Week 6 lecture 5 | Coconote

...intrastructural details of spongy bone before pivoting to the nervous system. So, first of all, let's talk about where we find compact bone. Obviously, we find it on the outer surface of all bones. thick because if it was bones would be really heavy and very hard to move so it's not that bones are solid compact bone in fact they're mostly spongy bone but in any case we find that bones all bones contain compact and spongy bone but they differ in the proportion of each meaning how much of one relative to another and location in the bone so again i'm giving you that in the notes i won't spend a whole lot of time on that but we still find compact and spongy bone in all bones and so this is really what i wanted to focus on so i'm going to be going through some details here then i'll be making some rough sketches on this bone sample that i have an image of and then we'll be talking specifically about about the details of osteons. And osteons are really interesting structures that we find only in long bones, and specifically only in the diaphysis of long bones, and they serve a really unique role. But before we get to that, let's talk about this compact bone in general. Compact bone, as a reminder, it could also be called cortical bone. That is a fine name for it. I don't use cortical bone, but in the future, your professor may. So let's talk about main functions of cortical or compact bone. So when we think about the main functions of this tissue, First of all, we're going to see protection. And by protection, I mean really helping preserve the function of the soft tissue within a bone. So when you think about protection, you can think about, you know, like skull bones, preserving the function of the brain, or bones of the thoracic cavity, preserving the functions of the heart-lung complex. But we could also talk about the compact bone, protecting the delicate tissue within the bone, like blood. vessels or stem cells or bone marrow. So protection. So when we think protection it would be fairly accurate to say of delicate tissue. And again we find delicate tissue both inside and outside of the bone. So compact bone because it has such a really dense hydroxyapatite formation, not many spaces at all in between that, very good at protecting. We also, sort of a little different, it is the anchoring. site for connective tissue. Remember CT stands for connective tissue. And when it comes to the type of connective tissue that we will form an attachment with or anchoring site, we're talking tendons or ligaments. What's the difference between these? Tendon attaches what to a bone? Muscle. That's right. Tendon is skeletal muscle to a bone. Ligament then of course would attach bone to bone. So we'd find those like in the carpal regions, the tarsal regions, ankles and wrists for example. So just make sure that when you see those words they they're fairly clear to you. When we think about compact bone, in general it is fairly similar. in most bones except in long bones and we think by shape you know we talked about short bones and flat bones long bones have a lot of tricks up their sleeves and that's because they do some amazing things in addition to protection anchoring they are really involved in locomotion so when it comes to long bones we find some unique compact bone features and those features have to do with how the bone tissue is formed and how it appears in those bones So we're going to talk about two different types or structures or patterns of bone tissue. And again, I'm keeping this specific to long bones because they are easy to see and they have some unique features that we don't see in any other type of bone. So first is something called circumferential lamella or lamellae, plural. So first of all, this word circumferential, if it looks like the word circumference, you're not wrong. It is the type of bone that's on the very outer aspect of compact bone. It's not super thick, but circumference on the very outer aspect, the surface of the bone. That's where we find circumferential lamella. So the very outer aspect or diameter of compact bone is called or formed into circumferential lamellae. Lamellae, by the way, it's kind of a weird word. It just means rings. They don't have to be concentric rings, like perfectly round. And over time, they're going to be more or less like rings. oval-shaped, round structure. And so circumference... Circumferential lamellae are oval shaped or round rings of bone on the circumference or outer aspect of bone. So this circumferential lamellae has a very unique composition and structure compared to the other type of bones. tissue that we find as far as compact bone goes. So again, we're only talking about compact bone, we're not talking about spongy bone. Compact bone itself has its own details. So we'll find that circumferential lamellae, lamellae means ring, so they form rings around the circumference on the surface of bone. And lamellae is a word that we'll see quite a bit today. but wherever we see it, lamellae means a ring of some type. Not necessarily perfectly round, but a ring of some type. Circumferential lamellae provides a great attachment site for this connective tissue. And that's really its main function is attachment site. Now, when we get into the weeds here of long bones, we find a really unique structure called an osteon and osteons are only found in the diaphysis of long bones. We do not find osteons as part of any other type of bone. You won't find osteons in flat bones or irregular bones or short bones. You only find osteons in this very particular area of long bones. Highly unique, very very specific purpose which is why we find them here and not in other parts of the bones. They are hugely important for allowing us to get tall, fairly large as far as animals go, and be able to move fast, turn, go the other way, and not snap our limbs. I think nine out of ten people are very interested in this. some people would agree that's probably a good thing. The other one was in the hospital because they had their leg snapped. So we have to really take some time and appreciate osteons. So osteons are made of, pay attention to this word right here, concentric. Concentric lamellae. Perfectly round, highly organized, very very rigid structure. And in the middle of these rings, these concentric rings, we find an interesting feature called a Haversion Canal. So all of this I will definitely illustrate for you here in a moment. So if you're thinking, wow, I really can't picture any of this, it's okay, we'll get to it. I just wanted to give you the basics, the details, so when we go into the drawing, you kind of have something to refer to as far. structure and basic vocab words. Alright, so before we get into the structural details of an osteon, I'm going to draw your attention to this cross-section of a long bone. So to make sure everybody got this down before I move the camera. Any questions on this sort of background detailed stuff yet before I apply it? Yes? Yeah sure, that's a good question. A virgin canal is a central canal. You can think of it as a space. Right in the middle of an osteon. Only see them in osteons. They're only part of an osteon. They form the center of the osteon. And what they contain and what they do, I will draw out for you here in a moment. So for now just know that Havergian canals are unique to Ossian because they're found smack in the center. And then around them are all these concentric lamellae. So good question there. Other questions before I go up into this drawing? Alright, so what you have up here is an image that I took of a fairly fresh bone. Mine is in black and white because that's the printer that we have here. I don't know if I gave it some sort of color. I try to like... the fence between a little bit of color so you can see distinction but not enough it's going to ruin your printer or like take out an entire e-cartridge try to print it off that's a balance but what I want to do is introduce you to parts of this book some of which we've talked about, some of which we are going to talk about. I'm going to be starting on the very outer aspect of the bone. In the middle here, this is the medullary cavity. So this would be the center medullary or marrow cavity. So we're obviously looking at a long bone. We're obviously looking at a diaphysis because that's the only one that's going to have a dedicated medullary or marrow cavity. Okay. On the outside here, this sort of rough piece piece of tissue that's periosteum and we've talked about periosteum before but as a reminder periosteum is an outer layer an outer tissue on the surface of the bone and you find it surrounding all parts of the bone except where there is what are we looking at to fill in the blank? Articular cartilage yeah so on the surface of the bone it's either periosteum or articular cartilage but we're in the diaphysis the mid shaft of the bone, so obviously no joints here. So we're looking at periosteum and sort of refracted or peeled back, your thumb is, or peeled back at periosteum just so you can see it is its own tissue. And underneath it was bone tissue. So when we think about periosteum, remember it has that inner osteogenic layer and it can make new bone. Okay, when we think about compact bone, and long bones really highlight the difference between compact and spongy bone so well, because you can take a cross section of a diaphysis and see both. So in this area right here, I'm just gonna sort of outline. This whole area right here, that's compact bone. and very hard, very dense, as you would expect. And then from here to about here, even though we're in the diaphysis, that's spongy bone. So we don't have a lot of spongy bone in the diaphysis, but we still have some. And you can see sort of the spongy cancellous bone as its other name, but these holes here make it look sort of spongy. So wherever we're looking in bone, we're going to have compact bone and spongy bone generally in these arrangements. Now I'm going to get into some details about circumferential lamellae and then osteons. And we can see their placement here. we can actually see some converging canals here. They don't really stand out too well, but I will at least do like a dotted line to help you understand the placement of circumferential amyloid. And kind of think about what the word means, circumferential amyloid. It runs on the circumference of this bone. So when we look at circumferential amyloid, it's going to be... just several rings of bone sort of in this area and you can almost see a color change even in this black and white photo. But here this is part of compact bone here but this area is where we would have the circumferential lamellae. And then obviously in this area here We're going to have osteons. So we see osteons have the concentric lamellae, but we can see there's already a difference between this type of compact bone based on placement alone. So circumferential lamellae will always be the outer aspect, the cross section that I have, so the outer aspect of that circumferential moving to the inside we have osteons. I'm going to just sketch in the details of what a circumferential lamella singular would look like and then compare that to an osseon. By the way, this is a converging canal, so that's how we know in the realm of the osseons. They are big enough to see with the naked eye on a brain, so they're pretty obvious if you get just the right cut. So when we think about circumferential lamella... I'm just going to be using brown for the bone cells. We still see osteocytes. I'll be drawing them a little bit bigger than they would be, obviously. We still see osteocytes living in lacuna, communicating. with the cannulae, but they don't run in a terribly organized, dedicated pattern. They just form the outer aspect of this bone. So I'm going to do that again. Here's the little cell body, its little extensions, sort of avoiding the area that I've written on. And again these would be living in lacuna and communicating with cannulae. So we just see osteocytes on the circumference. That's going to be really different compared to osteons. So osteons, I'm going to come down here where I already have some nice subversion canals that have made themselves known, and sort of highlight the area where we're going to find those osteons. So again, sort of on the inside. or deep to this dotted line. I'm going to circle this and say here is a Haversian canal, here's another Haversian canal, and Haversian canals are on the center or are in the center of an osseous. and then radiating out from that are concentric lamellae, or rings of bone cells. So compared to this, where you just have bone cells sort of running on the outer aspect, osteons have bone cells that form... very rigid concentric rings around the subversion canal so all these dashes that I'm drawing here these are osteocytes but I'm trying to at least get a few concentric lamellae sketched here so you can see what I mean by very very concentric very organized around that have version canal So that's kind of what we're looking at as far as details of compact bone. Questions before I continue? I'm going to sort of drill down and talk about the very super-duper details of an osteon to help you understand their significance. Yeah? So the osteons are only going to be overgrading to the excuse of being a compact bone? So they're going to fill the space between where the circumference of the leg end. and one of the spongy bones in. So you can have several in this region. Sometimes they look almost as though they are on top of each other. But we would have many, many layers of oxytocins. I just chose those because they're versioning out. Like for example, if it helps, I could put, I could just make another haversian canal here and we would still see rings of osteocytes surrounding that haversian canal. So osteomes exist between circumferential limilite and the start of cell canal. Other questions before we continue? All right, we're going to move into the details of an osteon again to help you understand the significance of them, why we only find them in long bones, and what would happen if we didn't have them, which is not good. So we're pretty happy that we have osteons. So we've talked about what they are. find them but I haven't really said much about their detail other than they have a center called nephrosian canal. So before we continue we're going to talk about structural details of an osteon. A single osteon will be presented but again bones have long bones and the diaphysis have many. So things we could say about osteons, long and cylindrical in structure. They almost look like, I don't know, those toy telescopes where you could pull them out and there's just like layer that comes out and layer and layer and layer. That's kind of what they remind me of. So they are very long and cylindrical in structure. The Haversian Canal runs their entire length. So every osteon has its own Haversian Canal. Runs the entire length and it houses vessels like blood vessels, lymph vessels. and neurons. So the Haversian Canal turns out to be a container in which we can place the soft tissue. Otherwise, the soft tissue has a hard time getting through that bone such that it can provide what osteocytes need, or osteoblasts. Osteoblasts are all near together. So the Haversian Canal is a hollow tube that houses soft tissue like vessels and neurons. Surrounding that Haversion Canal are rings of osteocytes. Just like we saw up here. So again, Haversion Canal, rings of osteocytes, concentric lamellae, very organized. I can even add that word here just to help contrast circumferential. And down here I'm going to add the word concentric, very organized. Oh, I already had that. I got excited. I guess I just really like the word concentric. But anyway, so concentric lamellae, concentric rings around the Havergian Canal. They look like they were almost made in a factory. They're so completely perfect. Very, very organized, that's for sure. I'm going to lift this up a bit. A few more details before we start sketching. The collagen fibers, so as part of the osteoid, we talked about this soft tissue called osteoid. It's what osteoblasts make directly. And the majority of osteoid is collagen, the most prevalent protein in the body. So these osteoblasts secrete these components of collagen fibers, which will become the actual mature collagen fiber. But here and only here do we find these osteoblasts deposit these collagen fibers at opposing angles. And so that turns out to be the evolutionary trick that allows us to allows osteons to be really great at preventing bones from snapping when you turn and run the other direction. This is the trick, and I'll explain it to help you visualize what this means, but it took a while for people to figure out. So osteons are very concentric, very organized. We think it has something to do with helping support these long bones. Think about how long a femur is and how much shearing force that's going to get on it if you turn and run the other direction. But what is it about these osteons? that are so important. And it turns out it's this trick right here, seemingly so not important, turned out to be super important. So talk about how the collagen fibers contribute a lot to the tricks of the osteon. So at this point, I'm gonna draw this out for you. I used to present this already done and it didn't really work for people. So instead I'm gonna do it with you. And I'm gonna be showing you how an osteon really looks like a telescope that you could pull. out like think about like a child's toy this plastic telescopes and there's just like a layer that comes out and layer and layer gets longer that's kind of what this looks like so I'm gonna be starting at the bottom and actually building this osteon from the bottom up and I'll explain why I do that here in a moment pretty good drawing field and at any point if you have a question about why it is the way it is feel free to speak up somebody's starting down here I'm going to be drawing a pretty big base to this osteon. And this osteon is made up of sort of ring after ring of bone. So I'm going to give this in this area here. You're going to see me add some osteocytes. And I'm not going to be terribly detailed with their structure. I'll just give them sort of a soma, a nucleus, and some processes. That's it. And that's good. That's all we really need. And so you'll see again, that's sort of the pattern here. I'm making very concentric. Lamilla, that one has a weird nucleus. But here's a ring of bone. And what we're going to see on the inside of that, and so this ring of bone would exist all the way down different osteocytes. organized. And so I'm going to sort of close this particular layer off and I'm going to go into a different layer. Still same osteon but I've just sort of pulled it apart so you can see the concentric rings of bone. And so again we're going to have osteocytes living very organized relationship to each other. Close that off. And before I do the Haversion Canal, I'm gonna do one more. Again, they would be flattened if you looked at them from the top, like I did in that drawing above, but it's hard to understand the concentric rings unless you kind of get down to this level. So this one's pretty small, but there we go. And so I have pulled apart this osteon and then in the middle... I'm going to add a Haversian canal. And that Haversian canal has a lot of features and a lot of significance to it. So first of all, this Haversian canal in the center here would run all the way down, but we're looking at solid bone here so obviously we can't see the Haversian canal all the way down. But what we could would see if we could look all the way through and we can see it on the top are blood vessels and those blood vessels can take sort of a networking approach and they're gonna be what these osteos... sites rely on for nutrients. So maybe an artery or an arteriole, maybe a vein. Usually where we see arterioles, we're going to see a venule, for example. So blue and red, I think most people are familiar with what that means. So two blood vessels, arteriole and venule, for example. We also often ignored, but too bad, we don't want to do that, lymph vessels. Lymph vessels are super thin, kind of built in a strange way. pattern. They have little trap doors that lets stuff in. So that's a limp vessel and I'll label that because that's probably less familiar. And then we also have, I'll use orange for neurons. We have neurons, and some of those neurons may be sensory, and some of those neurons may be motor, but we definitely have neurons. And so I'm just going to write neuron here. It doesn't really matter which type. So Haversian canals, it turns out, sort of give this a little bit more of a wall structure here. Haversian canals have a lot of stuff that's really important for the health and well-being of the osteocytes. living in very uniform concentric lamellae. We don't see this arrangement in any other type of bone. Nothing else looks quite like this. So that's kind of the introduction to Ossigan, but I have not talked about what makes them so dang special. Before I do that, questions on structure and function here? Can we see the concentric lamellae, how organized, how it's really all based around the haversian canal? These osteoblasts lay down the osteoid around the haversian canal, so the osteocytes have no choice but to form very concentrically because that's how the osteoblasts look. So we're so good. All right, I'm going to add in collagen fibers now, and these collagen fibers are going to be light blue, and I'm going to do it in a very particular way. So as I do this, maybe just watch for a little bit before you go into your drawing, because this is the trick of the osteon. So these osteoblasts making these collagen fibrils, which later become mature collagen fibers, they lay them down in such a way that they run always in an opposite angle to each other. So you could say it's running on the diagonal or maybe on the bias, but wherever we find neighboring concentric lamellae, they always have collagen laid down in opposite directions. And that was the evolutionary trick to the osteon. That is, principally, what allows... We need mammal really that runs on the earth to run fast in one direction, plant that pet go the other way, and not snap the long bone. So let's talk about, now that you've sort of got your collagen fibers and all the directions, how on earth could that matter? What do you think that does? I'll give you a hint. It has to do with torsion. Do you know what torsion is? I've heard that word before. It's like a twisting. Things maybe twist in relation to each other. So if you run in one direction, everybody's probably done that in high school. Like they had different names for it. like zippers or I don't know, it's been a long time. You had to run to one line and then, just for the heck of it, run back, and then go back, you know, just to like torture people and PE. But that's torsion. And so when you plant that foot, the femur gets some rotation, torsion, because you're changing direction, turns out the collagen fibers allow the osteons to absorb that torsion just a little bit because it allows just a tiny, tiny bit of rotational movement before it will lock. So they will come a time where these things sort of lock and that absorbs that torsional force that could otherwise snap the bone. And so it's just crazy. That alone allows a large animal with a lot of. velocity to go in a different direction. And humans aren't the only ones with this trick. All mammals that have had evolution on terrestrial ground. Rabbits are great. You ever chase a rabbit? I don't know why you would, but if you did, they'd zigzag. And so they're very good at that and they would have that same setup. So this is an old trick that nature developed and she put it in all of her terrestrial mammals. So that's pretty much what I want to talk about with Osteons. I'm going to give you a little break, just a couple minutes. I'll come back at 3.05. We're moving to spongy bone. So 3.05, not that long. Use it wisely. Yeah, she was having a hard time. Yeah, yeah, yeah. She was supposed to be awake. I had to wake her up. I had to wake her up. So, like, that is sort of, like, she was just getting her sleep. We got, like, a team of nine hours. What do we think about this question down here? Arteries run along the surface of a bone. So, for example... Thank you. Back up here. In order to get blood into a bone, it has to come from the external part of the bone. You're going to have really large arteries on the external surface of a bone. So here's a really large artery. So I'm outside of the periosteum. And my question is, how do you get it here? Because this is where your aversion canals are that would feed this particular area of compact bone, so important for the health of the bone, and eventually wind its way into the spongy bone. Any idea how you're going to get blood from here to here? It's in the reading. I haven't talked about it specifically. I don't know if you've read that part of the book or maybe Megan talked about it in lab. So if you haven't come across it, it's okay. I'm just wondering. Anybody have an answer? Name of a vessel or a canal. No takers. There. So when we think about large arteries in the surface of a bone, they will enter a bone. So here's a branch. It's going to branch off of that large artery, become a smaller artery arteriole, and then from there it will splay out at almost 90 degree angles until it reaches. this area of osteons and that's what feeds the Haversian canal blood vessels. These are called Volkmann's canals. Volkmann's canals and Volkmann's canals are really important because they feed a lot of bone tissue in a particular region. So the Volkmann's canals would eventually lead to the Haversian canal and feed that and it would continue to branch off. and feed the spongy bone. So I just wanted to mention that I've said before bone has a lot of water in it. Bone tissue itself has water but also there's a lot of blood and when you look at a preserved bone, a dried bone, it is easy to forget they are highly vascularized structures, highly vascularized. And then of course we talked about getting blood vessels to the spongy bone because that's where we see that endosteum, which is a blood rich lining that houses stem cells. So if you're going to have a blood rich lining here, the feeder vessels are out here, and you know it kind of lends the question, well how do you go from here to here? And it turns out bones have that figured out. So I just wanted to hit that really quickly before we move on to spongy bones. speaking of spongy bone. Before I do, are there any questions here that you would like clarification on or some different explanation of? Yeah. Does collagen lay between the legs? I'm sorry, you said it right. The collagen, is it, does it, which direction? Oh, that's a great question. What's your name? Aaron. Yeah, it's a great question. Aaron's question is, the collagen, is it on the outside or is it within the concentric glomerulus? Is that kind of your question? So yes to both, I guess. So you can see, if you're a really good artist, you could run that collagen fiber in the same direction through that entire lung blood. Does that answer your question? Yes. Okay. And then this one, you know, if you're a big artist, you can come down. So that collagen is an integral part of that osteoid, which is the bone tissue. I just put it on the surface because it's easier and it sort of helps you understand the interlocking features of that. But yes, these collagen fibers will run in the same direction interiorly as well. Good question. Any other questions? That's a good observation and a good question. I'm going to move into spongy bone. So we've talked a little bit about spongy bone already. Good to have some detail. Spongy bone is such a biologically, physiologically important tissue. I know compact bone looks really cool. It is, but honestly from a physiological perspective when we think about endocrine system, marrow, vasculature system, spongy bone really has a lot more important details. talk about. So just as a reminder, all bones have spongy bone and it's always located deep to or within that compact bone. If we didn't want to find spongy bone on the surface that would be very bad. It is strong in a way, but it still needs protection from the outer elements, the outer forces, I should say, by that compact bone. So some main functions of spongy bone. Three main functions, and I'll see if I can zoom in and not lose... Resolution, maybe. So three main functions. That's still not too good. One, it forms the internal, what I call supportive strets for bone. This actually is collectively, these internal strets, collectively are quite strong on their own. Very small, but collectively quite strong. So, support. Internal support against muscle tension. They also, because they have this blood-rich lining called endosteum, we'll find stem cells here. And just as important as the other two, we have their third function, main function, which is we consider spongy bone to be, we call it bioavailable. Bioavailable. I'll explain what that means here in a moment. All right, so for our first bullet point, internal supportive struts, these are more appropriately termed trabiculae, and you've seen this word before. It's always good to kind of review. Trabiculae is Latin for a little column of bone which acts like a supportive strut or brace inside the bone. Another main function house stem cells and that's because of this tissue called endosteum. Endosteum houses the stem cell niche or environment in which these do pretty well in. And then our third function it's a bioavailable mineral source. I'm going to explain that. All bone tissue is made of calcium and phosphate, hydroxyapatite. But spongy bone, and only really spongy bone, is considered bioavailable, which means it's usable. It's usable and accessible. Usable and accessible. The hydroxyapatite of compact foam is not accessible by and large. It's too dense. It's all locked up in this very dense tissue. You really can't get your osteoclasts in there to dislodge some of that. And that's really good for a spongy belly. You kind of look at that scanning electron microscope picture that I've attached in the upper right-hand corner. There's a ton of surface area and that allows osteoclasts to just really roll around and chew up what they want and liberate that calcium. So that's what we call spongy bone. We call it bioavailable. bone is not really very generous when it comes to helping balance blood calcium levels. All right, when we think about spongy bone, again still in all bones, we find it as, as I mentioned before, trabiculae, little columns of bone. That's the Latin meaning of it. But what we're going to look at here in a minute is the internal and external structure of trabeculae. And this has typically been a source of confusion for students, which is why I really wanted to take just a little bit of time and be very clear. about what's on the inside and what's on the outside. And so historically, when I went too fast through this, I think people for some reason equated the cross section of a trabecula singular with an osteon. And they thought they were the same. And so they are not the same. So that's why I want to be very clear about osteons are only compact bone. And trabecular are also little columns of bone, but they're totally different. So do not be that person that equates osteon with spongy bone. They don't exist. Osteons are only compact bone. So on the inside, on the inside of trabecular, We can find a couple of things and then on the surface we can find a couple of things. And there's some things that don't exist when it comes to spongy bone. So if we were to look at, come over here in this little microscope image, if we could zoom in and really see what's going on internally in this spongy bone. On the inside we would find osteocytes. Living in Lacuna, communicating with cannuliculi. But we would not find concentric lamellae. We would not find a Haversian canal. We would just find a little disc of ossified tissue. Osteocytes are still present. In fact, on the inside of spongy bone, we don't even find blood vessels. They're just too small. Too small. So instead, we find a lot of that on the surface. So it's like trabiculae of spongy bone are sort of formed inside out compared to osteons. On the surface of trabicula, we would find osteoblasts, lots of them, always remodeling, and then their counterpart, osteoclasts. We'd find vessels, we'd find stem cells. It's a very busy place on the surface of trabecula and that's why it's considered to be bioavailable. There's a lot of cell activity that can easily rearrange the structure, the shape, the angle. of trabiculae and so that helps the bone respond to external forces and allow that bone to be available for calcium. If you want to deposit calcium, super easy, just sort of smear it on the surface when you make your osteoid. You want that calcium? Let your osteoclast have a little bit more freedom and start to chew that up. Put it in a nearby vessel. So trabeculae are a lot more important from a physiological perspective because of their structure. So we're going to do a couple things on this. I'm going to sort of provide a drawing of the internal features of trabecula and then sort of remind you of the details of the external part, which we've seen a little bit, but not formally. So before I do that, just a little note about this image that you have in the upper right hand corner. Scanning electron microscopes are very powerful, much more than a light microscope. And so I purchased these images, I licensed them, and I just really thought this was worthy because it really shows you the detail of spongy bone. So everything you see here is an actual trabiculae. This is from a human. Little spongy bone columns. everywhere and all these holes here remember those are supposed to be there these would be filled with marrow so red marrow because it's not a large cavity like we see in the diaphysis spongy bone typically contains red marrow and so if you want to just sort of remind yourself that that these columns of bone house stem cells that make marrow and it's usually red i don't know how well that's going to show up but if you want to sort of color these little pockets in in a few places that show up for you with red marrow that might help you understand some other details that I'm going to talk about. And that is another reason why spongy bone is such a great tissue. It has a lot of places where we can store unique tissue like marrow. So just realize it's there. You can, you know, it might be worthy of your time after lecture to sort of compare what I talk about here to this actual drawing up there. But for now, we're going to move into internal features of a trabecula. trabecula singular, so if we could isolate a single column of bone there, we would find that bone cells, the osteocytes inside, still exist in lamella. They're still in rings, but they're not concentric. They just sort of take the random shape of this trabecula, and they can change. Bone cells still exist as rings or units that look ring-like. but not concentric, not like an osteon. And as I mentioned, the bone cells, the osteocytes, they're still living in the lacuna. That's the little pothole, the little lake that they live in full of fluid. So this is Latin for little lake, and it contains extracellular fluid. And that is a diffusional medium that those cells use to attain nutrients, get rid of waste. I have cannulae that radiate out from that lacunae, little canals, lets them communicate, lets them get nutrients from the outside. I'll draw this out here in a moment. You can see I'm going to provide a sketch of a trabecula, but let's get through some basics first. So even though we have osteocytes or bone cells and they are going to exist in units, ring-like units, they are not nearly as organized as osteons. So as a reminder, there are no osteons in spongy bone. That is only a compact bone feature, no osteons. Further, there's no blood vessels internally, so we won't find aversion canals, we won't find neurons inside, we won't find lymph vessels inside. Everything has been relegated to the surface and that actually opened up other opportunities like nourishing stem cells, providing a place for them to produce their product and have it accumulate. So it turns out this was pretty beneficial. When we think about supporting the cells within, it's diffusion. So we're going to see these cells that are in here need diffusion from the blood vessels out here in. And that's going to limit the number of osteocytes that we can have internally because diffusion is a slow process. And too far of a distance is going to really limit the ability of those cells to survive. So diffusion to these bone cells from the external blood vessels is key. And again, that really limits the number of lamellae present. you know, can't have rings and rings and rings of bone, the inner ones would not get enough nutrients. So we will see a small surface, not surface area, but cross-sectional area to this trabecula. Alright. So I'm going to sort of sketch out the details here. And so what I'll have you do, you cannot draw this incorrectly. This is going to be just a zoomed in image of a trabecula. It does not have to look just like this. You don't have to follow it. Trabecula are random, so feel free to let your hand do what it wants. I'll be talking about external features in the drawing below. So right now, when you do draw, draw yourself like you have this comb on the bone. And you cut it so that you can see the internal features of it. I'm going to do two different aspects to help you see this kind of random looking structure. But wherever we find it, we see some similarities. So on this, I'm going to color the surface because we're not going to be worried about what the surface has. And this is just ossified bone. We'll talk about what we'd find on that surface in my next drawing down below. But right now we're just going to have bone tissue ossified, pretty strong, good against tensional forces. put on it by muscle on the outside, but on the inside, that's where we're going to focus. So inside, I've got sort of a cut surface here, and I'm going to find, I'm just going to do really sort of small dashes for osteocytes, and they may... live in sort of units, but again it's not necessarily concentric looking. We don't see them in these nice organized rings. They kind of take different directions and that's really all that we find inside. So these would be osteocytes and they would be living in lacuna. I'm not going to draw that in, but they would be living in lacuna, those little lakes, and communicating with canuliculi. But notice the bone cells here. random smattering, nothing concentric at all. And they are really pretty short-lived because on the surface, which we'll see in a moment, are osteoblasts waiting to just chomp through that. So it doesn't make sense to make them in a very organized fashion because they're not going to be around as long. On the other sort of cut surface we'd see the same thing. We'd see osteocytes. Some sort of lamellae but nothing concentric. So same thing. Very different than osteons. No reversion canal, no concentric rings. That's pretty much what we found on the inside. Osteocytes and bone tissue. So pretty straightforward. I am gonna go down and talk about the external features. So questions before I. No questions. Alright, let's talk about external features. Very different. Spongy bone has a lot of tricks. It's not big, so it uses its surface well. External features of trabecula, again just working off of a single column of bone here. This is a very busy area internally. Trabecular are kind of plain, straightforward. On the surface though, that's where all the stuff happens. This is a very busy area. First of all, we're going to see there's a really good population of osteoblasts and osteoclasts present. And they are always present, and they are, to some degree, always active. Because you have to build bone all the time. damage you don't have to have a huge calcium deficit you get a new skeleton every seven years and again it's not like on you know you're six point nine you have an old skeleton and you wake up on your son's birthday and you're like I feel different what is it? Oh I know I got new bones. That's not how it works it's a continual process so if it's going to be a continual process the area of the cells are active continuously building and rebuilding chewing and rehashing this tissue These cells need a lot of blood. They need a lot of oxygen. These are very active cells. So, very conveniently, we find this to be a highly vascular area. A lot of blood vessels, lymph vessels, everything that we saw in a virgin canal. has been flipped is now on the outside to provide the nutrients and waste removal for these bone cells so lots and lots of blood vessels and those blood vessels are generally contained in an area called endosteum. And I've mentioned this before, endosteum is sort of a collective area. I will sketch out on the surface here where endosteum exists and what it will be comprised of. But as a reminder, as part of the endosteum, we usually find stem cells, HSCs and MFCs. What do HSCs stand for? Good job, that's a hard one. A lot of people are like, I don't want to say it out loud. Oh, good job there, very brave. So HSCs, hematopoietic stem cells. And so those would produce red blood cells, white blood cells, and something called a thrombocyte, which eventually makes platelets. Platelets are not living. Platelets are fragments of cells. So hematopoietic stem cells think blood cells. MSC, what does that stand for? Mesenchymal. Yeah, good job there. So mesenchymal. mesenchymal stem cells and these produce cells that end in the suffix blast and that's a lot so I'm going to say the blast cells osteoblasts Wherever we see the suffix blast, the suffix blast tells us a lot about that cell. So cells that enter the suffix blast, they are usually very active making a product. They are sometimes immature. I don't mean like they make stupid jokes and like prank people. I mean that they are, they could differentiate into something else yet. So, for example, osteoblasts, you know, they differentiate into osteocytes. and osteoblasts make osteoid. As another example, fibroblasts. Those are in the skin or really all over any part of connective tissue. Fibroblasts make collagen, and they could differentiate into fibrocytes. Myoblasts, these are immature muscle cells, and they could differentiate into skeletal muscle cells. So just a few things. When you see the cervix blast, even if you don't know anything else about that cell, you could guess fairly accurately. characteristics about that cell. So most bone cells are mesenchymal in origin. The only exception of course osteoclasts. Osteoclasts are hematopoietic in origin. They come from a macrophage. ...fage stem cell line and that's because they can undergo phagocytosis basically and degrade bone. When we think about the last part of endosteum, we see products. And those products come from these stem cells. And those products make marrow. Marrow is a soft, gelatinous tissue full of the products that came from these stem cells. So they accumulate and eventually will find their way into a blood vessel to be distributed to the rest of the body. It is itself sort of a, it's not fluid, it's just gelatinous type of tissue. And I will add that here in a moment as well. So some details about what we find externally. So hopefully you're starting to see, given this, all this stuff that happens here, that's important not just for bone but the rest of the body. Why sponge? bone is so physiologically important. Compact bone really doesn't have the ability to do this at least to the same degree. So over here I've made some very random trabecula. Again, you cannot draw this incorrectly. It doesn't have to look just like this, not at all. And what I'm going to be working on is the surface. I'm not going to talk about the internal features much. I'm going to remind you this is all ossified bone and it could be degraded fairly easily. Its components then return to the blood like calcium, phosphate. Other proteins that we would find within this ossified bone, like osteocalcin, hormone, proteoglycans, all of that, if it's degraded, is capable of entering the blood and it could go somewhere else for use. But when it comes to spongy bone being bioavailable, let's talk about how we could build or degrade this based on other needs of the bone or the body. So let's say over here we may have some osteoblasts, and I think I used this color before. And osteoblasts may be working in that mineralization front or seam. Maybe they've been directed to, so I'm just going to do OB for osteoblasts. Maybe they're making bone. And so over time, we would see this area here get reinforced. Maybe it gets a little bit thicker because we needed more strength on this aspect. So as osteoblasts lay down that bone, they themselves would be stuck in their own matrix and become osteocytes. Maybe we also, at the same time, needed that column just to be moved a little bit to support some new stress. So maybe over here we see osteoclasts. And maybe over here they're degrading that bone. They usually work as a team as well. That ruffled border, that apical surface, really chewing into that bone. And... And of course, if they're active, they're multinucleated. So I'm just going to add in some details we've talked about already just to help you connect concepts. So I'm going to label these osteoclasts, just OC. And feeding these cells, osteoblasts, osteoclasts are a lot of blood vessels. So I'm going to have blood vessels really just coming in and weaving around, maybe going in front of, coming back behind. And yes, osteoclasts can chew up blood vessels as well. They make no distinction. so lots and lots of features here that would be very useful for living cells that are very active so we have a lot of blood a lot of blood highly vascularized area also we have neurons in this area lymph vessels everything that you could possibly need to support living cells but we also have the endosteum and the endosteal stem cell niche which I'm going to highlight again I mentioned it before, but I think this is a good time to sort of go over it again. I'm going to be using this area here, this sort of cleaner area, to highlight this endosteum and endosteal stem cell niche. Just to be clear though, just because I do it here doesn't mean it isn't everywhere. I'm just showing you what it would look like, but realize the endosteum surrounds everything I've talked about here. Osteoblasts themselves are part of the endosteum. So I'm just going to do like a bracket and remind you that that. Endosteum is a region. It's a region that contains different structures, different components, endosteum. And so endosteum, let's do numbering here because that's a little bit easier, contains things like blood vessels. That's a really big part of endosteum. Sorry, I've got that tickle that just won't stop. The more you think about it, the worse it gets. So blood vessels. We also find hematopoietic stem cells and mesenchymal stem cells and I'll just be using sort of colored dots for that. We don't need to be too concerned about their structure but these are hematopoietic stem cells and we would find them in certain parts of this endosome. Sometimes they don't like oxygen as much as you think so sometimes they're a little bit farther removed from these blood vessels which is kind of strange because they make blood cells, but turns out during that developmental period they need to be a little bit farther away from oxygen. And then we would find mesenchymal stem cells sort of clinging to the surface here, maybe scattered a little bit out, but mostly on the surface here. So this is a very active area with hematopoietic stem cells, mesenchymal stem cells, blood vessels, and of course we would still have osteoblasts and osteoclasts, and I'm just trying to keep it pretty clean. So keeping with my numbering system, one, blood vessels, two, hematopoietic stem cells, three, mesenchymal stem cells, and these stem cells, hematopoietic stem cells, Poetic and mesenchymal are just making product like crazy and that product becomes the marrow and will fill all the spaces here. So when you think about endosteum, it's an area, a region that is... from the surface of your vitula, just a little bit, has all these features, all these cells, and then you can color it in with some sort of light red to indicate red marrow, and then you have a lot of very active hematopoietic stem cells in this area. Now, if this was a marrow cavity of a long bone, it would look different. It would likely be yellow marrow if we're talking about a juvenile to an adult. So this area again bioavailable because you have all these osteoclasts that I'm going to notice could be directed, controlled by osteoclasts to start degrading any of this. And now you can really see, if you treat your bone here, with the brain in calcium, how easy it would be to pick that up in an neighboring blood vessel and really fairly quickly correct the low blood calcium issue. Now the bones, I don't want to say suffer, but they donate that calcium and it's easily able to get into a transportation route nearby. So questions on this that I can help you with? Questions? Down here, I want to make sure you're aware of this question. What is marrow? What is the difference between red and yellow marrow? And the reason why, again, this is one of those that historically has been tricky for people, so I like to hit it frequently. remind you, hey, keep up on this. Make sure you still understand what it is. You know, information changes a little bit every time we have a lecture. Your understanding likely changes a little bit. So I just want to make sure that you're still clear with what marrow is. Also, it's probably good to remind yourself. what is the difference between yellow and red marrow? Who would have what marrow type and in what location? I'm going to stop there because I literally feel like I'm choking on this. You guys get that tickle every once in a while and you're like, I just cannot stop, so I'm going to spare you the coughing. We'll stop here for today. Please remember to take your quiz. I'll see you on Tuesday. If you have any questions, please let me know.

thick because if it was bones would be really heavy and very hard to move so it's not that bones are solid compact bone in fact they're mostly spongy bone but in any case we find that bones all bones contain compact and spongy bone but they differ in the proportion of each meaning how much of one relative to another and location in the bone so again i'm giving you that in the notes i won't spend a whole lot of time on that but we still find compact and spongy bone in all bones and so this is really what i wanted to focus on so i'm going to be going through some details here then i'll be making some rough sketches on this bone sample that i have an image of and then we'll be talking specifically about about the details of osteons. And osteons are really interesting structures that we find only in long bones, and specifically only in the diaphysis of long bones, and they serve a really unique role. But before we get to that, let's talk about this compact bone in general. Compact bone, as a reminder, it could also be called cortical bone.

That is a fine name for it. I don't use cortical bone, but in the future, your professor may. So let's talk about main functions of cortical or compact bone. So when we think about the main functions of this tissue, First of all, we're going to see protection.

And by protection, I mean really helping preserve the function of the soft tissue within a bone. So when you think about protection, you can think about, you know, like skull bones, preserving the function of the brain, or bones of the thoracic cavity, preserving the functions of the heart-lung complex. But we could also talk about the compact bone, protecting the delicate tissue within the bone, like blood. vessels or stem cells or bone marrow. So protection.

So when we think protection it would be fairly accurate to say of delicate tissue. And again we find delicate tissue both inside and outside of the bone. So compact bone because it has such a really dense hydroxyapatite formation, not many spaces at all in between that, very good at protecting.

We also, sort of a little different, it is the anchoring. site for connective tissue. Remember CT stands for connective tissue. And when it comes to the type of connective tissue that we will form an attachment with or anchoring site, we're talking tendons or ligaments. What's the difference between these?

Tendon attaches what to a bone? Muscle. That's right.

Tendon is skeletal muscle to a bone. Ligament then of course would attach bone to bone. So we'd find those like in the carpal regions, the tarsal regions, ankles and wrists for example.

So just make sure that when you see those words they they're fairly clear to you. When we think about compact bone, in general it is fairly similar. in most bones except in long bones and we think by shape you know we talked about short bones and flat bones long bones have a lot of tricks up their sleeves and that's because they do some amazing things in addition to protection anchoring they are really involved in locomotion so when it comes to long bones we find some unique compact bone features and those features have to do with how the bone tissue is formed and how it appears in those bones So we're going to talk about two different types or structures or patterns of bone tissue. And again, I'm keeping this specific to long bones because they are easy to see and they have some unique features that we don't see in any other type of bone.

So first is something called circumferential lamella or lamellae, plural. So first of all, this word circumferential, if it looks like the word circumference, you're not wrong. It is the type of bone that's on the very outer aspect of compact bone. It's not super thick, but circumference on the very outer aspect, the surface of the bone.

That's where we find circumferential lamella. So the very outer aspect or diameter of compact bone is called or formed into circumferential lamellae. Lamellae, by the way, it's kind of a weird word. It just means rings.

They don't have to be concentric rings, like perfectly round. And over time, they're going to be more or less like rings. oval-shaped, round structure. And so circumference...

Circumferential lamellae are oval shaped or round rings of bone on the circumference or outer aspect of bone. So this circumferential lamellae has a very unique composition and structure compared to the other type of bones. tissue that we find as far as compact bone goes. So again, we're only talking about compact bone, we're not talking about spongy bone. Compact bone itself has its own details.

So we'll find that circumferential lamellae, lamellae means ring, so they form rings around the circumference on the surface of bone. And lamellae is a word that we'll see quite a bit today. but wherever we see it, lamellae means a ring of some type. Not necessarily perfectly round, but a ring of some type.

Circumferential lamellae provides a great attachment site for this connective tissue. And that's really its main function is attachment site. Now, when we get into the weeds here of long bones, we find a really unique structure called an osteon and osteons are only found in the diaphysis of long bones.

We do not find osteons as part of any other type of bone. You won't find osteons in flat bones or irregular bones or short bones. You only find osteons in this very particular area of long bones. Highly unique, very very specific purpose which is why we find them here and not in other parts of the bones.

They are hugely important for allowing us to get tall, fairly large as far as animals go, and be able to move fast, turn, go the other way, and not snap our limbs. I think nine out of ten people are very interested in this. some people would agree that's probably a good thing.

The other one was in the hospital because they had their leg snapped. So we have to really take some time and appreciate osteons. So osteons are made of, pay attention to this word right here, concentric. Concentric lamellae. Perfectly round, highly organized, very very rigid structure.

And in the middle of these rings, these concentric rings, we find an interesting feature called a Haversion Canal. So all of this I will definitely illustrate for you here in a moment. So if you're thinking, wow, I really can't picture any of this, it's okay, we'll get to it. I just wanted to give you the basics, the details, so when we go into the drawing, you kind of have something to refer to as far.

structure and basic vocab words. Alright, so before we get into the structural details of an osteon, I'm going to draw your attention to this cross-section of a long bone. So to make sure everybody got this down before I move the camera.

Any questions on this sort of background detailed stuff yet before I apply it? Yes? Yeah sure, that's a good question.

A virgin canal is a central canal. You can think of it as a space. Right in the middle of an osteon. Only see them in osteons. They're only part of an osteon.

They form the center of the osteon. And what they contain and what they do, I will draw out for you here in a moment. So for now just know that Havergian canals are unique to Ossian because they're found smack in the center. And then around them are all these concentric lamellae. So good question there.

Other questions before I go up into this drawing? Alright, so what you have up here is an image that I took of a fairly fresh bone. Mine is in black and white because that's the printer that we have here.

I don't know if I gave it some sort of color. I try to like... the fence between a little bit of color so you can see distinction but not enough it's going to ruin your printer or like take out an entire e-cartridge try to print it off that's a balance but what I want to do is introduce you to parts of this book some of which we've talked about, some of which we are going to talk about.

I'm going to be starting on the very outer aspect of the bone. In the middle here, this is the medullary cavity. So this would be the center medullary or marrow cavity.

So we're obviously looking at a long bone. We're obviously looking at a diaphysis because that's the only one that's going to have a dedicated medullary or marrow cavity. Okay. On the outside here, this sort of rough piece piece of tissue that's periosteum and we've talked about periosteum before but as a reminder periosteum is an outer layer an outer tissue on the surface of the bone and you find it surrounding all parts of the bone except where there is what are we looking at to fill in the blank?

Articular cartilage yeah so on the surface of the bone it's either periosteum or articular cartilage but we're in the diaphysis the mid shaft of the bone, so obviously no joints here. So we're looking at periosteum and sort of refracted or peeled back, your thumb is, or peeled back at periosteum just so you can see it is its own tissue. And underneath it was bone tissue.

So when we think about periosteum, remember it has that inner osteogenic layer and it can make new bone. Okay, when we think about compact bone, and long bones really highlight the difference between compact and spongy bone so well, because you can take a cross section of a diaphysis and see both. So in this area right here, I'm just gonna sort of outline. This whole area right here, that's compact bone.

and very hard, very dense, as you would expect. And then from here to about here, even though we're in the diaphysis, that's spongy bone. So we don't have a lot of spongy bone in the diaphysis, but we still have some.

And you can see sort of the spongy cancellous bone as its other name, but these holes here make it look sort of spongy. So wherever we're looking in bone, we're going to have compact bone and spongy bone generally in these arrangements. Now I'm going to get into some details about circumferential lamellae and then osteons. And we can see their placement here.

we can actually see some converging canals here. They don't really stand out too well, but I will at least do like a dotted line to help you understand the placement of circumferential amyloid. And kind of think about what the word means, circumferential amyloid.

It runs on the circumference of this bone. So when we look at circumferential amyloid, it's going to be... just several rings of bone sort of in this area and you can almost see a color change even in this black and white photo.

But here this is part of compact bone here but this area is where we would have the circumferential lamellae. And then obviously in this area here We're going to have osteons. So we see osteons have the concentric lamellae, but we can see there's already a difference between this type of compact bone based on placement alone.

So circumferential lamellae will always be the outer aspect, the cross section that I have, so the outer aspect of that circumferential moving to the inside we have osteons. I'm going to just sketch in the details of what a circumferential lamella singular would look like and then compare that to an osseon. By the way, this is a converging canal, so that's how we know in the realm of the osseons. They are big enough to see with the naked eye on a brain, so they're pretty obvious if you get just the right cut.

So when we think about circumferential lamella... I'm just going to be using brown for the bone cells. We still see osteocytes. I'll be drawing them a little bit bigger than they would be, obviously.

We still see osteocytes living in lacuna, communicating. with the cannulae, but they don't run in a terribly organized, dedicated pattern. They just form the outer aspect of this bone. So I'm going to do that again. Here's the little cell body, its little extensions, sort of avoiding the area that I've written on.

And again these would be living in lacuna and communicating with cannulae. So we just see osteocytes on the circumference. That's going to be really different compared to osteons.

So osteons, I'm going to come down here where I already have some nice subversion canals that have made themselves known, and sort of highlight the area where we're going to find those osteons. So again, sort of on the inside. or deep to this dotted line.

I'm going to circle this and say here is a Haversian canal, here's another Haversian canal, and Haversian canals are on the center or are in the center of an osseous. and then radiating out from that are concentric lamellae, or rings of bone cells. So compared to this, where you just have bone cells sort of running on the outer aspect, osteons have bone cells that form...

very rigid concentric rings around the subversion canal so all these dashes that I'm drawing here these are osteocytes but I'm trying to at least get a few concentric lamellae sketched here so you can see what I mean by very very concentric very organized around that have version canal So that's kind of what we're looking at as far as details of compact bone. Questions before I continue? I'm going to sort of drill down and talk about the very super-duper details of an osteon to help you understand their significance.

Yeah? So the osteons are only going to be overgrading to the excuse of being a compact bone? So they're going to fill the space between where the circumference of the leg end. and one of the spongy bones in.

So you can have several in this region. Sometimes they look almost as though they are on top of each other. But we would have many, many layers of oxytocins. I just chose those because they're versioning out.

Like for example, if it helps, I could put, I could just make another haversian canal here and we would still see rings of osteocytes surrounding that haversian canal. So osteomes exist between circumferential limilite and the start of cell canal. Other questions before we continue?

All right, we're going to move into the details of an osteon again to help you understand the significance of them, why we only find them in long bones, and what would happen if we didn't have them, which is not good. So we're pretty happy that we have osteons. So we've talked about what they are. find them but I haven't really said much about their detail other than they have a center called nephrosian canal.

So before we continue we're going to talk about structural details of an osteon. A single osteon will be presented but again bones have long bones and the diaphysis have many. So things we could say about osteons, long and cylindrical in structure.

They almost look like, I don't know, those toy telescopes where you could pull them out and there's just like layer that comes out and layer and layer and layer. That's kind of what they remind me of. So they are very long and cylindrical in structure.

The Haversian Canal runs their entire length. So every osteon has its own Haversian Canal. Runs the entire length and it houses vessels like blood vessels, lymph vessels.

and neurons. So the Haversian Canal turns out to be a container in which we can place the soft tissue. Otherwise, the soft tissue has a hard time getting through that bone such that it can provide what osteocytes need, or osteoblasts.

Osteoblasts are all near together. So the Haversian Canal is a hollow tube that houses soft tissue like vessels and neurons. Surrounding that Haversion Canal are rings of osteocytes. Just like we saw up here.

So again, Haversion Canal, rings of osteocytes, concentric lamellae, very organized. I can even add that word here just to help contrast circumferential. And down here I'm going to add the word concentric, very organized.

Oh, I already had that. I got excited. I guess I just really like the word concentric. But anyway, so concentric lamellae, concentric rings around the Havergian Canal.

They look like they were almost made in a factory. They're so completely perfect. Very, very organized, that's for sure.

I'm going to lift this up a bit. A few more details before we start sketching. The collagen fibers, so as part of the osteoid, we talked about this soft tissue called osteoid. It's what osteoblasts make directly.

And the majority of osteoid is collagen, the most prevalent protein in the body. So these osteoblasts secrete these components of collagen fibers, which will become the actual mature collagen fiber. But here and only here do we find these osteoblasts deposit these collagen fibers at opposing angles.

And so that turns out to be the evolutionary trick that allows us to allows osteons to be really great at preventing bones from snapping when you turn and run the other direction. This is the trick, and I'll explain it to help you visualize what this means, but it took a while for people to figure out. So osteons are very concentric, very organized.

We think it has something to do with helping support these long bones. Think about how long a femur is and how much shearing force that's going to get on it if you turn and run the other direction. But what is it about these osteons? that are so important.

And it turns out it's this trick right here, seemingly so not important, turned out to be super important. So talk about how the collagen fibers contribute a lot to the tricks of the osteon. So at this point, I'm gonna draw this out for you. I used to present this already done and it didn't really work for people. So instead I'm gonna do it with you.

And I'm gonna be showing you how an osteon really looks like a telescope that you could pull. out like think about like a child's toy this plastic telescopes and there's just like a layer that comes out and layer and layer gets longer that's kind of what this looks like so I'm gonna be starting at the bottom and actually building this osteon from the bottom up and I'll explain why I do that here in a moment pretty good drawing field and at any point if you have a question about why it is the way it is feel free to speak up somebody's starting down here I'm going to be drawing a pretty big base to this osteon. And this osteon is made up of sort of ring after ring of bone.

So I'm going to give this in this area here. You're going to see me add some osteocytes. And I'm not going to be terribly detailed with their structure.

I'll just give them sort of a soma, a nucleus, and some processes. That's it. And that's good. That's all we really need. And so you'll see again, that's sort of the pattern here.

I'm making very concentric. Lamilla, that one has a weird nucleus. But here's a ring of bone. And what we're going to see on the inside of that, and so this ring of bone would exist all the way down different osteocytes.

organized. And so I'm going to sort of close this particular layer off and I'm going to go into a different layer. Still same osteon but I've just sort of pulled it apart so you can see the concentric rings of bone. And so again we're going to have osteocytes living very organized relationship to each other. Close that off.

And before I do the Haversion Canal, I'm gonna do one more. Again, they would be flattened if you looked at them from the top, like I did in that drawing above, but it's hard to understand the concentric rings unless you kind of get down to this level. So this one's pretty small, but there we go. And so I have pulled apart this osteon and then in the middle... I'm going to add a Haversian canal.

And that Haversian canal has a lot of features and a lot of significance to it. So first of all, this Haversian canal in the center here would run all the way down, but we're looking at solid bone here so obviously we can't see the Haversian canal all the way down. But what we could would see if we could look all the way through and we can see it on the top are blood vessels and those blood vessels can take sort of a networking approach and they're gonna be what these osteos...

sites rely on for nutrients. So maybe an artery or an arteriole, maybe a vein. Usually where we see arterioles, we're going to see a venule, for example. So blue and red, I think most people are familiar with what that means.

So two blood vessels, arteriole and venule, for example. We also often ignored, but too bad, we don't want to do that, lymph vessels. Lymph vessels are super thin, kind of built in a strange way. pattern. They have little trap doors that lets stuff in.

So that's a limp vessel and I'll label that because that's probably less familiar. And then we also have, I'll use orange for neurons. We have neurons, and some of those neurons may be sensory, and some of those neurons may be motor, but we definitely have neurons.

And so I'm just going to write neuron here. It doesn't really matter which type. So Haversian canals, it turns out, sort of give this a little bit more of a wall structure here. Haversian canals have a lot of stuff that's really important for the health and well-being of the osteocytes.

living in very uniform concentric lamellae. We don't see this arrangement in any other type of bone. Nothing else looks quite like this.

So that's kind of the introduction to Ossigan, but I have not talked about what makes them so dang special. Before I do that, questions on structure and function here? Can we see the concentric lamellae, how organized, how it's really all based around the haversian canal? These osteoblasts lay down the osteoid around the haversian canal, so the osteocytes have no choice but to form very concentrically because that's how the osteoblasts look. So we're so good.

All right, I'm going to add in collagen fibers now, and these collagen fibers are going to be light blue, and I'm going to do it in a very particular way. So as I do this, maybe just watch for a little bit before you go into your drawing, because this is the trick of the osteon. So these osteoblasts making these collagen fibrils, which later become mature collagen fibers, they lay them down in such a way that they run always in an opposite angle to each other.

So you could say it's running on the diagonal or maybe on the bias, but wherever we find neighboring concentric lamellae, they always have collagen laid down in opposite directions. And that was the evolutionary trick to the osteon. That is, principally, what allows...

We need mammal really that runs on the earth to run fast in one direction, plant that pet go the other way, and not snap the long bone. So let's talk about, now that you've sort of got your collagen fibers and all the directions, how on earth could that matter? What do you think that does?

I'll give you a hint. It has to do with torsion. Do you know what torsion is?

I've heard that word before. It's like a twisting. Things maybe twist in relation to each other.

So if you run in one direction, everybody's probably done that in high school. Like they had different names for it. like zippers or I don't know, it's been a long time. You had to run to one line and then, just for the heck of it, run back, and then go back, you know, just to like torture people and PE. But that's torsion.

And so when you plant that foot, the femur gets some rotation, torsion, because you're changing direction, turns out the collagen fibers allow the osteons to absorb that torsion just a little bit because it allows just a tiny, tiny bit of rotational movement before it will lock. So they will come a time where these things sort of lock and that absorbs that torsional force that could otherwise snap the bone. And so it's just crazy.

That alone allows a large animal with a lot of. velocity to go in a different direction. And humans aren't the only ones with this trick. All mammals that have had evolution on terrestrial ground. Rabbits are great.

You ever chase a rabbit? I don't know why you would, but if you did, they'd zigzag. And so they're very good at that and they would have that same setup.

So this is an old trick that nature developed and she put it in all of her terrestrial mammals. So that's pretty much what I want to talk about with Osteons. I'm going to give you a little break, just a couple minutes. I'll come back at 3.05.

We're moving to spongy bone. So 3.05, not that long. Use it wisely. Yeah, she was having a hard time. Yeah, yeah, yeah.

She was supposed to be awake. I had to wake her up. I had to wake her up. So, like, that is sort of, like, she was just getting her sleep. We got, like, a team of nine hours.

What do we think about this question down here? Arteries run along the surface of a bone. So, for example...

Thank you. Back up here. In order to get blood into a bone, it has to come from the external part of the bone.

You're going to have really large arteries on the external surface of a bone. So here's a really large artery. So I'm outside of the periosteum. And my question is, how do you get it here? Because this is where your aversion canals are that would feed this particular area of compact bone, so important for the health of the bone, and eventually wind its way into the spongy bone.

Any idea how you're going to get blood from here to here? It's in the reading. I haven't talked about it specifically.

I don't know if you've read that part of the book or maybe Megan talked about it in lab. So if you haven't come across it, it's okay. I'm just wondering. Anybody have an answer?

Name of a vessel or a canal. No takers. There.

So when we think about large arteries in the surface of a bone, they will enter a bone. So here's a branch. It's going to branch off of that large artery, become a smaller artery arteriole, and then from there it will splay out at almost 90 degree angles until it reaches. this area of osteons and that's what feeds the Haversian canal blood vessels. These are called Volkmann's canals.

Volkmann's canals and Volkmann's canals are really important because they feed a lot of bone tissue in a particular region. So the Volkmann's canals would eventually lead to the Haversian canal and feed that and it would continue to branch off. and feed the spongy bone.

So I just wanted to mention that I've said before bone has a lot of water in it. Bone tissue itself has water but also there's a lot of blood and when you look at a preserved bone, a dried bone, it is easy to forget they are highly vascularized structures, highly vascularized. And then of course we talked about getting blood vessels to the spongy bone because that's where we see that endosteum, which is a blood rich lining that houses stem cells.

So if you're going to have a blood rich lining here, the feeder vessels are out here, and you know it kind of lends the question, well how do you go from here to here? And it turns out bones have that figured out. So I just wanted to hit that really quickly before we move on to spongy bones.

speaking of spongy bone. Before I do, are there any questions here that you would like clarification on or some different explanation of? Yeah.

Does collagen lay between the legs? I'm sorry, you said it right. The collagen, is it, does it, which direction?

Oh, that's a great question. What's your name? Aaron. Yeah, it's a great question. Aaron's question is, the collagen, is it on the outside or is it within the concentric glomerulus?

Is that kind of your question? So yes to both, I guess. So you can see, if you're a really good artist, you could run that collagen fiber in the same direction through that entire lung blood. Does that answer your question? Yes.

Okay. And then this one, you know, if you're a big artist, you can come down. So that collagen is an integral part of that osteoid, which is the bone tissue. I just put it on the surface because it's easier and it sort of helps you understand the interlocking features of that.

But yes, these collagen fibers will run in the same direction interiorly as well. Good question. Any other questions?

That's a good observation and a good question. I'm going to move into spongy bone. So we've talked a little bit about spongy bone already. Good to have some detail.

Spongy bone is such a biologically, physiologically important tissue. I know compact bone looks really cool. It is, but honestly from a physiological perspective when we think about endocrine system, marrow, vasculature system, spongy bone really has a lot more important details.

talk about. So just as a reminder, all bones have spongy bone and it's always located deep to or within that compact bone. If we didn't want to find spongy bone on the surface that would be very bad.

It is strong in a way, but it still needs protection from the outer elements, the outer forces, I should say, by that compact bone. So some main functions of spongy bone. Three main functions, and I'll see if I can zoom in and not lose... Resolution, maybe. So three main functions.

That's still not too good. One, it forms the internal, what I call supportive strets for bone. This actually is collectively, these internal strets, collectively are quite strong on their own.

Very small, but collectively quite strong. So, support. Internal support against muscle tension. They also, because they have this blood-rich lining called endosteum, we'll find stem cells here. And just as important as the other two, we have their third function, main function, which is we consider spongy bone to be, we call it bioavailable.

Bioavailable. I'll explain what that means here in a moment. All right, so for our first bullet point, internal supportive struts, these are more appropriately termed trabiculae, and you've seen this word before.

It's always good to kind of review. Trabiculae is Latin for a little column of bone which acts like a supportive strut or brace inside the bone. Another main function house stem cells and that's because of this tissue called endosteum.

Endosteum houses the stem cell niche or environment in which these do pretty well in. And then our third function it's a bioavailable mineral source. I'm going to explain that. All bone tissue is made of calcium and phosphate, hydroxyapatite. But spongy bone, and only really spongy bone, is considered bioavailable, which means it's usable.

It's usable and accessible. Usable and accessible. The hydroxyapatite of compact foam is not accessible by and large. It's too dense.

It's all locked up in this very dense tissue. You really can't get your osteoclasts in there to dislodge some of that. And that's really good for a spongy belly. You kind of look at that scanning electron microscope picture that I've attached in the upper right-hand corner.

There's a ton of surface area and that allows osteoclasts to just really roll around and chew up what they want and liberate that calcium. So that's what we call spongy bone. We call it bioavailable. bone is not really very generous when it comes to helping balance blood calcium levels.

All right, when we think about spongy bone, again still in all bones, we find it as, as I mentioned before, trabiculae, little columns of bone. That's the Latin meaning of it. But what we're going to look at here in a minute is the internal and external structure of trabeculae. And this has typically been a source of confusion for students, which is why I really wanted to take just a little bit of time and be very clear. about what's on the inside and what's on the outside.

And so historically, when I went too fast through this, I think people for some reason equated the cross section of a trabecula singular with an osteon. And they thought they were the same. And so they are not the same. So that's why I want to be very clear about osteons are only compact bone.

And trabecular are also little columns of bone, but they're totally different. So do not be that person that equates osteon with spongy bone. They don't exist. Osteons are only compact bone. So on the inside, on the inside of trabecular, We can find a couple of things and then on the surface we can find a couple of things.

And there's some things that don't exist when it comes to spongy bone. So if we were to look at, come over here in this little microscope image, if we could zoom in and really see what's going on internally in this spongy bone. On the inside we would find osteocytes.

Living in Lacuna, communicating with cannuliculi. But we would not find concentric lamellae. We would not find a Haversian canal.

We would just find a little disc of ossified tissue. Osteocytes are still present. In fact, on the inside of spongy bone, we don't even find blood vessels. They're just too small. Too small.

So instead, we find a lot of that on the surface. So it's like trabiculae of spongy bone are sort of formed inside out compared to osteons. On the surface of trabicula, we would find osteoblasts, lots of them, always remodeling, and then their counterpart, osteoclasts. We'd find vessels, we'd find stem cells.

It's a very busy place on the surface of trabecula and that's why it's considered to be bioavailable. There's a lot of cell activity that can easily rearrange the structure, the shape, the angle. of trabiculae and so that helps the bone respond to external forces and allow that bone to be available for calcium. If you want to deposit calcium, super easy, just sort of smear it on the surface when you make your osteoid. You want that calcium?

Let your osteoclast have a little bit more freedom and start to chew that up. Put it in a nearby vessel. So trabeculae are a lot more important from a physiological perspective because of their structure.

So we're going to do a couple things on this. I'm going to sort of provide a drawing of the internal features of trabecula and then sort of remind you of the details of the external part, which we've seen a little bit, but not formally. So before I do that, just a little note about this image that you have in the upper right hand corner. Scanning electron microscopes are very powerful, much more than a light microscope.

And so I purchased these images, I licensed them, and I just really thought this was worthy because it really shows you the detail of spongy bone. So everything you see here is an actual trabiculae. This is from a human.

Little spongy bone columns. everywhere and all these holes here remember those are supposed to be there these would be filled with marrow so red marrow because it's not a large cavity like we see in the diaphysis spongy bone typically contains red marrow and so if you want to just sort of remind yourself that that these columns of bone house stem cells that make marrow and it's usually red i don't know how well that's going to show up but if you want to sort of color these little pockets in in a few places that show up for you with red marrow that might help you understand some other details that I'm going to talk about. And that is another reason why spongy bone is such a great tissue.

It has a lot of places where we can store unique tissue like marrow. So just realize it's there. You can, you know, it might be worthy of your time after lecture to sort of compare what I talk about here to this actual drawing up there. But for now, we're going to move into internal features of a trabecula. trabecula singular, so if we could isolate a single column of bone there, we would find that bone cells, the osteocytes inside, still exist in lamella.

They're still in rings, but they're not concentric. They just sort of take the random shape of this trabecula, and they can change. Bone cells still exist as rings or units that look ring-like.

but not concentric, not like an osteon. And as I mentioned, the bone cells, the osteocytes, they're still living in the lacuna. That's the little pothole, the little lake that they live in full of fluid.

So this is Latin for little lake, and it contains extracellular fluid. And that is a diffusional medium that those cells use to attain nutrients, get rid of waste. I have cannulae that radiate out from that lacunae, little canals, lets them communicate, lets them get nutrients from the outside. I'll draw this out here in a moment. You can see I'm going to provide a sketch of a trabecula, but let's get through some basics first.

So even though we have osteocytes or bone cells and they are going to exist in units, ring-like units, they are not nearly as organized as osteons. So as a reminder, there are no osteons in spongy bone. That is only a compact bone feature, no osteons. Further, there's no blood vessels internally, so we won't find aversion canals, we won't find neurons inside, we won't find lymph vessels inside. Everything has been relegated to the surface and that actually opened up other opportunities like nourishing stem cells, providing a place for them to produce their product and have it accumulate.

So it turns out this was pretty beneficial. When we think about supporting the cells within, it's diffusion. So we're going to see these cells that are in here need diffusion from the blood vessels out here in. And that's going to limit the number of osteocytes that we can have internally because diffusion is a slow process.

And too far of a distance is going to really limit the ability of those cells to survive. So diffusion to these bone cells from the external blood vessels is key. And again, that really limits the number of lamellae present. you know, can't have rings and rings and rings of bone, the inner ones would not get enough nutrients. So we will see a small surface, not surface area, but cross-sectional area to this trabecula.

Alright. So I'm going to sort of sketch out the details here. And so what I'll have you do, you cannot draw this incorrectly. This is going to be just a zoomed in image of a trabecula.

It does not have to look just like this. You don't have to follow it. Trabecula are random, so feel free to let your hand do what it wants.

I'll be talking about external features in the drawing below. So right now, when you do draw, draw yourself like you have this comb on the bone. And you cut it so that you can see the internal features of it. I'm going to do two different aspects to help you see this kind of random looking structure. But wherever we find it, we see some similarities.

So on this, I'm going to color the surface because we're not going to be worried about what the surface has. And this is just ossified bone. We'll talk about what we'd find on that surface in my next drawing down below. But right now we're just going to have bone tissue ossified, pretty strong, good against tensional forces. put on it by muscle on the outside, but on the inside, that's where we're going to focus.

So inside, I've got sort of a cut surface here, and I'm going to find, I'm just going to do really sort of small dashes for osteocytes, and they may... live in sort of units, but again it's not necessarily concentric looking. We don't see them in these nice organized rings. They kind of take different directions and that's really all that we find inside.

So these would be osteocytes and they would be living in lacuna. I'm not going to draw that in, but they would be living in lacuna, those little lakes, and communicating with canuliculi. But notice the bone cells here.

random smattering, nothing concentric at all. And they are really pretty short-lived because on the surface, which we'll see in a moment, are osteoblasts waiting to just chomp through that. So it doesn't make sense to make them in a very organized fashion because they're not going to be around as long.

On the other sort of cut surface we'd see the same thing. We'd see osteocytes. Some sort of lamellae but nothing concentric. So same thing.

Very different than osteons. No reversion canal, no concentric rings. That's pretty much what we found on the inside. Osteocytes and bone tissue.

So pretty straightforward. I am gonna go down and talk about the external features. So questions before I. No questions.

Alright, let's talk about external features. Very different. Spongy bone has a lot of tricks. It's not big, so it uses its surface well. External features of trabecula, again just working off of a single column of bone here.

This is a very busy area internally. Trabecular are kind of plain, straightforward. On the surface though, that's where all the stuff happens. This is a very busy area. First of all, we're going to see there's a really good population of osteoblasts and osteoclasts present.

And they are always present, and they are, to some degree, always active. Because you have to build bone all the time. damage you don't have to have a huge calcium deficit you get a new skeleton every seven years and again it's not like on you know you're six point nine you have an old skeleton and you wake up on your son's birthday and you're like I feel different what is it? Oh I know I got new bones. That's not how it works it's a continual process so if it's going to be a continual process the area of the cells are active continuously building and rebuilding chewing and rehashing this tissue These cells need a lot of blood.

They need a lot of oxygen. These are very active cells. So, very conveniently, we find this to be a highly vascular area. A lot of blood vessels, lymph vessels, everything that we saw in a virgin canal.

has been flipped is now on the outside to provide the nutrients and waste removal for these bone cells so lots and lots of blood vessels and those blood vessels are generally contained in an area called endosteum. And I've mentioned this before, endosteum is sort of a collective area. I will sketch out on the surface here where endosteum exists and what it will be comprised of.

But as a reminder, as part of the endosteum, we usually find stem cells, HSCs and MFCs. What do HSCs stand for? Good job, that's a hard one. A lot of people are like, I don't want to say it out loud.

Oh, good job there, very brave. So HSCs, hematopoietic stem cells. And so those would produce red blood cells, white blood cells, and something called a thrombocyte, which eventually makes platelets. Platelets are not living. Platelets are fragments of cells.

So hematopoietic stem cells think blood cells. MSC, what does that stand for? Mesenchymal. Yeah, good job there. So mesenchymal.

mesenchymal stem cells and these produce cells that end in the suffix blast and that's a lot so I'm going to say the blast cells osteoblasts Wherever we see the suffix blast, the suffix blast tells us a lot about that cell. So cells that enter the suffix blast, they are usually very active making a product. They are sometimes immature. I don't mean like they make stupid jokes and like prank people. I mean that they are, they could differentiate into something else yet.

So, for example, osteoblasts, you know, they differentiate into osteocytes. and osteoblasts make osteoid. As another example, fibroblasts.

Those are in the skin or really all over any part of connective tissue. Fibroblasts make collagen, and they could differentiate into fibrocytes. Myoblasts, these are immature muscle cells, and they could differentiate into skeletal muscle cells.

So just a few things. When you see the cervix blast, even if you don't know anything else about that cell, you could guess fairly accurately. characteristics about that cell.

So most bone cells are mesenchymal in origin. The only exception of course osteoclasts. Osteoclasts are hematopoietic in origin.

They come from a macrophage. ...fage stem cell line and that's because they can undergo phagocytosis basically and degrade bone. When we think about the last part of endosteum, we see products. And those products come from these stem cells.

And those products make marrow. Marrow is a soft, gelatinous tissue full of the products that came from these stem cells. So they accumulate and eventually will find their way into a blood vessel to be distributed to the rest of the body.

It is itself sort of a, it's not fluid, it's just gelatinous type of tissue. And I will add that here in a moment as well. So some details about what we find externally.

So hopefully you're starting to see, given this, all this stuff that happens here, that's important not just for bone but the rest of the body. Why sponge? bone is so physiologically important.

Compact bone really doesn't have the ability to do this at least to the same degree. So over here I've made some very random trabecula. Again, you cannot draw this incorrectly. It doesn't have to look just like this, not at all.

And what I'm going to be working on is the surface. I'm not going to talk about the internal features much. I'm going to remind you this is all ossified bone and it could be degraded fairly easily. Its components then return to the blood like calcium, phosphate.

Other proteins that we would find within this ossified bone, like osteocalcin, hormone, proteoglycans, all of that, if it's degraded, is capable of entering the blood and it could go somewhere else for use. But when it comes to spongy bone being bioavailable, let's talk about how we could build or degrade this based on other needs of the bone or the body. So let's say over here we may have some osteoblasts, and I think I used this color before.

And osteoblasts may be working in that mineralization front or seam. Maybe they've been directed to, so I'm just going to do OB for osteoblasts. Maybe they're making bone.

And so over time, we would see this area here get reinforced. Maybe it gets a little bit thicker because we needed more strength on this aspect. So as osteoblasts lay down that bone, they themselves would be stuck in their own matrix and become osteocytes.

Maybe we also, at the same time, needed that column just to be moved a little bit to support some new stress. So maybe over here we see osteoclasts. And maybe over here they're degrading that bone. They usually work as a team as well.

That ruffled border, that apical surface, really chewing into that bone. And... And of course, if they're active, they're multinucleated.

So I'm just going to add in some details we've talked about already just to help you connect concepts. So I'm going to label these osteoclasts, just OC. And feeding these cells, osteoblasts, osteoclasts are a lot of blood vessels. So I'm going to have blood vessels really just coming in and weaving around, maybe going in front of, coming back behind. And yes, osteoclasts can chew up blood vessels as well.

They make no distinction. so lots and lots of features here that would be very useful for living cells that are very active so we have a lot of blood a lot of blood highly vascularized area also we have neurons in this area lymph vessels everything that you could possibly need to support living cells but we also have the endosteum and the endosteal stem cell niche which I'm going to highlight again I mentioned it before, but I think this is a good time to sort of go over it again. I'm going to be using this area here, this sort of cleaner area, to highlight this endosteum and endosteal stem cell niche.

Just to be clear though, just because I do it here doesn't mean it isn't everywhere. I'm just showing you what it would look like, but realize the endosteum surrounds everything I've talked about here. Osteoblasts themselves are part of the endosteum.

So I'm just going to do like a bracket and remind you that that. Endosteum is a region. It's a region that contains different structures, different components, endosteum. And so endosteum, let's do numbering here because that's a little bit easier, contains things like blood vessels.

That's a really big part of endosteum. Sorry, I've got that tickle that just won't stop. The more you think about it, the worse it gets.

So blood vessels. We also find hematopoietic stem cells and mesenchymal stem cells and I'll just be using sort of colored dots for that. We don't need to be too concerned about their structure but these are hematopoietic stem cells and we would find them in certain parts of this endosome. Sometimes they don't like oxygen as much as you think so sometimes they're a little bit farther removed from these blood vessels which is kind of strange because they make blood cells, but turns out during that developmental period they need to be a little bit farther away from oxygen.

And then we would find mesenchymal stem cells sort of clinging to the surface here, maybe scattered a little bit out, but mostly on the surface here. So this is a very active area with hematopoietic stem cells, mesenchymal stem cells, blood vessels, and of course we would still have osteoblasts and osteoclasts, and I'm just trying to keep it pretty clean. So keeping with my numbering system, one, blood vessels, two, hematopoietic stem cells, three, mesenchymal stem cells, and these stem cells, hematopoietic stem cells, Poetic and mesenchymal are just making product like crazy and that product becomes the marrow and will fill all the spaces here.

So when you think about endosteum, it's an area, a region that is... from the surface of your vitula, just a little bit, has all these features, all these cells, and then you can color it in with some sort of light red to indicate red marrow, and then you have a lot of very active hematopoietic stem cells in this area. Now, if this was a marrow cavity of a long bone, it would look different.

It would likely be yellow marrow if we're talking about a juvenile to an adult. So this area again bioavailable because you have all these osteoclasts that I'm going to notice could be directed, controlled by osteoclasts to start degrading any of this. And now you can really see, if you treat your bone here, with the brain in calcium, how easy it would be to pick that up in an neighboring blood vessel and really fairly quickly correct the low blood calcium issue.

Now the bones, I don't want to say suffer, but they donate that calcium and it's easily able to get into a transportation route nearby. So questions on this that I can help you with? Questions? Down here, I want to make sure you're aware of this question.

What is marrow? What is the difference between red and yellow marrow? And the reason why, again, this is one of those that historically has been tricky for people, so I like to hit it frequently.

remind you, hey, keep up on this. Make sure you still understand what it is. You know, information changes a little bit every time we have a lecture. Your understanding likely changes a little bit. So I just want to make sure that you're still clear with what marrow is.

Also, it's probably good to remind yourself. what is the difference between yellow and red marrow? Who would have what marrow type and in what location? I'm going to stop there because I literally feel like I'm choking on this.

You guys get that tickle every once in a while and you're like, I just cannot stop, so I'm going to spare you the coughing. We'll stop here for today. Please remember to take your quiz.

I'll see you on Tuesday. If you have any questions, please let me know.

Transcript for:Week 6 lecture 5

Transcript for:
Week 6 lecture 5