Transcription
Hi everyone, Dr. Mike here. In this video, we're going to take a look at bony tissue.
So, remember, bone is just like cartilage, just like blood, just like tendons and ligaments. It is another connective tissue, and that means it's going to contain cells, gels, and fibers. Remember, cells, gels, and fibers make up connective tissue.
Now, the skeleton, which is going to be made up of bones, is there to provide structural support for the body, protection, but also is a housing unit for very important minerals of the body. And we're going to talk about those in a sec.
But first, let's have a look at the cells, gels, and fibers that make up bony tissue. So, the cells in bony tissue include osteoprogenitor cells. Osteo is a prefix that means bone. Progenitor means it's going to create something. So, these are basically the stem cells of our bony tissue. Then we've got osteoblasts. The "blast" tells you it's building. So, these are the bone-building cells. They release a substance called osteoid, which helps build bone. Osteoclast. Think of the "clast" result means crushing. This breaks bone down. And the last one is the osteocytes. That's the mature osteoblasts, the mature bone cell.
Now, our bone is constantly being remodeled. That means it's constantly building itself up, breaking itself down, building up, breaking down. The more you use bone, the more this remodeling process occurs. Also, the more you use bone, the more the building process occurs to make the bone stronger.
Now, osteoporosis is a disease in which this remodeling process of building and breaking is imbalanced, and you have more breaking and less building, and you get holes in your bones. I'll talk more about that later on.
Okay, so these are the cell types within bony tissue. What about the gels? So, the gels include the glycoproteins and also the glycosaminoglycans. Remember, they're the carbohydrates.
Now, if we look at bony tissue, 15% of its entire mass is water. Okay, 15 percent of bony tissue. The rest is what we call dry weight. Now, of that 75-odd 80 percent of that dry weight, what you're going to find is that 30% of it is going to be the gel portion. So, that extracellular matrix, the ground substance, what we call the glycoproteins and the glycosaminoglycans. That's 30% of the dry weight. The remaining 70% of the dry weight is actually something that bony tissue has that other connective tissues don't have. These are the mineral depositions. So, 70% of the dry weight of bone is made up of something called hydroxyapatite, and it's predominantly calcium and phosphate. Okay, so bone has all these minerals dissolved into it, and this is what makes bone hard. And these minerals are calcium and phosphate. So, that's very, very important.
You'll also find that we can break up the gels and fibers together. We call the organic components of bone, and the minerals are the inorganic components of bone. So, this is again important as well. I'll talk about that shortly.
Now, when it comes to fibers, the major fiber type within bone is collagen. So, together, the organic components of bone make up 30% of its dry weight, and the inorganic components of bone make up 70% of its dry weight. All right.
Now, what we're going to do is look at the anatomy of a long bone. So, we've got a long bone here, and you can see that this long bone, I've sort of cut, and we're looking inside of that long bone. And you can see that it's made up of two bone types. This is what we call cortical bone and spongy bone. Okay. This cortical bone is a very dense bone. This spongy bone, by name, is a spongy in appearance looking bone. It's not spongy to feel, still very hard, but not as hard as the cortical bone. Okay.
So, what you're going to find is you're going to have the cortical bone sitting on the very outside of a long bone, and then you're going to have the spongy bone sitting more so in the middle of the long bone.
Now, deep within the middle of a long bone is actually hollow, and we call that the medullary cavity. So, there is a cavity deep inside of long bones that's hollow, and it's called the medullary cavity. And what is inside of that hollow area? A lot of our blood vessels, but also, in the adult, it's going to be what we call yellow bone marrow. Okay. So, you've got two types of bone marrow: yellow bone marrow, that's fat, and red bone marrow, which helps us create blood.
Now, in adults, the medullary cavity is yellow bone marrow. Okay. But in an infant, the long bones have red bone marrow. So, that means in infants, their long bones are the primary site for red blood cell production. So, the question may be, where's our primary site for red blood cell production now that we're adults? It's predominantly in our flat bones. So, flat bones are going to be bones of our skull, for example, and other sternum, and there are some other places that you'll find what we call the red bone marrow.
Now, the other thing is that if we were to take a look at the spongy bone and the cortical bone, a couple of things. Firstly, with the spongy bone, you can see that there's these holes in them. And you'll find the endosteum is the most inside portion of a bone. The periosteum is the most external portion of a bone. And the endosteum, you're going to have certain cell types. That's these cell types. You're going to have the osteoprogenitor cells producing osteoblasts. Then you're going to have osteoblasts building the bone up. So, that means you can build bone from the inside. Okay. Osteoblasts. Then you have the osteoclasts, which start to eat and break the bone up. Okay.
When we look down at the cortical bone, which is sitting more on the outside, you'll find that you can peel away the outside layer of the bone. That's called the periosteum. And underneath that peeled layer, you can see that some of the cells sit. So, you can build bone from the outside as well. So, from the endosteum, you can build bone. From the periosteum, you can build bone. For adults, most of it happens on the endosteum, inside the bone.
You can also see that as the osteoblasts start releasing all these substances, an actual fact, that osteoblasts, they secrete the organic components, they secrete the gels and the fibers. The inorganic components, well, that comes from when we ingest foods, for example. All right. So, we've got the osteoblasts secreting all this osteoid, the gels and fibers, and laying it down, and it's like putting setting concrete around it. And once it sets concrete around it, it's stuck. And once it's stuck, it matures into an osteosarcoma. So, you can see the osteocytes sitting between these layers, like the layers of a tree. Okay.
Now, another thing that you're going to find is that bone is highly vascularized. There's a lot of blood vessels. Cartilage, on the other hand, does not have any blood vessels. So, there's a big difference between two connective tissue types.
If you look at what we call the macroscopic anatomy of the long bone, you've got two ends of the bone. They're called the epiphysis. Okay. So, you've got the proximal epiphysis, that's closest to the side of attachment, and the distal epiphysis, that's farthest away from the side of attachment. Then you've got the long middle portion of the bone, which we call the diaphysis. And then in between the diaphysis and the epiphysis, we've got the metaphysis.
The other thing is, you can see this green line that I've drawn here, basically between the metaphysis and the epiphysis, that we call the epiphyseal line. This epiphyseal line is all in all of our long bones as an adult, and it's hardened. But it used to be cartilage. It's actually when we grow from an infant to an adult, there's cartilage in here that actually starts to lay down tissue and then hardens over time to form bone. So, to our growth plates, if you break in here as an infant, the growth plate, it may stunt or alter the way that an individual grows.
So, what we've got here overall is an outlook or a general outlook or overview of bony tissue.