Gross Anatomy of Bone: Short, Flat, and Irregular Bones - Online Tutor, Practice Problems & Exam Prep

Short, flat, irregular, and sesamoid bones feature spongy bone at their core, surrounded by compact bone for strength and protection. The endosteum lines the spongy bone, housing bone stem cells, while the periosteum covers the compact bone, consisting of an osteogenic layer and a fibrous layer. Sesamoid bones differ as they lack periosteum, being entirely enclosed in tendons, which affects their growth and physiology. Understanding these structures is crucial for grasping bone anatomy and function.

Short, Flat, and Irregular Bones

We've been talking about the gross anatomy of bone in general, but now we want to take what we've learned and apply it to specific types of bone. We're going to apply it first to short, flat, and irregular bones, and also at the end of the video, sesamoid bones. You'll see what's missing from this list is the long bones. Long bones have a unique structure that we're going to go into more detail in a future video. We're starting with these bones because what we've learned so far applies pretty much directly to these types of bones. And that is that these bones on the inside, well, they have spongy bone. And to illustrate this, we have a diagram of a vertebra here. A vertebra is an irregular bone, and we've pulled one vertebra out of the spine, and we see it right here, a transverse view that we see in cross-section. And if we look first at the body of the vertebra, the anterior portion of the vertebra here, we can see that the center of this bone that I'm going to color in green is filled with all these little holes because that middle of that bone is the spongy bone. Now the posterior portion of the vertebra, on these processes, it almost looks like a separate bone here. They are connected, but this irregular bone, when you take a cross-section of it, you're going to see 2 different sections separately here. But you can see here, even though it's thinner, we still have this thin line of spongy bone that's making up the middle of this bone here. Now that spongy bone is going to be lined with endosteum because remember, endosteum is the connective tissue that lines the inside of bone. It's that real thin layer that has the bone stem cells in it. Now, in contrast, the outside of the bone is going to be compact bone. And again, we can see this in the diagram. I'm going to color it in blue, just all the way around the outside. There's this thin layer of compact bone. Now remember spongy bone, each individual strut well, when you put them together, it's quite strong, but each individual strut may not be that strong on its own. So when you wrap them all with compact bone, it gives a nice strong outer layer that can give protection and structure to the bone, and it also just sort of defines the edge of the bone really well. So that outside surface, compact bone, and of course, the outside of it is going to be lined with periosteum. And periosteum, remember, has 2 layers, the osteogenic layer, which is right next to the bone, which has the bone stem cells, and then the fibrous layer, which is all that collagen wrapped around the bone. Okay. So that's the pattern that goes for short, flat, and irregular bones. It also is true for sesamoid bones, with one exception. Sesamoid bones have no periosteum, and that's because the sesamoid bone, remember, is entirely enclosed in a tendon, so there's just no place for the periosteum to develop. Now that does, down the road, create some differences for how sesamoid bones grow and some physiological aspects of them. You usually don't need to worry about that that much, though. Just know the sesamoid bone is inside the tendon, and so it doesn't have a periosteum. Again, this does not all apply to long bones. We're going to talk about that in more detail coming up, and I'll see you there.

Gross Anatomy of Bone: Short, Flat, and Irregular Bones Example 1

This example wants to know what type of bone, spongy or compact, would you likely find on the inside lining of the cranial cavity closest to the brain? To help us think about this, we have a picture of a skull and a mid sagittal section of the skull, so we can see inside the cranial cavity. I'm just going to draw little arrows pointing to the part of the skull that we're interested in, the part of these flat bones that make up the cranial cavity, that inside lining there. Now remember, flat bones are the sandwich of compact bone, spongy bone, compact bone, where spongy bone is in the middle and spongy bone is that bone that's filled with all sorts of holes and compact bone is this dense, fairly solid bone that makes up the sides. So the inside lining, well, that's still the outside of the bone. And so that is still going to be compact bone.

All right. With that, like always, we have practice problems to follow. I'll see you there.

You are shown an image of a cross section of a bone. You decide it is likely a flat bone based on the structure. Without seeing the overall shape of the bone, how could you tell?

• A. You see yellow marrow in the spaces within the bone.
• B. There is no spongy bone present, only compact bone.
• C. You see a long thin sandwich of compact and spongy bone.
• D. The layer of spongy bone is at least twice the thickness of the compact bone.