. So let's get started. Slide 1 The material for this online lecture can be found in chapter 6 of your textbook, specifically in section 6.3 through to 6.7. Slide 2 So we just finished talking about bone tissue and the matrix and cellular components that make up bone tissue, so now we're going to move on to how we classify different types of bone tissue. So one of the ways that we classify bone tissue is how the collagen fibers that are within the bone matrix are organized. So the first type of bone tissue is woven bone. This is where the collagen fibers are randomly oriented around the cells. This is the type that we first made when we're making our skeleton as an embryo in many of the different parts of the body, as well, this is the type of bone that we make when we first repair a bone that has been broken. So this is sort of a quick and fast way of making bone. We lay out some college and fibers in no particular pattern, and then basically we ossify the areas around those fibers with that hydroxyapatite. However, woven bone doesn't stay this way and all of woven bone eventually is remodelled. So that means that those osteoclast cells are going to come in and break it down, and then the osteoblast cells will build it back up in a much more organized fashion. The more organized type of bone tissue is known as lamellar bone. And that's because now our bone is organized into sheets or layers, and these layers are known as lamellae. So lamellar bone is much more organized, and this is the type of bone that we see in all the bones of our body. Slide 3 So lamellar bone can actually be classified into two types based on the amount of matrix we have, relative to the amount of space around that matrix, as well as the actual organization of the matrix itself. The first type of lamellar bone is known as spongy bone or cancellous bone or trabecular bone. In spongy bone we have less bone matrix and much more space. So this is what gives spongy bone it's porous appearance. It's found on the inside of bones, and it completely fills spaces of bones that are irregularly shaped like our vertebrae, or flat bones like our sternum and our skull, or the ends of long bones as you can see in this image here. It will also form a thin layer in regions where we have cavities inside of bones. For example, in the shaft of a long bone, the majority of the space on the inside is actually a cavity that's filled with bone marrow, however, there is a very thin layer of spongy bone on the inside of the other type of bone known as compact bone. It makes up only 20% of our total skeleton in terms of mass, and that's actually a good thing because we need this spongy bone to provide strength to our bones without providing extra mass. All of our spongy bone is protected by a layer of compact bone, so when you look on the outside of the bone, you'll see that it's quite smooth and that's because the outside of all bones is made of compact bone. So compact bone is also known as cortical bone, is quite dense and it has very few spaces within it. So as I mentioned, it is found on the outside of all bones, and it makes up the majority of the bone tissue that's found in the shafts of long bones. This type of bone makes up a much larger proportion of the mass of our skeleton because of its density, about 80% of the total mass, but it does allow for this important strength that we need for our bones in order to support the weight of the body and withstand the force produced by the muscles. Slide 4 So the first type of bone that we're going to look at is the spongy or cancellous bone. In spongy bone, the bone matrix is organized into connecting rods or plate-like structures that are known as trabeculae. So if we blow up that porous nature of this spongy bone, this is what it would look like. All these individual little branches or rods, and those are known as trabeculae. The spaces in between the trabeculae are filled with bone marrow and blood vessels in the majority of the ends of long bones and in irregularly shaped bones that bone marrow would be red bone marrow, but we can also have yellow bone marrow as well. The trabeculae, while they appear that they're organized in a random fashion, are actually formed along lines of stress. So that allows for the less dense bone to have a lot of strength to it. So if there's a muscle pulling in a specific direction, the trabeculae line up to provide the greatest amount of strength in that line of pull of the muscle so that it is providing strength without the extra mass. If we blow up one of these cross-sections of the trabeculae, this is basically what it looks like. So remember, lamellar bone is organized into sheets or layers, so within each trabeculae you can see the individual sheets of matrix. In between each of the sheets is where the osteocytes are going to be sandwiched. So as we mentioned before, are osteocytes are found in little spaces known as lacunae, and they're connected to each other via these small canals known as the canaliculi. So if you imagine our blood vessels are on the outside of the trabeculae here, if we have nutrients or gases that need to reach these osteocytes on the insides of this lamellae or these layers of matrix, they're going to have to diffuse through all of these little canaliculi to reach the inner most osteocytes within the bone. On the outside surface of each and every trabeculae, we will have a layer of cells. So those cells will be our osteoclast cells, which we'll break down the bone when they're active, and we have very few of those relative to the amount of osteoblast cells we have. But again, these cells are making up this layer doesn't necessarily mean that they're active at any given time, so they sort of go in cycles. We will also have mixed in here some osteochondral progenitor cells to rebuild any of the osteoblast cells, if those cells have become trapped in matrix and turned into osteocytes. So while this seems like it is fairly organized in terms of its structure, compact bone is even more organized. So let's take a little break now and then we'll do some practice questions and come back and talk about compact bone. Video 2 Slide 5 So this is an image showing the differences between spongy bone and compact bone. So spongy bone again is found on the inside of the bones and looks quite a bit more porous than our compact bone, which is found on the outside of the bones. Surrounding the outsides of all bones, we have a layer of dense fibrous connective tissue, as well as that cell layer, and those together make up a layer known as the periosteum. The periosteum and the perichondrium are very similar in their structure and shape, tt's just the cell types that are found in that inner layer that differ. In cartilage they're chondroblasts and in bone they're osteoblasts and osteoclasts. When we look at bones, like the shafts of long bones, there's also space within the bone and the space is known as the medullary cavity. So in this image down at the bottom here, they've taken a little section out of this region of long bone and blown it up so that we can look more closely at the features of the compact bone. So this area over here represents the space on the inside of the bone, the medullary cavity, and then you can see that spongy or trabecular bone that we talked about on the last slide. Compact bone has very organized sheets of lamellar bone, qnd essentially that lamellar bone is organized in three different ways, and we'll talk about each of those three different types of lamellae in a moment. So on the outside of the compact bone we have that double layer are known as the periosteum, and the periosteum, as I mentioned, is quite similar to what we see in the perichondrium. There's an outer layer that's made up of dense fibrous connective tissue, and again, it has those fibroblasts in there secreting the collagen fibers that make up that dense fibrous connective tissue. And then the inner layer which your textbook labels as the osteogenic layer, is the cell layer that contains the osteoblasts and osteoclasts, as well as the osteochondral progenitor cells. So if you think back to the image that we just looked at with the trabeculae and the cell layer surrounding that. You can imagine that's exactly the same thing that's happening on this inner layer, and then surrounding those cells we have an outer layer of dense fibrous connective tissue. We also have a layer surrounding the inside of bone as well in the medullary cavity side, And this is known as the endosteum. So it's very similar to the periosteum, except it only has the cell component or the inner osteogenic layer that it's referring to out here. So it looks like what we saw on the last slide, it just has the osteoblasts and osteoclasts cells and the osteochondral progenitor cells without having that second layer that would be the fibrous dense connective tissue. You can see it's the outer layer of the periosteum where we have the blood supply to our bone that will then be able to enter or penetrate into the bone. You can see the periosteal vein and artery are just the ones that are associated with the periosteum. The blood vessels then enter into the bone through canals that run perpendicular to the length of the bone, and those are known as perforating canals or Volkmann's canals. Once they reach certain depths within the bone, they are within these very organized units of bone, and in the center of those units of bone, the blood vessels will then branch off and run in a parallel direction to the bone. These canals that form these parallel tunnels within the bone are known as central canals or Haversian canals. So that's where our blood vessels are going to be able to run along the length of the bone, but you can see they enter the perforating canals, run up and down through the central canals, and then they will continue through the bone through further perforating canals to reach other central canals. If we blow up one sort of unit of bone here, you can see there's a central canal with the blood vessels and then surrounding it are some lamellae, that form these concentric circles. The lamella that form these concentric circles are known as the concentric lamellae, and that's because they actually form a series of circles around those blood vessels that are found in that central canal. Within those concentric lamellae circles, it looks very much like what we saw with our trabecular bone. We're going to see our osteocytes cells sandwiched between layers of lamellar bone, we can see canaliculi connecting those different layers together. This organization of bone into these concentric lamellae around a central canal forms a structure within the compact bone known as an osteon, and these osteons, as you can see, repeat throughout the compact bone, such that you're getting these circular units of bone that are compressed towards each other, compacting that bone. So here again, you can see a blown-up image of the osteocyte trapped within the matrix, it's located in the lacuna and it has those canaliculi that allow for nutrients and gases that move from the blood supply to reach the osteocytes. Unlike what we saw with the spongy bone, where the blood vessels around the outside of the bone and all of our nutrients and gases diffuse towards the center of the bone. In compact bone because the blood vessels are found in the central canals, the nutrients and gases are actually moving outwords from the central canal towards the cells. So you can see how the blood gases and nutrients are going to move through the canaliculi to reach each of the layers of cells found within these concentric lamellae. The next take-up of lamellae that form the compact bone are known as the circumferential lamellae. Circumferential lamellae are basically bundling all of the osteons together within the compact bone. So imagine the circumferential lamellae is running around the outside of the entire bone, and specifically it's the outer circumferential lamellae that runs around the entire outside surface of the bone. So this would be, few layers thick, just underneath that periosteum layer. So it's wrapping around in strengthening all of those osteons that are found within that bone. We also have some inner circumferential lamellae, so that would surround the inside cavity of the bone, and it's really that inner circumferential lamellae where the trabecular or spongy bone is forming off of that sort of layer on the inside of the cavity. The third type of lamella is known as interstitial lamellae. So each of our osteons again is in a circular orientation, and when you stack a bunch of circles together, you'll notice there's some spaces in between those circles. Those spaces we want to fill with bone in order to make this bone rigid and strong, so those are filled with these very shortened pieces of lamellar bone known as the interstitial lamellae. And really what these layers of bone are are leftover remnants of osteons that had been broken down by osteoclast cells. So if you can imagine you had an osteoclast cell over here breaking down the bone, it didn't quite break the entire osteons down, but then it rebuilt a new osteon next to it, you would have some leftover remnants of those osteons, and basically that's what's making up these interstitial lamellae. So the three layers or lamellae that we find in compact bone, again are the concentric lamellae, the circumferential lamellae that run around the outside and inside of the bone, and then the interstitial lamellae that are run between the osteons to fill in those empty spaces. Slide 6 So this slide is highlighting again, a lot of the things I just said on the last slide. So the blood vessels enter into the bone and are found within the bone instead of surrounding the bone, they also surround the bone, but you can find them within compact or cortical bone, unlike what you see with spongy bone, where the blood vessels only surround the bone. The blood vessels run parallel to the length of the bone through the central or Haversian canals, and there's several layers or sheets of bone within compact or cortical bone, each with a different role. We have the concentric surrounding the central canals, the circumferential surrounding the entire bone and the interstitial filling in the gaps between the osteons. And an osteon is also referred to as a Haversian system, and this is where we have the central canal and all of its contents, including the blood vessels and all of the associated concentric lamellae, as well as the osteocytes that are sandwiched between those layers of lamellar bone. Again, the central canals are connected to the outside of the bone and to each other via these perforating or Volkmann canals, and this allows for the blood vessels to move perpendicular through the bone. And we talked about the periosteum having two layers, the outer layer being dense, fibrous connective tissue, and the inner layer being the osteogenic layer, which is basically osteoblast cells and osteoclasts cells and the osteochondral progenitor cells. On the inside of the bone we have again, a connective tissue lining of all the internal surfaces of the bone. Really this is just the cellular layer of the periosteum without the fibrous layer. So they're actually the same thing, it's just they don't have that extra fibrous layer of dense fibrous connective tissue on the, on the outside surface of the cells like we do in the periosteum. So the periosteum is on the outside of the bone and the endosteum on the inside of the bone. So let's take another pause here and then we'll come back and we'll talk about the structure of long bones as well as bone development. Video 3 Slide 7 So now that we know about the different types of bone, compact and spongy bone, let's look at how we organize these types of bone within a long bone in our body. So long bones, of course, are bones that are longer in length than they are in width. And there's some general features that are common to all long bones in the body. In this particular image we're looking at a partially section femur or thigh bone at the proximal end. So the region of the bone that's closest to the end of the bone is something known as the epiphysis. When the epiphysis is pointed towards the trunk of the body, it's known as the proximal epiphysis. But we also have an end of the bone on the other end of the bone, and this would be known as the distal epiphysis. So this is the epiphysis with that would be farther away from the trunk of the body. The shaft portion of the bone is known as the diaphysis, and then connecting the diaphysis and the epiphysis, we have regions known as the metaphysis. Surrounding the outside of our epiphysis, we have that hyaline cartilage layer known as the articular cartilage, and this is because this region is going to form an articulation or a joint with another bone. In the ends of our long bones, we have primarily spongy bone filling all of the space and within the spaces between the trabeculae, we will often have red bone marrow, although sometimes depending on which bone it is and whether we're talking about the proximal epiphysis or the distal epiphysis, it will be filled with yellow bone marrow. Forming most of the shaft of our long bone, is compact bone, as well, compact bone will surround all of the spongy bone, creating a nice strong layer of bone on the outside of our bone. As I mentioned, on the outside of all of our bones, we have this double layer known as the periosteum. On the inside of the shaft region of bones, we have a space, and this space is known as the medullary cavity. And again, in babies and children, it starts out containing red bone marrow, but as we grow into adults, that red marrow will turning to yellow marrow, which is basically adipose tissue or fat. Lining the medullary cavity, we have that single layer of cells known as the endosteum, containing our osteogeneic cells, which would be our osteoblasts and osteoclasts cells. In between the epiphysis and the diaphysis, we have this region of bone known as the epiphyseal line. In this particular image, it's showing an adult bone. So as we grow, we actually have a layer of cartilage at this portion of the bone and that allows the bone to grow in length. But when we reach adulthood and we reach our final height, this layer of cartilage gets ossified and turns into bone. So the epiphyseal plate, which is cartilage as we're growing from a baby into adulthood, gets ossified and turns into the epiphyseal line. Slide 8 So the diaphysis is the shaft region of our long bone, it's mostly compact bone where there's a sort of thin layer of spongy bone on the inside. The epiphysis is the end of the long bone and it's mostly spongy bone. And then we have the metaphysis which is between the epiphysis in the diaphysis. The epiphyseal plate is are also referred to as our growth plate and that's hyaline cartilage, and that would be present between the epiphysis in the diaphysis, and is present until our bone stops growing in length. And eventually the epiphyseal plate will become the epiphyseal line when that hyaline cartilage finally becomes ossified, and that was the line that we just saw in that last image. And again, the medullary cavity in children contains red marrow, but as we get older, that changes to yellow marrow in the long bones as well as in the skull. Some of the regions of spongy bone will keep that red marrow, for example, the proximal epiphysis of long bones as well as things like our pelvic bones. And it's in those regions that we can produce our cells for our blood as an adult. Slide 9 So next we're going to talk about bone development or basically how do we form the bone tissue in the first place? The process of forming bone tissue is known as osteogenesis or ossification. Bone formation happens in our embryo at about 8 weeks. It's because around the 8 week mark is when blood vessels are starting to invade into areas where we're going to form bones, and that's going to stimulate these cells known as the mesenchyme cells to become osteochondral progenitor cells. So mesenchyme cells are basically going to be the stem cells that will form all the different connective tissues in the body, but specifically around this 8 week mark, the blood vessels will stimulate those mesenchyme cells to become osteochondral progenitor cells. Once we get the osteochondral progenitor cells, the bone will form in one of two ways. The first way is known as intramembranous ossification. Here we get bone formation in connective tissue membranes, and this is the type of bone formation that happens in our skull bones, in our mandible or our jawbone, and some parts of our clavicle as well. So when intramembranous ossification, we start with our mesenchyme cells and they create a membrane of connective tissue that's primarily made of collagen fibers. And then the osteochondral progenitor cells will turn into osteoblast cells, and those osteoblast cells will start to ossify that membrane starting at about 8 weeks. So basically we create a framework with the collagen and then we ossify that framework using the osteoblast cells in intramembranous ossification. The other type of bone development as known as endochondral ossification. In this type of bone formation, we actually start as cartilage. So I've mentioned this before in class where most of our skeleton forms as cartilage first, and then we turn that cartilage over into bone. This is the process of endochondral ossification. Then the base of our skull formed this way, the other part of the clavicle that wasn't formed via intramembranous ossification, and basically the rest of the bones of the body are formed using this endochondral ossification method. So endochondral ossification is a little bit of a complicated process, so I'm going to simplify it. So in this case our mesenchyme cells become osteochondral progenitor cells just like I mentioned before, except instead of turning into osteoblasts, they turn into chondroblasts. And the chondroblasts then form the hyaline cartilage skeleton. So the hyaline cartilage skeleton is formed by about 8 weeks, but at this point we don't actually have any bone yet, we just have a model of our skeleton made out of hyaline cartilage. So around that 8 week mark, again, blood vessels are going to invade into the area where we have that hyaline cartilage. Specifically, they're going to enter into the perichondrium surrounding that cartilage and that stimulates the osteochondral progenitor cells to become osteoblasts. Before this, they were actually chondroblasts. So we need the invasion of the blood vessels in order to stimulate the conversion of those osteochondral progenitor cells forming chondroblasts into those osteochondral progenitor cells forming osteoblasts. Once the osteoblasts are formed in this region of the perichondrium, the perichondrium actually becomes the periosteum, and that's why the perichondrium and the periosteum are very similar in structure, because essentially the periosteum started as perichondrium, the only difference is the osteochondral progenitor cells are no longer making chondroblasts, they're now making osteoblasts. So now the osteoblasts will invade into the area where the cartilage has been formed and remodel that into lamellar bone. Once the bones are formed you can't actually tell which way the bone was created, so there isn't really any difference in the structure of these bones once they're formed. Both of these methods produce woven bone first, and then that woven bone gets remodelled into either spongy bone or compact bone. So let's take another pause here and then we'll come back and talk about bone growth and bone remodeling. Video 4 Slide 10 So once we have our bone, we need to be able to change the shape of that bone as we grow. So then the next thing that we're going to talk about is bone growth. Bone, unlike cartilage, can't undergo interstitial growth, and that's because the matrix is solid and you can't have osteocytes cells dividing and pushing each other away from each other because that matrix is too rigid. So the only type of bone growth that bone can actually undergo is appositional growth. So we get the formation of bone on the surface of old bone. However, we can take advantage of the interstitial growth that happens within cartilage, and that happens at the cartilage layer that I mentioned before, known as the epiphyseal plate, and we'll talk about that more in a second. So bone growth occurs in two ways. We have bone growth in length and we have bone growth in thickness, and there are actually different mechanisms for growing in length compared to growing in thickness. So we're going to look at each of those now. Slide 11 First we're going to look at growth in bone length, so in this image here we can see a radiograph or an X- ray of the epiphyseal plate. So down here we have the tibia, which will form part of the leg of the individual. And then up here we have the femur, which is the thigh bone. And in between the diaphysis and the epiphysis, we have the epiphyseal plate which is made of hyaline cartilage. So if we take a section and blow this up to look at what the cartilage looks like in this region, this is what we would see. It forms several layers and there's different activity of the cells going on within each of these layers that allows for growth in length. So this represents the epiphyseal side,so this is the side attached to the epiphysis. And then this is representing the diaphyseal side or the side that is attached to the diaphysis. Essentially the bone is forming on the diaphyseal side and the cartilage remains on the epiphyseal side. The region of cartilage that is closest to the epiphyseal is known as the zone of resting cartilage. So in this region of zone of resting cartilage, we have very slowly dividing chondrocytes, so there are undergoing interstitial growth, but it's very, very slow, and it's considered at rest because it provides an anchor plate for the epiphysis so that the epiphysis stays connected to the diaphysis. The next region is known as the zone of proliferating cartilage. Here we have rapidly dividing chondrocytes, so they're undergoing interstitial growth, and they actually divide into stacks that look very much like stacks of coins. So these newer cells are on the resting side and the older cells wind up being closer to the next zone, which is known as the zone of hypertrophy. And here are chondrocytes are enlarging and maturing, and they actually start to secrete matrix vesicles that contain hydroxyapatite. And what happens is they start to calcify the matrix in the zone next to the zone of hypertrophic cartilage, known as the zone of calcified cartilage. So here we get a calcified cartilage layer formed and the chondrocytes in this layer actually die off. Blood vessels as well as osteoblasts invade into this area from that layer of endosteum that lines the inside of the medullary cavity within the diaphysis. So then these osteoblast cells will come in, start to develop bone on the side of the diaphysis. So we maintain these zones as our individual bone is growing in length, but as we reach our genetic potential for height, eventually these cartilage layers will all become ossified and our epiphyseal plate will become the epiphyseal line. Slide 12 So we can see how these layers move in this particular image. So here we can see the zone of resting cartilage, which is that anchor for our epiphysis, and underneath that we have the zone of proliferating cartilage where we have rapidly dividing cartilage cells, and then hypertrophic cartilage where these cells mature and enlarge, and then eventually will become calcified as they start secreting hydroxyapatite. And that calcified cartilage will eventually become bone as our osteoblasts invade from the medullary cavity. So you can see now how the zone that was once calcified cartilage is now replaced by bone and it moves upwards as our zone of resting cartilage is moving up or getting more proliferating cartilage below it, and it continues in an upwards fashion as these layers move up and grow the bone in length. Slide 13 So for growing the bone in length, we're really taking advantage of that interstitial growth that happens within the cartilage, and that's allowing us to grow the bone sort of within the bone at the ends of the long bones in the region of the epiphysis and diaphysis. But when we want to grow the bone in thickness, we have to use oppositional growth. So in this case, what happens is we have our periosteum layer that's surrounding the outside of our compact bone. And the osteoblast cells that are found beneath the outer layer of the periosteum begin to lay down matrix or form bone. And they tend to do this around regions where we have a periosteal blood vessel. So what happens is they begin to lay down the bone and the bone begins to form some ridges around that blood vessel. Eventually the ridges create a groove in which the blood vessel is sitting. When the ridges meet, they form a tunnel. What happens then is the periosteum basically gets pinched off and the layer that's left on the inside becomes the endosteum. Now what happens is the osteoblasts that are part of the endosteum start to fill in the concentric lamellae towards the blood vessel. So we pinch this off and now our osteoblast or working in a direction that it's moving towards the blood vessel, forming those layers of concentric lamellae that will surround that blood vessel. So as the osteoblasts fill in the space around this blood vessel, they're making new layers of concentric lamellae, and eventually what they create is a new osteon. Within that new osteon, eventually we get that central or Haversian canal where our blood vessels are running parallel to our bone. The periosteum on the outside of the bone remains and the osteoblast cells that are part of the periosteum are continually laying down bone on the outside as well, except in this case, instead of creating new osteons, they're creating this circumferential lamellae that are surrounding all of those osteons. So as long as the osteoblast cells are activated or stimulated to grow the bone, it will continue this process of building ridges, building osteons, and then surrounding those osteons with circumferential lamellae. Slide 14 So we've talked about how the bone grows in length and we've talked about how it grows in width, but if you can imagine if you were an infant and you only grew your bone in length and you only grew in width our bones would get very, very thick. So instead of just growing in length and width, we actually have to undergo the process of bone remodelling, and as I mentioned last, a bone remodelling is a continuous function of the osteoblast and osteoclast activity. Basically the osteoclasts will break the bone down and the osteoblasts will build the bone back up. So when we're a child and we are growing, we actually need our bones to change in size. So what happens is we need to increase the size and the medullary cavity. Otherwise, our bone would just get very, very thick and very long and will become very heavy. So what happens is the inside of the bone gets destroyed by the osteoclast cells, while the outside of the bone is being formed by the osteoblast cells. So you can see how the medullary cavity actually gets quite a bit larger, but the thickness of the bone stays relatively constant. It does get a little bit thicker and it will depend on the individual genetics as well as physical activity and things along those lines as well. But eventually our osteoclast cell will continue to remodel the inside of the bone, creating the trabecular bone that we find on the inside as well as that space, the medullary cavity and the outside of the bone will become that compact bone that's being formed by the osteoblast in the periosteum layer. So in addition to the growth that happens at the epiphyseal plate, there's also similar growth going on underneath the cartilage that's covering over the epiphysis. So we're getting growth of the epiphysis as well. The difference is that as you get to adulthood, that articular cartilage will remain and the rest of the cartilage becomes completely ossified. In addition to that, you can see highlighted on here that epiphyseal line that's been calcified from the epiphyseal plate. And you can also see how the spongy bone has filled in into the epiphysis and gotten more dense in the adult bone. So once we reach adulthood, we have our more mature structure of our bone, but it doesn't mean that this bone remodelling process ends. Basically, our bone is constantly being remodelled, and it takes about ten years, but by ten years our entire skeleton will have been broken down and rebuilt by osteoblasts and osteoclasts cells will also will stimulate bone to undergo remodelling during times of stress on the bone, or if we had a fracture, if we have changes in the minerals in our body, we may need to break some of the bone down, and then things like exercise can also stimulate the osteoblast cells to build bone. Slide 15 In last image I just wanted to show you an example of this. So this is an individual there, left and the right arms. The right arm is their dominant arm and their left arm is their non-dominant arm. They are an elite athlete involved in racket sports. So you can imagine in an individual that's involved in racket sports uses their dominant are more predominantly then their non-dominant arm. In the average individual, we see about 10% difference between our dominant and non dominant arms, but in this individual we have quite a dramatic difference in our bone density. You can see here there's much smaller bone in their left forearm than there is in the right forarm. And essentially that's because as the muscle pulls on the bone, it's stimulating that bone to grow back stronger so that it can withstand greater forces. So that exercise model will actually stimulate the osteoblast cells to build the bone more strongly and more densely so that it can withstand that greater muscle force that's constantly happening on that bone. So that's just an interesting example for you. So let's take another pause here, we'll first watch an animation of bone growth in width. And then we'll do some more practice questions. Animation No caption file Video 5 Conclusion So that's what we're going to end this online lecture. So today we began by looking at woven bone versus lamellar bone, and then we looked more closely at the types of lamellar bone, spongy bone, and compact bone. Then we began to look at how we develop bone and how we grow bone in both length and width. And finally, we ended with the remodelling process. So how we break bone down and build it back up over our lifespan. In the next online lecture, we're going to start to look at the bones of the body by looking at the skull. So until then, take care.