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.