In this video, we're going to focus on the microscopic anatomy of bone, specifically the compact or lamellar bone, as well as spongy bone. So the structural unit of compact bone is called the osteon, or some textbooks will refer to it as the Haversian system. And basically the osteon is these concentric circles called lamellae that are dominated by...
collagen. And each of these concentric circles have collagen that run in different directions. So you can see this outer layer of the collagen is going in this direction, and then the deeper layer of the collagen is kind of going in the sort of opposite orientation. And so this sort of mishmash running of collagen fibers between the various layers of lamellae allows for the osteon to resist stress and twisting.
Now within the central area of this sort of cylinder, you have what's called the central canal, and that's where you have the blood vessels as well as the nerves. So if we kind of zoom out and look at compact bone sort of in a larger picture, we've already talked about the canals. that have the blood vessels and nerve fibers. And let's kind of eliminate that. That's a mist type.
For those blood vessels to actually be connected to the circulatory system, you have these canals that sort of run 90 degrees, and they emanate from the endosteum, which remember the endosteum lines the medullar cavity of the diaphysis. So these 90 degree canals are known as perforating or Volkmann's canals, and they also contain blood vessels as well as nerves. So it allows for this connection between the osteon, the endosteum, and the periosteum.
Next structure we want to look at are the lacunae. And we'll talk a little bit more about the lacunae in the next slide, but when bone begins to form, there are these small little cavities where the osteocytes are. So here, if you look at a specific osteon, you have one lamellar layer here, you have a second here, and you have a third here.
So between these layers, you have these little cavities where the osteocyte was secreting. bone matrix. Remember that osteoid that we mentioned before.
Now as that osteoid begins to harden, they become trapped in that lacunae. Now they do form connections amongst themselves, so both within the lamellae and between the lamellae, and those hair-like connections are called caniculi. So really it's the connection between those osteocytes via gap junctions that forms the basis of the canaliculi.
So if we look at canaliculi formation, the osteoblasts, when they secrete the bone matrix, they're maintaining this contact amongst themselves and with osteocytes via gap junctions. And as I mentioned before, When the matrix hardens, the cells can't move anymore, thus they are trapped. But you still have those gap junctions, which allows communication between the osteoblasts and the osteocytes. And they can exchange then nutrients and waste from one osteocyte to another until you reach a blood vessel or a capillary.
So when we look at the lamellae, we've talked about the lamellae as a functional unit within the osteon, but there are other types of lamellae that are not part of the osteon. So if I kind of jump back over here, you'll notice that the most superficial part of the compact bone, let me kind of use red here, so the most superficial part of the compact bone here, is not made up of osteons. It's the more deep region of the compact bone that has osteons.
So you have a full osteon here. You have another full osteon here. Again, the full osteon here.
So these particular lamellae are known as circumferential lamellae. So let's get to the appropriate side now. So here are the circumferential lamellae.
So when we look at the sort of perimeter of the compact bone, they're going to be just deep to the peristeum, but they're superficial to the endosteum. And it's going to extend along the entire surface of the diophysis. So again, forgive me, I'm going to jump back here. So this little layer here in light blue, that's the peristeum. You have the circumferential lamellae.
And then separating that from the osteon region, that's where you have your endosteum. A third type of lamellae is known as the interstitial lamellae. So you see this when the osteon has begun to be broken down. So it's going to form the gaps between one osteon to the next. So we have that image over here where you have a complete osteon and then over here these osteons are incomplete.
So we refer to this as the interstitial lamella. In this image, it's also clear. You can see the lacuna, these little dark spots where the osteocytes are. And then if you kind of squint, you can kind of see the connections between the various lacuna. That's the coniculae.
So let's examine spongy bone microscopic anatomy. So spongy bone is also known as cancellous bone. And when you look at it sort of superficially, it looks very poorly organized, right? You just see sort of this mishmash of bone fragments that are kind of fused together.
And that's what the trabeculae are. However, the trabeculae actually align along points of stress to help resist that stress that that bone might be subjected to. If we actually zoom in and look at the individual trabeculae, we find that there are no osteons like you have in compact bone.
Instead, the lamellae are sort of irregularly arranged, so it's not a specific arrangement like the circumferential or the lamellae in an osteon. They're just kind of like mish-mashed all over the place. However, you do have the coniculi that allows the connection between the various osteocytes, and you see the endosteum that allows you to supply nutrients to the bone tissue. So if we look at the blood and nerve supply, we've talked about how you have blood vessels that run in between Volkmann's canals.
the central canal. The spongy bone and the medullary cavity need to receive nutrients from arteries that pass through compact bone. These areas through which the blood vessel passes are found right over there. These are called nutrient foramen.
Foramen are basically small holes or small openings that are found in bone specifically in the diophysis here along with these blood vessels you also have nerves to pass into the compact bone itself as well as the medullary cavity