Gross Anatomy of Bone: Periosteum and Endosteum - Online Tutor, Practice Problems & Exam Prep

Bone, a type of connective tissue, is enveloped by the periosteum and endosteum. The periosteum, a dense irregular connective tissue, contains blood vessels and nerves, serving as an attachment point for tendons and ligaments. Perforating fibers anchor it tightly to the bone. The endosteum lines the inner surfaces of bones, particularly in spongy bone and the medullary cavity, consisting mainly of osteoprogenitor cells, or bone stem cells. Understanding these layers is crucial for grasping bone structure and function, including bone remodeling and repair processes.

Periosteum and Endosteum

Bone is a type of connective tissue, but it's also wrapped by other connective tissue. Specifically, it's going to be wrapped by the periosteum and the endosteum. These connective tissues provide bone with the blood, the nerves, the attachment points for tendons and ligaments, and also the bone stem cells. So, we're going to go over these now. We're going to 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, and it's a covering on the bone that's really on there tightly, 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.

Now that periosteum, we can break it 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, that's going to be the superficial layer, the layer on the outside, and that'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. Now, this is also going to serve a bit 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.

So a lot of times you might think of the connection with a tendon on a bone sort of like it's super glued to 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. Now, 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. So, it's not just that this is wrapped on, it's woven into the bone. It's on there really tightly. So if you look over here on our image, we can see a cross-section of a long b

Gross Anatomy of Bone: Periosteum and Endosteum Example 1

This example tells us that it's important for muscles and bone to maintain extremely strong connections, and I agree with that statement. It's going to ask us though, what material is most responsible for the strength of this connection. Alright. So remember, a tendon connects a muscle to a bone, and a tendon is largely comprised of collagen fibers. Collagen is a protein and a fiber that we've been talking about, and we're going to talk about a lot in this chapter.

Collagen is strong like a rope. It's really flexible, but if you pull it, it has really high tensile strength. It's very difficult to break. So now it's going to ask us what structural features aid in it performing this function. And here, I want to focus on the structural features of bone because that's the chapter we're in.

And to help us out here, though, we have this little cutaway of bone, this picture over here, and the main thing that you see here is this bit of tissue that's coming off the bone. This covering on the bone is being peeled away, and you can see these collagen fibers on the inside there. So the first thing that I just want to note in the structural features are the collagen fibers themselves. As we said, the collagen fibers are extremely strong. You can pull on them, and they're just really hard to break, even though they're very flexible.

Now where the tendon attaches to the bone, there's usually a sort of rough spot there to give lots of surface area because those collagen fibers come in, and it's not just like they're glued on. They actually get woven into everything around them. One of the things that they get woven into that we've been talking about here is going to be that periosteum. And that's the thing that we can see sort of pulled away from the bone right there. That periosteum is also largely made of collagen fibers, and it is wrapped around the bone really tightly.

So as those collagen fibers from the tendon are woven into the periosteum, when the tendon pulls, it's not just pulling on the bone, it's pulling on the periosteum that surrounds the bone. Another structural feature here that we can sort of see in these collagen fibers here is going to be the perforating fibers. Remember, these perforating fibers are the collagen fibers of the periosteum that are actually woven down into the bone itself. Now that's also going to be true of the collagen fibers, at least some of the collagen fibers in the tendon. They get woven into the bone too.

So it's wrong to think about a tendon just being superglued onto a bone. A tendon is collagen, and that collagen gets woven into the bone itself. It gets woven into the periosteum. The collagen of the periosteum is woven into the bone. There's no real place where you can say the collagen of one ends and the collagen of the next one begins.

That means when you pull on a tendon, it's attached really strongly to the bone. In fact, if you're likely to damage something by pulling too hard, you're more likely to damage the muscle than you are the tendon. So, with that, collagen is going to be a really important feature in bone going forward, and I'll see you there.

True or False; if false, select the answer that best corrects the statement.

Endosteum is most similar to the fibrous layer of the periosteum.

• A. True, endosteum and fibrous layer have the same composition.
• B. False, the endosteum is most similar to the osteogenic layer of the periosteum.
• C. False, the endosteum contains thicker and more numerous collagen fibers than the fibrous layer.
• D. False, the endosteum is connective tissue while the periosteum is cartilage.

True or False; if false, select the answer that best corrects the statement.

A function of the fibrous layer of the periosteum is to house osteogenic cells.

• A. True, the osteogenic cells reside in the fibrous layer of the periosteum.
• B. False, a function of the fibrous layer is to provide attachment sites for ligaments.
• C. False, a function of the fibrous layer is to provide a site for hematopoiesis.
• D. False, a function of the fibrous layer is to house osteocytes.

Here’s what students ask on this topic:

What is the periosteum and what are its functions?

The periosteum is a dense irregular connective tissue that covers the outer surface of bones. It consists of two layers: the fibrous layer, which is rich in collagen and contains blood vessels and nerves, and the osteogenic layer, which contains bone stem cells (osteoprogenitor cells). The periosteum serves several functions: it provides nourishment to the bone through its blood vessels, supplies nerves for sensation, and serves as an attachment point for tendons and ligaments. Additionally, the periosteum plays a crucial role in bone growth and repair due to the presence of osteoprogenitor cells.

What is the endosteum and where is it located?

The endosteum is a thin membrane of connective tissue that lines the inner surfaces of bones. It is primarily composed of osteoprogenitor cells, which are bone stem cells. The endosteum is found lining the medullary cavity of long bones, as well as the trabeculae of spongy bone. Unlike the periosteum, the endosteum does not contain a fibrous layer and is not as tough. Its primary function is to facilitate bone growth, remodeling, and repair by providing a source of new bone cells.

How do the periosteum and endosteum contribute to bone repair?

Both the periosteum and endosteum play vital roles in bone repair. The periosteum contains osteoprogenitor cells in its osteogenic layer, which can differentiate into osteoblasts to form new bone tissue. Additionally, the periosteum's blood vessels supply essential nutrients and oxygen to the repair site. The endosteum, lining the inner surfaces of the bone, also contains osteoprogenitor cells that contribute to the formation of new bone during the repair process. Together, these layers ensure that bone repair is efficient and effective by providing the necessary cells and nutrients.

What are perforating fibers and what role do they play in the periosteum?

Perforating fibers, also known as Sharpey's fibers, are collagen fibers that extend from the fibrous layer of the periosteum into the bone matrix. These fibers play a crucial role in anchoring the periosteum tightly to the bone. By weaving into the bone matrix, perforating fibers ensure that the periosteum remains securely attached, providing stability and support. This tight attachment is essential for the periosteum to effectively serve as an attachment point for tendons and ligaments, and to withstand the mechanical forces exerted on the bone.

What is the difference between the fibrous layer and the osteogenic layer of the periosteum?

The periosteum consists of two distinct layers: the fibrous layer and the osteogenic layer. The fibrous layer is the outermost layer and is composed of dense irregular connective tissue, rich in collagen fibers. This layer provides structural support, contains blood vessels and nerves, and serves as an attachment point for tendons and ligaments. The osteogenic layer, located beneath the fibrous layer, contains osteoprogenitor cells, which are bone stem cells. These cells are crucial for bone growth, remodeling, and repair, as they can differentiate into osteoblasts to form new bone tissue.