 So we just went through the components or how to build cartilage and the different kinds of cartilage we can have. Now we're going to do the same thing with bone. So does it make sense that, of course, we're going to have cells in bone tissue? What's one of the types of cells? We've got to have stem cells coming from somewhere. So we've got our mesenchymal cells. Remember, these are the stem cells that are going to give rise to if the stem cell differentiates, it can become a chondroblast in bone tissue. Chondroblasts are found in cartilage. Osteo. Blast doesn't osteo make you think of bone, it's because that's what it means. Osteo blasts. And go ahead, take a wild guess. Chondroblasts built cartilage. Osteo blasts, what do they do? They build bone. They are the fellas or the ladies who are producing the ground substance and fibers. When osteoblasts are active, they're going to be building bone tissue. Do you think that osteoblasts can paint themselves into a corner? Yes, if an osteoblast is barfing out bone matrix all around themselves and they get trapped, what do you suppose? Here, I'm going to draw you a picture because I can't help it. They're barfing. Okay, we've got to have a cell first. Here's our osteoblast and it's barfing out bone matrix. But it's cooler if the bone matrix is blue and pretty soon they painted themselves into a corner. Now what? What are they living in? Of course they are. I hope you just yelled that. I hope you sang that. Sing it out, friends, sing it out. They're living in the lacuna. Lacuna matata. What a wonderful place. The osteoblasts build their own lacuna. Osteoblasts and chondroblasts build themselves a lacuna. And once they live in the lacuna dog bounds, now, what are they called? They're not osteoblasts anymore. I mean, they could keep building bone if they're idiots because then they build bone all around themselves and pretty soon they'd be like, I've got no room. And they'd be crunched inside their lacuna. Who wants to live like that? No one. So they're going to become osteocytes. Hallelujah. And they just get stuck in there and they live out their retirement inside the bone tissue. How great is that? Guess what? You might be like, oh, we're done. Except there's something that's really important about bone. And bone tissue is dynamic. There's one more cell type that we're going to add into this mix. It doesn't come from osteoblasts or osteocytes or even mesenchymal cells. It's called an osteoclast. And I'm making it red because I actually feel like I love osteoclasts. I know you hear that from me a lot. They're so cool. Number one, they've got lots of nuclei. They look like little, I was about to say cyclopses, but they're not cyclopses. They're like the many-eyed, remember the Greek guys that are related to, I mean, cyclopses that have lots of eyes. They've got like a hundred eyes. I'm drawing you an osteoclast right now. And I'm going to draw all their nuclei in there so that you can see that, oh my gosh, how cute are these things? They really do have all those little nuclei in there. So, they're fantastic. They're actually a type of macrophage. What does that mean to you? Macrophages are your body's amoebas. And they travel around and they yumptialize. I already told you about macrophages in the respiratory system. They yumptialize stuff that gets in the way. And so their osteoclasts are a type of macrophage going around and yumptializing. I think they yumptialize. Bone. What? They yumptialize bone? They do. They go around and eat away bone matrix and enable bone remodeling. They can change the bone tissue, which is really an interesting physical, it has a lot of physiological applications because your bones actually store minerals. Calcium, phosphate. When your body needs calcium, when your cells need calcium, they can hopefully get it from your diet and then you don't have to worry about it. But if you don't have enough calcium in your diet, then they'll get it from your bones. And that's how osteoporosis happens. Your bones get thin because your osteoclasts are getting the message, hey, break down that bone, send us some calcium. What do they do? They eat the bone and like barf out the calcium and the bone gets thinner and smaller. So I think of it as osteoclasts crush bone, clasts crush, blasts build. I'm sure someone has a much better explanation, but that allows for change in the bone tissue itself. Okay, we also have ground substance. Our ground substance has a couple of components and these two components are different, but the concentrations that we throw in there are going to tell us about the nature of the bone. So the two substances we're going to find in the ground substance are osteoid and osteoid makes me think gooey. Doesn't osteoid sound like a gooey word? It's opposed to hydroxyapatite, really. Hydroxyapatite is brittle and hard. It's a mineral. Your bones have a lot of hydroxyapatite in them, which is why they are hard because they've got this mineral in them. If you have too much hydroxyapatite, your bones become brittle and can shatter. If you have too little hydroxyapatite, then your bones become bendy and you can't actually fold them up. And then, of course, you have fibers. And you tell me, go ahead, take a wild guess what kind of fiber is found in... Hopefully, that wasn't a hard guess. Collagen fibers. They're big. They're thick. They're everywhere. The more collagen you have, the more elastic your bones are. The more... It's not really elastic because obviously you can't bend them, but that's because you've got a lot of hydroxyapatite. If you remove the hydroxyapatite, you can actually make a bendy bone. What else do you need to know? We have two types of bone. And we're in cartilage. The fiber composition told us what kind of cartilage we were looking at. In bone, it's more the structure, the way that the pieces get fit together in terms of the type of bone that you have. And there are two flavors, two bone flavors. Which one are you going to order? We have compact bone and spongy bone. And I'm going to show you an image of both of them so that you can visualize the difference between them. Now, you... Let's see. So look, compact bone might be the most lovely histological tissue that you're going to find. I mean, it's almost more obvious. Compact bone is almost more easy to identify than adipose tissue. And is adipose tissue like the most easy histology lesson to have? Check out this thing. I can't draw on it, but this is an osteon. An osteon is this circular structure that's kind of the base unit of the compact bone. When you see an osteon, you're in compact bone tissue. If, right down the middle, is a central canal, the central canal contains blood supply. So what does that tell you about bone tissue when compared to cartilage tissue? Bone tissue is vascular. Look, guess what that is right there. Name this structure. That is a lacuna. That's where your cell is going to live. And you can see that we have several lacuni hanging out inside an osteon. And look at the little spider web network that is coming out of my lacuna. Those little canals are called canaliculi. And they bring blood supply to the osteocyte that is now trapped in the lacuna from the central canal. So we actually get blood, food, waste, delivery and removal to our individual osteocytes in the lacuna. Isn't that cool? Okay, so let's go look at spongy tissue. Spongy tissue, dude, this is a finger. Are you kidding me? Yeah, this is a baby finger. Okay, here's a baby finger. You would expect the bone in the middle, right? That's the bone. But I just want to take you up here really fast to say, what is this? Please tell me true. What is that? Dude, look, it's strata granulosa. That is skin at the top of my baby finger. But let's go back to the baby finger and take a look at the tissue. Spongy tissue, spongy bone. Look, do you see my lacuna? It almost looks like cartilage. The difference between cartilage and spongy bone is that my spongy bone, like shards, it looks almost like I'm going to draw you a picture because I want you to see what I'm talking about. Spongy bone has little, they're called spicules or trabeculae or funky shaped bloops, bloops, blobs, flups, flippets of bone tissue. And then inside that spongy zone, you're going to have other stuff going on. You've got a blood supply, you've got fat, you've got all sorts of nerve information coming in and out. But these things, these weird shaped pieces of bone tissue, that's how you identify spongy bone. What was the thing that made the compact bone so clear? We actually had the osteons, remember? And the osteons are how I knew that I was dealing with compact bone tissue. And it has its lacuna, the lacuna, this looks, oops, that looks like an atom with electrons going around it, but it's not, it's compact bone. You can see the difference right here. This is bone tissue, this is bone tissue. We just have different types. Shall we look at how bone forms? Where did it come from? Haven't you always wondered, where did bone come from?