Hi guys, today we are going to talk about the processes involved in bone formation. We've been making some great progress so far on the skeletal system and this lesson will hopefully be no different. There are two distinct ways in which the body can form bone.
This tutorial will focus on the type that is first present after conception occurs, and we refer to it as intramembranous ossification. If we start by breaking those two terms down quickly, intramembranous and ossification, we can get a bit of an idea of how it happens from the start. So, intramembranous meaning from within a membrane, and ossification meaning The deposition or formation of bone. So formation of bone from within a membrane.
And as you can see that I've already started writing up here, in utero before week eight, all bone is formed from hyaline cartilage and fibrous membranes. So that's how it's going to start. And when it's formed from those fibrous membranes, we call that intramembranous.
So that's our term here, and that's what the focus of this lesson is going to be right now. Now before we get straight into the actual process, I'll just point out quickly that cranial bones and the clavicle are formed by this method. Mostly, so mostly flat bones are formed by intramembrane ossification.
So just writing that down quickly. Cranial bones and the clavicle are formed this way, so mostly flat bones. Now moving on to the part of this lesson that matters. Formation. How does it happen?
How does it work? Well, we're going to go through in a step-by-step method, and I'll show you guys what's going on. So now, really early in development, we have...
mesenchymal stem cells and stem cells are just cells that can differentiate or can turn into many different types of cells. You have mesenchymal stem cells that are going to aggregate and differentiate, so change their nature. They're going to differentiate into a type of cell called an osteoblast.
Now the But osteoblasts will be the main type of cell responsible for bone deposition in your body. And blast means to produce, to grow. So another way to think of this differentiation that's happening here with these mesenchymal cells, they're becoming committed to this cell type, the osteoblast.
Now these bone forming osteoblasts. are going to come together and form what we call an ossification center. And that ossification center is really the start point of the bone.
And just all these blue lines you see around as well are just collagen fibers in that matrix around the bone, or around the ossification center. And now those osteoblasts, they're going to begin to start to secrete something called osteoid. Now, osteoid is a substance that is going to be responsible for the actual properties of the bone, the hardness of the bone. So it's an unmineralized bone at this point.
So just drawing that up on here as well, just all these little purple dots that I'm drawing are the osteoid that is being secreted into this area. Now if we move onto the second step of our intramembranous ossification, we can see that the peripheral mesenchymal cells are still transforming into the osteoblasts. So they are continuing to differentiate. Now I'll draw that up on here as well, just so we're getting a really good idea of what's going on. So those mesenchymal cells are still differentiating.
They're still transforming into osteoblasts and becoming committed to that cell line. Now, our osteoblasts, we can see that they're taking on a slightly different shape. What we can see that's happening here is the osteoid now is only being secreted inward. It's being secreted towards that ossification center. And as that secretion continues inward, we can see that some of those osteoblasts are becoming trapped.
So we've got osteoid going towards the ossification center, and the osteoblasts are becoming trapped within that space. The osteoblasts that remain trapped in that central space in there, they differentiate further. to become an osteocyte.
The osteocytes are going to be matured bone cells that are just responsible for maintaining that matrix that's going to be forming around the bone. So we've written up here that they become trapped and it will cause that differentiation to the osteocyte. What's going to happen now is that osteoid that is in this space here that's been continually secreted by all those osteoblasts. After a couple of days, it's going to calcify and harden.
Now when it calcifies and harden, that makes our bone. And I'll just draw, well I'll fill that in quickly here as well, to show you guys that it's now hardened. Moving on to the next step that we're going to see, actually I'll quickly say as well that this is the hardened bone matrix, just so there can be no mistakes. So we've got the hardened bone matrix, and what happens next?
Well, we can see that we're getting quite a change happening now, and I've changed this color of the bone here. I didn't think gray was an appropriate color for the bone, so I've moved more to a bone color. The osteoblasts that we have, they're going to continue depositing that osteoid, but it's going to start to assemble in quite a random manner.
And the reason it's going to assemble quite randomly like this and form this trabecular structure is because it's doing it around the embryonic blood vessels that are forming there as well. So those embryonic blood vessels are going to be twisting and curving through that woven bone and causing the formation of that finely woven trabeculae. So we've got these blood vessels.
We can see them going in all directions and like I just said that will cause the formation of that trabeculae. So finely woven trabeculae. The last thing that we can see happening here on the outer surface is that these mesenchymal cells are joining together much more densely, and these collagen fibers are joining together more densely as well. And what's going to happen is they're going to differentiate and condense into what will be the periosteum of the bone surface.
So they begin to differentiate into periosteum. And that will bring us on to our last phase of intramembranous ossification. What's going to happen in this last step is this lamella or compact bone that we've learnt about when we looked at the gross anatomy of the bone, is going to start forming toward the outer edges of those trabeculae.
So towards where those osteoblasts that we can still see are lying. So I'll just draw that down quickly for you as well so you can see what that lamellar bone is going to look like. Now that lamellar bone gets deposited into layers like this. So it's different to the spongy bone.
So it gets deposited in layers and it's a layered structure. Now that internal spongy bone, it's going to remain there. The reason that... Internal spongy bone is going to remain there, because we have something very important in our body happening in those spaces.
Something called hematopoiesis. You've heard the term red bone marrow before, and I discussed red bone marrow in our gross anatomy as well. And that vascular tissue is going to be forming that red bone marrow. So we're going to have this area now that's going to be filled up with that red bone marrow. So I'm just writing this all down.
Vascular tissue within the trabecular spaces is going to be forming that red marrow. And we'll put that up on here now. And I'll just point out that this whole area now is just filled with that marrow. Now we can see one more thing going on here. There's still osteoblasts there.
But if that bone is completely formed... Why do we need the osteoblasts to remain there? That question can be answered by the fact that at different stages in our life we have different stresses that are going to be placed upon the bone, so repeated impact or repeated stress is going to cause the deposition or remodelling of the bone to become stronger, or we can take away the nutrients and use them elsewhere when we don't need it there. So we can see those osteoblasts are just waiting on that surface to remodel the shape of that bone when needed.
So we have learnt all the steps involved in this intramembranous ossification, but we're going to have to conclude and just bring it back all together so we really remember it. We know that the bone is going to form from a membrane or cartilage. Now it's going to mostly be flat bones. And it's going to be caused by those mesenchymal cells differentiating into those osteoblasts.
And once they've differentiated into that osteoblast, that osteoblast can then direct the growth of the bone by depositing that osteoid. Woven bone will start to form. And once that woven bone forms, we start to push outwards, where the lamellar bone and the outer periosteum can form. from those mesenchymal cells that remain in that area.
So that'll be the outer surface. And those surface osteoblasts that remain there are going to be used for remodeling when required. And that's our intramembranous ossification, guys.
In the next video, we'll have a look at endochondral ossification and how it differs. I hope that was helpful, and thanks for watching.