Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson on bone structure and function.
Today we'll be talking about the structure and function of bones, as well as how bones develop.
Bones are basically the building blocks of the skeletal system, and they're a type of connective tissue. If you were to actually take a look at a bone, the outside of the bone, the part that you're looking at, is the periosteum. The periosteum is a double layer membrane that makes up the outer covering of bone.
Two cells that we're going to talk about today that are important in bone growth and bone structure are osteoblasts and osteoclasts. We're first going to talk about osteoblasts.
Osteoblasts are a type of cell that are found in the second layer of the periosteum Osteoblasts are responsible for building bone. So their job is to help build bone. These osteoblasts help to build bone, and when the area around them has mineralized-- and for a bone to mineralize, it requires calcium-- so when that area around the osteoblast has mineralized, it becomes an osteocyte. So osteoblasts will form bone when the area around the osteoblast mineralizes. It becomes an osteocyte. And now osteocytes will no longer form bone matrix, and so they are basically just a mature bone cell. They do not form bone anymore. So osteoblasts build bone.
Osteoclysts, second type of cell, are cells that break down bone. So we have cells that build bone, and cells that break down bone. Now both of these types of bone cells are really important in bone remodeling. Bone remodeling allows our bodies to maintain calcium levels. We need certain levels of calcium in our blood. If calcium levels in our blood are too low, osteoclasts will break down bone, and release calcium into the blood. And then osteoblasts will help build bone if there's access calcium in the blood. So these two are both very important in bone remodeling which maintains, as I said, the calcium levels in the blood, and also helps keep bones resilient.
There are two types of bone tissue, spongy and compact. The names kind of give you an idea of what they're like. Spongy bone tissue is found inside the shaft and at the ends of bones, and it's made up of tiny flattened struts that are fused together. So looking at it, you wouldn't think that it's very strong, but spongy bone is actually a fairly strong type of bone because it has all these tiny flattened struts all fused together in this web.
Compact bone is dense bone, and it's found at the shaft and the outer parts of long bone. If we take a look here, this is going to show us compact bone. Compact bone forms in circular layers. You can see our circular layers right here. And each of the circular layers is called an osteon. So it forms in these circular layers, and in the middle of each circular layer is a hollow canal. In this hollow canal is room for blood vessels and nerves. So you can see some blood vessels in our diagram right here. Blood vessels will carry substances to and from osteocytes, which we discussed up here, which are those bone cells. So we have the osteons, with the canal in the middle, for service for blood vessels and nerves. So this is a little bit of the structure of a compact bone.
Bones also contain bone marrow, and there are two types of bone marrow. Yellow and red. Bone marrow is found in the shaft cavity of a bone. We'll get to that a little bit more in just a moment here. But the two types of bone marrow, as I mentioned, are yellow and red. In adults, most of our bone marrow is actually yellow bone marrow, which serves for fat storage. Red bone marrow actually produces red blood cells and white blood cells, but most adults do not have a whole lot of red bone marrow. It can be found in a few bones within our body, a few of our flat bones, but most adults actually have more yellow bone marrow than red.
But for example, if you were in some sort of accident, you lost a lot of blood, your bone marrow would need to produce more red blood cells to replace the blood that you've lost. So yellow bone marrow can actually convert back to red bone marrow, if necessary. So if your body detects that needs to happen, the yellow bone marrow will actually convert back to red bone marrow. Which is actually really interesting.
Let's take a look at our diagram here, just a little bit of the structure of a bone. First we have our periosteum. We discussed up here, which is our two layer membrane. And then we have compact bone. And spongy bone. So you notice our spongy bone in here. It gets its name because it's kind of spongy looking. Than the compact bone is the hard bone. And then this long part, the long center part here, is called this bone shaft. Within our bone shaft, as I mentioned, is where we'll find bone marrow. Our yellow or red bone marrow.
Next we're going to talk about how bones develop on the cartilage model. The cartilage model is basically just a model that shows how bones develop from being cartilage to a full grown adult bone. Over time, the cartilage that composes bone will be turned into actual bone by osteoblasts. And osteoblast, remember, are those cells that build bone. And is building of bone starts from the middle of the bone, and will work its way outward. So the cartilage, starting in the middle of the bone, will become mineralized and turn into bone, and then work its way outward as the bone grows.
The end of a bone is called an epiphysis. The epiphyseal plate is a part of the bone that's made of cartilage, that separates the shaft from the epiphysis. We're going to talk a little bit more about these features here, as we look at this diagram.
This diagram is basically showing how a bone develops over time. A bone of a developing embryo will be made of cartilage. And then what's going to happen is that osteoblasts are going to become active. They're going to form this bony collar, and then the bone will start to mineralize. And from there, it'll grow, as I mentioned outwards, and then blood vessels will start to invade. More bone tissue will form, and then bone remodeling will continue to take place, building more bone, starting from the middle working outwards. And then secondary bone-forming centers will appear at the end. So we have bone-forming areas at the ends, and then working their way out from the middle of the bone, as well.
And the epiphyseal plate, as I mentioned, is this cartilage plate that separates the end of the bone-- the epiphysis-- from the shaft. So this part of the ball, the epiphyseal plate, is not going to calcify, or it's not going to turn into bone, until the person is done growing. So the human growth hormone, HGH, prevents this epiphyseal plate from calcifying. And what this does is it allows the bones to lengthen until they're done growing. this will continue to happen. The bone will continue to lengthen, bone remodeling happens, and then eventually we'll end up with a mature adult bone. And at this point the epiphyseal plate well calcify, and will become regular bone tissue. This is how bones develop on the cartilage model.
This lesson has been an overview on bone structure, function and development.
