Full transcript
0:00- So let's talk about how bone forms.
0:02Like, where does your bone come from in the first place?
0:05And this is relevant, because there are actually
0:08two different processes by which bone organs form.
0:13And they don't come out of nowhere.
0:15It's not like a random mesenchymal
0:18cell is going to say,
0:19"Oh, gee, I feel like we need a bone here,
0:21so let's build some bone."
0:24In the process of development, like fetal development,
0:29when you are just a little slime inside your mama's belly,
0:32you at a certain point,
0:35about eight weeks along, your body says,
0:40"We now have structures in place to build bone."
0:43And there's two ways that that happens.
0:45The first one that we're going to talk about is called
0:48intramembranous ossification.
0:55Doesn't ossification make you think of bone?
1:03Yeah, we're making bone, dog pound.
1:06So all bone formation,
1:10you have to start with some sort of connective tissue precursor.
1:15You can't make bone out of a random cell,
1:20or out of a muscle cell, or out of epithelial tissue.
1:24Bone has to come from an existing
1:29connective tissue model almost.
1:32And in the case of intramembranous ossification,
1:35there is a dense,
1:37irregular, connective tissue, for like of a better word,
1:42membrane where the bone will form.
1:52So visualize.
1:54Intramembranous ossification happens with flat bones,
1:57so your skull bone, like your frontal bone.
2:00There is a dense-- this is back in the day
2:03when you're eight weeks in your mama's belly.
2:06There is a flat, dense, irregular,
2:08connective tissue structure in your forehead.
2:13And if you--
2:14somebody went in and, like, squished your head,
2:16they'd squish it, because it's just dense,
2:18irregular, connective tissue. It is not yet bone.
2:22So the first thing that happens is that this dense,
2:25irregular, connective tissue membrane has mesenchymal cells
2:30in there. Right?
2:35It makes perfect sense.
2:37And the mesenchymal cells are going to differentiate
2:41ultimately into osteoblasts.
2:46And this happens in, like,
2:48an ossification center.
2:51Like, there are places in the dense, irregular,
2:54connective tissue membrane
2:57where there's going to be a thickening.
2:59And the mesenchymal cells
3:01there are going to differentiate and become osteoblasts,
3:04and the osteoblasts are going to start doing their thing.
3:09As they produce bone matrix,
3:13or bone tissue,
3:15of course they're going to get trapped and become osteocytes,
3:18and of course they're going to build
3:20their little lacunas around themselves,
3:22surrounded by bone matrix.
3:24And eventually they actually are going to make spongy bone.
3:31This would be step three.
3:33The osteoblasts make spongy bone.
3:39And now you have all this spongy bone,
3:43and the osteoblasts-- oops.
3:47We don't go backwards in our numbers.
3:48We go forward most of the time.
3:53The osteoblasts
3:54in the spongy bone keep building bone, and eventually,
3:57can you visualize how this spongy bone could,
4:01if we kept building bone tissue all around ourselves,
4:05if we're the osteoblasts, we could--
4:08eventually that spongy bone could turn into compact bone.
4:13And the end model that you end up
4:16with is a layer of compact bone.
4:20So this is my compact bone.
4:22And then what's in-between your compact bone?
4:26Spongy bone. A little bit of a mess.
4:30You get what I'm saying right there.
4:31And that's like if you chop a bone in half,
4:34and you look on the inside, and you look at the bone,
4:36like, the medullary cavity in the middle of a bone,
4:38the space where the bone marrow is,
4:42it, like, kind of looks spongy.
4:46And that's because there is spongy bone tissue in there.
4:50One thing that I did not tell you about bone tissue,
4:53and this is also true about cartilage.
4:56Let's do it in-- oh, gosh, sure.
5:01Bone tissue is surrounded by a dense, irregular,
5:05connective tissue layer called the periosteum.
5:16So the tissue that it's made out of is dense,
5:18irregular, connective tissue.
5:20The structure is called the periosteum.
5:23The periosteum houses different cells
5:26that are going to help maintain and continue building
5:30and growing this bone,
5:32which originally was formed through the process
5:35of intramembranous ossification.
5:38How does that work for y'all?
5:41Let's talk about the other form of ossification,
5:44which is endochondral ossification,
5:46and that's how we end up with our long bones.