Full transcript
0:00This is the second video over chapter seven. We will be going over the
0:06histology of osseous tissue. There are four types of bone cells. Three of those
0:12are related. The osteogenic cells are stem cells. They
0:17do mitosis, and they will develop into osteoblasts. The osteoblasts are the bone
0:24forming cells. How they make bone, is that they are going to produce collagen and
0:32carbohydrate protein complexes. They release it to the outside of their cell.
0:38They make a very soft fibrous matrix. Then minerals will deposit into this
0:46fibrous matrix. So the two organelles that osteoblasts have to have, is rough
0:54ER and Golgi complexes. Once those osteoblasts have the matrix completely
1:00around them, then they are called osteocytes, and so they're trapped, now in
1:07a little pocket within the matrix. That pocket is called a lacunae. These
1:13osteocytes are actually going to be connected to each other. In the matrix
1:18are going to be little cracks, called canaliculi, and so the osteocytes can
1:24touch each other and form gap junctions. So if one cell gets some nutrition, it
1:30can pass it along to the next cell through those gap junctions. These
1:35osteocytes are maintaining the matrix, and they're very sensitive to stress.
1:42Let's look at an osteoblast. So this is just a basic diagram. When the
1:48osteoblast forms the matrix, you have to have rough ER; you're going to produce
1:54proteins. That is sent to the Golgi, and once you have the Golgi, you're going to
2:00release it to the outside. And so you have these fibers, collagen fibers, and
2:06other protein carbohydrate fibers. Then minerals deposit around
2:13those fibers. Once you have the matrix completely around your osteoblast, that
2:21osteoblast stops making the matrix, and now the osteoblast is called an
2:27osteocyte. The last type of bone cell is called an osteoclast, these are bone
2:36dissolving cells, and you find it on the bone surface, whether on the external or
2:41internal surface. They're derived from multiple white blood cells, or sometimes
2:47from stem cells that have fused together, and they're usually multinucleate. They
2:54have lysosomes, and so they secrete enzymes and acids to break down bone. We
3:01call breaking down bone, bone resorption. So the enzymes are there to break down
3:08the collagen fibers into amino acids. Those amino acids move into the blood
3:14and we recycle it. The acids are dissolving the minerals, and again, once
3:21you dissolve the minerals, that calcium and phosphate moves into the blood. So
3:28the three cells that are related, you have the osteogenic cell that produces
3:33the osteoblasts. The osteoblasts, once they make the matrix are mature, and now
3:41they're called osteocytes. For the osteoclasts, stem cells are fused together and
3:48then these lysosomes, in here, release enzymes and acids to break down bone.
3:56Let's look at the matrix of bone. One third of the matrix is organic; two-thirds is
4:03inorganic. The organic part of it is produced by the osteoblasts; these are
4:09the collagen and the carbohydrate protein complexes. This is responsible
4:14for the flexibility of bone. Your bone has to give a little bit. You see much
4:21more flexibility of the bone in a child, than an elderly person. The
4:28organic portion is the calcium phosphate. We call it hydroxyapatite. We also have a
4:37little bit of calcium carbonate and some other minerals. This is responsible for
4:42the hardness of bone. So let's look at two disorders. What if there's a mineral
4:48deficiency? What happens to the bone? In a child, that is called Rickets, and so this
4:55results in very soft deformed bones. What if there's a defect in the collagen, what
5:03happens to the bone? Then the bone has no flexibility. It's very brittle. This is
5:10brittle bone disease. Now let's look at the histology of compact bone. Compact
5:19bone is made out of osteons, old name is haversian systems. So you see these
5:26circles, they're very close together, and when we open it up, you can see down
5:33that central canal, down the center of it, is going to be blood vessels and nerves.
5:38And they're connected to each other, and they're connected to the outside of the
5:44bone. Looking at this close-up, you can see these perforating canals that
5:50connect up all the blood vessels and nerves. So on the osteon, you can see that
5:56the matrix is in circles. These small dots, that is where the osteocytes are.
6:03Those are the lacunae. Down the center, that's the central canal. Let's look at
6:09some terminology. Lamellae are the circles of the matrix. So those circles that are
6:17on the osteon are called concentric lamellae. Then you have lamellae that go
6:24around the whole outside of the bone. So the circumference of the bone, and then
6:30in between the osteons are the interstitial lamellae. Down the center, the
6:35central canal, that's where the blood vessels and nerves are.
6:38Then you have the side canals, the perforating ones, and that connects the
6:44blood and nerves from the outside of the bone on the periosteum, to each central
6:51canal. Let's look at this close-up. You can see that we have the pockets in the
6:59matrix called the lacunae. Inside the lacunae, are the osteocytes. You have
7:05these little cracks through the matrix called canaliculus, and the osteocytes
7:11can extend through these cracks and touch each other. Each of these
7:17osteocytes are connected to the next one by gap junctions. If we were going to
7:23look on an osteon on real bone, here's the central canal. Here are the lamella,
7:30the circles, the little lacunae, the pockets are where the osteocytes would
7:36be. Spongy bone is a little bit different. So you have these big wide areas. Inside
7:44of this is going to be lined by the endosteum. These little plates here are
7:49called trabeculae. The spaces in here are going to be filled with red bone marrow.
7:54You don't have central canals, very few osteons,
8:00and so this lightens up the bone, but it still provides strength. You will develop
8:08more trabeculae, if there's more stress in that area. Now let's look at the bone
8:17marrow. That's that soft tissue that fills up the cavities of the long bones
8:22and your spongy bones. You have two types, red marrow and yellow marrow. Red marrow
8:30is red because you're producing blood. So it's called hemopoietic cells. You are
8:38producing red blood cells, white blood cells, and platelets. Yellow marrow is
8:44found in adults; this is storing triglycerides.
8:49So fat has a yellow color to it. If we look at a patient who has had chronic
8:58anemia, anemia is where you do not have enough red blood cells, their body can
9:04actually change some of that yellow marrow back to red marrow. Let's look at
9:13normal hemopoietic tissue. Where is the red marrow? In infants, it's in almost
9:20every bone. It's in the medullary cavity of the long bones. It's in all the areas
9:26of the spongy bones. In adults, we see it in the head of the femur and the head of
9:33the humerus, where you have the spongy bone at the epiphysis. We find it in the
9:39diploe of all flat bones, and then some irregular bones, such as the hip and the
9:46vertebra, also have red marrow. If we were going to look at the distribution, what
9:52is red here, is producing blood. Everything else is storing fat. This is
10:00the end of the second video. The next video is looking at bone development.