This chapter is going to be on the skeletal system. It's going to predominantly concentrate on bone and tissue of bone, so we're going to talk a lot about histology. Now, the skeletal system is actually made up of two separate parts.
We have the axial skeletal system and the appendicular skeletal system. Altogether, in the adult, we have approximately 206 bones. It may vary between individuals, but in general, the average number is 206. The axial skeleton, think of the word axis, is going down the middle of the body.
That's going to be your skull, mandible, vertebral column, and thoracic cage, as well as a tiny bone called the hyoid bone. The appendicular skeleton is going to be basically the outer portions of your body. Your pectoral girdle, so your shoulder girdle, your upper appendages, your lower appendages, and your pelvic girdle, so your hips.
Now within the skeletal system, you are going to have different shapes of bones. So you can see in this picture we have long bones, and long bones get their name because they're going to be longer than wider. We have short bones.
Short bones are going to be a more compact shape. It's not really safe to call them cube-like because some might have this variance of a shape, but they're basically much more compact, relatively equal on all sides. We have flat bones. Flat bones are going to be thin and broad.
So you have the sternum, scapula, and the parietal bones, which are top bones of your skull. Irregular bones. Those are bones that don't have any particular shape.
So vertebrae are a good example. Your ethmoid bone, which is within your skull. Your zygomatic bone, which is your cheekbones. And then sesamoid. Sesamoid gets its name for the sesame seed shape.
And everyone's going to have the patella for the sesamoid bone. And then you have your pisiform, which is a tiny bone in your wrist. If people form extra bones based off of manual labor, many times the shape will be a sesamoid bone. Now regardless of the shape, all bones in the body are going to contain both types of bone. So two types of bones are going to be spongy and compact.
As you're looking in this picture, what you can see is that the compact bone is very solid and dense, while the spongy bone is really porous. So regardless of the bone shape, we will see both types of bones. So this is a long bone.
We can see that the spongy bone is predominantly at the top. The compact bone is predominantly through what we call the shaft of the bone. So let's talk a little bit more about compact and spongy.
Some of this will be a review from Chapter 4. Compact bone, again, gets its name because it's very dense and solid. It's going to be made up of units called osteons. So we see this kind of tunnel that is lifted.
That is one osteon. So in compact bone, these osteons will repeat themselves. Osteons are going to be made up of rings of salt called lamella.
Within the lamella, we're going to have our pits. Our pits are the lacuna. Inside the lacuna, we're going to have osteocytes, the cells that maintain bone. And those osteocytes, since they're living, they're going to need nutrients and they will produce waste. So capillaries are going to branch off from these blood vessels within the perversion canal, also known as the central canal.
And those capillaries are going to actually channel themselves into the canaliculis, which are tiny, tiny channels that allow osteocytes and blood vessels to meet. Now, spongy bone is not going to have as compact of a disposition. It's going to be very porous, very light. And with spongy bone, it is going to be made up of units called trabecula. Trabecula, I call it...
lattice work. It's basically these little bony spurs or columns. And when you open up a trabecular, when you slice it open, you'll see that a lot of the same components that are in compact bone are present in spongy bone. So we can see that inside this little trabecular box, we have our rings of salt. We have the lamella.
We have our pits, the lacuna. You have the cells within the pits. the osteocytes. And then you have the tiny little canals that bring nutrients and take away waste from the osteocytes, the canaliculis.
So really they have very similar parts. What a trabecula is missing is that uniform circular shape. And in the center, we don't have that reversion canal.
So trabecula, since they're not as compact, they're much lighter, less dense. And since the structure is these columns going in different directions, we're really strong in all the different directions because the trabecula are putting tension and force in those different directions. If you go back up and look at the osteons, the osteons are running lengthways of the bone. So even though osteons are pretty strong, they're going to be strongest in the direction that they run. Whereas trabecula, since they go in all different directions, they give you a little bit more strength and stability.
It's very similar to when you think about dense irregular and dense regular. Now, what we have to remember is that bone is a connective tissue. And like all connective tissues, it's going to be made up of two major things, the matrix and the cells.
So think for a second. What are the cells that produce the matrix? If you said osteoblast, you're correct. So with bones, what's going to happen is we're going to have a precursor cell.
We call this precursor cell an osteogenic cell. If you think of genesis, it means to begin. So what are we beginning? We're beginning bone. The osteogenic cells will become osteoblasts, and remember blasts secrete, so the osteoblasts are secreting the matrix, and then the osteoblasts will become the osteocytes.
We also are going to have the cells that break down bone, and these are called osteoclasts. But you can see I don't have it in a linear pattern because osteoclasts do not differentiate from osteocytes. They come from... basically your bone marrow.
Now once we have our osteoblast secreting the matrix, our matrix is going to have two major parts, the organic part and the inorganic part. The organic part just means that carbon is present. So bone is mostly going to be made up of collagen and then salts. The collagen, other fibers such as elastic and reticular, and then some sugars are all going to be found within bone. And that's going to be the organic matrix.
Now what's really important about the collagen is that collagen's function is strength. So bone's strength comes primarily from the collagen fibers. But we also have the inorganic portion and this is going to be those calcium salts.
Bone is going to be made of quite a few different minerals. Obviously, calcium and phosphorus are going to be the most abundant, but we'll have some magnesium, some manganese. We'll also have a little bit of fluoride, so there's other minerals present besides calcium and phosphorus.
But calcium and phosphorus are the most abundant. What happens is the calcium salts will actually crystallize on the collagen. So collagen has given us the strength, but we have to remember it's relatively flexible. So we have to have something to make bone also hard, and that's going to be those inorganic salts.
The calcium phosphorus salts are actually known as hydroxyapatite. So hydroxyapatite is the combination of the different types of the calcium salts. So let's go over the actual cells again so we can talk a little bit more about the matrix. So the bone lineages, we're going to start off with osteogenic cells.
Basically, these are mesenchymal cells that are differentiating to become bone. The osteogenic cells are going to form into an osteoblast. Osteoblasts are the parts of the cells that are going to secrete the matrix.
Osteoblasts are capable of producing collagen fibers, but they're also going to form the hydroxyapatite and remember that's just basically calcium salts. As soon as the osteoblasts have secreted the matrix and kind of cocooned themselves or isolate themselves in a pit, that is when they become osteocytes. Now we have to remember that sites jobs are to maintain the tissue so they do daily upkeep, nutrient exchange, waste exchange.
We do not see osteocytes until they are fully isolated within a pit, within a lacuna. Now, at this point, the lineage will stop, but we haven't covered all the cells yet. We've gone over the mesenchymal cells that develop into osteogenic cells.
Osteogenic cells form blasts, and once osteoblasts are surrounded in a lacuna, they're going to turn into osteocytes. So our last cell are going to be osteoclasts. And as we can see, they're going to be from your bone marrow, basically modified white blood cells.
And their major function is to help break down, remodel bone. So what osteoblasts secrete bone's matrix, osteoclasts are actually going to break it back down. So they basically have opposite jobs. So this is just another way to view how bone cells arise. We can see that over here we have osteogenic cells, which turn into osteoblasts.
Osteoblasts will eventually become osteocytes. When do osteocytes show up? If you said when they are isolated in the lacuna, you're correct. But we also see that we have osteoblasts. the bone marrow cells right here that are going to produce our osteoclast.
Osteoclasts are basically 50 monocytes merged together to make up a relatively large cell. And what happens is if you look at the osteoclast's border, kind of looks like a ghost from Pac-Man, that border is going to be very ruffled. So we can see the ruffles better in this picture. And Those ruffles are going to be able to cover a lot of surface area.
So the osteoclast will release enzymes. Those enzymes will go onto the matrix of the bone. These ruffled edges will work those enzymes in, which helps actually break down the matrix. So the calcium salts will go back into your intracellular fluid and then will go into the plasma.
Now the next thing is the collagen. What we can see in this picture is that collagen is going to be forming our lamella. So what happens is we have those calcium salts and they're going to deposit on the collagen. We can see that the collagen is going to form the lamella in different directions. So this outside one is basically going from left to right.
The inside one is going from right to left at an angle. Then the middle or the most inner lamella is almost going straight up and down since the collagen fibers are All facing different directions it helps give us more strength and support But what's also great is that since collagen fibers are relatively flexible Even with the calcium salts crystallized on it it allows bone to have different types of force created. So because bone can actually be moved out, basically stretched a little bit, not like Gumby or Pokey or Silly Putty, but it does give a little bit of movement.
We call that tensile force, basically being pulled longer. Because bone can be compressed, we call that compressional force. And because these collagen fibers do run in different directions, bone does allow some twisting, which is called torsional force. Remember it's not moving in large increments but it does give some flexibility which helps aid in more strength, more force. So just for another little recap, I just want to talk one more time about the makeup of bone.
Bone is going to be made up of collagen fibers and calcium salts. The salts will crystallize on the fibers in rings that we call lamella. Since the fibers are going to always be going in a different direction with each layer of lamella, it helps add to different types of strengths.
So remember, the calcium salts are kind of like the candy that will harden on the sticks, which are like the collagen fibers.