Gross Anatomy of Bone: Compact and Spongy Bone: Study with Video Lessons, Practice Problems & Examples

Bone anatomy consists of two primary types: spongy bone and compact bone. Spongy bone, resembling a sponge, contains trabeculae that provide strength while reducing weight and housing bone marrow. In contrast, compact bone appears solid and is designed for strength and protection, forming the outer layer of bones and the shafts of long bones. Understanding these structures is crucial for grasping how bones support the body and facilitate movement.

Compact and Spongy Bone

When talking about the anatomy of bone, we need to realize that all bone is made of 2 main structural arrangements, spongy bone and compact bone. We'll talk about spongy bone first. Spongy bone is called spongy bone because it looks like a sponge. Makes sense. Right?

It's filled with all sorts of holes, and we can see that in this image here. We have the end of a femur. And if you look at the middle of the bone here, you can see that it's filled with all sorts of these little holes with little pieces of bone connecting around those holes, much like a sponge. Now, really importantly, spongy bone is not soft like a sponge. That's a misconception some people get when they hear the name.

Spongy bone is hard just like bone. It just, in a cross-section, is filled with all sorts of holes like a sponge. Now we're going to say rather than, likening it to a sponge, we're going to say it's built more like scaffolding. And when you think of scaffolding, right, you put scaffolding up the outside of a building when you're doing construction on it, and it's built with all these pipes and bars that are connected in different ways. Altogether, that scaffolding is going to be really strong.

But when you look at it, it's mostly open space. That's like spongy bone. The struts connecting in different ways are what give it its strength. Spongy bone, we're going to call those struts trabeculae. So trabeculae are the bone struts of spongy bone.

Building spongy bone this way with these struts that connect the space in between is going to reduce weight. Bone is really heavy tissue. And if your bones were solid bone, your skeleton would weigh a lot more. So by making spaces inside, you lighten the skeleton and you also give room to put stuff inside the bone. The stuff you put inside the bone is bone marrow.

We'll talk about bone marrow more in another video. Spongy bone is going to be found in the middle of all bones. If you take a bone, cut it in cross-section, you should see spongy bone in the middle. The only exception to this is the shaft of a long bone. But even long bones at the ends, just like the end of the femur that we see here, will have spongy bone on the inside.

Now in contrast, we have compact bone. Compact bone is called that because it looks solid. Now I say look solid because there are no visible spaces. If you look at a skeleton, you're looking at compact bone. We say no visible spaces because there actually are lots of little spaces and canals through the compact bone, but it's all at the microscopic level.

At least it's mostly at the microscopic level. So compact bone is really much more like solid bone, and that's going to be optimized for strength and hardness. And because it is so strong and so hard, it's going to be on the outside edge of all bone. So we put spongy bone in the middle, and then you can see here on the end of this femur here, all along the edge, there's a layer of compact bone. Now if you want if you have a bone, so you get an impact near the impact on your bone, you want that compact bone on the outside because it is so strong and hard.

It's going to present, a lot of protection for the bone. The other place you're going to find compact bone is in the shaft of long bones. That's the only place where you don't have spongy bone in the middle, and it's only compact bone. Okay. So with that, remember compact bone looks solid, spongy bone filled with holes like a sponge, but not soft like a sponge.

We're going to go into more detail in both for both bones going forward, and I'll see you there.

Gross Anatomy of Bone: Compact and Spongy Bone Example 1

This example tells me that spongy bone and compact bone are sometimes compared to scaffolding in a brick or concrete wall, and it wants me to explain that analogy. So let's take a look. Alright. Over here on the left, we have scaffolding, and we have a little picture of scaffolding, and it has all these bars or pipes that are connected in different directions. It has a little ladder going up too.

But all these bars and pipes, they're individually quite strong, but when you connect them in all these different ways, you can create a structure that together is incredibly stable and strong, stronger than any one pipe or bar could be on its own. And yet, most of the structure is empty space. That reminds me a lot of spongy bone. Spongy bone, we said, are these bony struts that are pointed in all different directions and connected to each other with space in between them. Together, they create this quite rigid and quite strong structure even though most of the space is filled with marrow.

So my reasoning is that both of them are struts surrounding space. Now we can compare that to this brick wall over here, and this brick wall shows a little picture of some bricks. And brick wall is hard. It is heavy. It is solid.

That reminds me a lot of compact bone. Compact bone is all those things. It's really hard. It's solid. Now technically, there's microscopic space in there, and we're going to learn all about those microscopic spaces in future videos.

But compared to spongy bone, it's much, much more solid, which also makes it heavier. But because it's heavy and hard, that's why it's on the outside of the bone, protecting the scaffolding, the spongy bone on the inside that's a little bit more delicate. So for my reasoning, I'm just going to say those things. It is solid. It's hard.

It's heavy. Have to spell 'heavy' correctly, though. Alright. So we're going to rely on these two analogies a little more going forward, so keep them in mind. For now, there are a couple more problems to solve.

Give them a try.

If a person's spongy bone were all replaced by compact bone, how may that affect the function of their bones and skeleton?

• A. The skeleton would be stronger but also heavier.
• B. The interior of the bones would lose their softness meaning breaks would happen more easily.
• C. The skeleton would have less room for marrow meaning the function of joints would be affected negatively.
• D. Long bones would become heavier, but other bones would not be affected.

Here’s what students ask on this topic:

What is the difference between spongy bone and compact bone?

Spongy bone, also known as cancellous bone, resembles a sponge with its porous structure filled with trabeculae, which provide strength while reducing weight and housing bone marrow. It is found in the middle of bones and at the ends of long bones. In contrast, compact bone appears solid and is designed for strength and protection. It forms the outer layer of bones and the shafts of long bones. Compact bone has microscopic spaces and canals, but no visible spaces, making it much denser and harder than spongy bone.

Why is spongy bone important in the human skeleton?

Spongy bone is crucial because it reduces the overall weight of the skeleton, making it easier for the body to move. Its trabecular structure provides strength and support while allowing space for bone marrow, which is essential for producing blood cells. Additionally, the porous nature of spongy bone helps absorb impact and distribute stress, protecting the more delicate inner structures of the bone.

Where is compact bone found in the human body?

Compact bone is found on the outer layer of all bones, providing strength and protection. It is especially prominent in the shafts of long bones, such as the femur, where it forms a solid, dense structure optimized for bearing weight and withstanding impact. This dense arrangement helps protect the inner spongy bone and bone marrow from damage.

What are trabeculae in spongy bone?

Trabeculae are the struts or rods of bone tissue found within spongy bone. They form a lattice-like network that provides structural support and strength while reducing the overall weight of the bone. The spaces between trabeculae are filled with bone marrow, which is vital for producing blood cells. This arrangement allows spongy bone to absorb impact and distribute stress efficiently.

How does the structure of compact bone contribute to its function?

The structure of compact bone, with its dense and solid appearance, contributes to its primary functions of providing strength and protection. The microscopic spaces and canals within compact bone allow for the passage of blood vessels and nerves, ensuring the bone remains nourished and functional. This dense arrangement makes compact bone ideal for withstanding mechanical stress and protecting the inner spongy bone and bone marrow.