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.