 Hey everybody, Dr. O'Hare. Let's talk about bone growth. So bone growth occurs until you reach skeletal maturity, maybe between the ages of 23 and 25, somewhere in there. The process of turning another tissue into bone is ossification and that's going to be, or specifically you can call it osteogenesis, the creation of bone if you want. And that's going to be what happens here because we start with a cartilage framework and we turn it into bone. Ossification is the adding of calcium salts that mineralizes bones during this ossification process. So that's the difference between ossification and calcification. So there are two main forms of ossification or bone formation. There's intramembranous ossification which impacts a few bones, but we're going to focus on endocondral ossification here. We'll go through the steps, but I think that people overthink this and they make this way more confusing than they need to be. We start with, so our bones, in our mother's wombs, our bones started as a hyaline cartilage framework. So remember cartilage is avascular, bone is vascular. So you start with a cartilage framework and then blood vessels show up. As blood vessels show up, cartilage is slowly converted into bone, into bone tissue and then we'll talk about what happens at the end there. So just try not to overthink this. Let's go ahead and dive in with endocondral ossification. So we started with that hyaline cartilage framework and there's actually some remnants of that when you see a bone. We'll cover this more when we talk about joints, but the ends of your long bones are still covered with hyaline cartilage. So it's not like hyaline cartilage was laid down at the end of your long bones. Your long bones were hyaline cartilage and everything but the tips and everything but the end became bone. So you see this cartilage here and then as we travel to the next picture, the cartilage is going to slowly start to become a calcified matrix and that's going to open these spaces and these spaces are going to allow blood vessels to show up. So we started with cartilage. The cartilage starts to become calcified and opens up a bit and then we have blood vessels showing up. That blood vessel showing up there in image C is going to be what begins the formation of the primary ossification center. So that's an important term. Primary ossification centers are going to be in the shafts of your long bones. So as blood vessels show up, what they do is they bring fibroblasts with them and these fibroblasts become osteoblasts and that's going to be where bone starts to develop and the first bone to develop just like with fracture healing, the first type of bone to develop is going to be spongivone and then it's going to slowly become dense, compact bone in some areas. All right, so next we see so remodeling during this process is going to create this medullary or marrow cavity, which is going to be where your red bone marrow is and then later some of this is going to become yellow marrow as well. And then we start to see what's happening at the metaphysis. Remember the metaphysis is the space between the shaft of a long bone and the ends. The bone is going to start to replace cartilage there. That's how our bones are going to get longer. That's going to play a big role in bone lengthening. So what we have next, so what's happening next is now new blood vessels, new capillaries are going to show up at the epiphasies, the ends of your long bones and that's going to create secondary ossification centers. So remember primary ossification centers are going to be in the shaft of a long bone. Secondary ossification centers are going to be at the epiphasies, the ends of a long bone. So now we have bone growth occurring from the inside out and then now from both ends as well. So next we're near the end here. You notice it's starting to look like a bone. So a lot of that spongivone has now been remodeled into compact bone there at the shaft and we now have the ends of the bone are now becoming spongivone. The key now is this epiphasial plate. So at the metaphysis, the junction where the shaft and the ends of long bones connect, we're going to have cartilage. So new cartilage cells are being laid down. They're becoming bone and that happens again and again. So imagine it being a race. You lay down a layer of cartilage and behind it the layer of cartilage becomes bone and that happens as long as your bones continue to grow. Once bone lengthening has ended, the bone is going to catch up. There won't be any more cartilage, sorry, there will be plenty of cartilage. There won't be any more cartilage. The cartilage is all going to be turned into bone and our growth plate will now be gone. So you can actually see this on an X-ray, I'll show you an image, but the growth plate is called an epiphasial plate and you see that as an opening or in an X-ray because you don't see the cartilage on an X-ray. But then once bone lengthening has ended, the epiphasial plate will become an epiphasial line. So let me show you a closer image of what's actually happening there at the metaphysis because this is really the key as far as the length of bones. So remember we have our primary ossification center in the shaft, our secondary ossification centers at the ends, but between the two, the bone is growing longer because as you see here, we have calcium being, or cartilage, I keep saying calcium, I apologize. We have cartilage, new cartilage cells being laid down. They're being calcified, they're turned into bone and you have, so just imagine this is actually happening from the top down here so you see new cartilage cells being laid down and as those cell layers are being laid down, the old ones are being calcified. And as long as that happens, as long as you keep having new cartilage laid down, then bones will get longer and longer. Once the cartilage stops growing and all the cartilage is there, it's replaced with bone, the growth plate is now gone. You'd see a bony joint called a synostosis there, the epiphasial line and bone lengthening has ended. So once you see an epiphasial line on a bone, it means that bone lengthening has ended. That bone can't get any longer. Bone can change. It can remodel. It can get bigger if you start lifting weights or gain weight, but it cannot get any longer once that epiphasial line is there. All right. Here you see it just, if you still have growth plates, those where those, what is that, fuchsia, where those fuchsia colors are, those would be, there'd still be cartilage there. So on an x-ray, you'd see them as open spaces. Once you see a bony line there, then we know the bone is mature. Bone lengthening has ended. All right. So that's bone growth in a nutshell or at least endocondral ossification. I hope this helps. Have a wonderful day. Be blessed.