Hello students. In this lecture, we're going to focus on the microscopic structure of bone tissue. This is really important in understanding the physiological aspects of bone growth, bone repair, um, and calcium regulation. So, understanding the different types of bone, the compact and spongy bone will help you in understanding these processes. So when we talk about bone um we need to understand the extracellular matrix and what it's made up of and the different types of cells that are within that matrix. So remember bone is a connective tissue. So there are cells scattered within a matrix and that matrix is a hard matrix. It is solid and it is made up of inorganic components and organic components and the inorganic components are minerals mostly calcium and phosphorus but there's some magnesium and some other minerals in there but calcium and phosphorus so make up most of the hydroxy appatite which is that mineral. So, one of the hardest substances in the body um except for the enamel on our teeth and so very firm and that provides resistance against compression and helps support our body and that gives the bones their strength and and and helps hold us up, right? But the organic matrix is very important in the bone as well. This is the um soft components of the bone so to speak. It's about 35% of the bone's weight. But this is the collagen fibers and the extracellular matrix that kind of the proteoglycans that hold water also preventing compression and preventing the the torsion and the um the twisting um forces that happen to bone as well. So organic matrix also very important. Think of it like when you're mixing cement and or or pouring pouring cement um at your house. They usually put something within the cement like rebar. This is kind of the rebar of the of the process. And this is this is the cement and the stuff with them the minerals so to speak. um also of the cement. This would also this would be the water and maybe some of the other things you put in the cement. So if that helps a little bit. So organic matrix again calcium phosphate um is is mostly what's in there. The hydroxy appatite is very very hard. It's resistant to compression. It's very strong. The organic matrix we've got collagen. We've got proteoglycans and glycosaminoglycans and glyoproteins. Remember all of these things attract water. This helps in um the proteins, glycosaminoglycans and glyoproteins. This helps resist compression. Collagen fibers are strong and they help with that tensile strength and that resisting pulling and twisting. And so this organic component is very very in protein very very important in protein. Um so collagen predominant fiber in the matrix. So we do have lots of fibers but collagen is the most prominent one and this this helps with the torsion and and the pulling on bone. So when you when someone grabs your arm and pulls your arm away, this prevents your your the bone from from breaking um and twisting so to speak. And this this helps the glycosaminoglycans and proteoglycans. This is helping with that water. The glyoproteins help bind the mineral and the the um the organic matrix together. So this is helping put everything together. So keeping it sticky and stuck together. And so note like normal bones, this is this is what you get with if you remove the organic matrix, the bones get too brittle. This is what happens when we when we age. Remember collagen is less we produce less collagen and so our skin gets kind of saggy. We have less firmness. um in our bones there's less collagen and our bones become more brittle and this is why we're more prone to fractures in bones without the mineralized matrix. So without the calcium phosphate with less mineral let's say if you have a vitamin D deficiency um they get rubbery so there's less mineral so you need both you need the organic matrix and you need the mineralized matrix to have adequate bone formation both are so important when we look microscopically we have different cell types that are very important you have your osteoggenic cells which are the stem cells. These are going to differentiate and form your osteoblast. Your osteoblast are your building cells. These are going to lay down this organic matrix that will then become mineralized and then this hardens around the osteoblast and they become osteocytes which then now maintain that matrix around them until it gets broken down by the osteoclass and we rebuild in that area. And so osteoclast are cells that release enzymes and chemicals that will break down the the the organic matrix and dissolve the mineralized matrix so that we can reuse those components and build new bone or use the components elsewhere. So this is a dynamic process. We're always building. I'm breaking down. Once once development is over, once we've finished puberty and we finish growing, we should kind of be at a maintenance process for all of this. So growth. So So re re remodeling should be pretty equal as far as breakdown should be the same as building. And so we pretty much maintain until we get to a certain point in our age where hormones start to decrease. And when hormones start to decrease, we start to see an a relative decrease in the building and an increase in the breakdown. And that is from loss of hormones like estrogen which inhibits the osteoclass which inhibits the the breakdown. And so if you lose the estrogen now you've got an increase in the breakdown and now you've got a kind of relative deficit and so you're breaking down a lot. Um and so we can see that happen less so in men but in women that's a big deal and that's why we're at increased risk for osteoporosis. So here you can see the osteogenic cells differentiating into osteoblast. They're going to lay down this this matrix and now they've become osteocytes within this matrix. And then here you've got the osteoclast. They're releasing hydro um hydrogen ions. This is going to help um degrade the extracellular matrix. And um this re and they release enzymes. this this um releases calcium and phosphate and sugars and everything that's within that bone and then now we can use these things um particularly when we talk about calcium regulation this is very important in releasing calcium into the blood so it can be used for the body so if calcium is low the bone will be a site where where we will resorb calcium so we will break down the bone and release that calcium into the blood to get it for the rest of the body. So we can use that as a storage site. So when we look at the hisystologology of bone, remember the outer portion of the bone is usually compact bone. So the cortex is made up of compact bone and there's always an inner the indostium usually there's a little bit of spongy bone at least um lining the inner portion of that. And there's different functions for each of these. So when we look at the structure of the compact bone, it's it's made to withstand a lot of stress. So the columns of the oons, they're um layers arranged in a circle. Okay? So kind of like a target around a central canal. Okay? So, it's layers of the mineralized matrix and the osteoccytes are sitting in little spaces between the layers and they're communicating through the layers through their little canals and they're getting nutrition from the blood vessel in the center and they're in columns. And so those columns are tightly packed and you can imagine like if I had a a building and I had a whole bunch of columns tightly packed together that is very very strong. also heavy but very very strong and so it's very hard to like if you were to hit it from the side it would be it would it would hold up and so that provides a lot of strength and so this is these are those columns picture them extending up and down so as the blood vessels come in they dive up and down along with the nerves and and and they're the central become the central vessels to these um oons and then the lamea are laid around these vessels. And so you can see here this is an osteon forming. So lamelea are being laid around this this set of vessels and we're going to form a column there and this makes it very strong. And then you can see towards the inner portion of the bone. So the meillary cavity you've got a little a small portion of spongy bone and this is where you have layers of bone but they're form they form arches in different directions and this provides strength in different directions but provides space for the red marrow and doesn't carry the weight that this has. So strength in many directions. So different kind of a complimentary function without the weight. And so this allows us to have bones not be too heavy and still provide strength and in a in a number of directions rather than just you know preventing if if my columns are like this. It's mostly preventing compression this way. Although you do get some the more columns you have stacked side by side, you do get some protection from the side as far as traum traumatic or m injury protection from mechanical injury from the the side. Okay. So um compact bone again the unit of compact bone is the osteon. This is the functional unit. They're not permanent. they get they get put down and they'll get broken down and rebuilt because that matrix can get get micro fractures and cracks and so they have to be able to be rebuilt and it does depend on the needs of the body. So if I need calcium I'm going to break things down. If I if I don't I won't I can I can build also if I'm not using my bones I'm not going to build them. So remember you have to use like weightbearing exercise promotes bone building. Nonweightbearing exercise or no exercise does not promote bone building. So the density of my compact bone and my spongy bone will be less if I'm not exercising and especially if it's not weightbearing. So this is an example of this is when astronauts go into space. They do not have any um gravity to work against. And so we actually have to devise exercises for them to try to help them with that because when they come back to Earth, they have lost bone mass. No matter what we do, they've lost bone mass and it's and it's very difficult for them and and it's damaging to their body. Okay. So remodeling, we had the interstitial lame. You can see the little spaces between those con those oons. Those are the old oons that got broken down. And then you got the circumferential lamea which is the stuff that's laid around the outside just under the berryium kind of holding everything together. These are the circumferential lamea that are outside and around just under. And then the perforating canals. These are the ones that go horizontally that come in. This is the blood vessels coming in. And the they will then divide and go up and down. And then the bone will be laid around them. the lamelea to form each oion. So here this is an example. So we have our um perryioium here. You've got your underlying blood vessels. You've got your perforating canals or vulman's canals coming in. Then they divide. They kind of separate and form your central canal or herversion canal. And that is the central canal for each oion. And then the bone is laid around that. You can see here these are the what you call circumferential lamelea. They go all the way around the whole the whole bone your um oion lamea and then you've got in here between the oons you've got old um basically the remains of lamelea. These were um interstitial lamea but those are the what was left and remember the osteocytes are sitting in spaces called lacun and then they're communicating between the the layers of matrix through little canals and that's all these little lines that you're seeing here in in the in the matrix. Okay. So, spongy bone um not directly weightbearing in that we're not touching it from the outside in. It's it's with it's always within some some um compact bone. It's much less dense. This allows for our bones not to be so heavy. Think of the amount of muscle we would have to have if our all our spongy bone was was actually compact bone. Much more muscle. We would look like the Hulk. Okay? And these struts or these arches are called tbecula. So they look they are lamelea. They're um layers of bone and they're like think of a cinnamon roll like you would roll a cinnamon roll just looped around but there's no central canal. So now the blood the blood supplies on the outside these these lamelea are literally sitting within the red marrow. So the blood supply is outside of the tacula um outside of the layers and the osteoccytes are getting their nutrition from outside in. Okay. And these are resisting forces in multiple directions. The beauty in these is you'll put the tbacula wherever you need them. So, if you're um if you're, you know, doing jumps, a specific type of jump, and that put puts a certain amount of force on your feet, you're going to strengthen your feet by putting more tbacula on those bones where you have the most force, and you'll actually over time put more tbacula in the areas where you have more force. And so, you change your bone based on that. Another example is think of dancers, ballerinas. They're on their toes. So their bones in their feet change based on the force that they have on those toes. So if they're always on their toes, the toes widen, the tones become more dense because of the force that is on the toes. And so um you can see that and that's they they strengthen based on that. And so again we have lamelea we have lacun we have kennoliculi we just do not have central canals no oons. So you see here it's again layers of bone but no central canal. Okay. And in between these tbacula are is is the red marrow all the hematopoetic stem cells and the the blood cells being produced here. So if you have a moment pause the screen go talk with somebody and explain the functions of compact bone versus spongy bone. What does it do for your body? how is it working to help you um as part of bone functions. Okay? And then when you're done, think it over and um come to class and we'll talk about it in class. All right? I'll see you on the next lecture. Our next lecture is going to be bone formation. So, we're going to talk about how does this bone form and when