As a reminder, we picked flat bones and long bones as our sort of representative bone to talk about. There are different categories, you know, we talked about them on Tuesday. There's short bones and regular bones, but most bones of the body are...
long or flat or resemble these so it makes sense to talk about these in more detail. So when we think about bone tissue internally anyway moving into the details of these bones we're going to see that bones have a really specific arrangement of compact versus spongy bone tissue and everything about this arrangement has a really particular purpose. So generally when we think about tissues and organs in the body they are structured a certain way. Rarely is it random and this is certainly the case.
Nothing within a bone is that all random even though spongy bone looks like a ranch. them replaced, smattered tissue, everything about it is also purposeful. So we're going to see how in certain regions of these bones, like the epiphysis and metathesis, these come together to form some pretty important structures.
Additionally, we'll find CT internally in certain areas. Can you remind me what CT stands for? Connective tissue. Connective tissue.
Now, connective tissue found internally inside the stone, so not just on the outside like articular cartilage, but also on the inside. So as a reminder, when we think about connective tissue, that is a broad heading. That is a general name.
And then in that heading, we have different... types of connective tissue like hyaline cartilage we talked about that on Tuesday other types of connective tissue include dense regular connective tissue dense regular we'd find that in tendons for example ligaments and then we have all sorts of other types of connective tissue like elastic tissue that you find in your ears or your nose so I only bring those up to help you understand CT is a broad heading and we have specific types of CT like hyaline cartilage When we think about hyaline cartilage, spongy bone, compact bone, there's really a particular area in long bones that we could use to highlight this functional relationship. So that is why I have specifically chosen the epiphysis and metaphysis junction to highlight these three different types of tissues come together to form some sort of really important structure. So there are other areas we could have chosen but I think this really gets the job done quite well. So on this drawing we're gonna see just a specific area and I'm gonna let you draw this out.
You don't need to write this down right now. I'm gonna go through all of these in time but you are looking at for example the proximal epiphysis and this is going to be the metaphysis and you have sort of a larger external sketch of these internal features just to the left so these drawings the left and the right go together now in this particular aspect what what you're looking at is if I had taken a long bone and I cut it right down that frontal plane and I opened it up to look at the internal features that's what we're looking at so we're looking at functional relationship between three very different at least in structure type of tissues and I will go through all of those in time but for now as you're sketching this first of all don't obsess about how I've arranged this you can arrange it however you want just realize got spongy bone here. We're gonna have a line in the sand and then spongy bone down here. I also really want you to focus on this seemingly thin yet very important crest of bone. That's gonna be our compact bone.
So as you draw this think about that spatial relationship between compact bone and spongy bone and then I'll introduce this mysterious blue line sort of in the middle. Give you a little bit of time to work on that and then I'll get on with labeling. So let's talk about some things on the labeling side.
First of all, We're looking at the proximal epiphysis. So this whole edge right here, I'm just going to do some dotted lines. We can usually kind of put a line in the sand and say, well, everything... Above this area here is the epiphysis or epiphysis.
So this whole area here, this sort of large flanged out knotty looking area, that's the proximal epiphysis. Proximal because it's closest to the trunk or midline. Pyphysis, epi, outside, physis, part of a bone, outer part of a bone. So we got that.
Now, the metaphysis or a metaphysis, I'm going to sort of do a dotted line here, just like we did on the other side, you know, on the left-hand side. But this is going to be our proximal metaphysis. or metaphysis. It is small in, you know, sort of spatial area, but it's going to contain a really important structure right here.
So this brings me to our next thing to talk about, areas of connective tissue. In this particular drawing, all we will have is a particular type of connective tissue called hyaline cartilage. So this sort of glassy blue line here is a strip of of hyaline cartilage and its thickness or length changes based on the age of a person, we'd also find as we talked about on Tuesday, you know the ends of long bones have articular cartilage as well. And as we'll see in our next drawing about synovial joints, this is going to be really important for reducing friction, helping two bones move in relation to each other. When we think about areas of CT, everything here that sort of has this glassy blue appearance, that would be where we find CT and just as a reminder this type of CT is called hyaline cartilage hyaline cartilage a type of CT everything here also came from that fetal remnants of the fetal skeleton so very early in life you don't have ossified tissue you have what's called a fetal cartilage model and it is completely made of hyaline cartilage and as you age even before you are born parts of that fetal cartilage model are replaced with bone by the time you are about some people it takes that you have to be 21 22 years old before you're you're fully ossified that whole skeleton so it's kind of interesting it's usually takes longer in taller males shorter females usually ossify their skeleton earlier but here we have this age I'm gonna put this person at about 12 13 14 and at that point you know junior high early teenage years most people their large long bones still have remnants of the growth plate and that's what this is the growth plate in long bones is made of highland cartilage when you're very young this would have been much thicker and as you age and this ossifies your cartilage shrinks and is replaced with bone and that's actually what causes this bone to elongate So when we think about long bones, they actually elongate from this metaphysis, sort of mid part of the bone, and it elongates from the end out, so that the epiphyse get pushed away from each other.
And that happens because... of this growth plate which starts to a lot of cartilage ossifies which causes it to elongate push up reason for that is you have to preserve the joint capsules kids like to move around you can't add bone to just the top of the bone because that would not be too good for the joints so what a challenge this would be you need to elongate a bone while it's currently in use talk about flying the plane while you build it you ever heard that that term before flying the plane it's kind of like That's what we do. And so in any case, we're going to see as a person ages, this gets thinner and thinner and thinner to the point where when you're fully ossified skeleton, this is just a microscopic rim of hyaline cartilage, but it is still there throughout life. It's just that over time, it's not active. You don't continue to get taller, for example.
So those are our two areas of connective tissue hyaline cartilage in this particular area. Real quick while I'm here, this growth plate, in younger people, when this growth plate is thicker, that's kind of a vulnerable place for the bone. And you can actually see growth plate fractures, and that's because this bone internally is not made of oxygen. tissue, it's made of connective tissue, and all it takes is a kick just the right angle, or actually what does it the most is a hockey puck or like a hockey stick right there at that growth plate, and you can fracture that growth plate, and if it's not fixed correctly, then that bone will stop growing, and then you would have disproportionately lengthened long bones in that person, and so Young males especially because they're growing so quickly and that birth plate is really active they seem to be more vulnerable to fractures at the birth plate.
As you age that is replaced with ossified bone and it's a less vulnerable area. Okay so let's talk about areas of compact bone and spongy bone that should be pretty straightforward now so I'm just gonna color in areas of compact bone again that is very dense stuff it's not very thick if your entire skeleton was made of compact bone, that would be super heavy and that would be hard to move around. So we have areas of compact bone, just draw a line here, and we have several areas of spongy bone.
The inside of the epiphysis is mostly spongy bone and parts of the metathesis are also spongy bone and that region changes based on the age of the person and the shrinkage of that growth plate. So spongy bone is found in multiple areas. areas, multiple areas, very biologically active bone tissue. Compact bone is not really considered as biologically active as spongy bone.
Spongy bone really is where most of the magic happens. Other things to talk about while we are on this drawing, areas of periosteum. So as I mentioned on Tuesday, periosteum covers the entire surface of the bone except where there is articular cartilage or hyaline cartilage. cartilage on the external surface.
So I'm just gonna add some periosteum in here. So wherever that articular cartilage ends we would pick right back up in periosteum and I've already talked about its value but I think the more practice you can get with where is it, what is it, the better. introduce this new term called endosteum and I put those together because I want you to look at these words in relation to each other.
Periosteum, the prefix peri usually means surrounding, so that makes sense. It's a tissue that surrounds a bone. Endosteum, the prefix endo means within, and so it turns out that spongy bone itself is lined with a membranous tissue and that's called endosteum. So I will put endosteum, I'm kind of out of... contrasting colors.
But wherever we have spongy bone, up here or in here, as best you can, line it with some color that's sort of contrasting. But this is endosteum. So wherever we have spongy bone, it is going to be lined with a blood-rich membranous-like tissue, and that's called endosteum.
So just sort of sketch a few areas here. But all spongy bone, all that trabeculae that we talked about. in a previous lecture, is lined or covered with endocyan. So long story short, bone tissue is never really bare or exposed to any particular environment. It's always covered with something.
And that is a great example of how nature means the best use of all the space available. So endocyan actually houses the majority of our endosteal stem cell niche. So you've seen this before, but I just wanted to add some sort of examples to it. some extra information. Endosteum is this blood-rich lining and it houses the endosteal stem cell niche.
And you've seen this before. We've talked about, for example, hematopoietic stem cells. They are found clinging to the trabeculae and that whole area. This is the endosteal stem cell niche and that is part of the endosteum. So notice the words here, endosteum, endosteal.
The endosteum would be more broad, have more features, and as part of the endosteum, we have the endosteal stem cell niche, a really particular microenvironment that we find as part of the endosteum. Periosteum outside, endosteum on the inside. So most of the things you see here, the distribution of compact bone and spongy bone, periosteum, endosteum, those things would be true of any bone, any category, short, irregular, long, flat.
We see the same categories, it's just that longbows have that growth weight and that is unique to longbows. So that is sort of goes with what we talked about on Tuesday. So I think when you study, you know, these two drawings really go pretty well together.
All we've done here is just take a look at this area right in here. simplified view of that epiphysis and a simplified view of the growth plate. So I think these go together. Questions on anything so far before we sort of move into a different area which is a synovial joint?
Questions on tissues, arrangements, endosteum versus stem cell niche? So far so good. Alright, I'm going to move into synovial joints because since we're talking about long bones, this is the most appropriate time to talk about a synovial joint. So joints are where bones come together.
There's different types of joints. There's some joints that move a lot, like synovial joints. They permit really fluid-free movements, your elbows, your knees. Those are synovial joints. There's some joints that don't permit any movement, like the gonfosis joint.
Where do you find those, the gonfosis joint? Any pre-dental people out there? Pre-dents? It's the tooth socket. So the tooth sits in a joint cavity and that's called the gonfosis joint and we really hope that doesn't move very much you know especially once you get your adult teeth.
So some joints provide a lot of movement some joints don't provide hardly any movement and so we see and we see a wide variety in between but since we're talking about long bones this is an appropriate time to talk about synovial joints so that's what we're going to see on this next drawing we're going to talk about how long bones come together and what it takes to keep them lubricated and allow movement without friction and heat and destruction of that joint. So we're going to talk about synovial joint anatomy. So on this you have you know a couple things you've got you've got this printed off if you don't you could draw it I think it would be a challenge but you can you could do it. So a couple of pointers about synovial joints bulleted points I guess They are unique amongst the joints because they are fluid filled and that fluid is synovial fluid.
So other joints should not have fluid in them. If they do, that's damage. So this one purposefully has fluid in it. It's not a lot. The knee is the biggest synovial joint and I think it has on average like two milliliters of synovial fluid.
So not a lot, just enough to lubricate the joint. So if you're going to have a fluid filled cavity, you have to have a way of keeping that fluid in the right place so it doesn't leak out. And so we'll also see as part of this complicated joint an interesting structure it's called a fibrous capsule or a fibrous joint capsule. Upon gross anatomical inspection it's not super impressive but histologically we're moving deeper deeper into the cellular level it has a lot of secrets it's pretty cool and we're going to take a look at how it makes the fluid.
This capsule contains a membrane that makes that fluid. And believe it or not, that process where we take a membrane and create fluid is the same process that the body uses, the same sequence of events, to make pericardial fluid that surrounds the heart or pleural fluid that surrounds the lungs. So we will take a little moment to sort of investigate here a process called ultrafiltration, which is how we can take blood plasma and we can very carefully create another fluid like synovial fluid or pericardial fluid.
So once you learn the process sort of in one area, you'll see it recapitulate or repeat in other areas. And then you're like, wait, I think I've seen this before. And indeed you have.
So take a look at that. And then I will remind you that synovial joints are the most common type of joint in the body. They permit a lot of movement. They're also easily damaged.
If you've ever damaged a knee or an elbow, you know all about it. And they take a while to heal. And that's because part of this structure is avascular. So it doesn't...
doesn't even get direct blood flow, part of it is vascularized, but not as much as we probably would like, because without blood flow, it's really hard to fix a structure. It's really hard for leukocytes or white blood cells to reach the damaged area and clean it up. So this is why synovial joints are kind of tricky. So on this, we're going to be looking at these things.
So again, if you've got this printed, you have access to this picture. If not, you can probably sketch that out. But these are the things I want to label and talk about. And as you can tell, it's an appropriate time to do that because we just got done talking about long bones, articular cartilage, so it makes sense to talk about this. So I'm going to be using the knee as the proxy or my example here.
Now, obviously, this picture is not showing the complexities of like the ACL or the meniscus or the lateral or medial leg. It's not what we're after, timeout stability here. I'm talking about just keeping these bones when they move from rubbing on each other and creating heat.
So that's why I like this picture. It has sort of the extra stuff stripped away. It truly helps us focus on the task at hand, which is how do we keep these bones from rubbing together? And what happens when they do?
So we're going to talk about normal, all good stuff, but every once in a while we do see a disruption. talk about what happens, sort of the progression, the destruction that happens in a synovial joint as a result of osteoarthritis, usually just referred to as OA. And that is something that, you know, is pretty common, especially in aging people. You can have it, sometimes I get people that are vet school bound, animals just as vulnerable to osteoarthritis as people.
And so sometimes we can actually see medicines work between multiple species. So that's kind of cool. Um, So anyway, we're going to get right into labeling this thing. And as I do, I will sort of walk you through the process of ultrafiltration or the production of a new fluid from an existing fluid, which would be plasma. All right.
So first of all, the fibrous joint capsule, the fibrous joint capsule. So we're just going to be moving our way from the outside in. So the fibrous joint capsule is just this outer connective tissue. tissue layer, so sort of sketch it in the outer connective tissue layer.
So it's a loose CT. It permits a little bit of movement and flexibility, but it's to hold it together because connective tissue is really really great at, as you might guess, connecting things. And so here we're going to see the ends of these bones, the visceral lepidoplasm and the proximal lepidoplasm really be held together and stabilized by this joint capsule. Now again, ligaments like ACL, medial and lateral ligaments would be really helpful here as well.
But still, this acts like kind of a sock or some sort of wrapping that holds this capsule together. So fibrous joint capsule on the outside. Right on the inside, so it's sort of a two-layered structure, right on the inside is our synovial membrane. It is also called a synovium.
Synovial membrane or synovium. Also, if you need to take a break, feel free. I'm going to keep going since we had that presentation.
I want to get you through the material, but if you need to take a break, feel free. The synovial membrane or synovium is highly vascularized and is the site that contains the vessels that will produce synovial fluid. So very thin structure if you just look at it, outer fibrous, tough, kind of protective, kind of a sealant, inner synovial membrane, also known as the synovium, and it is fragile, it is highly vascularized.
So we see two layers with very different jobs. Synovial membrane. The synovial membrane is responsible again for producing synovial fluid.
And the synovial fluid lines or fills this entire cavity. So if you want to color all this in, that's fine. That's synovial fluid.
And generally in the adult knee, as a very broad average, we may see at any one time, two milliliters of synovial fluid in there. So not a ton, really just. enough to keep these bones from rubbing on each other.
Synodial fluid has the viscosity or sort of feeling of olive oil. It's not water. It's a sort of oily, viscous substance because that really helps reduce friction.
Alright, so I'm going to take a minute and talk about how the seemingly thin, featureless synovium or synovium grain produces synovial fluid. And again, the reason why I spend this level of detail or this amount of time on it is because once you understand... this process when we get to the heart and I talk about the production of pericardial fluid you already have the basis down it's the same process and so that's the nice thing about learning you know physiology is you'll see these processes processes repeat again and again.
And you're like, wait a minute, I think I already know this. So what I'm going to do is sort of sketch out like an inset of what is happening here every time we make synovial fluid. It's made somewhat continuously, turns over, has to be drained somewhat continuously so you don't accumulate. So in this synovial membrane, and I think I used green for that, I'm just going to draw, here's our synovium or synovial membrane.
I'm just drawing it really big so I'm, you know, put a dot. right there but this is much bigger so the synovial membrane highly vascularized also highly innervated so if it's damaged you'll know it in this synovial membrane i'm going to draw and tell you what i'm going to draw before i do this is going to look a little weird I'm going to draw a very particular type of capillary. And this capillary is called fenestrated, which means the walls purposefully have holes or fenestrations that allow some fluid to leave.
And that's really the base. of ultrafiltration. So I just want to watch you draw this for a moment and then do it.
That might be helpful. Otherwise, this is going to look a little weird. So if I was just to sort of very much intensify or exaggerate a fenestrated capillary as part of our synovial membrane, I'm going to draw these little endothelial cells, which are what make up the capillary membrane or wall.
Just these red lines, that's my endothelial cells. And notice they are purposefully not touching each other. So endothelial cells here can kind of give them a little feature.
Purposefully here have holes or fenestrations. So there's one right there. So that is called a fenestration.
And a fenestration is a tiny pore or hole that exists between neighboring, I don't want to say touching because I really don't, neighboring endothelial cells. And so having these holes or junctions permits the passage of items in pretty good volume that otherwise could not get out of the blood. So these fenestrated capillaries, that's what this is because of the fenestrated, so I can write that out as well. These fenestrated...
capillaries are very different in structure than what we generally see in the body. Most capillaries are continuous, meaning their endothelial cells touch, but not these on purpose. So wherever we see the production of a fluid from blood plasma, this is the process of ultrafiltration. Ultrafiltration.
I'm trying to keep it all in the same view, but I'm not sure if that's going to work. Ultrafiltration permits the creation of a fluid, in this case synovial fluid, directly from blood plasma. So in other words, we could say that synovial fluid, so a lot of times you'll hear this, synovial fluid, like in this joint cavity here, is an ultrafiltrate.
of blood. That's what that means. And a lot of times that's all they say, like in, you know, scientific literature or textbooks. Synovial fluid is an ultra-filtrated blood. But if you're not really sure what that process means, you're like, okay, I could have replaced synovial fluid with pericardial fluid, the fluid that surrounds the heart, or pleural fluid that surrounds the lungs.
Or, believe it or not, this is the same mechanism that allows urine to be formed in the kidneys. So you learn it once and you see it. So it's a really good sort of concept to think about, to get down, ask questions about, because once you do, then you understand a lot about other organ systems and it will just come back to you. You're like, oh, there it is again. So ultrafiltration of blood.
ultra-filtrated blood. So that tiny little seemingly simple membrane just on the inside of our fibrous capsule does all this stuff as long as you don't damage your knee or your elbow and there's lots of ways you can do that. So this synovial fluid that we have here in this capsule about two milliliters if you want to color it in that's fine is produced continuously and it must drain continuously otherwise you're going to get too much fluid in here, stretch that synovium and that's gonna hurt.
So synovial fluid. Questions on sort of the inner workings of this before I continue? Questions on anything you see up here?
So far so good. I'm talking about articular cartilage. So right now I've talked about how do we make the fluid that lines this joint.
Now let's talk about structural things that help prevent these bones. also from running together so it takes like a whole conglomerate of items to keep a joint functioning joints are just complicated so articular cartilage i mentioned before that articular cartilage just refers to a location. An articulation is where two bones move in relation to each other.
An articulation, also known as a joint. So articular cartilage, this is a locational term. I'm going to color in articular cartilage here. I'll just use a dark blue since the synovial was light blue.
So articular cartilage here refers to the location. It's in the joint. the articulation and more precisely we would say this is if I ask you know what type of CT is this that's when you would say well that's a type of hyaline cartilage because hyaline cartilage is a type of CT just like fiber cartilage or dense regular CT.
Articular cartilage is not a type of CT it is a location. So helping permit rubbing or friction on our distal epiphysis and proximal epiphysis, synovial fluid, and this hyaline cartilage in our articular joint area. So when we put all this together, we should have some pretty interesting features. One of which is highly vascularized, and that would be the synovial membrane. Because we just came down here and talked about, you know, the synovial membrane is not just vascularized.
It has fenestrated capillaries, which makes it even cooler. So very, very vascularized. Lots of blood flow in neurons, nerves. You know if it's damaged.
Avascular areas include... the hyaline cartilage. This is avascular. So avascular, as a reminder, means no blood vessels there.
So this is interesting. If this is avascular and for some reason is damaged, it is very hard to fix because you cannot directly deliver leukocytes or white blood cells that help repair, clean up the damage, and start the repair process. Avascular means you're also not going to get a lot of nutrients to this area.
nutrients to our hyaline cartilage is delivered through diffusion through the synovial fluid. So the synovial fluid isn't just like a lubricating medium, it is also a diffusional medium to deliver nutrients, but that is a slow process. So diffusion through fluid is super slow. So in other words, the hyaline cartilage is sort of on its own. Once it is created during fetal development, good luck because it's going to take a lot of abuse and it doesn't have a lot of options for...
healing unless you of course see a surgeon. And so that is an interesting sort of thing to think about. Speaking of damage, I want to talk about areas in this particular area that could be impacted by something called osteoarthritis. And there's a lot of different types of arthritis. So first of all, arthritis, we'll block this part out, an inflammation of the articulation.
That's what arthritis means. An inflammation, itis, inflammation, as we should tell you, like what is in plain. So arthritis, osteoarthritis, this is an E by the way, a petrileuridic.
But this just means this inflammation in this joint is specific to damage of the bone. And I want to sort of walk you through what could happen as a result of osteoarthritis. And I'm going to keep it pretty specific to the articular cartilage has been damaged and now the bones are rubbing on each other. This would be a different lecture if I was talking about rheumatoid arthritis that is also damaging here, but that's a different. that's a different thing.
So I'm going to talk about this specific osteoarthritis. And it is an uncomfortable thing for people and animals for sure. So I'm going to walk you through sort of the progression of osteoarthritis and then help you understand why it's so painful and why it is hard to fix with medicine.
It's usually something you may have to fix with surgery just due to the damage of the joint. So I'm going to zoom in a little bit till I lose my resolution. And And for this, I'm gonna be doing a couple things in the area of the hyaline cartilage on this articular surface.
And we're gonna say that where use and tear age, something happened and this hyaline cartilage in certain areas was lacking. It got rubbed on, friction will kill, you know, friction equals heat equals tissue degradation. So we're gonna start by saying we have missing areas of our tissue.
cartilage. So I just sort of took a brown pen and just sort of, you know. So in that particular area there is no hyaline cartilage left and if it's a really small area it might it might be fine but generally a small area becomes a larger and a larger and a larger. So what's going to happen is as this wears we get Christian we get heat and the bone tries to help and it does the absolute worst possible thing bone is is strange it wants to help it says oh I think I'm damaged I know what I should do do, I should grow more bone. And on the one hand, you're like, that's not bad.
You're damaged. Let's grow more bone. But it doesn't do it in a very good way. Instead, it starts to grow these little spicules. And these are called osteophytes.
So wherever we have damage and this bone's trying to heal itself, you see these little spikes hanging down or projecting upwards? Those are called, let me see where I could write that, I'll just write it over here, osteophytes. And osteophytes remind me of, um, you ever been in a cave and there's like these mineral deposits and some hang down from the cave ceiling and some stick up?
That's kind of what this is like. They're mineralized deposits. They're like spicule bony mineralized needles, and that's coming from the bone as a result of damage.
Do you think this is a good idea? Nope, this is the worst possible thing that bones could do. But they are are doing their best. So what's going to happen is as we get these bony spicules growing, they're going to rub on whatever articular cartilage is around them, and that's going to degrade, and we're going to get more bony spicules. So over time, this is really just, I'll put number two, so now we've got more bony spicules projecting into that synovial cavity, into the synovium there.
Over time, this is just a self-feedforward kind of process, more spicules, more damage. damage, more spicules. And from what people have reported, that feels like sand in the synovial joint.
So just imagine you go walk or anything that requires movement, and you don't have this nice wood joint, it feels like sand grain. Just rubbing on that. And since the synovium is highly innervated, that's going to hurt a lot. So at this point, you're kind of past being able to treat this nutritionally.
or with some sort of pharmaceutical. This is going to require some heavy intervention, surgery likely. But that's the start of osteoarthritis, which, of course, can be very sort of horrible for quality of life if you cannot get that to fix or get help because that means all movement is excruciatingly painful.
So that's a little bit about the joints, synovial joints, anatomy, and what can... go wrong. Questions on this before we continue?
Leaving the issue of joints and moving into structure of flat bones. It is avascular. Yeah, it's mostly just a connecting capsule.
So, you know, it's going to get, it's right next to the synovium, which is highly vascular. So it doesn't really matter so much because it's right next to the vascular tissue. Other questions?
So far so good. We're moving to flat bones and flat bones are relatively structurally more simple for sure. They don't have to move so that helps. That kind of changes things. So let's take a look at flat bones.
Let me kind of get this set up a little bit differently here. Take a look at flat bones and introduce you to their sort of overall structure before we take a look at some of their components. So the nice thing about flat bones, whether we're talking the skull or we're talking ribs, is that they are structurally simpler compared to long bones. at least internally. There's not a marrow cavity.
There's not a growth plate. It's either compact bone or spongy bone. So it's pretty straightforward. They are still super important, both anatomically and physiologically.
It has to do with a large amount of spongy bone. So what you're gonna see me talk about, especially, I'm gonna call this, let's go crazy, let's call it a frontal bone. That's a pretty good one. I'm going to be working with all this called the outer table in the...
So this is going to be the scalp and down here you will see the frontal lobe of the brain. So I chose this specifically because I want to talk about not just the internal structure of flat bones, but I hope you see how some flat bones, like in the skull... have additional functions beyond just being a bone and protecting the viscera. Bones of the skull on the inner table, on the inner table have an additional job and that's to connect and work with something. called the dura mater.
So some flat bones have additional jobs compared to others. So first things first, just some basic labeling to get this out of the way. I'm going to call this the outer table of compact bone. And the compact bone here is very dense, it is very strong. it's relatively thin still compared to the inner spongy bone.
So we've got an outer table and then down here I'm going to talk about an inner table of compact bone. So still compact bone but the outer and inner helps us know you know which end is up I guess. And then in here This is all spongy bone.
Lots and lots of spongy bone relative to the overall size of these of these bones and spongy bone here collectively, spongy bone tissue collectively known as trabiculae. Columns of spongy bone, trabiculae. So just helping you sort of word associate I guess.
So just a couple things to get out of the way here. Next I'm going to come over here and talk about my formal labeling list. I can fit all this in, move it up so people in the back can see that's as much as I'm gonna get. So we talked about areas of compact bone so I got that the outer and inner tables of compact bone.
Areas of spongy bone I'm going to talk about areas of periosteum and endosteum because remember all bones on the surface have to be lined with something, either articular cartilage or periosteum. So in this case, in this particular aspect here, it's going to be all periosteum. So I'm going to put some periosteum on the outer table here.
Same tissue, same exact tissue, periosteum. We're going to have that inner. osteogenic layer.
And that inner osteogenic layer would be able to produce more compact bone if asked to. So a small fracture, for example, could be handled, could be repaired by our inner osteogenic layer. And we're going to have that on the inner part as well.
Generally not damaged to the same degree, but good. Okay, so you got that taken care of, periosteum. And then I've introduced endosteum to you, but it's probably helpful to see it again in a different aspect. So endosteum, look at your prefix there in the inner part.
So over here, I don't have... like a lot of ink on this, I can easily outline the trabiculae with endosteum. So of course it would be all over.
Wherever we have the surface of spongy bone, we would have endosteum. It's just that some areas are a lot easier to draw on. And the endosteum here would still contain that endosteal stem cell niche.
So wherever we find endosteum, endosteal stem cell niche will be part of it. Flatbones, because they have so much trabiculae, I mean they are mostly spongy bone. The disproportionate amount of a flat bone is spongy bone. So they have a lot of trabiculae, thus they have a lot of endosteum, thus they have... the ability to produce a lot of bone marrow.
So the endosteal stem cell niche here, just due to the virtue of space, surface area, this is really going to be an active area of bone marrow production throughout life. We still see bone marrow production in long bones, but relatively speaking, because you have so much trabeculae, so much area for endosteum and the stem cells, relatively speaking, you produce a lot of bone marrow in your flat bones. And in the adult, it's the...
Flat bones that still contain a relatively active population of hematopoietic stem cells. So that's why flat bones throughout life still make red marrow. So if you take a biopsy from the ischium, the point of the hip, for example, you will find red marrow.
That's because that's where that is pretty much isolated to the sternum, but also the aniridia. Alright, so we've seen most of that before, but I just wanted to put it in perspective in a different bone. But as I mentioned, some flat bones have additional jobs. Some are really great for muscle attachment, like the ribs.
a lot of muscle attachment. And some, like bones of the skull, the cranial vault, also have an additional job. They may have muscle attachment on the outside and on the inside, on the inner table, you're going to see that these These flat bones help support something called the mater. I know it looks like mater.
You can pronounce it mater. I won't giggle at you, but it's really pronounced mater. And there's three types of mater. There's going to be dura mater, arachnoid mater, and something called pia mater. So all of these are supported in some way, shape, or form by flat bones of the skull.
So all of these end with a pia mater. the word mater. Dura mater, arachnoid mater, and pia mater. So time for our Latin lesson. I don't want to disappoint.
Dura means tough. It's literally so mater means mother. This literally translates to tough mother.
So I always think about like you know some lady in a leather vest smoking a cigarette. like a tough mother. Anyway, it's very tough. It is very protective. It's almost hard to cut through even in a fresh specimen.
Duramotor. Deep to that we find the arachnoid mater and arachnoid from spiderweb. This is the spiderweb mother that's literally what it translates to so again fun with Latin and then pia means soft it is the soft mother and it is the actual covering of the brain it is so thin that sometimes you can't even find it to get your probe up and under there and it just sort of peels off like a really fine layer of cellophane or serine wrap but these modders really helpful for the brain's health and completely dependent upon the flat bones of the skull to support them they need something to stick to so I'm going to talk about that extra jobs that flat bones and the skull have as far as supporting these tissues.
So as I mentioned some flat bones support muscle attachment and you would have if you think about the flat broad muscles of the head that attach to the frontal bone so we could see you know areas of connective tissue attaching here but inside so I'm gonna put the brain down here I'm just gonna draw in some what are called gyri and sulky just the curvy areas of the brain. that enhances surface area and you're starting to see why you can't just really study one system it is all connected so yes I know we're talking about bones but it's also connected to the nervous system so this is just the bumpy region of the of the brain so here's our brain tissue And to protect that brain, obviously bones do a lot, but they don't do enough. They don't do it all. So let me talk about the tough mother, the dura mother, dura mater. It's gonna be this tough fibrous connective tissue that is almost seemingly intertwined with our periosteum.
So the blue here, the blue here, I would sketch it kind of fibrous-like. It is a tough connective tissue. I'm going to abbreviate this is the dura mater.
Dura mater, tough stuff. It is hard to cut. It's hard to peel away.
Deep to the dura mater or DM is the arachnoid mater. And the arachnoid mater gets its name from its spiderweb-like appearance, so you really can't draw this wrong. It is stringy, it is fibrous, and these fibers connect the dura mater on the top to the very delicate pia mater below.
But all the stringy things I'm drawing right now is a representation of the arachnoid mater. And the arachnoid mater has some spaces that you can can see here a lot of area in between these little connective tissue sort of tendrils and in these areas here we're going to find a lot of blood vessels like a lot of blood vessels and so that is a benefit of having room here this arachnoid The motor is fairly spacious and open. We can put a lot of blood vessels.
So these circles here are representations of blood vessels. Lots and lots of blood vessels. So the vasculature of the brain Somewhat tied up with the arachnoid monitor so lots and lots of blood vessels and then we get back to the surface of the brain But I'm going to talk about the pia mater.
And the pia mater, again, it looks like the surface of the brain. It is very thin, sits right on top of the actual cerebral cortex. And all of this is in one way or the other connected to that flat bone. It is all connected. That's why you can't just study a system in isolation.
It wouldn't really help. So PM, that shows up for pia mater. So, in addition to the brain being protected by this flat bone, the modders also protect that brain and are dependent on that flat bone for a connection point. The dura mater is connected to the periosteum, which of course is the surface of the brain.
Now, let me talk about, this is a good time to talk about... the physiology of a hangover. Not that you would, but maybe you have friends and you should educate them, you know, as they're partaking.
It's a really good time. That's when people want to know what's happening, so you'll make friends. But when a person is dehydrated from maybe making questionable decisions, this area here, which should be full of fluid, gets dehydrated. And this area here, when it gets dehydrated... sinks, stretches, and all these little things here stretch as well, and they're full of neurons that let you know, ow.
So as you dehydrate, this area of blood volume shrinks, this area of fluid shrinks, the brain starts to retract a little bit. So pain, dehydration, and what happens when that person trips and falls and seemingly lightly bumps their head on an average day causes causing no trouble, what do you think they're more vulnerable for now? A concussion.
A concussion, yeah. And so it turns out all of these structures play a real role in things that happen to people all the time. So a well hydrated person, nice blood volume flowing through these vessels here.
We see some nice... areas of cerebrospinal fluid, also in ultrafiltrated blood, and that provides not just a watery protective layer between the brain, the skull, and the brain, but also the brain itself sits. sort of in a capsule of this.
So this is just at the top of the brain. The entire brain sits in this little depression filled with cerebrospinal fluid, and as you dehydrate, that causes the brain basically to sink in its own cavity, which causes further issues. So there you go.
If you have friends, I don't know, is there a game this weekend? Probably. Just think about that.
If you're going to do it, just go forth and be educated about it. A couple questions here. Finishing up some stuff. External surface of bones is mostly covered by periosteum. What is its purpose?
Periosteum, what is its purpose? Give me one thing periosteum does. Say it louder. Well, the tendons may go through there, but we probably wouldn't rely on it for attaching.
It would actually come off. But what does this mean? Osteo... there we go.
So covered by periosteum. So it's going to contain osteogenic cells. And osteogenic cells have a name, and I'll zoom in so you can... have a fighting chance of seeing that. Osteogenic cells have a name.
These are called osteoblasts. And osteoblasts in the periosteum are quiescent and less stimulated. And when they are called into action, they produce new bone tissue. And that new bone tissue is called the periosteum. has a purpose.
And so the second part of my question over here, what is its purpose? So periosteum contains its osteoblasts. produces new bone tissue.
Its purpose is to provide a couple, it does a couple things. It provides an opportunity for apositional growth. In apositional growth, wherever we find that, long bones principally, is an increase in thickness.
So that compact bone gets thicker. That is reinforcing. Helps that compact bone deal with added tension from tendons that are carrying the tension of skeletal muscle. So if you ever look at a great study done with the swinging arm of tennis professional tennis players versus the non swinging arm and when they looked at the radiographs looking specifically at this the non swinging arm looked like it didn't even belong to the same person as the swinging arm because the swinging arm was getting so much tension from the muscle that that apositional growth over time have really made that compact bone much thicker and this non-swinging arm not getting that muscle tension never got stimulated to grow in width and so it's quite amazing to look at how much difference this can allow makes that bone a lot stronger for not just muscle attachment but also resistant to fractures so apositional growth something that can happen throughout life it is done mostly through exercise so anytime you stress a bone and usually you know good stress, not like crazy stress, but regular stress will actually make bones stronger.
So this is not just about exercise. This isn't just for muscle or the heart. It's for bone health as well.
And of course, B, you can do some small amount of fracture repair. Fracture repair can be handled sort of, I call it in-house, by the periosteum. That's because if you have a very small fracture, so it's going to come up here.
Let's say that you bumped your head and it wasn't terrible, but you... still had sort of these microscopic cracks in here that can be healed by your your periosteum the inter osteogenic layer will allow that healing process to occur pretty quickly too so fracture repair pair also a benefit of the periosteum. So we are almost out of time. Good stopping point because I'm done with this page. This leads me right to the next set of notes talking about bone cells.
So they go together. Please remember to take your quiz. Have a good weekend.