Understanding Microscopic Bone Structure

Okay, microscopic bone structure. So now we're going to be looking at the bone a little more in depth. So in those organic components, remember, which is about a third of the bone, is the collagen fibers as well as the cells. So there are different types of bone cells. The first is osteoprogenator cells. These sound terrible, but if you break it down, osteo means bone. If you've ever heard the word progeny, that means babies or offspring. So this literally means bone babies. And these cells are osteogenic. Osteo means bone. Gene. Genic means to make. So these are little baby cells that are making the bone. Okay, osteoprogenator cells can undergo mitosis. So these cells are a type of stem cells. So some of them become osteoblasts. Some stay stem cells. So I think of it like kids. When little kids are running around and we say, what do you want to be when you grow up? Some of them say doctor. Some of them say lawyer. Some of them say space cowboy. Whatever. The point is, a child, in theory, can be anything they want to be. especially when they're like five. Now when you ask like a 15 year old what do you want to be and they say a doctor you kind of go yeah maybe not or you're like okay that's possible. By then you could kind of tell. Osteoprogenator cells can be any type of bone cell they want to be. So a stem cell is basically a cell that hasn't chosen what it's going to be when it grows up. So these guys at least know they're going to be bone cells. So we constantly get new osteoprogenator cells. So some children grow up to be bone builders. They grow up to be because construction workers. Some osteoprogenitor cells grow up to be osteoblasts. Others stay as stem cells, just like a lot of people don't know what they want to be when they grow up. So sometimes there's those, you know, 30-year-old kids like sleeping on their mom's couch because they don't want to get a job, which really I don't know why the parents let them do this. I'd be like, get a job. They're found in the endosteum, the inner layer of the periosteum. So you're constantly building new bone. So the only source of new cells in the bone. bone has to come from osteoprogenator cells. Just like if we need doctors or we need construction workers, the only place we're going to get those is if a child grows up and decides to be a doctor or a construction worker. So some of these osteoprogenator cells decide, okay, I'm going to be a construction worker, and they become osteoblasts. Osteoblasts'job is to build bone. That's all they do. So blast means to build. So they're making that matrix by secreting those collagen ropes, and then the calcium phosphate has something to do with it. to attach to. Now these guys can't go through mitosis. They're stuck in that G0, that kind of rest area of mitosis that we talked about in lab. They can't go through mitosis. They are stuck. The only way we get new osteoblasts is those osteoprogenator cells have to grow up and become them. Osteocytes then are mature bone cells. So osteoblasts work so hard that sometimes they're building, they're building, they're building, they're building, they're building, and they trap themselves. Like this picture I show here of that guy kind of standing in the concrete there. He's stuck. He's kind of painted himself into a corner, if you will. So he's got nowhere to go. So these guys build, build, build, build, build. They build this bony, hard matrix all around them, and then they have nowhere to go. So they might as well retire. So these are your mature bone cells. They're not. building bone anymore. They're just living their lives. They live inside the little lacunae. Remember when we saw tissues? They need that little space because otherwise if I fill the room with concrete, you're going to drown. You have to have a little tunnel so you can breathe. So those caniculi were those little tunnels. Remember, we've got to ultimately get back to the central canal, back to the blood, so we can get our oxygen and our nutrients and get rid of our waste. So these guys do not undergo mitosis. So the only way we get new osteocytes is osteoblasts have to paint themselves into a corner. They have to build themselves into a corner. The only way we get new osteoblasts is those osteoprogenator cells have to grow up. Then we have osteoclasts. These do not come from the osteoprogenator cells. Osteoclast, clast, kind of clash means to destroy. So these guys destroy the bone, which sounds awful when you think about it. But the skeleton, remember, is your savings account for these inorganic salts, for calcium and phosphorus. So if your blood needs some calcium, you're going to chew up your skeleton and free that calcium and get it into your blood. So these guys are constantly destroying the bone and releasing the minerals back into the blood. And this is a... important for repairing bone. Like say you broke a bone. Well, you may pull some calcium out of another bone. So you have some to fix this bone that's in distress, but also for your muscles and your nerves to fire. We've got to have calcium. Multinucleated. So remember skeletal muscle was multinucleated. And I said that was a really rare thing because most cells just have one nucleus. Technically skeletal muscle was a bunch of embryonic cells. fused together and brought their little nucleus with them. So technically they used to be individual cells, but now they're one big cell with lots of nuclei. Well, the same thing kind of happened with osteoclasts. A bunch of white blood cells fused together and bring their nuclei. So if you remember, white blood cells are phagocytes. Wah, wah, wah. The little Pac-Man thing. They're constantly gobbling up and chewing things up. So that's what the clasts do. They're chewing up your skeletal system. They produce an acid that helps dissolve the inorganic stuff and an enzyme to digest the organic stuff, like that example we started with in the last lecture with the meat tenderizer. So their job is to destroy. It sounds awful, but there are times when you need to rob your skeleton. There are times when you need to pull money out of your savings account. So their job is to do. So osteoblasts and osteoclasts are in the periosteum, the outer layer, and the endosteum. Like we had said before, You can build a house by working on the outside and the inside at the same time. The organic component, collagen. Collagen is also called osteoid. We're just going to call it collagen. Remember from the integument system, we saw vitamin C was to make collagen, and that's what helps keep your skin all tight. Well, collagen fibers are connective tissue that are found in the bone matrix, giving that bone its bendability. Because like we said before, we don't want to tap our knee on a desk and have it shatter. You want a little bit of flexibility. The collagen provides some strength as well. Hydroxyapatite, we don't really need that word in our lives because we're really just talking about calcium phosphate. So it's just a specific structure of it. So calcium phosphate gives bone its hardness and rigidness, about 65%. As we've been told our whole lives, calcium make our bones strong. Microscopic structure, we've already done. Remember, it looks like soda cans or beer cans from the top. So if you look at the compact bone picture there on the right, you have the little red center, that little central canal. You have the osteocytes living inside their little lacunae. So if you look at the picture then that's farther to the right, you have the little yellow osteocyte and the little lacunae that it's in, and all those little caniculae, so they ultimately kind of get back to the blood supply. So we've seen these all before. The osteon is the whole top of the soda can, so that includes the central canal and the lamella. The lamella, remember we don't really need that word, but that's just the rings. They lay down around the blood vessel, which is in the central canal. You can also call it a Haversian canal. I imagine somebody named Haversian thinks they're quite fancy. I just prefer osteon. Lacunae is the little space. Osteocytes, the mature bone cells, because the builders trapped themselves in that matrix. Those osteoblasts work too hard, and then the cell retires. Caniculi, important to get nutrients and waste. They've got to connect back to that blood. like little driveways. So an osteon or Haversian system, this is a nice picture of it. So you have that Haversian canal, where the blood is, which is the central canal. And then you have all those osteocytes and see all the little, little caniculi kind of getting, getting back to there. So the osteocytes, remember they lay themselves down in these concentric rings. You can't get too away from that blood. And the caniculi are like driveways connecting every single one of those cells back to the blood. So bone is crazy complicated. When we look at a bone in lab, it just doesn't look that complicated, but surely there's a lot going on. One thing we haven't seen with tissues, we just talked about the central canal, but we also have these perforating or Volkmann's canal. Again, I imagine it's named after somebody, Volkmann, who's very impressed with himself. I prefer perforating because I think about perforations like the little holes in a notebook that you can rip the paper out easier. This perforating canal, if you look, it looks like the letter H. You have the two central canals and you have the perforating canal in the middle. Well, blood is all connected, so the perforating canals are just to link two central canals together. So, our skeleton has cartilage and bone, but also this material called dentin. DAC doesn't have a dental hygiene program. I wish we did. And so, we don't do a lot with teeth, but I did want to at least mention dentin. So, cartilage, remember, was a vascular. It doesn't hurt to get a cartilage piercing. It doesn't bleed. It hurts when you're going through the skin because there's nerve and blood there, but once you're deep inside the cartilage, it doesn't hurt. We have perichondrium covering the cartilage, just like we have periosteum covering the bone. Chondrocytes were the mature bone cells that, remember, look like eyeballs staring back at you in lab. There's no calcification, so there's no calcium. There's no caniculi. Remember, the cartilage is very, very slow to heal because it doesn't have a direct blood supply. It just has to rely on diffusion to feed it. And so to heal, it would never, ever get enough nutrients to be able to handle a major repair. Denton. Denton is... Also avascular. If you've ever had your teeth drilled, yowza, doesn't bleed when they're getting through the outer layer. It's bone-like. It's actually stronger than bone, but there's no osteoblasts, there's no osteoclasts, and there's no Haversian canals. Bone is alive. It's very bloody. It's very vascularized. The osteocytes are the mature bone cells, and there's a lot of calcium involved. So this is kind of comparing them. So as bones develop, we call this osteogenesis, because osteo means bone, genesis means to make. and ossification, which is really the hardening of bones. So these are two very similar words. But it's getting the bony skeleton and embryos when we're growing and then as we're growing in height until early adulthood. after that, if you're done growing in height, everybody else is doing the last one all the time. Our bones change in thickness. We're constantly remodeling and occasionally we have to repair. So this is a little growing fetus. Now, I did not add the milkalobe. A student did that, although it's really funny. We just thought that this picture looked like he was holding something, so somebody came up with milkalobe. Get it. So in this picture, this is a fetus that's pretty far along. So about six to seven years old. to seven weeks into gestation, we start this process. But this is a lot further on than that. If you look, he's got some intermembraneous bones forming in the skull, which we'll kind of see that next. So you can see some of those plates coming together in the skull. You can see the vertebrae. You can see his cute little humerus there and his little radius and ulna, his little femurs, those little coxal bones. So the kind of the map work of the skeleton is pretty laid out here. But early on in development, usually again, six to seven weeks, and this is probably more more like 20 or so, we've got fibrous connective tissue and hyaline cartilage. So we get these, as it says there in quotes, quote unquote skeleton, because it's not bone yet. What your body does is it plans the bone by laying down those hyaline cartilage jello molds again. So that way the bone has something to attach to. So bone development and changing is very different in newborns versus adults. In newborns, the hematopoietic tissue, remember heme is blood, the point means to make. So the tissue. that's making the blood, the bone marrow, is found in almost all of your bones, in almost all of the spongy bones. There's lots of red bone marrow. Because if you think about it, a newborn is growing so fast, like just constantly growing, constantly needing new blood to support this larger organism that it's becoming. Whereas when you're an adult, you just have to maintain your blood supply. So most of our long bones, instead of having red marrow, have just become yellow marrow, which is fat. Because fat is good for cushioning and and good for energy storage. So really, our bone cell production is just in a few bones. The heads of the femur, that big ball joint, and in the humerus in our arm, and in our flat bones, such as our sternum, and our coxal bones, the irregular bones. So the sternum and the coxal bones are huge places for making blood. So the point is, I don't usually lose a lot of blood, and I'm growing out, but I'm not growing up. And so we just don't need to make the mass production of blood cells that newborns need to make. So newborns have lots of red... marrow, adults have lots of yellow. So the flat bones, the sternum and the coxal bones are much more active in hematopoiesis for adults. And this is where they do sampling to see if your marrow is okay. So to diagnose cancers or certain anemias or pain cytopania, which is just all of your blood is being reduced, they would do a bone marrow sample. And they usually take it from the hip bones. They'd go in there and drill it out. Yowza, looks painful. Now, if you go through some kind of trauma and you need lots of blood, if you you become anemic or something happens to you, your red bone marrow can actually convert back to red. So that's pretty cool. Formation of the bony skeleton. So as we grow, about six to seven weeks in, we start convertiling cartilage into bone. We're not going into a lot of crazy details here, so don't be scared by this. As we get into it, I'm going to kind of say what do we need to know. So there's two types of converting our skeleton into bone. Intramembraneous ossification and endochondral ossification. The issue with this is we're going from connective tissue, like cartilage, into another connective tissue of bone. So we have to remodel. We have to take that pre-existing connective tissue, cartilage, converted into bone. These are two very different tissues. Now, intramembraneous ossification is the formation of our skull bones and some of our flat bones. So this is not the most common one. Basically, bones are formed within a fibrous connective tissue. We don't really care about intramembraneous ossification. intramembraneous ossification that much on that. So the stages of intramembraneous ossification. In the fibrous connective tissue with all the stringy collagen, which is shown there, a cluster of osteoblasts become activated. So basically... These mesenchymal cells, these embryonic cells that we don't need to know about mesenchymal cells, grow into osteoblasts in these builders, and so they start building the bone. You can see that osteocyte, how he got trapped by that matrix, and now he has nowhere to go, so he might as well retire. And the builders keep building, and they get trapped and create osteocytes. Eventually, blood kind of weaves its way in there, and then the osteoblasts are still building on the outside, so you can see the trabeculae left room for the butycine. kind of come in there. So at birth, your skull bones aren't completely fused. They've got this fibrous membrane in between that we call fontanelles. You don't need to know the word fontanelle and you don't need to know the different fontanelles, but they begin to ossify about two months in and the last one is finally done, the anterior, the soft spot, if you've ever held a baby at two years. It's important that the skull, remember, is not solid at birth. This has to fit through the vagina. Poor vagina. We don't want her to be abused. So the head needs some flexibility. When I was born, I looked like I had a cone head because my skull bones were so pointy. And of course, then they flatten out. We also, up until age two, are learning to walk and we're going to fall down a lot. And so this gives room for flexibility swelling as well. But the anterior fontanelle is definitely the biggest. So this is showing a fetal skull. They will all grow completely together by age two. Endochondral ossification. This begins in the second month of development, so six to eight weeks in. We have these hyaline cartilage bones as models, so we start with the cartilage. So now we have a problem. Before we can make bone, we have to break down that cartilage. So almost all of your bones are formed in this manner, so I kind of want you to know the general steps of endochondral ospiction, and like I said, I'll tell you what to focus on. So the first thing, we've got a problem. We've got to get from cartilage to bone. And the biggest difference is, remember, cartilage doesn't have a blood supply. Bone does. So we start with these little pre-bones. See how they kind of look like a bone shape, but not really. So we start with this jello mold of hyaline cartilage. Right now, this cartilage is keeping bone out. The way, or keeping, sorry, keeping blood out. The way that cartilage keeps blood out is it makes a chemical called anti-angiogenesis factor, which you need to know. That's a terrible word, I know, but anti means against. Angio, like an angiogram for the heart, means blood. Genesis means to make. So this literally means against blood making. So although the term is terrible, once we get familiar with some of these roots, we can break it down. So this cartilage is releasing a chemical saying blood you can't come in. Just like if you sprayed Raid in your house to keep out ants. The ants can't come in because the chemical is present. The ants really want to come in. That's what ants do. The chemical keeps them out. So as these cartilage bones get bigger, the cells inside get so big that they can't feed themselves through diffusion anymore, which is kind of sad. So they die. As soon as they die, they can't produce the anti-angiogenesis factor anymore. And the rest of your body is producing angiogenesis factor, which is saying blood men. So immediately blood comes in. The only thing that keeps blood out is the fact that that cartilage makes that chemical. If that chemical is not present, say you go on vacation and you're not spraying. rate anymore, the ants come in. So now all of a sudden the blood comes in. So it comes in through the center there, through the medullary cavity, then it also starts in the ends kind of independently as well. So the diaphysis is doing its thing and the epiphysis is doing start doing the same thing. But you can see on that third picture those epiphyseal plates, those growth plates. As long as that's still cartilage, the bone is still growing. This picture is the best because it shows kind of the whole thing on one page. So four months to birth, you have that cartilaginous model there. And I like that second picture in on the left where the cartilage cells are getting so big. So they get so big, they can't feed themselves through diffusion anymore. They can't make that anti-angiogenesis factor And so in the third picture, the blood migrates in. Blood always wants to come in. So when the blood comes in, it brings in osteoblasts, which immediately start building bone. And the osteoblasts, remember, become osteocytes when they kind of corner themselves, when they paint themselves into a corner. So this happens 12 to 25 years is usually when, you know, your growth plates fuse when the bone is completely dead. So osteoblasts are the ones building the bone. Periosteum. is the outer kind of layer of the bone, you have osteoblasts in there as well. And so we call this apositional growth. Longitudinal growth is the growth we just saw, where the bone was getting longer. Apositional growth is the bones getting thicker. And that's something that happens for the rest of your life. It seems so. It's unfair. It's like you could get strong bones, but then if you don't take care of yourself for like six months, your bones get weak. Same thing with muscles. It's like you use it or you lose it kind of thing. It's really depressing. I have no patience for tasks like this. It's kind of like doing dishes. Every time you do dishes, there's more dishes. Every time you do laundry, there's more laundry. You're never done strengthening your bones. because they can constantly change because the medullary cavity has the osteoclast breaking it down and then you have the osteoblast building it back so if the osteoblasts work well you have strong bones if the osteoclasts work too hard then you have weak bones. It's just a constant yin and yang. So from 12 to 25 years, the primary and secondary ossification centers come together, which just means the center in the diaphysis and the center in the epiphysis on the ends kind of meet. Then you get your growth plate is when that's completely ossified. It's no longer cartilage, which means longitudinal growth is over. So we all have this little leftover line. We keep cartilage on the ends of our bones though, and we call it articular cartilage. Articular cartilage is the part of the brain that's going to be the most important cartilage is just where two bones articulate, where two bones come together. We'll have a lab where we talk about joints and we'll see that this cartilage is essential because these bones are grinding together all the time. So we want to keep a little knobby, rubbery protection on the end. But most of the time, by age 25, your bones are all done growing longitudinally. So this little video here. Bones grow through the process of appositional growth, the formation of new bone on the surface of older bone or cartilage. Osteoblasts beneath the periosteum lay down bone to form ridges around a blood vessel. The blood vessel lies in a groove between the ridges. The groove is transformed into a tunnel when the bone built on the adjacent ridges meets. The periosteum of the groove becomes the endosteum of the tunnel. Osteoblasts from the endosteum lay down bone to form a new concentric lamella. The production of additional concentric lamellae fills in the the tunnel and completes the formation of a new osteon. Although cheesy, I kind of like the visual of how it shows the tissue kind of fold and then the osteoblast kind of fill it in. Because they want to be close to that blood supply so they can get their little caniculae and that way they can be fed.

So there are different types of bone cells. The first is osteoprogenator cells. These sound terrible, but if you break it down, osteo means bone. If you've ever heard the word progeny, that means babies or offspring.

So this literally means bone babies. And these cells are osteogenic. Osteo means bone. Gene.

Genic means to make. So these are little baby cells that are making the bone. Okay, osteoprogenator cells can undergo mitosis.

So these cells are a type of stem cells. So some of them become osteoblasts. Some stay stem cells. So I think of it like kids. When little kids are running around and we say, what do you want to be when you grow up?

Some of them say doctor. Some of them say lawyer. Some of them say space cowboy.

Whatever. The point is, a child, in theory, can be anything they want to be. especially when they're like five.

Now when you ask like a 15 year old what do you want to be and they say a doctor you kind of go yeah maybe not or you're like okay that's possible. By then you could kind of tell. Osteoprogenator cells can be any type of bone cell they want to be.

So a stem cell is basically a cell that hasn't chosen what it's going to be when it grows up. So these guys at least know they're going to be bone cells. So we constantly get new osteoprogenator cells.

So some children grow up to be bone builders. They grow up to be because construction workers. Some osteoprogenitor cells grow up to be osteoblasts.

Others stay as stem cells, just like a lot of people don't know what they want to be when they grow up. So sometimes there's those, you know, 30-year-old kids like sleeping on their mom's couch because they don't want to get a job, which really I don't know why the parents let them do this. I'd be like, get a job. They're found in the endosteum, the inner layer of the periosteum.

So you're constantly building new bone. So the only source of new cells in the bone. bone has to come from osteoprogenator cells. Just like if we need doctors or we need construction workers, the only place we're going to get those is if a child grows up and decides to be a doctor or a construction worker. So some of these osteoprogenator cells decide, okay, I'm going to be a construction worker, and they become osteoblasts.

Osteoblasts'job is to build bone. That's all they do. So blast means to build.

So they're making that matrix by secreting those collagen ropes, and then the calcium phosphate has something to do with it. to attach to. Now these guys can't go through mitosis.

They're stuck in that G0, that kind of rest area of mitosis that we talked about in lab. They can't go through mitosis. They are stuck. The only way we get new osteoblasts is those osteoprogenator cells have to grow up and become them. Osteocytes then are mature bone cells.

So osteoblasts work so hard that sometimes they're building, they're building, they're building, they're building, they're building, and they trap themselves. Like this picture I show here of that guy kind of standing in the concrete there. He's stuck.

He's kind of painted himself into a corner, if you will. So he's got nowhere to go. So these guys build, build, build, build, build. They build this bony, hard matrix all around them, and then they have nowhere to go. So they might as well retire.

So these are your mature bone cells. They're not. building bone anymore.

They're just living their lives. They live inside the little lacunae. Remember when we saw tissues? They need that little space because otherwise if I fill the room with concrete, you're going to drown. You have to have a little tunnel so you can breathe.

So those caniculi were those little tunnels. Remember, we've got to ultimately get back to the central canal, back to the blood, so we can get our oxygen and our nutrients and get rid of our waste. So these guys do not undergo mitosis. So the only way we get new osteocytes is osteoblasts have to paint themselves into a corner.

They have to build themselves into a corner. The only way we get new osteoblasts is those osteoprogenator cells have to grow up. Then we have osteoclasts. These do not come from the osteoprogenator cells. Osteoclast, clast, kind of clash means to destroy.

So these guys destroy the bone, which sounds awful when you think about it. But the skeleton, remember, is your savings account for these inorganic salts, for calcium and phosphorus. So if your blood needs some calcium, you're going to chew up your skeleton and free that calcium and get it into your blood.

So these guys are constantly destroying the bone and releasing the minerals back into the blood. And this is a... important for repairing bone.

Like say you broke a bone. Well, you may pull some calcium out of another bone. So you have some to fix this bone that's in distress, but also for your muscles and your nerves to fire.

We've got to have calcium. Multinucleated. So remember skeletal muscle was multinucleated.

And I said that was a really rare thing because most cells just have one nucleus. Technically skeletal muscle was a bunch of embryonic cells. fused together and brought their little nucleus with them. So technically they used to be individual cells, but now they're one big cell with lots of nuclei.

Well, the same thing kind of happened with osteoclasts. A bunch of white blood cells fused together and bring their nuclei. So if you remember, white blood cells are phagocytes. Wah, wah, wah.

The little Pac-Man thing. They're constantly gobbling up and chewing things up. So that's what the clasts do.

They're chewing up your skeletal system. They produce an acid that helps dissolve the inorganic stuff and an enzyme to digest the organic stuff, like that example we started with in the last lecture with the meat tenderizer. So their job is to destroy. It sounds awful, but there are times when you need to rob your skeleton.

There are times when you need to pull money out of your savings account. So their job is to do. So osteoblasts and osteoclasts are in the periosteum, the outer layer, and the endosteum.

Like we had said before, You can build a house by working on the outside and the inside at the same time. The organic component, collagen. Collagen is also called osteoid. We're just going to call it collagen. Remember from the integument system, we saw vitamin C was to make collagen, and that's what helps keep your skin all tight.

Well, collagen fibers are connective tissue that are found in the bone matrix, giving that bone its bendability. Because like we said before, we don't want to tap our knee on a desk and have it shatter. You want a little bit of flexibility.

The collagen provides some strength as well. Hydroxyapatite, we don't really need that word in our lives because we're really just talking about calcium phosphate. So it's just a specific structure of it. So calcium phosphate gives bone its hardness and rigidness, about 65%.

As we've been told our whole lives, calcium make our bones strong. Microscopic structure, we've already done. Remember, it looks like soda cans or beer cans from the top.

So if you look at the compact bone picture there on the right, you have the little red center, that little central canal. You have the osteocytes living inside their little lacunae. So if you look at the picture then that's farther to the right, you have the little yellow osteocyte and the little lacunae that it's in, and all those little caniculae, so they ultimately kind of get back to the blood supply. So we've seen these all before.

The osteon is the whole top of the soda can, so that includes the central canal and the lamella. The lamella, remember we don't really need that word, but that's just the rings. They lay down around the blood vessel, which is in the central canal.

You can also call it a Haversian canal. I imagine somebody named Haversian thinks they're quite fancy. I just prefer osteon. Lacunae is the little space. Osteocytes, the mature bone cells, because the builders trapped themselves in that matrix.

Those osteoblasts work too hard, and then the cell retires. Caniculi, important to get nutrients and waste. They've got to connect back to that blood.

like little driveways. So an osteon or Haversian system, this is a nice picture of it. So you have that Haversian canal, where the blood is, which is the central canal. And then you have all those osteocytes and see all the little, little caniculi kind of getting, getting back to there.

So the osteocytes, remember they lay themselves down in these concentric rings. You can't get too away from that blood. And the caniculi are like driveways connecting every single one of those cells back to the blood.

So bone is crazy complicated. When we look at a bone in lab, it just doesn't look that complicated, but surely there's a lot going on. One thing we haven't seen with tissues, we just talked about the central canal, but we also have these perforating or Volkmann's canal. Again, I imagine it's named after somebody, Volkmann, who's very impressed with himself.

I prefer perforating because I think about perforations like the little holes in a notebook that you can rip the paper out easier. This perforating canal, if you look, it looks like the letter H. You have the two central canals and you have the perforating canal in the middle.

Well, blood is all connected, so the perforating canals are just to link two central canals together. So, our skeleton has cartilage and bone, but also this material called dentin. DAC doesn't have a dental hygiene program.

I wish we did. And so, we don't do a lot with teeth, but I did want to at least mention dentin. So, cartilage, remember, was a vascular.

It doesn't hurt to get a cartilage piercing. It doesn't bleed. It hurts when you're going through the skin because there's nerve and blood there, but once you're deep inside the cartilage, it doesn't hurt. We have perichondrium covering the cartilage, just like we have periosteum covering the bone.

Chondrocytes were the mature bone cells that, remember, look like eyeballs staring back at you in lab. There's no calcification, so there's no calcium. There's no caniculi.

Remember, the cartilage is very, very slow to heal because it doesn't have a direct blood supply. It just has to rely on diffusion to feed it. And so to heal, it would never, ever get enough nutrients to be able to handle a major repair. Denton.

Denton is... Also avascular. If you've ever had your teeth drilled, yowza, doesn't bleed when they're getting through the outer layer.

It's bone-like. It's actually stronger than bone, but there's no osteoblasts, there's no osteoclasts, and there's no Haversian canals. Bone is alive.

It's very bloody. It's very vascularized. The osteocytes are the mature bone cells, and there's a lot of calcium involved. So this is kind of comparing them. So as bones develop, we call this osteogenesis, because osteo means bone, genesis means to make.

and ossification, which is really the hardening of bones. So these are two very similar words. But it's getting the bony skeleton and embryos when we're growing and then as we're growing in height until early adulthood.

after that, if you're done growing in height, everybody else is doing the last one all the time. Our bones change in thickness. We're constantly remodeling and occasionally we have to repair. So this is a little growing fetus.

Now, I did not add the milkalobe. A student did that, although it's really funny. We just thought that this picture looked like he was holding something, so somebody came up with milkalobe. Get it. So in this picture, this is a fetus that's pretty far along.

So about six to seven years old. to seven weeks into gestation, we start this process. But this is a lot further on than that. If you look, he's got some intermembraneous bones forming in the skull, which we'll kind of see that next. So you can see some of those plates coming together in the skull.

You can see the vertebrae. You can see his cute little humerus there and his little radius and ulna, his little femurs, those little coxal bones. So the kind of the map work of the skeleton is pretty laid out here. But early on in development, usually again, six to seven weeks, and this is probably more more like 20 or so, we've got fibrous connective tissue and hyaline cartilage. So we get these, as it says there in quotes, quote unquote skeleton, because it's not bone yet.

What your body does is it plans the bone by laying down those hyaline cartilage jello molds again. So that way the bone has something to attach to. So bone development and changing is very different in newborns versus adults.

In newborns, the hematopoietic tissue, remember heme is blood, the point means to make. So the tissue. that's making the blood, the bone marrow, is found in almost all of your bones, in almost all of the spongy bones. There's lots of red bone marrow. Because if you think about it, a newborn is growing so fast, like just constantly growing, constantly needing new blood to support this larger organism that it's becoming.

Whereas when you're an adult, you just have to maintain your blood supply. So most of our long bones, instead of having red marrow, have just become yellow marrow, which is fat. Because fat is good for cushioning and and good for energy storage. So really, our bone cell production is just in a few bones.

The heads of the femur, that big ball joint, and in the humerus in our arm, and in our flat bones, such as our sternum, and our coxal bones, the irregular bones. So the sternum and the coxal bones are huge places for making blood. So the point is, I don't usually lose a lot of blood, and I'm growing out, but I'm not growing up. And so we just don't need to make the mass production of blood cells that newborns need to make.

So newborns have lots of red... marrow, adults have lots of yellow. So the flat bones, the sternum and the coxal bones are much more active in hematopoiesis for adults. And this is where they do sampling to see if your marrow is okay.

So to diagnose cancers or certain anemias or pain cytopania, which is just all of your blood is being reduced, they would do a bone marrow sample. And they usually take it from the hip bones. They'd go in there and drill it out. Yowza, looks painful.

Now, if you go through some kind of trauma and you need lots of blood, if you you become anemic or something happens to you, your red bone marrow can actually convert back to red. So that's pretty cool. Formation of the bony skeleton. So as we grow, about six to seven weeks in, we start convertiling cartilage into bone. We're not going into a lot of crazy details here, so don't be scared by this.

As we get into it, I'm going to kind of say what do we need to know. So there's two types of converting our skeleton into bone. Intramembraneous ossification and endochondral ossification.

The issue with this is we're going from connective tissue, like cartilage, into another connective tissue of bone. So we have to remodel. We have to take that pre-existing connective tissue, cartilage, converted into bone. These are two very different tissues. Now, intramembraneous ossification is the formation of our skull bones and some of our flat bones.

So this is not the most common one. Basically, bones are formed within a fibrous connective tissue. We don't really care about intramembraneous ossification. intramembraneous ossification that much on that. So the stages of intramembraneous ossification.

In the fibrous connective tissue with all the stringy collagen, which is shown there, a cluster of osteoblasts become activated. So basically... These mesenchymal cells, these embryonic cells that we don't need to know about mesenchymal cells, grow into osteoblasts in these builders, and so they start building the bone.

You can see that osteocyte, how he got trapped by that matrix, and now he has nowhere to go, so he might as well retire. And the builders keep building, and they get trapped and create osteocytes. Eventually, blood kind of weaves its way in there, and then the osteoblasts are still building on the outside, so you can see the trabeculae left room for the butycine.

kind of come in there. So at birth, your skull bones aren't completely fused. They've got this fibrous membrane in between that we call fontanelles. You don't need to know the word fontanelle and you don't need to know the different fontanelles, but they begin to ossify about two months in and the last one is finally done, the anterior, the soft spot, if you've ever held a baby at two years.

It's important that the skull, remember, is not solid at birth. This has to fit through the vagina. Poor vagina.

We don't want her to be abused. So the head needs some flexibility. When I was born, I looked like I had a cone head because my skull bones were so pointy.

And of course, then they flatten out. We also, up until age two, are learning to walk and we're going to fall down a lot. And so this gives room for flexibility swelling as well. But the anterior fontanelle is definitely the biggest. So this is showing a fetal skull.

They will all grow completely together by age two. Endochondral ossification. This begins in the second month of development, so six to eight weeks in. We have these hyaline cartilage bones as models, so we start with the cartilage.

So now we have a problem. Before we can make bone, we have to break down that cartilage. So almost all of your bones are formed in this manner, so I kind of want you to know the general steps of endochondral ospiction, and like I said, I'll tell you what to focus on. So the first thing, we've got a problem.

We've got to get from cartilage to bone. And the biggest difference is, remember, cartilage doesn't have a blood supply. Bone does. So we start with these little pre-bones.

See how they kind of look like a bone shape, but not really. So we start with this jello mold of hyaline cartilage. Right now, this cartilage is keeping bone out.

The way, or keeping, sorry, keeping blood out. The way that cartilage keeps blood out is it makes a chemical called anti-angiogenesis factor, which you need to know. That's a terrible word, I know, but anti means against. Angio, like an angiogram for the heart, means blood.

Genesis means to make. So this literally means against blood making. So although the term is terrible, once we get familiar with some of these roots, we can break it down. So this cartilage is releasing a chemical saying blood you can't come in.

Just like if you sprayed Raid in your house to keep out ants. The ants can't come in because the chemical is present. The ants really want to come in.

That's what ants do. The chemical keeps them out. So as these cartilage bones get bigger, the cells inside get so big that they can't feed themselves through diffusion anymore, which is kind of sad.

So they die. As soon as they die, they can't produce the anti-angiogenesis factor anymore. And the rest of your body is producing angiogenesis factor, which is saying blood men.

So immediately blood comes in. The only thing that keeps blood out is the fact that that cartilage makes that chemical. If that chemical is not present, say you go on vacation and you're not spraying.

rate anymore, the ants come in. So now all of a sudden the blood comes in. So it comes in through the center there, through the medullary cavity, then it also starts in the ends kind of independently as well.

So the diaphysis is doing its thing and the epiphysis is doing start doing the same thing. But you can see on that third picture those epiphyseal plates, those growth plates. As long as that's still cartilage, the bone is still growing.

This picture is the best because it shows kind of the whole thing on one page. So four months to birth, you have that cartilaginous model there. And I like that second picture in on the left where the cartilage cells are getting so big. So they get so big, they can't feed themselves through diffusion anymore.

They can't make that anti-angiogenesis factor And so in the third picture, the blood migrates in. Blood always wants to come in. So when the blood comes in, it brings in osteoblasts, which immediately start building bone.

And the osteoblasts, remember, become osteocytes when they kind of corner themselves, when they paint themselves into a corner. So this happens 12 to 25 years is usually when, you know, your growth plates fuse when the bone is completely dead. So osteoblasts are the ones building the bone. Periosteum.

is the outer kind of layer of the bone, you have osteoblasts in there as well. And so we call this apositional growth. Longitudinal growth is the growth we just saw, where the bone was getting longer. Apositional growth is the bones getting thicker.

And that's something that happens for the rest of your life. It seems so. It's unfair.

It's like you could get strong bones, but then if you don't take care of yourself for like six months, your bones get weak. Same thing with muscles. It's like you use it or you lose it kind of thing. It's really depressing.

I have no patience for tasks like this. It's kind of like doing dishes. Every time you do dishes, there's more dishes.

Every time you do laundry, there's more laundry. You're never done strengthening your bones. because they can constantly change because the medullary cavity has the osteoclast breaking it down and then you have the osteoblast building it back so if the osteoblasts work well you have strong bones if the osteoclasts work too hard then you have weak bones. It's just a constant yin and yang. So from 12 to 25 years, the primary and secondary ossification centers come together, which just means the center in the diaphysis and the center in the epiphysis on the ends kind of meet.

Then you get your growth plate is when that's completely ossified. It's no longer cartilage, which means longitudinal growth is over. So we all have this little leftover line. We keep cartilage on the ends of our bones though, and we call it articular cartilage.

Articular cartilage is the part of the brain that's going to be the most important cartilage is just where two bones articulate, where two bones come together. We'll have a lab where we talk about joints and we'll see that this cartilage is essential because these bones are grinding together all the time. So we want to keep a little knobby, rubbery protection on the end. But most of the time, by age 25, your bones are all done growing longitudinally.

So this little video here. Bones grow through the process of appositional growth, the formation of new bone on the surface of older bone or cartilage. Osteoblasts beneath the periosteum lay down bone to form ridges around a blood vessel. The blood vessel lies in a groove between the ridges. The groove is transformed into a tunnel when the bone built on the adjacent ridges meets.

The periosteum of the groove becomes the endosteum of the tunnel. Osteoblasts from the endosteum lay down bone to form a new concentric lamella. The production of additional concentric lamellae fills in the the tunnel and completes the formation of a new osteon. Although cheesy, I kind of like the visual of how it shows the tissue kind of fold and then the osteoblast kind of fill it in.

Because they want to be close to that blood supply so they can get their little caniculae and that way they can be fed.

Transcript for:Understanding Microscopic Bone Structure

Transcript for:
Understanding Microscopic Bone Structure