Bone is a type of connective tissue, but it's also wrapped by other connective tissues. Specifically, it's going to be wrapped by the periosteum and the endosteum. These connective tissues provide bone with blood, nerves, attachment points for tendons and ligaments, and also the bone stem cells. So, we're going to go over these now. We will start with the periosteum. The periosteum well, peri means around and osteum means bone. So, literally, that means the connective tissue that is around the bone. And so we're going to say this covers the outside of the bone. It's a covering on the bone that's really on there tight, but it's something that you can see in bones. If you cut up a chicken bone, for example, it's easier to see on a raw chicken bone. You can get a sharp knife and you can get in there and start to peel that periosteum away, and you can see a very thin layer of connective tissue around it. The periosteum can be broken down further into 2 smaller layers, though those 2 smaller layers you can't really see without a microscope. The first layer, the fibrous layer, is going to be the superficial layer, the layer on the outside, and it's going to be dense irregular connective tissue. Remember, dense irregular connective tissue is mostly collagen just running in all sorts of different directions. So this fibrous layer is the part that's going to be vascular. It has blood vessels, and it's also going to be innervated. It has nerves. So that means that if you break the periosteum, it bleeds and it hurts. This is also going to serve as an attachment point for tendons and ligaments. Remember, tendons are the connections between muscles and bones, and ligaments are the connections between bones and bones. And those connections are mostly collagen. So, we're going to say here that the collagen from tendons and ligaments actually comes down and it weaves into the periosteum. A lot of times you might think of the connection with a tendon on a bone sort of like it's super glued on the bone, but that's not the way it works. That collagen actually weaves well, it weaves into the bone itself, but it also weaves into the periosteum. So when a muscle pulls on the bone, it doesn't just pull on the bone, it pulls on the connective tissue that's surrounding the bone as well. Keeping this connective tissue on really, really tightly are these things called perforating fibers. Perforating fibers are collagen fibers from this fibrous layer that actually weave down through and enter the bone matrix. So remember, collagen is a really, really strong rope-like protein, so it's woven into the bone. It's not just that this is wrapped on, it's woven into the bone. It's on there really tightly. If you look over here on our image, we can see a cross-section of a long bone and we have a section of the shaft of the bone, the compact bone on the outside taken out. And we can see up here, it's zoomed in and we can see this tissue being pulled off the outside. That tissue, because it's being pulled off the outside of the bone, would be the periosteum. And you can see here that you have some of that collagen there. That's that white. Those are those perforating fibers. The other thing that you see here that you wouldn't see that well in actual tissue, but you can see these cells. Those cells are part of the osteogenic layer. The osteogenic layer is a layer of osteoprogenitor cells. These are the bone stem cells, and they are going to be closer to the bone and it's going to be a really thin layer. These line the bone surface, and they're going to go on to become the different types of bone cells. And we'll learn about that more in detail in a future video.
Our second type of tissue is going to be the endosteum. Endosteum, well, endo means within, and again, osteum means bone. So this literally means the connective tissue that is within the bone. All the inside surfaces are going to be lined with endosteum. So we're going to say here, it lines the inner surfaces of bones. Now, that's going to be especially true, for example, in the spongy bone where you have all that space and all the trabeculae, all those different struts, all of that is going to be wrapped by this endosteum. But it's also going to be in the shaft of a long bone in that medullary cavity, in that hollow part, of the long bone. There will be an endosteum lining as well. This is going to really just be the same composition as that osteogenic layer. So it's not going to have all the collagen and that toughness that the periosteum has. It's just a really thin, membrane of these osteoprogenitor or bone stem cells. If we look over here in our image, again, we see the cross-section of bone. Now we're taking a little zoom in on the inside of that bone. Since it's the inside, we can see a layer that's the endosteum. And you can see just the little cells that are lining that bone. These are going to be those bone stem cells. So periosteum, really tough. A lot of collagen on the outside. Endosteum, just those bone stem cells on the inside of the bone. With that example and practice questions to follow. I'll see you there.