Microscopic Anatomy of Bones - Bone Cells - Online Tutor, Practice Problems & Exam Prep

Bone is a dynamic tissue composed of cells and extracellular matrix (ECM). Key cell types include osteoprogenitors, which differentiate into osteoblasts that build bone matrix by secreting collagen and enzymes for hydroxyapatite formation. Osteocytes, derived from osteoblasts, maintain the matrix and monitor stress, while osteoclasts, originating from white blood cells, break down bone for remodeling and calcium homeostasis. This continuous process of bone remodeling ensures strong, healthy bones, adapting to stress and growth needs.

Introduction to Bone Cells

When talking about bone, I've really tried to stress the idea that bone is living dynamic tissue, and as such, there are cells that live inside bone, and bone has its own very specific cell types. We're going to talk about those cell types now. We'll start out just by saying that bone tissue, or sometimes just called osseous tissue, is comprised, well, like all connective tissue, of two things, cells and ECM, or extracellular matrix. We've already talked about that ECM in some detail, so let's talk about the cells of bone tissue. There are going to be four cell types in bone tissue, and we see them labeled and illustrated here.

But before we do that, you'll notice they're in two groups. I just want to say we'll explain why they're in two groups at the end. First, let's just go through each cell type one by one. First off, we have the osteoprogenitors or sometimes just called the osteogenic cells. The osteoprogenitors, the way I remember what these do, well, in that word progenitor, I see progeny, and osteoprogenitor cells make progeny.

They make new cells. These are the bone stem cells of the periosteum and the endosteum. Remember the osteogenic layer of the periosteum is that very thin layer of the periosteum that's on the outside of the bone, and the endosteum lines all of the inside of the bone. And what is there is a thin membrane of these, as you can see in the picture, these very flat little cells, they're these stem cells that are basically just hanging out until they are needed. Now when they're needed, when they get the call, they develop into osteoblasts, and that's why we have this arrow going from one to the other.

The osteoblasts, the way I remember what they do is I say blasts build. I have that alliteration going on. Blasts are osteoblasts that secrete bone matrix. They secrete the collagen of the bone, and they secrete enzymes, which lay down the hydroxyapatite crystals. Now when you look at them, these, osteoblasts are these osteoprogenitor cells that are developed, so they're sort of thicker.

They're more like a cube-shaped cell that goes around and lays down new matrix. Now some of these osteoblasts, once they've laid down the matrix, they're kind of done, and they'll shrink back down until they're needed again. But others will actually lay down matrix until they're completely surrounded by bone, and they've built matrix all the way around them. Those cells are going to go on to become what we call osteocytes. Now remember osteocytes by saying that the sites are the mature cells.

Again, I have that alliteration going on. What these are there for is to maintain the matrix. These osteocytes, you can see in this picture, they're sort of a typical cell type, but then they have these extensions that reach out through the bone and they reach out so that they can connect with other bone cells because they're completely trapped in the matrix. And without these extensions, they wouldn't be able to get the nutrients they need and they wouldn't be able to communicate. But what they're doing is they're in the bone and they're monitoring for stress.

And if there's anything wrong with the bone, they're going to send out the call and say, Hey, this bone here needs fixing. So the way we get the fixing going, we have the osteoclasts that come in. The osteoclast, the way I remember this, I say clasts cut. Again, I have that alliteration there. Clasts cut, what I mean by that is they break down the matrix for remodeling.

If there's something wrong with the matrix or you need to fix it or change it in some way, the first thing that you need to do, you need to remove the old matrix and then you can send out the call and the osteoblasts can come in, and they can lay down the new matrix and build it better than it was before. And that's something that is happening all the time in your skeleton. Now when you look at this image, you see the osteoclasts have all these nuclei. They have multiple nuclei. They're actually giant cells formed from multiple cells fusing, and they also have this big ruffle border.

That's how you can recognize them. Now you'll see here I have them in different groups. We have the osteoprogenators, the osteoblasts, and the osteocytes in one group, and that's because they're all part of one cell lineage. One develops into the next. The osteoclasts come from a completely different cell lineage.

These arise from white blood cells, so they're made in the bone marrow. With that, we've talked a little bit about the bone cell types. We're going to go into more detail on each one in upcoming videos, and I'll see you there.

Microscopic Anatomy of Bones - Bone Cells Example 1

This example tells me that calcium homeostasis is important for muscle contraction and nerve functioning. Bone cells help maintain blood calcium levels between 8.6 and 10.3 milligrams per deciliter. If the blood calcium level drops to 8 milligrams per deciliter, which type of bone cell would become more active and which type of bone cell would become less active? Alright. Then it shows me a picture of 4 different bone cells, so I just want to identify these real quickly.

First up, we have this sort of flattened bone cell. Those flattened bone cells are the bone stem cells. We call those osteoprogenitor cells. Next, we have this sort of cube-shaped one here, an osteoprogenitor cell that bone stem cell may go on to become an osteoblast. And remember that B, B stands for build.

Osteoblasts build new bone. Some osteoblasts, once they've built new bone, will go on to become this spidery shaped bone cell here. This is a mature bone cell. The mature bone cells are osteocytes. And then finally, we have this giant bone cell down here.

In reality, it would be much bigger than these other ones, it has this real ruffled border and multiple nuclei. This is going to be the osteoclast. And remember, clast, C is for cut. Osteoclasts cut into the bone and they remove bone when that's necessary. So in this case, our blood calcium levels have dropped, and we need to raise blood calcium levels.

To do that, we need to remove some bone. To remove bone, what type of bone cell do you want to use? An osteoclast. Osteoclasts with that ruffle border are going to release acids and enzymes that are going to break down the bone. They're going to take that calcium.

It's going to go into the blood and raise that blood calcium level. So what's going to become more active? The osteoclast. Now, what's going to become less active? Well, if you're trying to take calcium out of the bone, you don't want to be building new bone.

Blast B stands for build, osteoblasts will become less active. Alright. With that, remember that bone is not just there for structure, but also for calcium homeostasis, and I'll see you in the next video.

Osteoblasts

Now we're going to talk about osteoblasts in more detail, and I always remember this by saying blasts build. The osteoblasts are the bone cells that are responsible for producing new bone matrix. They build new bone. And to illustrate this, down here, we have an illustration of an osteoblast. It's wearing a hard hat, and it's a work in progress, and it's building something out of bricks, a brick wall or a brick structure here.

And those bricks are supposed to represent that new matrix that it's laying down. Alright. The way it actually does this is the osteoblast is going to secrete collagen, collagen fibers. Remember, collagen fibers are really strong like a rope. They're flexible, but they're really hard to break.

And the other thing it's going to secrete is calcium. I'm going to write calcium here as Ca2+ binding enzymes. So the osteoblasts don't actually release the calcium. They release enzymes, which bind to calcium, and that produces those hydroxyapatite crystals, which give bone its hardness. So the osteoblasts, remember, arise from the osteoprogenitor cells.

These are those bone stem cells that are in the periosteum and the endosteum. The periosteum is that connective tissue lining on the outside of the bone. The endosteum, the connective tissue lining on the inside of the bone. So when you need to lay down new bone, some of these progenitor cells are going to develop into these osteoblasts. They'll start laying down new bone matrix.

Some of them will just finish up their job and they'll shrink right back down until they're needed again, but some others will actually build themselves into the bone matrix. They lay down matrix all around themselves until they're sort of stuck in a little cell, and then they are a mature osteoblast, which we are going to say is an osteocyte. So, once that osteoblast builds itself into the bone matrix and it's stuck there, then it's called an osteocyte. Because osteoblasts are so active in laying down new bone, this means that they are going to be active in bone growth. Now, we talked a little bit about bone growth when we talked about the structure of the long bone.

Remember we talked about the epiphyseal plate, that growth plate, that cartilage line where long bones grow longer. We're not talking about that now. We're talking about how bones grow longer and how other bones grow. The osteoblasts will just go around and they'll lay new bone on the outside of the bone. That will make those bones wider.

As you add more bone to the outside of the bone, the bone is just going to get bigger. Other cells will then go in and sort of reshape the inside of the bone to fit its new size. Part of that will be other osteoblasts laying down some new bone to match the structure of the new size of the bone. Alright. But for now, remember, osteoblasts come from these osteoprogenitor cells, and then some of them grow into the osteocytes.

So we'll talk about osteocytes next.

Osteocytes

Now it's time to talk about the osteocytes. And I remember these by saying the osteocytes. The sites are the mature bone cells. So these are the mature bone cells that maintain the matrix. Remember, osteocytes come from osteoblasts.

Osteoblasts build new bone. Sometimes they build themselves into the matrix, and then they become the osteocyte. So I think of it as a trapped osteoblast that becomes an osteocyte. But they're not just like in the matrix. They build themselves a little room or a little chamber in which to live in, and that little room is called a lacuna, or the plural is lacunae.

These lacunae are the rooms that osteocytes are housed within. Now they're there for a reason, though. They are there to monitor bone stress and also to contribute some to calcium homeostasis. But let's talk about the stress first. Your bone is always being replaced.

You're always removing some bone and building it new again. One of the ways your body knows where to do that is from the osteocytes. They're monitoring the bone around them. And if it's showing signs of stress, they'll send out a little message, and it will say, hey, replace this bone. The osteoclasts will come in, remove the bone.

Osteoblasts will come in and build new bone, and some of those osteoblasts will get stuck in the matrix and become the new osteocytes. Now for calcium homeostasis, you have a ton of calcium in your bones, but you also need calcium in other parts of your body. So it's really easy for your blood to stash a little calcium in your bones or just to take a little bit out. Most of that job is done by the osteoclasts and the osteoblasts, but the osteocytes contribute to it as well. Alright.

Finally, I want you to think if you're living in bone, if you're living in this chamber inside bone, well, you still need to get the nutrients. You still need to get rid of waste. You need to communicate. So these cells can't be completely trapped in the bone. They're not.

They have small projections that reach out and allow them to communicate and diffuse with other cells via gap junctions. So these cells have these projections that reach out to the smallest cracks in the bones called canaliculi, which we'll talk about in a later video, and they sort of hold hands with the cells around them. And these gap junctions remember, gap junctions are ways that diffusion can happen between cells really easily. So that's the way that they can get the nutrients they need. They can get rid of waste, and they can communicate as long as some of those cells are in contact with the capillaries where the blood is going to be.

So to illustrate all this, we have this illustration of a brick structure down here, and we have six osteocytes living in their lacunae. But what I want you to see here is these projections that are reaching out and connecting with all the cells around them, almost like these sort of spider webby, almost octopus-like arms reaching out and touching the cells around them so they can exchange materials and also communicate, so they're not completely stuck in that bone matrix. Alright. Finally, I just want to remind you, osteocytes start out as osteoprogenitor cells. They become osteoblasts.

Some of those then become osteocytes. That's the end of that cell line. So when these sites are done, well, they're just going to die, but they can live a long time. Some of them live for decades until that bone is replaced. Alright.

With that, our final bone cell type is going to be the osteoclast. We'll do that next.

Osteoclasts

To finish up the bone cells, we're going to talk about the osteoclast. And I remember osteoclast by saying osteoclast. The clast cuts into the bone. And by that, I mean that they are specialized for osteolysis or dissolving or breaking up bone. We can look at that word, osteolysis.

Osteo means bone, and lysis means to break up. So it literally means just breaking up the bone. But they're not just breaking up the bone and leaving a mess. They're good citizens. They clean up after themselves.

So they're also doing bone reabsorption. They break up the bone matrix, but then they take that matrix into the cell. And inside the cell, they digest it further, and anything that's left, they sort of just shoot out the back. Now to do this, they have a lot of specialized structures. They are multinucleate.

These are cells with many nuclei in them, and that is because they are large. And hazard to say they're even giant cells. They're derived from white blood cells. Remember, they come from a different cell lineage than all our other bone cells, and they're formed by many cells coming together and fusing to make this uber cell with many nuclei inside. They also have this really distinctive ruffled border just on one edge.

That ruffled border is going to be against the side of the cell that's up against the bone, and that is there to increase surface area. Whenever you see a membrane that's ruffled or has lots of folds in it, it's usually there to increase surface area because that means you can get more stuff across the membrane. The more membrane you have, the more transport across a membrane you can do. What are they trying to transport? They're trying to secrete acid, and that acid is going to dissolve the inorganic matrix, the hydroxyapatite or the calcium phosphate we've been talking about.

Remember, in an example before we did this, I put a bone in vinegar and it broke up all that hydroxyapatite. Remember that bone got really soft because there are no more hard crystals inside it. Of course, you guys, they don't want to just make soft bone. They want to get rid of it entirely, so they also secrete enzymes. And these enzymes are what's going to digest the organic matrix.

And by organic matrix, we mostly mean that collagen. That collagen, it's really strong, that rope-like fiber that keeps bone from breaking. And, again, when we did this in the example before, we used bleach to do it. To illustrate this, we have this drawing here of a bulldozer plowing through a brick wall, and we've been using these bricks to represent our bone matrix.

And what's driving the bulldozer? We have an osteoclast. And we can see that osteoclast. It has multiple nuclei, and it also has that ruffle border that's going to secrete those enzymes and the acid and also pick up the matrix that it breaks down. So why is it doing this?

You want strong bones. Why do you have cells that are designed to break down your bones? That seems a little counterintuitive. Well, it's for something that we're going to define here as bone remodeling. Bone remodeling is when osteoclasts and osteoblasts work together to maintain the skeleton.

We said your bone is always being replaced, and it's being replaced for a few reasons. It's being replaced for maintenance, for growth. When a bone grows bigger, you now need to go in and reshape the inside of the bone so it matches that new size. These cells are going to do that. It's going to be a response to stress.

If a part of the bone is too stressed, these cells can come in and build it back better than it was. And it's also for calcium homeostasis. Right? We said you have a ton of calcium in your bones, but you also need it in other places in your body. It's really easy.

Just have an osteoblast put down a little bit of bone. That's going to lower the amount of calcium in your system, or you can have an osteoclast pick up, dissolve just a little bit of bone, and that's going to up the amount of calcium in your system. So to illustrate this, we have this humerus here and clearly not to scale. We have the osteoclast and the osteoblast, and the osteoclast is going along in this direction, and it's leaving space in the bone because it's breaking up that bone behind it. And the osteoblast is coming in, and it's laying down that new bone.

It's laying down new bone matrix. It's going to be newer, better than what was there before. That finishes our tour of the bone cells, and I'll see you in the next video.

Microscopic Anatomy of Bones - Bone Cells Example 2

This question tells me that osteoporosis is a common condition in older adults, especially women. Bones become weak due to an imbalance between osteoblast and osteoclast activity. Then it wants to know which type of cell would be more active and which type of cell would be less active in an individual with osteoporosis and explain your reasoning. It also gives us a little picture here. It's the epiphysis of the femur.

And on the left, we can see that the spongy bone looks normal. There are all these really small holes between the struts of the spongy bone. And on the right, this is an image of what it might look like for someone with osteoporosis. That spongy bone seems to be hollowed out as there are fewer struts connecting the spongy bone. So what would be more active to produce something like this?

So remember, osteoblasts, blasts build. They lay down new bone matrix. While osteoclasts, clasts cut. They remove the bone matrix. So if the bone's getting hollowed out like this, it sounds to me that the osteoclast is going to be relatively more active.

And my reasoning, osteoclasts remove bone matrix. Well, in contrast, what's going to be less active? Well, if it's building less bones, blast build, so osteoblasts will be less active. The reason? Blasts produce bone matrix.

You don't need a big imbalance for this to add up over time. Right? Osteoporosis isn't something that just happens overnight. It's years of a slight imbalance, and over those years, this bone just becomes more and more hollowed out. The spongy bone gets fewer struts connecting it, and because there's less matrix, the bone becomes more fragile.

Alright. With that, more questions to follow, and I'll see you there.

How does the structure of an osteocyte allow communication with adjacent cells?

• A. Osteocytes have cellular projections that meet and form gap junctions with adjacent osteocytes.
• B. Osteocytes have ruffled borders to release signaling molecules that communicate with adjacent osteocytes.
• C. Osteocytes have significant surface area to allow for rapid diffusion across the cell membrane.
• D. Osteocytes are multinucleated allowing them to make more protein rapidly and pass information quickly.

A central theme in anatomy is the relationship between structure and function. What structural feature of osteoclasts aids in their function of osteolysis?

• A. Their multiple nuclei allow for more rapid production and deposition of collagen.
• B. Their cellular projections allow cell to cell communication.
• C. Their flattened shape allows them to move through the bone more easily.
• D. Their ruffles border increases surface area between the osteoclasts and the matrix.

Which pathway correctly identifies the relationship between bone cells?

• A. Osteoprogenitor → osteoblast → osteoclast
• B. Osteoprogenitor → osteoblast → osteocyte
• C. Osteoclast → osteocyte → osteoblast
• D. Osteoclast → osteoprogenitor → osteoblast

What is the major function of bone cells found in lacunae?

• A. Bone reabsorption.
• B. Bone deposition.
• C. Monitoring bone stress.
• D. Laying down new organic matrix.

Here’s what students ask on this topic:

What are the four main types of bone cells and their functions?

The four main types of bone cells are osteoprogenitors, osteoblasts, osteocytes, and osteoclasts. Osteoprogenitors are stem cells found in the periosteum and endosteum that differentiate into osteoblasts. Osteoblasts are responsible for building new bone matrix by secreting collagen and enzymes that facilitate hydroxyapatite formation. Osteocytes, derived from osteoblasts, are mature bone cells that maintain the bone matrix and monitor mechanical stress. Osteoclasts, which originate from white blood cells, break down bone tissue through osteolysis, aiding in bone remodeling and calcium homeostasis.

How do osteoblasts contribute to bone formation?

Osteoblasts contribute to bone formation by secreting collagen fibers and enzymes that bind to calcium, forming hydroxyapatite crystals. These components give bone its strength and rigidity. Osteoblasts originate from osteoprogenitor cells and are active in areas of bone growth, such as the periosteum and endosteum. Some osteoblasts become embedded in the bone matrix they produce, transforming into osteocytes, which maintain the bone tissue. Osteoblasts play a crucial role in bone growth, repair, and remodeling.

What is the role of osteocytes in bone tissue?

Osteocytes are mature bone cells that originate from osteoblasts. They reside in small cavities called lacunae within the bone matrix. Osteocytes maintain the bone matrix and monitor mechanical stress. They communicate with other bone cells through extensions called canaliculi, which allow for nutrient and waste exchange via gap junctions. Osteocytes also play a role in calcium homeostasis by signaling osteoclasts and osteoblasts to resorb or deposit bone as needed, ensuring the bone remains healthy and strong.

How do osteoclasts function in bone remodeling?

Osteoclasts are large, multinucleated cells that break down bone tissue through a process called osteolysis. They secrete acids and enzymes that dissolve the inorganic and organic components of the bone matrix, respectively. This breakdown releases calcium and other minerals into the bloodstream, aiding in calcium homeostasis. Osteoclasts are essential for bone remodeling, a continuous process that maintains bone strength and integrity by removing old or damaged bone and allowing osteoblasts to lay down new bone tissue.

What is the difference between osteoblasts and osteoclasts?

Osteoblasts and osteoclasts have opposite functions in bone tissue. Osteoblasts are responsible for building new bone matrix by secreting collagen and enzymes that facilitate hydroxyapatite formation. They originate from osteoprogenitor cells and are active in bone growth and repair. In contrast, osteoclasts break down bone tissue through osteolysis, secreting acids and enzymes that dissolve the bone matrix. Osteoclasts originate from white blood cells and play a crucial role in bone remodeling and calcium homeostasis by resorbing old or damaged bone.