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.