Understanding the Skeletal System and Cartilage

Hello everyone. So we are breaking up chapter six into two parts. This first part will focus just on a very quick introduction to the skeletal system along with cartilage. Cartilage and an understanding of cartilage will be really important for when it comes to bone because a specific type of bone formation relies on cartilage being laid down first as a framework. That will make more sense once we get through the bone lecture. But for today let's just focus on cartilage, how it grows, and the components of cartilage. So as far as the skeletal system, notice that it is in fact a system. When we talk about the skeleton, it's not just the bone, but we're also going to have all kinds of other components. We have cartilage, which will help us to make up our articulations. We will learn that articulations are the joints where bones meet within the body. We also have within the bone, we have various stem cells that will allow for different cells we have throughout the body such as leukocytes and erythrocytes. We have nervous tissue. So there is all kinds of different organs and tissues that come together to make this skeletal system. We are actually not born with all of the skeleton that we're going to have in our lifetime. For instance, the soft spots that you find on a baby's skull are actually hyaline cartilage that will eventually be replaced by bone to make strong articulations. You don't want those bones that make up the skull to shift at all or to move. But again, when we're born, we have what are called fontanelles or fontanelles. Either pronunciation is fine. Those are the soft spots that you find on a fetal skull. Our skeleton is always changing, responding to stresses that we oppose upon it during our life. The distal end of your femur, so closest to where your knee is. That actually gets replaced about every six months in a normal human skeleton because there's so much stress that occurs there. Where other bones that don't experience as much stress, maybe we only replace them or parts of them once in our lifetime. So the skeleton is ever evolving and ever changing throughout our life. So we will focus on cartilage first. We know that the cartilage is part of the skeletal system and one of the things that we can associate with cartilage right away is being part of articulations, which are joints. We already know that cartilage has three main flavors, or there are three different types of cartilage that we find within the body. The first is hyaline cartilage. Remember that that has a ground substance that looks a little bit like a watercolor painting, so very light, not a lot of fibers there. We have elastic cartilage. which is characterized by, we'll just leave elastic there, which is characterized by the elastic fibers which stain nice and dark purple, very thin. And then we have our fibrocartilage. Lots and lots of collagen fibers in this which provide it with lots of strength, which ties right into its function. So while we know these different types of cartilage and we can identify them visually, We also know some of the cells that contribute to cartilage. For instance, we should automatically associate at this point in our understanding chondrocytes and lacuna. However, what we're going to learn now is that chondrocytes are technically cells that have made cartilage, but are not making it actively. We're going to layer on a couple different names in addition to chondrocytes. One of the other types of cell names you're going to find are chondroblasts. What I'd like you to associate in your mind is that chondroblasts are actively making the cartilage. So making the cartilage. And we will associate blast with many cells as they are actively making something. For instance, later we'll see osteoblasts. Osteoblasts are actively making bone. But what you previously knew is maybe just osteocytes. So chondroblasts actively making cartilage. Chondrocytes are cartilage cells that are not actively making cartilage. So imagine they're just resident cells hanging out after they've done their job. They're kind of lazy, just hanging out until they may need to be used again to make more cartilage. So not actively making cartilage. Okay, so again, when we think about cartilage and now maybe more specifically comparing it to bone. We know that it is connective tissue. We also know that the ground substance is semi-rigid, and the rigidity of this ground substance really depends on the type of cartilage we're talking about. But where does most of the rigidity come from? Most of the rigidity or strength that's going to be found within a specific type of cartilage is actually going to be due to the fibers that are within that cartilage. So a lot of the, for instance, flexibility and elasticity we have of elastic cartilage, which you're seeing here, that comes from the elastic fibers that are housed within this cartilage. Same thing with fibrocartilage. We find that a lot of that strength comes from the collagen fibers that are within the fibrocartilage. So cartilage is not quite as hard as bone, but That allows for flexibility, which depending on, again, the type of cartilage and its function is really what we're going for. We want to have a little bit of give or flexibility where these cartilages are found within the body. Another thing that we find with cartilage is that it's avascular. So just like our epithelial tissue, we really rely on diffusion into the cells to provide us the nutrients that we need. Just like epithelial tissue doesn't have vascularity, In that case for protection, same thing here. Cartilage goes through quite a bit of stress, and so we do not house vessels here, but rather we rely on diffusion into the ground substance and then into the cells to provide these chondrocytes and chondroblasts with the nutrients they need. We are going to find that once that diffusion is not available to these cells, such as when we make bone from a cartilage framework, those cells die. So it's essential that we have diffusion available to us through that ground substance to get the nutrients to these cells that they need. Now I just mentioned to you in the previous slide, we're going to add one more cell on top of our understanding so far with cartilage. We have chondroblasts and chondrocytes. What I want you to understand is that chondroblasts and chondrocytes are the same cells. A chondrocyte, if it's resting and not actively making cartilage, can be sent signals to start making cartilage. And in that case, it becomes a chondroblast because it's actively making cartilage. Vice versa, if a chondroblast surrounds itself with cartilage and is essentially stuck in its lacuna and no longer needs to make cartilage, a chondroblast becomes a chondrocyte. So please note it's the same cell, just being given a different name depending on whether it's actively making cartilage or not. Whether or not it is making cartilage or not, These cells will find themselves eventually into a lacuna. Now when I say actively making cartilage or not, chondroblasts are actually building cartilage all around themselves and eventually trap themselves in a lacuna. The E here tells us plural. So lacuna with an E is plural. Lacuna without an E is singular. Once a chondroblast has trapped itself in a lacuna, it no longer makes that cartilage and so it becomes a chondrocyte. So the lacuna is kind of like a little house. It's where the chondroblast essentially gets itself stuck. And we're going to learn a little bit more and understand that a little bit more in just a second when we talk about cartilage growth. But characteristic, as we know now from lab, of cartilage is that lacuna that surrounds the chondrocytes. The chondroblasts are actually the ones that made that house, and we'll talk about that more in a second. As we know, there are three different types of cartilage. There is hyalin, fibrocartilage, and elastic cartilage. So we're going to talk about these individual types in a little bit more detail than we previously covered now. The first one is hyalin cartilage. Again, you probably have the familiarity here with its histology. You know that there is a lot of ground substance around the chondrocytes within lacuna and that ground substance gets stains that light color. Not a lot of fibers here. Because there's not a lot of fibers here, such as collagen to provide strength, hyaline cartilage is the weakest of our different cartilage types. Where would that be valuable? Well, we're going to find that cartilage, and very specifically hyaline cartilage, it is going to serve as a template for bone in a lot of different parts of our body. Here are those fontanelles or fontanellies that I talked about previously. There's actually several, two of which you can't see in this image, but this is a fetal skeleton or a fetal skull, and you can see those different little soft spots. Once the brain comes to full size, there's no longer need to have these gaps within the cranial bones, and so we'll fill those in with bone. But you want a little bit of flexibility as the brain grows, specifically from being a fetus and into a young child. And so in the meantime, you will have hyaline cartilage there available. Now again, this provides flexibility so that the brain can grow as the cranial bones expand to allow for that brain size to expand as well. But maybe you've heard before you really don't want to press on these soft spots on a baby's head because they are quite fragile. In addition to finding hyaline cartilage in the fetal skeleton, we also actually have hyaline cartilage at the end of all of our articulating bones. As a reminder, articulating bones are bones that touch other bones. So an articulation is a joint. For instance, if we are talking about our femur, you have your fibula and your tibia, which are going to articulate with that. Now at those edges, where they meet, you will have them surrounded by hyaline cartilage. You may have seen this, or have commonly, most commonly visualized it on chicken bones. At the very edge, it's kind of almost a little bit softer than the bone. If you've ever accidentally tried to munch on it, it's kind of gross, but at the end of that chicken bone, you would see hyaline cartilage. What is the hyaline cartilage doing there? It's preventing friction between the two bones, so we're minimizing the damage of the bone itself, which is more brittle than the cartilage. We also have hyaline cartilage supporting our trachea. We need some flexibility here. If you're not familiar, the trachea is the windpipe. This is going to be our passageway down to our lungs, and we need to make sure that we keep our windpipe open. Just posterior to the trachea is our esophagus, which is essentially the passageway for our food down into our stomach. Because they're in such close proximity to one another, when you swallow and that food stuff goes down the esophagus, if we didn't have hyaline cartilage to support the windpipe and to keep it open, it would collapse every time we swallowed food. So the hyaline cartilage provides flexibility but support to the trachea. Our larynx is essentially the housing for our vocal cords, and so hyaline cartilage there protects our voice box. And then also as you know our nose, we have a little bit of flexibility and give there. It's not quite bend and snap but we have that movement of our nose which thankfully doesn't break all the time as long as we don't hit it too hard. That flexibility we have in our nose is thanks to hyaline cartilage. Fibrocartilage as you know has a bunch of fibers in it and those fibers specifically are collagen fibers. Hopefully now you associate collagen with strength. So the fibrocartilage is going to be very, very sturdy. When we talk about that sturdiness and the function that that strength provides, we are going to find fibrocartilage in areas of the body where we experience great stress. And most of the time we're going to see it kind of like a little cartilage pad. So one example of what that might look like, where we find fibrocartilage is in an intervertebral disc. Intervertebral disc is this little cartilage pad. that is between the bodies of two vertebrae. So this is one vertebrae here and here is another vertebrae right here. When I say vertebrae, technically that is plural. So vertebrae is plural. Vertebra is singular. So vertebra and vertebra. I can also tell you right now, even though we haven't gotten here yet, we'll get a chance to focus on this in lab, these kind of like look like moose. To me, this looks like a moose and this looks like a moose to me from the side. So I know these to be lumbar vertebrae. Now imagine where your lumbar is. You know your lumbar region now. And all the stress that you accrue from standing up tall, from sitting, there's a lot of stress between those vertebrae. And so this fibrocartilage is there to help withstand that stress. Now because there is so much stress, usually in cartilage we have an outer covering called a perichondrium. Peri means periphery, chondral means cartilage, and so perichondrium is an outer covering that most cartilage has, which is dense irregular connective tissue. One of the ways that you can compare and contrast the different types of cartilage is specifically fibrocartilage does not have a perichondrium. Why does it not have a perichondrium? Because it undergoes so much stress, if we did have a perichondrium, it would constantly rip and tear. So there is no perichondrium that is formed on the outside of fibrocartilage. With that said, we will see that there is a perichondrium on hyaline cartilage, so you can associate perichondrium with hyaline cartilage, and we will find that our elastic cartilage has a perichondrium as well, but fibrocartilage does not. So where can we find fibrocartilage? We'll find it in the intervertebral discs, like I told you just now. Also, the pubic symphysis, so our hip bones where they come together anteriorly, there is another little pad of cartilage there, specifically fibrocartilage, that's our pubic symphysis, and the menisci of the knee. P is plural for meniscus. And so really what we're seeing here is it's all about shock absorption and dealing with stress and compression in these areas of the body. Our third one is elastic cartilage. Go ahead and write yourself a note now that elastic cartilage also has a perichondrium. We can recognize the elastic cartilage by the elastic fibers which help to provide for its function of elasticity or flexibility. Really what we get with elastic cartilage is that bend and snap kind of function. So you can do this with your ear if you bend it forward and then let it snap back into place you can really see that. The external ear, also known as the auricle, that's one place we find elastic cartilage, but the other place is the epiglottis. We actually share some of the passageway between our mouth and our nose, making our way down before we get to either the trachea or to the esophagus. This shared passageway is called the pharynx, and depending on where it's housed or what part that is posterior to, We can have the nasopharynx or the oropharynx or the laryngopharynx, but we have the pharynx that we share, and again, that shares air and food. At some point, we need to be able to diverge between going down the trachea to our lungs or down the esophagus to our stomach. What's going to help us to keep food from going down the trachea and into our lungs? That's going to be the epiglottis. We will get to see it. It almost looks like a little tongue feature, but that bend and snap, that allows for us to divert food and make sure it goes the right way, that is the epiglottis. So as far as the functions of the cartilage, what we are doing with them overall is supporting different soft tissues. So you've already seen, or I've mentioned to you the trachea. So here's the trachea here. And all of the cartilage are these little blue rings. So that's tracheal cartilage. So the trachea itself is soft tissue, but we support it and give it structure with the tracheal cartilage. And more specifically now, hopefully you can associate that tracheal cartilage with being hyaline cartilage. So we support it throughout the respiratory system. We decrease in the amount of cartilage we have as we make our way towards the lungs and into the lungs. We'll talk more about that a little bit later when we get to the respiratory system. But also in our ear. So our ear isn't going to be just elastic cartilage, even though we have that bend and snap. You have adipose tissue. You have areolar connective tissue. Of course, epithelial tissue, right? It's layered with skin. But we support those soft tissues with the cartilage there. We also have the cartilage available to us for articulations. Again, those are going to be joints. Over here, we can see our first example of some of the joints we're going to be talking about. Just to give you an idea, we have our clavicle bone right here. This is our scapula, and this is part of the scapula coming up here. And then this is the very... most proximal part of our humerus, so this would be within our arm. The glenohumeral joint is named by, first you see humeral, that's named for the humerus. The gleno is associated with a specific feature of the scapula called the glenoid fossa. That's where the head of the humerus fits. So the glenoid fossa of the scapula and the humerus, where they articulate or come together, is called the glenohumeral joint. In addition to this major joint within your shoulder, you actually have another one. Here it's called the AC joint, but this one is called, more specifically, the acromioclavicular joint. Clavicular you have for the clavicle, and the acromion process of the scapula is where these two bones are meeting. So you get often, if you know the features of the bones, that helps you to memorize the articulation names as well. We will talk more about articulations in detail in lab. In addition, I've mentioned that we can use cartilage, and we'll find that it's specifically hyaline cartilage, as a precursor or framework for some of our bone growth. Not all of the bone growth, but some. So these are the three functions we associate with cartilage. Now, how do we grow the cartilage first before we make the bone? So if we ever need to make cartilage, There are two types of cartilage growth that we can have. We can have interstitial growth of cartilage and we can have appositional growth of cartilage. What I want you to notice right now is I associate the I with being long or tall. So interstitial growth is growth in length. So interstitial growth is growth in length. I think of the A as being really wide or I think of like a stout little apple. So wide, that A is being wide, appositional growth is growth in width. So that's how we can first distinguish the two types. Next we are going to go over the specific steps in both interstitial growth and appositional growth of cartilage. So first we'll talk about interstitial growth. One of the things that I do want you to note is that the same cartilage can go through both interstitial and appositional growth. So I don't want you to think that this is different types of cartilage that do one type of growth versus the other. So I'm going to try my best to kind of give you a 3D representation. Here's cartilage. And so the very first type that we're talking about is going to be growing in length. So that means then that if this is going towards maybe the proximal part of the body and this is distal, we would be growing this direction or this direction. In width, we would go this direction or this direction. So let me just first focus on the interstitial growth. Again, growth in length. In order to do this, we are going to see, again, at proximal to distal, chondrocytes, as we know, they are not actively making cartilage. So say that these chondrocytes that are housed within the cartilage, they're in their little lacuna. Okay, so here's my little chondrocytes and lacuna. They receive a signal to go through mitosis, to make more cartilage. So each one of those then is going to go through mitosis, and hopefully now we have a good level of comfort with what mitosis looks like and all of the different steps. And these are then going to have two little cells within. And I should have started bigger, so let me make them bigger. There we go. So each one, these are each chondrocytes. They're going to go through mitosis, and as a result of mitosis, we now have two of these chondrocytes within the single lacuna. Now, what that means then is that these chondrocytes are sharing a lacuna. So that's this Rumi step. So two chondrocytes after mitosis are sharing a lacuna. Now, these chondrocytes are going to start making... cartilage. And when they do that, they become chondroblasts. So chondroblasts start laying down cartilage. When chondroblasts lay down cartilage, they lay it down all around them. So what you're seeing here is they are sharing a lacuna, and each one of them now is going to start to build it around itself. So this particular cell right here, this chondroblast, is going to be building cartilage around itself. And this little chondroblast right here is building cartilage all around itself. And in doing so, they start to fill in the gaps around the periphery of them, but also in between them. and so in doing so the chondroblasts lay down cartilage including in between one another and when they do that they essentially then house themselves in two separate lacuna so the chondroblasts separate And after they lay down all of the cartilage that they can, they get stuck, they get trapped. And the signals that they receive from being stuck or trapped in their cartilage that they've made tell them, okay, we're done making cartilage, we can go back to rusting. So after the chondroblasts have separated, the chondroblasts then become chondrocytes once more. So we've now essentially separated these into separate ones and in doing so we've increased the length because remember they built all around themselves. So let me again walk you through the process. Chondrocytes go through mitosis and become two chondroblasts sharing a lacuna. They're chondroblasts because they're now going to be actively making cartilage. The chondroblasts lay down that cartilage all around themselves and do so until they increase the cartilage length, so interstitially, and once they're separated into their separate lacuna, they stop making cartilage and become chondrocytes. That is essentially what's happening in interstitial growth. Now remember, oppositional growth is growth in width. So I'm going to try and draw this same little one, but we're going to focus on the periphery here. So the edges do have growth in width. So one major difference... is that where we just had previously with interstitial growth, just chondrocytes received signals to go through mitosis and then became active chondroblasts. Here instead with appositional growth, we're actually starting with stem cells or mesenchymal cells. And where are these mesenchymal cells housed or stem cells housed? So they're on the periphery. So I'm going to just give a slightly different color here. Let's see if this doesn't get too annoying. I want you to imagine all along the periphery. Along the width, you can't see it on the other side, but you have them there too, you have stem cells. Well, what are these stem cells going to do? They're going to receive signals to divide. So these stem cells are going to go through mitosis, okay? And on either side, so here you're seeing them go through division and make their chondroblasts. Here I'm going to show you this stem cell is going to go through mitosis. This stem cell is going to go through mitosis. Remember that stem cells can divide to make more stem cells. or the cells that result from the mitosis can differentiate to be a specific type. In this case, we are making ourselves chondroblasts, so they are destined to specifically make cartilage. So these are our chondroblasts. So stem cells go through mitosis and the resulting daughter cells differentiate into chondroblasts. So again, you would see that on both sides as we are growing in width. Chondroblasts lay down cartilage. So they lay down cartilage or produce cartilage. And in doing so, remember, as chondroblasts lay down cartilage, they build it all around themselves. So they're building cartilage all around themselves. Okay. They continue to do this again until they build all around themselves and get themselves stuck in the lacuna. And so again, they... The major difference here is that they resulted from stem cells or came through the process of stem cells dividing. But they get trapped in their lacuna. And then those chondroblasts, when they are no longer making cartilage, become chondrocytes. So that is essentially the major difference. So please note interstitial versus appositional. Interstitial is growth in length. Appositional is growth in width. Interstitial is the division and mitosis of chondrocytes to become actively to be active chondroblasts and they are going to divide so that we can increase the length from proximal to distal. For appositional Instead, we are utilizing stem cells. Those stem cells go through mitosis, divide towards the outside or to the periphery, and those daughter cells differentiate into chondroblasts, which lay down cartilage, get stuck in their lacuna, and then become chondrocytes. So that is it for the cartilage. We are going to rely on this growth when it comes to specific types of bone. And so you'll want to be able to not only understand these steps but remind yourself of these steps when we talk about bone growth as well. If you have any questions, feel free to reach out.

But for today let's just focus on cartilage, how it grows, and the components of cartilage. So as far as the skeletal system, notice that it is in fact a system. When we talk about the skeleton, it's not just the bone, but we're also going to have all kinds of other components.

We have cartilage, which will help us to make up our articulations. We will learn that articulations are the joints where bones meet within the body. We also have within the bone, we have various stem cells that will allow for different cells we have throughout the body such as leukocytes and erythrocytes.

We have nervous tissue. So there is all kinds of different organs and tissues that come together to make this skeletal system. We are actually not born with all of the skeleton that we're going to have in our lifetime.

For instance, the soft spots that you find on a baby's skull are actually hyaline cartilage that will eventually be replaced by bone to make strong articulations. You don't want those bones that make up the skull to shift at all or to move. But again, when we're born, we have what are called fontanelles or fontanelles.

Either pronunciation is fine. Those are the soft spots that you find on a fetal skull. Our skeleton is always changing, responding to stresses that we oppose upon it during our life.

The distal end of your femur, so closest to where your knee is. That actually gets replaced about every six months in a normal human skeleton because there's so much stress that occurs there. Where other bones that don't experience as much stress, maybe we only replace them or parts of them once in our lifetime. So the skeleton is ever evolving and ever changing throughout our life.

So we will focus on cartilage first. We know that the cartilage is part of the skeletal system and one of the things that we can associate with cartilage right away is being part of articulations, which are joints. We already know that cartilage has three main flavors, or there are three different types of cartilage that we find within the body.

The first is hyaline cartilage. Remember that that has a ground substance that looks a little bit like a watercolor painting, so very light, not a lot of fibers there. We have elastic cartilage.

which is characterized by, we'll just leave elastic there, which is characterized by the elastic fibers which stain nice and dark purple, very thin. And then we have our fibrocartilage. Lots and lots of collagen fibers in this which provide it with lots of strength, which ties right into its function. So while we know these different types of cartilage and we can identify them visually, We also know some of the cells that contribute to cartilage. For instance, we should automatically associate at this point in our understanding chondrocytes and lacuna.

However, what we're going to learn now is that chondrocytes are technically cells that have made cartilage, but are not making it actively. We're going to layer on a couple different names in addition to chondrocytes. One of the other types of cell names you're going to find are chondroblasts. What I'd like you to associate in your mind is that chondroblasts are actively making the cartilage. So making the cartilage.

And we will associate blast with many cells as they are actively making something. For instance, later we'll see osteoblasts. Osteoblasts are actively making bone.

But what you previously knew is maybe just osteocytes. So chondroblasts actively making cartilage. Chondrocytes are cartilage cells that are not actively making cartilage.

So imagine they're just resident cells hanging out after they've done their job. They're kind of lazy, just hanging out until they may need to be used again to make more cartilage. So not actively making cartilage. Okay, so again, when we think about cartilage and now maybe more specifically comparing it to bone.

We know that it is connective tissue. We also know that the ground substance is semi-rigid, and the rigidity of this ground substance really depends on the type of cartilage we're talking about. But where does most of the rigidity come from? Most of the rigidity or strength that's going to be found within a specific type of cartilage is actually going to be due to the fibers that are within that cartilage. So a lot of the, for instance, flexibility and elasticity we have of elastic cartilage, which you're seeing here, that comes from the elastic fibers that are housed within this cartilage.

Same thing with fibrocartilage. We find that a lot of that strength comes from the collagen fibers that are within the fibrocartilage. So cartilage is not quite as hard as bone, but That allows for flexibility, which depending on, again, the type of cartilage and its function is really what we're going for.

We want to have a little bit of give or flexibility where these cartilages are found within the body. Another thing that we find with cartilage is that it's avascular. So just like our epithelial tissue, we really rely on diffusion into the cells to provide us the nutrients that we need. Just like epithelial tissue doesn't have vascularity, In that case for protection, same thing here. Cartilage goes through quite a bit of stress, and so we do not house vessels here, but rather we rely on diffusion into the ground substance and then into the cells to provide these chondrocytes and chondroblasts with the nutrients they need.

We are going to find that once that diffusion is not available to these cells, such as when we make bone from a cartilage framework, those cells die. So it's essential that we have diffusion available to us through that ground substance to get the nutrients to these cells that they need. Now I just mentioned to you in the previous slide, we're going to add one more cell on top of our understanding so far with cartilage. We have chondroblasts and chondrocytes. What I want you to understand is that chondroblasts and chondrocytes are the same cells.

A chondrocyte, if it's resting and not actively making cartilage, can be sent signals to start making cartilage. And in that case, it becomes a chondroblast because it's actively making cartilage. Vice versa, if a chondroblast surrounds itself with cartilage and is essentially stuck in its lacuna and no longer needs to make cartilage, a chondroblast becomes a chondrocyte.

So please note it's the same cell, just being given a different name depending on whether it's actively making cartilage or not. Whether or not it is making cartilage or not, These cells will find themselves eventually into a lacuna. Now when I say actively making cartilage or not, chondroblasts are actually building cartilage all around themselves and eventually trap themselves in a lacuna.

The E here tells us plural. So lacuna with an E is plural. Lacuna without an E is singular.

Once a chondroblast has trapped itself in a lacuna, it no longer makes that cartilage and so it becomes a chondrocyte. So the lacuna is kind of like a little house. It's where the chondroblast essentially gets itself stuck.

And we're going to learn a little bit more and understand that a little bit more in just a second when we talk about cartilage growth. But characteristic, as we know now from lab, of cartilage is that lacuna that surrounds the chondrocytes. The chondroblasts are actually the ones that made that house, and we'll talk about that more in a second.

As we know, there are three different types of cartilage. There is hyalin, fibrocartilage, and elastic cartilage. So we're going to talk about these individual types in a little bit more detail than we previously covered now. The first one is hyalin cartilage.

Again, you probably have the familiarity here with its histology. You know that there is a lot of ground substance around the chondrocytes within lacuna and that ground substance gets stains that light color. Not a lot of fibers here. Because there's not a lot of fibers here, such as collagen to provide strength, hyaline cartilage is the weakest of our different cartilage types. Where would that be valuable?

Well, we're going to find that cartilage, and very specifically hyaline cartilage, it is going to serve as a template for bone in a lot of different parts of our body. Here are those fontanelles or fontanellies that I talked about previously. There's actually several, two of which you can't see in this image, but this is a fetal skeleton or a fetal skull, and you can see those different little soft spots.

Once the brain comes to full size, there's no longer need to have these gaps within the cranial bones, and so we'll fill those in with bone. But you want a little bit of flexibility as the brain grows, specifically from being a fetus and into a young child. And so in the meantime, you will have hyaline cartilage there available. Now again, this provides flexibility so that the brain can grow as the cranial bones expand to allow for that brain size to expand as well. But maybe you've heard before you really don't want to press on these soft spots on a baby's head because they are quite fragile.

In addition to finding hyaline cartilage in the fetal skeleton, we also actually have hyaline cartilage at the end of all of our articulating bones. As a reminder, articulating bones are bones that touch other bones. So an articulation is a joint.

For instance, if we are talking about our femur, you have your fibula and your tibia, which are going to articulate with that. Now at those edges, where they meet, you will have them surrounded by hyaline cartilage. You may have seen this, or have commonly, most commonly visualized it on chicken bones. At the very edge, it's kind of almost a little bit softer than the bone.

If you've ever accidentally tried to munch on it, it's kind of gross, but at the end of that chicken bone, you would see hyaline cartilage. What is the hyaline cartilage doing there? It's preventing friction between the two bones, so we're minimizing the damage of the bone itself, which is more brittle than the cartilage. We also have hyaline cartilage supporting our trachea. We need some flexibility here.

If you're not familiar, the trachea is the windpipe. This is going to be our passageway down to our lungs, and we need to make sure that we keep our windpipe open. Just posterior to the trachea is our esophagus, which is essentially the passageway for our food down into our stomach.

Because they're in such close proximity to one another, when you swallow and that food stuff goes down the esophagus, if we didn't have hyaline cartilage to support the windpipe and to keep it open, it would collapse every time we swallowed food. So the hyaline cartilage provides flexibility but support to the trachea. Our larynx is essentially the housing for our vocal cords, and so hyaline cartilage there protects our voice box.

And then also as you know our nose, we have a little bit of flexibility and give there. It's not quite bend and snap but we have that movement of our nose which thankfully doesn't break all the time as long as we don't hit it too hard. That flexibility we have in our nose is thanks to hyaline cartilage. Fibrocartilage as you know has a bunch of fibers in it and those fibers specifically are collagen fibers. Hopefully now you associate collagen with strength.

So the fibrocartilage is going to be very, very sturdy. When we talk about that sturdiness and the function that that strength provides, we are going to find fibrocartilage in areas of the body where we experience great stress. And most of the time we're going to see it kind of like a little cartilage pad.

So one example of what that might look like, where we find fibrocartilage is in an intervertebral disc. Intervertebral disc is this little cartilage pad. that is between the bodies of two vertebrae. So this is one vertebrae here and here is another vertebrae right here.

When I say vertebrae, technically that is plural. So vertebrae is plural. Vertebra is singular.

So vertebra and vertebra. I can also tell you right now, even though we haven't gotten here yet, we'll get a chance to focus on this in lab, these kind of like look like moose. To me, this looks like a moose and this looks like a moose to me from the side. So I know these to be lumbar vertebrae. Now imagine where your lumbar is.

You know your lumbar region now. And all the stress that you accrue from standing up tall, from sitting, there's a lot of stress between those vertebrae. And so this fibrocartilage is there to help withstand that stress.

Now because there is so much stress, usually in cartilage we have an outer covering called a perichondrium. Peri means periphery, chondral means cartilage, and so perichondrium is an outer covering that most cartilage has, which is dense irregular connective tissue. One of the ways that you can compare and contrast the different types of cartilage is specifically fibrocartilage does not have a perichondrium.

Why does it not have a perichondrium? Because it undergoes so much stress, if we did have a perichondrium, it would constantly rip and tear. So there is no perichondrium that is formed on the outside of fibrocartilage. With that said, we will see that there is a perichondrium on hyaline cartilage, so you can associate perichondrium with hyaline cartilage, and we will find that our elastic cartilage has a perichondrium as well, but fibrocartilage does not. So where can we find fibrocartilage?

We'll find it in the intervertebral discs, like I told you just now. Also, the pubic symphysis, so our hip bones where they come together anteriorly, there is another little pad of cartilage there, specifically fibrocartilage, that's our pubic symphysis, and the menisci of the knee. P is plural for meniscus.

And so really what we're seeing here is it's all about shock absorption and dealing with stress and compression in these areas of the body. Our third one is elastic cartilage. Go ahead and write yourself a note now that elastic cartilage also has a perichondrium. We can recognize the elastic cartilage by the elastic fibers which help to provide for its function of elasticity or flexibility. Really what we get with elastic cartilage is that bend and snap kind of function.

So you can do this with your ear if you bend it forward and then let it snap back into place you can really see that. The external ear, also known as the auricle, that's one place we find elastic cartilage, but the other place is the epiglottis. We actually share some of the passageway between our mouth and our nose, making our way down before we get to either the trachea or to the esophagus.

This shared passageway is called the pharynx, and depending on where it's housed or what part that is posterior to, We can have the nasopharynx or the oropharynx or the laryngopharynx, but we have the pharynx that we share, and again, that shares air and food. At some point, we need to be able to diverge between going down the trachea to our lungs or down the esophagus to our stomach. What's going to help us to keep food from going down the trachea and into our lungs? That's going to be the epiglottis. We will get to see it.

It almost looks like a little tongue feature, but that bend and snap, that allows for us to divert food and make sure it goes the right way, that is the epiglottis. So as far as the functions of the cartilage, what we are doing with them overall is supporting different soft tissues. So you've already seen, or I've mentioned to you the trachea.

So here's the trachea here. And all of the cartilage are these little blue rings. So that's tracheal cartilage.

So the trachea itself is soft tissue, but we support it and give it structure with the tracheal cartilage. And more specifically now, hopefully you can associate that tracheal cartilage with being hyaline cartilage. So we support it throughout the respiratory system. We decrease in the amount of cartilage we have as we make our way towards the lungs and into the lungs.

We'll talk more about that a little bit later when we get to the respiratory system. But also in our ear. So our ear isn't going to be just elastic cartilage, even though we have that bend and snap. You have adipose tissue. You have areolar connective tissue.

Of course, epithelial tissue, right? It's layered with skin. But we support those soft tissues with the cartilage there.

We also have the cartilage available to us for articulations. Again, those are going to be joints. Over here, we can see our first example of some of the joints we're going to be talking about. Just to give you an idea, we have our clavicle bone right here. This is our scapula, and this is part of the scapula coming up here.

And then this is the very... most proximal part of our humerus, so this would be within our arm. The glenohumeral joint is named by, first you see humeral, that's named for the humerus.

The gleno is associated with a specific feature of the scapula called the glenoid fossa. That's where the head of the humerus fits. So the glenoid fossa of the scapula and the humerus, where they articulate or come together, is called the glenohumeral joint.

In addition to this major joint within your shoulder, you actually have another one. Here it's called the AC joint, but this one is called, more specifically, the acromioclavicular joint. Clavicular you have for the clavicle, and the acromion process of the scapula is where these two bones are meeting.

So you get often, if you know the features of the bones, that helps you to memorize the articulation names as well. We will talk more about articulations in detail in lab. In addition, I've mentioned that we can use cartilage, and we'll find that it's specifically hyaline cartilage, as a precursor or framework for some of our bone growth. Not all of the bone growth, but some.

So these are the three functions we associate with cartilage. Now, how do we grow the cartilage first before we make the bone? So if we ever need to make cartilage, There are two types of cartilage growth that we can have. We can have interstitial growth of cartilage and we can have appositional growth of cartilage.

What I want you to notice right now is I associate the I with being long or tall. So interstitial growth is growth in length. So interstitial growth is growth in length. I think of the A as being really wide or I think of like a stout little apple.

So wide, that A is being wide, appositional growth is growth in width. So that's how we can first distinguish the two types. Next we are going to go over the specific steps in both interstitial growth and appositional growth of cartilage. So first we'll talk about interstitial growth.

One of the things that I do want you to note is that the same cartilage can go through both interstitial and appositional growth. So I don't want you to think that this is different types of cartilage that do one type of growth versus the other. So I'm going to try my best to kind of give you a 3D representation. Here's cartilage.

And so the very first type that we're talking about is going to be growing in length. So that means then that if this is going towards maybe the proximal part of the body and this is distal, we would be growing this direction or this direction. In width, we would go this direction or this direction. So let me just first focus on the interstitial growth. Again, growth in length.

In order to do this, we are going to see, again, at proximal to distal, chondrocytes, as we know, they are not actively making cartilage. So say that these chondrocytes that are housed within the cartilage, they're in their little lacuna. Okay, so here's my little chondrocytes and lacuna.

They receive a signal to go through mitosis, to make more cartilage. So each one of those then is going to go through mitosis, and hopefully now we have a good level of comfort with what mitosis looks like and all of the different steps. And these are then going to have two little cells within.

And I should have started bigger, so let me make them bigger. There we go. So each one, these are each chondrocytes.

They're going to go through mitosis, and as a result of mitosis, we now have two of these chondrocytes within the single lacuna. Now, what that means then is that these chondrocytes are sharing a lacuna. So that's this Rumi step. So two chondrocytes after mitosis are sharing a lacuna.

Now, these chondrocytes are going to start making... cartilage. And when they do that, they become chondroblasts. So chondroblasts start laying down cartilage. When chondroblasts lay down cartilage, they lay it down all around them.

So what you're seeing here is they are sharing a lacuna, and each one of them now is going to start to build it around itself. So this particular cell right here, this chondroblast, is going to be building cartilage around itself. And this little chondroblast right here is building cartilage all around itself.

And in doing so, they start to fill in the gaps around the periphery of them, but also in between them. and so in doing so the chondroblasts lay down cartilage including in between one another and when they do that they essentially then house themselves in two separate lacuna so the chondroblasts separate And after they lay down all of the cartilage that they can, they get stuck, they get trapped. And the signals that they receive from being stuck or trapped in their cartilage that they've made tell them, okay, we're done making cartilage, we can go back to rusting.

So after the chondroblasts have separated, the chondroblasts then become chondrocytes once more. So we've now essentially separated these into separate ones and in doing so we've increased the length because remember they built all around themselves. So let me again walk you through the process. Chondrocytes go through mitosis and become two chondroblasts sharing a lacuna.

They're chondroblasts because they're now going to be actively making cartilage. The chondroblasts lay down that cartilage all around themselves and do so until they increase the cartilage length, so interstitially, and once they're separated into their separate lacuna, they stop making cartilage and become chondrocytes. That is essentially what's happening in interstitial growth. Now remember, oppositional growth is growth in width. So I'm going to try and draw this same little one, but we're going to focus on the periphery here.

So the edges do have growth in width. So one major difference... is that where we just had previously with interstitial growth, just chondrocytes received signals to go through mitosis and then became active chondroblasts.

Here instead with appositional growth, we're actually starting with stem cells or mesenchymal cells. And where are these mesenchymal cells housed or stem cells housed? So they're on the periphery. So I'm going to just give a slightly different color here. Let's see if this doesn't get too annoying.

I want you to imagine all along the periphery. Along the width, you can't see it on the other side, but you have them there too, you have stem cells. Well, what are these stem cells going to do?

They're going to receive signals to divide. So these stem cells are going to go through mitosis, okay? And on either side, so here you're seeing them go through division and make their chondroblasts. Here I'm going to show you this stem cell is going to go through mitosis.

This stem cell is going to go through mitosis. Remember that stem cells can divide to make more stem cells. or the cells that result from the mitosis can differentiate to be a specific type.

In this case, we are making ourselves chondroblasts, so they are destined to specifically make cartilage. So these are our chondroblasts. So stem cells go through mitosis and the resulting daughter cells differentiate into chondroblasts.

So again, you would see that on both sides as we are growing in width. Chondroblasts lay down cartilage. So they lay down cartilage or produce cartilage. And in doing so, remember, as chondroblasts lay down cartilage, they build it all around themselves. So they're building cartilage all around themselves.

Okay. They continue to do this again until they build all around themselves and get themselves stuck in the lacuna. And so again, they...

The major difference here is that they resulted from stem cells or came through the process of stem cells dividing. But they get trapped in their lacuna. And then those chondroblasts, when they are no longer making cartilage, become chondrocytes.

So that is essentially the major difference. So please note interstitial versus appositional. Interstitial is growth in length.

Appositional is growth in width. Interstitial is the division and mitosis of chondrocytes to become actively to be active chondroblasts and they are going to divide so that we can increase the length from proximal to distal. For appositional Instead, we are utilizing stem cells.

Those stem cells go through mitosis, divide towards the outside or to the periphery, and those daughter cells differentiate into chondroblasts, which lay down cartilage, get stuck in their lacuna, and then become chondrocytes. So that is it for the cartilage. We are going to rely on this growth when it comes to specific types of bone.

And so you'll want to be able to not only understand these steps but remind yourself of these steps when we talk about bone growth as well. If you have any questions, feel free to reach out.

Transcript for:Understanding the Skeletal System and Cartilage

Transcript for:
Understanding the Skeletal System and Cartilage