Lecture on Bone Tissue Overview

Hello everyone and welcome to another recorded lecture of Anatomy and Physiology 1 online. Today we will discuss chapter 7 which is on bone tissue. Bone is one of the major components of the skeletal system and we'll be talking about the skeletal system in the upcoming two chapters. In addition to bones, we also have cartilage, joints, and ligaments as part of the skeletal system. The skeletal system has many functions. One of the major ones is support. So our limb bones and our vertebrae can support our body. Our jaw bones can support our teeth. So the skeletal system functions as a support system. Our skeletal system also protects our organs such as our brain, our spinal cord, hearts, and lungs. Our skeletal system helps us move around. So whether it's limb movements and walking or even breathing. All these kinds of movements depend on the skeletal system. The skeleton can also play a role in acid and base balance and electrolyte balance to maintain homeostasis. So depending on how much calcium and phosphate is in the blood, the skeletal system can either contribute more or can take away to, again, make sure there's the right concentration of these minerals in our body. Similarly, bone tissue can buffer the blood against excessive pH changes. So this is the skeleton also functions to maintain homeostasis. And finally, the skeletal system is very important in blood formation because the bone marrow, the red bone marrow specifically, is the chief producer of blood cells including cells of the immune system. So bone is made up of osseous tissue and bones are not just these brittle, dry, cracked things that we see in the laboratory. They're actually alive. So bones are living organs and they're mostly composed of connective tissue. Many different kinds of different types of tissues are found within bone. So we not only have bone tissue, but we have bone marrow, we have cartilage, adipose tissue, nervous tissue, and dense regular connective tissue. So all these different types of tissues we discussed already are found in bone. And like I said, bone is alive. So there has to be a living component. And those living components are osteocytes. So osteocytes are cells of the bone. Osteo always means bone and site always means cell. So osteocyte is a bone cell. They're embedded in the non-living extracellular matrix. Extracellular matrix is just what's outside of the cells. So this matrix of bone is a composite of minerals. and proteins. Composite meaning it's a combination. So the matrix of bone is not just minerals, not just proteins. It's approximately 35% organic protein and about 65% inorganic mineral. And the organic component is collagen specifically. So we know that collagen provides a lot of flexible strength, which is really important. The inorganic component is composed of calcium, and minerals. And calcium phosphate called hydroxyapatite provides the weight-bearing strength required for bone. Also, calcium carbonate is found as part of the inorganic component and other minerals like fluorine, sodium, potassium, and magnesium. So, so far we said bone tissue has connective tissues plus cells. And There's importance to both the inorganic and the organic components of bone matrix. So what do you think would happen if we didn't have any of those minerals, any of those hard minerals? Then bone would be pretty flexible. It wouldn't have a lot of the strength required for bone. And you can actually do this. You could take a chicken bone and put it in vinegar, and it will seep out all the minerals. And you'll see that with just collagen, bone is very flexible. not very strong. Without collagen, however, bone would be very brittle and it would crack in response to even very tiny amounts of stress. So it's important that we have both this hardening, this mineral, the mineral component, but also the collagen, the flexible component. So we need both of these working together. We could classify bones according to their shape. So Starting with flat bones such as the sternum, these are thin curved plates and generally function to protect soft organs. So flat bones also are part of the skull bones, for example. The skull bones are flat bones as well, and they protect our brain. We also have long bones, which are bones of the limb, such as the humerus, the radius, the ulna. So these are longer than they are wide, and these serve as levers acted upon by muscles to enable movement. So those are long bones. We also have short bones. like those found in the wrist with the carpal bones. Those are approximately equal in length and width, and they glide across each other in multiple directions. We'll talk more about short bones when we discuss joints. Finally, we have irregular bones that are kind of elaborately shaped that don't fit into the other categories. So those can include like these vertebrae or this sesamoid bone as part of the patella. It kind of looks like a sesame seed. That's why it's called a sesamoid bone. So each vertebra is an irregular bone. A sesamoid bone of the patella is an irregularly shaped bone. So bones come in two different types when we're talking about the tissue itself. Bone can be either compact or spongy. Compact bone is very dense and very strong. It's also called cortical bone because it forms the outer cortical layer of bone material. So compact bone is very, very compact and very dense. Spongy bone, also called cancellous bone, is a lot different. This is more of a latticework, and these have what's called trabeculae. These are these little spaces in between, so this gives a lot more of like a spongy appearance. And this is less dense. than compact bone, but it's still very rigid. So it still allows us to have the strength, but it's less dense, so it doesn't take up as much, doesn't weigh as much. It's not as heavy to carry around. The spaces in the spongy bone are filled with bone marrow, and we see the spongy bone in the center and the ends of long bones and in the middle of nearly all other bones. Right, so overall we're seeing the spongy bone can reduce the weight of bone. So we still get the strength, but it's not as dense. So this is the spongy bone with those trabeculae, and this is the compact bone. So when we look at flat bones, such as let's say the parietal bone of the skull, we see a sandwich-like construction. We see two layers of compact bone. So here's compact bone, and here's more compact bone. But in the middle, there's what's called diploe. It's like a spongy middle layer. of that spongy bone. And again, that's good for absorbing shock and that also stores bone marrow. There's also this outer lining called periosteum. So periosteum, peri means outside, right? And ostium is bone. So periosteum is the surface of the bone. And we'll talk more about that in a bit. So let's talk about long bones next. So long bones are mostly composed of an outer shell of this dense compact bone. Outside of that compact bone we have the periosteum, which is a fibrous outer membrane made of dense irregular connective tissue. And then on the inside we have the spongy bone we said, the spongier bone. Let's just talk about some parts of bone. So the shaft of a long bone is called the diaphysis. So this part is called the diaphysis. And the two enlarged ends are called the epiphyses. Singular is epiphysis, both are epiphyses. So these are the heads of the long bone that could form joints or articulations with other bones. The articular cartilage is a layer of cartilage that covers and protects the joint surfaces to enable very smooth movements. Within here, Within the long bone, we have a space called the medullary cavity. And in the medullary cavity is where we see bone marrow. Specifically, we find yellow bone marrow inside the medullary cavity. We'll talk a little bit more about that. The endosteum is the inner membrane lining the medullary cavity. So the endosteum, endo, means inside. So the endosteum lines the inside. of the medullary cavity over here. The endosteum has a very important role in helping to form and repair bone. And it's actually a form of reticular connective tissue. So again, this forms like a mesh for the bone marrow to live in. So far, we spoke about here's the diaphysis. Within the diaphysis, we have the medullary cavity. And that's where we're going to have yellow bone marrow, as we'll soon see. Within the epiphysis, we have spongy bone. So spongy bone is in the epiphysis. So in here we're going to have spongy bone, spongy bone, and then again we have this compact bone forming the cortical layer. Within the epiphysis of long bones we have red bone marrow. So red bone marrow is found in the epiphysis of long bones. Also in the pelvis sternum, ribs, and the skull. So flat bones also have red bone marrow in them as well. So flat bones have red bone marrow, and the epiphyses of long bones have red bone marrow. And red bone marrow can make blood cells, right? So it makes our blood cells, which include all the cells of our immune system. The yellow marrow is found in the medullary cavity, and that functions to store fat. So it's a very different function as the red bone marrow. So yellow bone marrow is meant to store fat. Children have more red bone marrow than adults do. And because as we age, our red bone marrow gets converted into more yellow marrow. Interestingly, yellow marrow can become red bone marrow if more blood cells are needed. So the cells can basically turn into the yellow bone marrow cells can turn into red bone marrow if more blood cells are needed for hematopoiesis. So in this picture we can see the distribution of bone marrow in adults and we said that in adults most of the red bone marrow is replaced by the yellow bone marrow which is usually found in the endosteum of long bones. So that leaves adults with red bone marrow just in their skull and their vertebrae and their ribs, sternum, and the pelvic girdle, which is the hip. And of course, the proximal heads of long bones, such as the humerus and the femur, also have red bone marrow. So that's red bone marrow. We spoke about the functions and locations. And again, here is the red bone marrow. in the proximal epiphysis of this bone. So again, proximal, I've defined that proximal epiphysis means the one closer to the point of attachment. This would be the distal epiphysis of the humerus. This is the proximal epiphysis of the humerus. So the proximal heads of the humerus and the femur have red bone marrow. So try to answer this question and pause the recording here. So the answer is D, right? A long bone consists of a diaphysis that extends between two epiphyses. So read this question and please pause here. And the answer is C, red bone marrow, right? The spaces in spongy bone of an adult proximal epiphysis of humerus and femur, like we just said, has red bone marrow in it. So now let's talk about the histology of compact bone. So when we look at compact bone, and again, that's in the cortical layer of long bones, we see subunits called osteons. So osteons are the subunits of compact bone, and here's one osteon over here. And the osteon has many layers to it, and within those layers, in the very center, we have what's called the central canal. Over here you can see this is the central canal and that's where blood vessels and nerves can run through. So one osteon always has a central canal running through it. So around the central canal of an osteon, so here's one osteon again, and here we have a central canal. So within the osteon we have concentric rings of bone cells and bone matrix. So let's zoom in. Here, here's one ostinion. Let's zoom in. We have different concentric rings. So we have a layer of bone and layer means a lamella. Lamella literally means layer. lamellae is plural. So we have a layer of bone and then we have a layer of bone cells, osteocytes, and then we have more bone and then we have another layer of osteocytes. So it's almost like rings of a tree in a way. So they go in concentric circles. And we said that blood vessels go through each central canal, but they also can connect to neighboring osteons via perforating canals. So here you can see this and here's one osteon and here's another osteon but these perforating canals can go from one central canal to another. In addition there are also small holes in the outside of the bone through the periosteum that allows blood vessels to deliver nutrients and remove wastes from the medullary cavity. So we need to get a way we need the bloodstream to have a way inside of the deepest part of the bone, even in the spongy bone. And that's where we have these nutrient foramina. They're just holes. Foramen is singular. Nutrient foramina is a singular hole. Nutrient foramina are plural. And again, these are holes in the periosteum, in that outer layer surrounding bone that allows blood vessels to go inside. And again, we need a lot of blood supply for our bones to function. So we get a half a liter of blood supplied to our skeletal system every minute, which is very significant. So we have all these components. We said each compact bone is composed of osteons. Osteons have lamellae of bone with osteocytes, which are the bone cells. Within each osteon, we have a central canal, perforating canals. can connect adjacent osteons and nutrient foramina allow blood vessels to go from outside into the bone. So while it might not seem like it, bones are constantly being broken down and being rebuilt at a microscopic level. So this is called bone remodeling and we need specialized cells within the bone tissue to constantly build and break down bone as needed. On average, Your entire skeleton is remodeled over about 10 years. Now we have to introduce some very important types of cells found in bone tissue. The first we'll call osteogenic cells, which are just bone stem cells. And these bone stem cells can become mature bone cells or cartilage cells. So that's the first. So osteogenic cells are stem cells that can give rise to most bone cell types. Then we have osteocytes. Osteocytes are active bone cells. They're responsible for maintaining the existing matrix. They don't build, they don't break down. So the osteocytes are all connected to each other so they can pass nutrients and chemical signals from one another. So those are the active bone cells, osteocytes. Osteoblasts are bone forming cells that build new bone matrix. So you can remember the build and B for blast builds new matrix. So these osteoblasts are non-mitotic, they don't divide. So the only source of new osteoblasts are those osteogenic cells. So those can become more osteoblasts. Osteoblasts also secrete a hormone called osteocalcin, which can stimulate insulin secretion by the pancreas. which can limit the growth of adipose tissue and make sure our homeostasis is maintained. So this could basically help us store or help us not store excess sugar as fat. Osteoblasts eventually get trapped in the matrix and become osteocytes. So osteocytes are former osteoblasts that have been trapped in a matrix that they deposited themselves. Osteoclasts dissolve and break down old bone matrix. So osteoclasts destroy or dissolve bone in a process called osteolysis. So osteoclasts dissolve and degrade bone, osteoblasts build bone, osteocytes maintain bone, osteogenic cells can become any of these guys. So what type of cell is responsible for building new bone? Pause here. The answer is C, osteoblast. What about breaking down bone? So you can pause here. The answer is A, osteoclasts. So to build new bone, it is important that you do weight-bearing exercise. Weight-bearing exercise stimulates bone remodeling. You're basically telling your body that you're going to have some more weight to withstand, so you better build the bone stronger. Very similar to how the body builds muscle. When you use your muscles more, you're going to build it. So use it or lose it. The more you use your bones, the more they will be built. So for example, for astronauts who are in space, they actually lose a lot of their bone density because they don't have the ability to exercise the way that we do. So people in space have to do certain kinds of exercises in order to maintain their bone remodeling. Otherwise, their bones would just decompose. So now let's talk about ossification, which is the process of bone formation that begins during the sixth or seventh week of embryonic life. And I'm going to talk very briefly about ossification. There are two types of bone formation and development. There's intramembranous. That's how we get certain flat bones of the skull, parts of the collarbone and part of the mandible. And then we have endochondral ossification. I'll talk about each of those. First, intramembranous ossification forms flat bones, and flat bones forms... between connective tissue membranes of the embryo. So what this means is that there are connective tissues inside the membrane that become osteoblasts and make spongy bone. Remember that flat bones have spongy bone in the inside. Osteoblasts outside of the membrane deposit compact bone. So again, we have the inner part making spongy bone. The outer part makes compact bone. and it's called intramembranous because the bone forms between two membranes. That's what we call intramembranous. It's formed in between membranes. The next kind of ossification is endochondral ossification. In endochondral ossification, we see that bone forms over a hyaline cartilage model. So it starts off as hyaline cartilage is replaced by bone. And again, this happens around the sixth week of field development and continues into a person's 20s. And this is how most bones of the body develop. So most of your long bones, your vertebrae, your ribs, sternum, scapula, are all formed by endochondral ossification. Osteoblasts of the periosteum can make a collar around the diaphysis that starts to form compact bone. So cartilage becomes calcified, which hardens it. And around the diaphysis, we start seeing compact bone developing. And as the cartilage is calcified, it is being killed. So chondrocytes, which are cartilage cells, are being killed. And then osteoblasts will replace that inner cartilage with spongy bone. As the cartilage is being calcified, we have spongy bone being formed instead of the cartilage. And the osteoclasts remove spongy bone to form the medullary cavity in the center. And of course, the medullary cavity allows us to store bone marrow. So in this image, you can see endochondral ossification of the long bones forming and intramembranous. ossification of the flat bones of the skull. So pause here and try to answer this rapid response question. The answer is B, chondrocytes. Remember, endochondral ossification starts off with cartilage. So the chondrocytes, the cartilage cells, end up being killed by the deposition of calcium salts by osteoblasts. So again, I went through that very, very quickly. You don't need to know all the information contained in the textbook on ossification. So try this one more question before we move on. So you can pause here. And the answer to this question is D. So ossification does not stop at birth. It continues throughout our life with the growth and remodeling of bones. And bones need to grow in two directions, both in length and in width. So as we age up until a certain point, about in our mid-20s, our long bones continue to grow in length and we grow taller as a result. This is in response to the growth plate. The growth plate, also called the epiphyseal plate, is a thin band of cartilage between the diaphysis and the epiphysis of long bones. And this allows the shaft of the long bone to grow as well as the head of the long bone to grow. And the way that this happens is at this epiphyseal plate, it's made of cartilage. So we have chondrocytes growing, so cartilage cells being grown, and as the cartilage cells grow, they are then replaced by bone. So those chondrocytes, those cartilage cells, could then be calcified, and then bone gets deposited as the chondrocytes die. So again, The way that bones grow is first it starts off as cartilage and at this epiphyseal plate we see cartilage grow in length that will help us grow taller. Also the bones could grow in diameter around the periosteum. Bones can be built by osteoblasts and that gets our diameter of the diaphysis bigger. So again all is because of cartilage. What happens at around age 25 is that the growth plate hardens into bone, and that's when growth permanently ends. And we can see that in an adult bone as the epiphyseal line. That's where the fusion of diaphysis and epiphysis happens. So this was once the epiphyseal plate where that cartilage was that allowed the bone to grow in length as well as in diameter. There are many factors that can affect bone growth and repair. So just nutrition, what you eat, provides the raw materials to build bone. So the calcium, phosphorus, and the proteins in your diet have a lot to do with how your skeletal system can be remodeled. Vitamin D promotes calcium absorption in the intestines. So in order for our calcium to be taken from our intestines into our bloodstream, we need vitamin D's help. That's another... dietary requirement we need and we also need the sunlight to get proper vitamin d we also rely on growth hormones and sex hormones to promote the proper building of bone and like i mentioned before weight-bearing exercise is the stimulus to make bones stronger so bone stores the majority of calcium in our body our body contains about 1 100 grams of calcium and 99 of that is in our bones and Calcium is required for many functions outside of the skeletal system, right? A lot more than just the bone structure. We need calcium to maintain our heart rhythmicity. We need calcium for muscle contraction. We need calcium for our nerves to function properly. We need calcium also for our blood to clot. And again, this is a small subset. Calcium is also required in many other physiological processes. And calcium is continuously exchanged between the solid form in bones and the dissolved form in blood. The normal calcium concentration in blood plasma is about 9.2 to 10.4 milligrams per deciliter, and that's a very narrow margin of safety. If you have hypocalcemia, that means you are deficient in blood calcium levels, and that can cause excessive excitability of the nervous system. and what's called tetany, which are muscle spasms. And if you had tetany of the muscles of the larynx, that could cause death by suffocation. So if you have a vitamin D deficiency or excessive diarrhea or thyroid tumors, this could impact your ability to store calcium, and that could cause a calcium deficiency. And pregnancy and lactation increase the risk of hypocalcemia. hypercalcemia, a calcium excess, could make ion channels less responsive. And thus, nerve and muscle cells are less excitable. They're less able to respond to our brain signaling. And that is a lot rarer. So hypercalcemia is rare, but hypocalcemia is a lot more common. And again, like I said, pregnancy and lactation puts women at risk of hypocalcemia. because they need to have a lot of calcium to make milk and also to ossify the fetal skeleton. So now that we understand just how critical blood calcium level is, let's try to explain how the body controls calcium homeostasis. How do we make sure that we have the right amount of calcium in our blood at all times? Well, we have three important hormones that help our body regulate calcium homeostasis. The first is calcitriol, also known as active vitamin D. And calcitriol functions to increase calcium in the blood. So active vitamin D increases calcium in the blood. Vitamin D is produced by the skin, liver, and kidneys. There's actually a multi-step process to get active calcitriol. We have a precursor to vitamin D in our skin. We need UV light to activate it. Then there's a little step in the liver and a final step in the kidneys to make it fully functional. And once it's fully functional, it can raise our blood calcium by increasing absorption by the kidneys and the small intestines. What this means is absorption is helping keep calcium in the body. So calcitriol can go over to the small intestines and make sure that it's keeping the calcium in. It's not going to excrete it. So calcitriol goes to the small intestines and say, okay, do not let it. be excreted, keep it inside the blood, keep it in the body. Same idea at the kidneys. It's telling the kidneys do not let the calcium be eliminated in the urine. Keep it in the body, right? So absorption means keep it in the body, and that's what calcitriol is doing. Calcitriol can also stimulate osteoclasts to increase calcium resorption from the skeleton. What resorption means is we're going to take the calcium from the bone and put it into the blood. Remember, calcitriol will be released when we need more calcium in the blood. So what that calcitriol can do is it can activate osteoclasts to break down bone so the calcium could be then released into the bloodstream where we need it more. So this is how calcitriol acts to increase the concentration of calcium in the blood. right? It could act on bone resorption, reduced excretion of calcium, and increased absorption of calcium in the small intestines. Next, we could talk about calcitonin. Calcitonin is produced by the thyroid gland, and this is when blood calcium levels are high. So this is another way, so when blood levels are too high, then we want to have calcitonin be secreted from the thyroid gland. So calcitonin inhibits osteoclasts from degrading the bone matrix, and it stimulates bone deposition by osteoblasts. So this is the opposite of calcitriol. Calcitonin inhibits osteoclasts so they can no longer break down bone. So this will make sure that our blood doesn't get any more calcium in it. right? We don't want any more calcium if it's already high enough. So calcitonin will inhibit osteoclasts, making sure they don't degrade any bone matrix, and they will stimulate osteoblasts. So the osteoblasts will start taking the calcium from the blood and putting it into bone. So remember, calcitonin we need when blood calcium levels are too high. Calcitriol we needed when the calcium levels were too low. So these are doing opposite things. And again, calcitonin is released from the thyroid. So he said it inhibits osteoclasts and stimulates osteoblasts. Finally, the third hormone we'll talk about is parathyroid hormone, and this, like calcitriol, functions when blood calcium levels are low. Parathyroid hormone is named so because it is secreted from the parathyroid gland. I should have shown this before, but right in front of the larynx, we have the thyroid gland. That's where the calcitonin is released from. And then we have these four parathyroid glands where PTH is secreted from. Again, those are just these little glands on the posterior surface of the thyroid glands. So when blood calcium is low, the parathyroid glands release parathyroid hormone. And parathyroid hormone will stimulate osteoclasts to degrade bone matrix and release calcium into the blood, very similar to what calcitriol did. All right. Parathyroid hormone also helps with retention of calcium from the intestines and the kidneys. So calcium isn't excreted. So very similar to calcitriol, active vitamin D, you know, is parathyroid hormone. So let's review. If we have too much calcium in our blood, we could have calcitonin be secreted from the thyroid. And what that will do, well, inhibit osteoclasts. So they'll stop raking down bone. They'll also activate osteoblasts. So the osteoblasts will take the excess calcium from the blood and put it into the bone. And together, that will return the blood calcium back to normal. We said if there is too little blood, too little calcium in the blood, PTH gets secreted into the bloodstream by the parathyroid glands. And that does several things. increases osteoclast activity. So they'll continue to break down bone so more calcium can get into the blood. They will stop the osteoblasts from building more bone. And they will also make sure that calcium is conserved, making sure that calcium does not leave in the urine, does not get excreted and gets absorbed by the small intestine. And altogether, that will make sure that our blood calcium is maintained. We want to make sure that we have enough. So let's answer a few of these rapid response questions. So pause here. The answer is C, bone deposition is the building of bone. What is the effect of vitamin D on the skeletal system? So pause here. So the answer is A. So vitamin D functions to raise the blood calcium concentration, and it does so by making sure the small intestine does not get rid of any of that calcium. What is the effect of calcitonin on the skeletal system? So you can pause here, and the answer is D. Calcitonin inhibits the activity of osteoclasts because calcitonin is secreted when there's already enough calcium in the blood. So we don't want osteoclasts to break down bone and put even more calcium in the blood. So calcitonin inhibits the activity of osteoclasts. In response to low blood calcium, which would not occur? So pause here and try to use process of elimination for this one. So the answer is A. In response to low blood calcium, osteoclasts would not build new bone. There's not enough calcium in the blood to begin with to even build new bone. But in response to low blood calcium, parathyroid hormone would be active, and that would actually activate osteoclasts to break down bone so we could get more calcium inside the blood. And calcitonin would not be active because calcitonin inhibits osteoclasts, and we want osteoclasts to break down bone so we can get more calcium in the blood. So the answer is A. So to conclude, orthopedics is the branch of medicine that deals with the prevention and correction of injuries and disorders of bones, joints, and muscles. Osteoporosis is a disorder you've probably heard of already, and that's a decrease in bone density. And that leads to bones that are very weak and can be more easily fractured. So osteoporosis really looks like pores within the bone. And the risk of osteoporosis increases with age, and women are at greater risk than men. And osteoporosis is not just a calcium deficiency that can be fixed by taking calcium pills or having more milk. It has a lot to do with the endocrine system. And in women, estrogen levels decrease after menopause. And estrogen would normally help stimulate osteoblasts to build bone and would usually help to absorb more calcium. So when you have less estrogen, women have less osteoblast functioning to build bone and less ability to absorb calcium. So there's always a dietary approach to make sure that there's adequate intake of calcium, protein, vitamins, and minerals, but there's also the exercise approach. And weight-bearing exercise is known to promote stronger bones. And that's often the suggestion a doctor might give to somebody with osteoporosis. So you can watch a video contained in the ebook. on Connect about osteoporosis. So finally, I just want to talk about types of fractures. So a fracture is a break in the bone that can be caused by one of two things. You can have a stress fracture. That's when you kind of you fall or something happens in an accident. So it's an abnormal trauma to the bone. It's a stress fracture. You can also have a pathological fracture that's due to disease, and that's more of a break that's due to bone cancer osteoporosis. something that would not have normally broken a healthy bone. So if the fracture breaks, if a bone breaks when it normally would not have, it's pathological because it was probably due to the weakened bone itself. Fractures can be classified by their structural characteristics. For example, the direction of the fraction line, whether the skin is broken or not, and how many pieces. the fracture is broken into. So you can look in the book a little bit more about different types of fractures and they can be repaired either closed or open. So closed reduction is a procedure in which the bone fragments are manipulated without surgery, so with a cast for example. Open reduction involves surgical exposure of the bone and you can pin the bones back into position. So you realign the fragments using plates or screws surgically. So there's closed reduction versus open reduction. So an uncomplicated fracture can heal in about 8 to 12 weeks, but more complex fractures take longer, and in older people, all fractures heal more slowly. An uncomplicated fracture heals in about 8 to 12 weeks, but complex fractures take a lot longer, and in In older people, all fractures heal a lot more slowly. But fractures can be healed in a basic four steps. So first we have hematoma formation, which is a blood clot. So a bone fracture would have severed a lot of those blood vessels of the bone and the periosteum, which caused the bleeding, and that formed a blood clot. Then we start having some osteogenic cells becoming very abundant within 48 hours of the injury. So osteogenic cells start coming in and those osteogenic cells start to become chondroblasts and they start making a fibrocartilage and fibroblasts start making collagen. So this is called soft callus formation. We have collagen being made by fibroblasts and chondrocytes are making this fibrocartilage. So we have this soft callus being formed. Then we have the formation of a hard callus. So the soft callus is converted to a hard callus as osteogenic cells differentiate into osteoblasts, which deposit a bony collar around the fracture to unite all the broken pieces. And that hard callus can be there for about three to four months. And during that time, we have bone remodeling, where small bone fragments are removed by osteoclasts, where osteoblasts continue to deposit spongy bone. and then convert it into compact bone. So again, this is the four basic steps to fractures being healed. And that is the end of chapter seven. We will continue with talking about the skeletal system in the next chapters, and then followed by that we'll talk about joints. So that is it for now. I'll see you next time.

In addition to bones, we also have cartilage, joints, and ligaments as part of the skeletal system. The skeletal system has many functions. One of the major ones is support. So our limb bones and our vertebrae can support our body. Our jaw bones can support our teeth.

So the skeletal system functions as a support system. Our skeletal system also protects our organs such as our brain, our spinal cord, hearts, and lungs. Our skeletal system helps us move around.

So whether it's limb movements and walking or even breathing. All these kinds of movements depend on the skeletal system. The skeleton can also play a role in acid and base balance and electrolyte balance to maintain homeostasis.

So depending on how much calcium and phosphate is in the blood, the skeletal system can either contribute more or can take away to, again, make sure there's the right concentration of these minerals in our body. Similarly, bone tissue can buffer the blood against excessive pH changes. So this is the skeleton also functions to maintain homeostasis. And finally, the skeletal system is very important in blood formation because the bone marrow, the red bone marrow specifically, is the chief producer of blood cells including cells of the immune system. So bone is made up of osseous tissue and bones are not just these brittle, dry, cracked things that we see in the laboratory.

They're actually alive. So bones are living organs and they're mostly composed of connective tissue. Many different kinds of different types of tissues are found within bone. So we not only have bone tissue, but we have bone marrow, we have cartilage, adipose tissue, nervous tissue, and dense regular connective tissue. So all these different types of tissues we discussed already are found in bone.

And like I said, bone is alive. So there has to be a living component. And those living components are osteocytes.

So osteocytes are cells of the bone. Osteo always means bone and site always means cell. So osteocyte is a bone cell. They're embedded in the non-living extracellular matrix.

Extracellular matrix is just what's outside of the cells. So this matrix of bone is a composite of minerals. and proteins. Composite meaning it's a combination. So the matrix of bone is not just minerals, not just proteins.

It's approximately 35% organic protein and about 65% inorganic mineral. And the organic component is collagen specifically. So we know that collagen provides a lot of flexible strength, which is really important.

The inorganic component is composed of calcium, and minerals. And calcium phosphate called hydroxyapatite provides the weight-bearing strength required for bone. Also, calcium carbonate is found as part of the inorganic component and other minerals like fluorine, sodium, potassium, and magnesium.

So, so far we said bone tissue has connective tissues plus cells. And There's importance to both the inorganic and the organic components of bone matrix. So what do you think would happen if we didn't have any of those minerals, any of those hard minerals? Then bone would be pretty flexible.

It wouldn't have a lot of the strength required for bone. And you can actually do this. You could take a chicken bone and put it in vinegar, and it will seep out all the minerals.

And you'll see that with just collagen, bone is very flexible. not very strong. Without collagen, however, bone would be very brittle and it would crack in response to even very tiny amounts of stress. So it's important that we have both this hardening, this mineral, the mineral component, but also the collagen, the flexible component. So we need both of these working together.

We could classify bones according to their shape. So Starting with flat bones such as the sternum, these are thin curved plates and generally function to protect soft organs. So flat bones also are part of the skull bones, for example.

The skull bones are flat bones as well, and they protect our brain. We also have long bones, which are bones of the limb, such as the humerus, the radius, the ulna. So these are longer than they are wide, and these serve as levers acted upon by muscles to enable movement. So those are long bones. We also have short bones.

like those found in the wrist with the carpal bones. Those are approximately equal in length and width, and they glide across each other in multiple directions. We'll talk more about short bones when we discuss joints. Finally, we have irregular bones that are kind of elaborately shaped that don't fit into the other categories. So those can include like these vertebrae or this sesamoid bone as part of the patella.

It kind of looks like a sesame seed. That's why it's called a sesamoid bone. So each vertebra is an irregular bone. A sesamoid bone of the patella is an irregularly shaped bone. So bones come in two different types when we're talking about the tissue itself.

Bone can be either compact or spongy. Compact bone is very dense and very strong. It's also called cortical bone because it forms the outer cortical layer of bone material. So compact bone is very, very compact and very dense. Spongy bone, also called cancellous bone, is a lot different.

This is more of a latticework, and these have what's called trabeculae. These are these little spaces in between, so this gives a lot more of like a spongy appearance. And this is less dense.

than compact bone, but it's still very rigid. So it still allows us to have the strength, but it's less dense, so it doesn't take up as much, doesn't weigh as much. It's not as heavy to carry around. The spaces in the spongy bone are filled with bone marrow, and we see the spongy bone in the center and the ends of long bones and in the middle of nearly all other bones. Right, so overall we're seeing the spongy bone can reduce the weight of bone.

So we still get the strength, but it's not as dense. So this is the spongy bone with those trabeculae, and this is the compact bone. So when we look at flat bones, such as let's say the parietal bone of the skull, we see a sandwich-like construction. We see two layers of compact bone. So here's compact bone, and here's more compact bone.

But in the middle, there's what's called diploe. It's like a spongy middle layer. of that spongy bone.

And again, that's good for absorbing shock and that also stores bone marrow. There's also this outer lining called periosteum. So periosteum, peri means outside, right? And ostium is bone. So periosteum is the surface of the bone.

And we'll talk more about that in a bit. So let's talk about long bones next. So long bones are mostly composed of an outer shell of this dense compact bone.

Outside of that compact bone we have the periosteum, which is a fibrous outer membrane made of dense irregular connective tissue. And then on the inside we have the spongy bone we said, the spongier bone. Let's just talk about some parts of bone.

So the shaft of a long bone is called the diaphysis. So this part is called the diaphysis. And the two enlarged ends are called the epiphyses.

Singular is epiphysis, both are epiphyses. So these are the heads of the long bone that could form joints or articulations with other bones. The articular cartilage is a layer of cartilage that covers and protects the joint surfaces to enable very smooth movements.

Within here, Within the long bone, we have a space called the medullary cavity. And in the medullary cavity is where we see bone marrow. Specifically, we find yellow bone marrow inside the medullary cavity. We'll talk a little bit more about that.

The endosteum is the inner membrane lining the medullary cavity. So the endosteum, endo, means inside. So the endosteum lines the inside. of the medullary cavity over here.

The endosteum has a very important role in helping to form and repair bone. And it's actually a form of reticular connective tissue. So again, this forms like a mesh for the bone marrow to live in. So far, we spoke about here's the diaphysis.

Within the diaphysis, we have the medullary cavity. And that's where we're going to have yellow bone marrow, as we'll soon see. Within the epiphysis, we have spongy bone. So spongy bone is in the epiphysis.

So in here we're going to have spongy bone, spongy bone, and then again we have this compact bone forming the cortical layer. Within the epiphysis of long bones we have red bone marrow. So red bone marrow is found in the epiphysis of long bones. Also in the pelvis sternum, ribs, and the skull.

So flat bones also have red bone marrow in them as well. So flat bones have red bone marrow, and the epiphyses of long bones have red bone marrow. And red bone marrow can make blood cells, right? So it makes our blood cells, which include all the cells of our immune system.

The yellow marrow is found in the medullary cavity, and that functions to store fat. So it's a very different function as the red bone marrow. So yellow bone marrow is meant to store fat. Children have more red bone marrow than adults do.

And because as we age, our red bone marrow gets converted into more yellow marrow. Interestingly, yellow marrow can become red bone marrow if more blood cells are needed. So the cells can basically turn into the yellow bone marrow cells can turn into red bone marrow if more blood cells are needed for hematopoiesis. So in this picture we can see the distribution of bone marrow in adults and we said that in adults most of the red bone marrow is replaced by the yellow bone marrow which is usually found in the endosteum of long bones.

So that leaves adults with red bone marrow just in their skull and their vertebrae and their ribs, sternum, and the pelvic girdle, which is the hip. And of course, the proximal heads of long bones, such as the humerus and the femur, also have red bone marrow. So that's red bone marrow.

We spoke about the functions and locations. And again, here is the red bone marrow. in the proximal epiphysis of this bone.

So again, proximal, I've defined that proximal epiphysis means the one closer to the point of attachment. This would be the distal epiphysis of the humerus. This is the proximal epiphysis of the humerus.

So the proximal heads of the humerus and the femur have red bone marrow. So try to answer this question and pause the recording here. So the answer is D, right?

A long bone consists of a diaphysis that extends between two epiphyses. So read this question and please pause here. And the answer is C, red bone marrow, right?

The spaces in spongy bone of an adult proximal epiphysis of humerus and femur, like we just said, has red bone marrow in it. So now let's talk about the histology of compact bone. So when we look at compact bone, and again, that's in the cortical layer of long bones, we see subunits called osteons. So osteons are the subunits of compact bone, and here's one osteon over here. And the osteon has many layers to it, and within those layers, in the very center, we have what's called the central canal.

Over here you can see this is the central canal and that's where blood vessels and nerves can run through. So one osteon always has a central canal running through it. So around the central canal of an osteon, so here's one osteon again, and here we have a central canal. So within the osteon we have concentric rings of bone cells and bone matrix.

So let's zoom in. Here, here's one ostinion. Let's zoom in.

We have different concentric rings. So we have a layer of bone and layer means a lamella. Lamella literally means layer.

lamellae is plural. So we have a layer of bone and then we have a layer of bone cells, osteocytes, and then we have more bone and then we have another layer of osteocytes. So it's almost like rings of a tree in a way. So they go in concentric circles.

And we said that blood vessels go through each central canal, but they also can connect to neighboring osteons via perforating canals. So here you can see this and here's one osteon and here's another osteon but these perforating canals can go from one central canal to another. In addition there are also small holes in the outside of the bone through the periosteum that allows blood vessels to deliver nutrients and remove wastes from the medullary cavity. So we need to get a way we need the bloodstream to have a way inside of the deepest part of the bone, even in the spongy bone.

And that's where we have these nutrient foramina. They're just holes. Foramen is singular.

Nutrient foramina is a singular hole. Nutrient foramina are plural. And again, these are holes in the periosteum, in that outer layer surrounding bone that allows blood vessels to go inside. And again, we need a lot of blood supply for our bones to function.

So we get a half a liter of blood supplied to our skeletal system every minute, which is very significant. So we have all these components. We said each compact bone is composed of osteons.

Osteons have lamellae of bone with osteocytes, which are the bone cells. Within each osteon, we have a central canal, perforating canals. can connect adjacent osteons and nutrient foramina allow blood vessels to go from outside into the bone. So while it might not seem like it, bones are constantly being broken down and being rebuilt at a microscopic level. So this is called bone remodeling and we need specialized cells within the bone tissue to constantly build and break down bone as needed.

On average, Your entire skeleton is remodeled over about 10 years. Now we have to introduce some very important types of cells found in bone tissue. The first we'll call osteogenic cells, which are just bone stem cells.

And these bone stem cells can become mature bone cells or cartilage cells. So that's the first. So osteogenic cells are stem cells that can give rise to most bone cell types.

Then we have osteocytes. Osteocytes are active bone cells. They're responsible for maintaining the existing matrix.

They don't build, they don't break down. So the osteocytes are all connected to each other so they can pass nutrients and chemical signals from one another. So those are the active bone cells, osteocytes.

Osteoblasts are bone forming cells that build new bone matrix. So you can remember the build and B for blast builds new matrix. So these osteoblasts are non-mitotic, they don't divide.

So the only source of new osteoblasts are those osteogenic cells. So those can become more osteoblasts. Osteoblasts also secrete a hormone called osteocalcin, which can stimulate insulin secretion by the pancreas.

which can limit the growth of adipose tissue and make sure our homeostasis is maintained. So this could basically help us store or help us not store excess sugar as fat. Osteoblasts eventually get trapped in the matrix and become osteocytes. So osteocytes are former osteoblasts that have been trapped in a matrix that they deposited themselves.

Osteoclasts dissolve and break down old bone matrix. So osteoclasts destroy or dissolve bone in a process called osteolysis. So osteoclasts dissolve and degrade bone, osteoblasts build bone, osteocytes maintain bone, osteogenic cells can become any of these guys.

So what type of cell is responsible for building new bone? Pause here. The answer is C, osteoblast.

What about breaking down bone? So you can pause here. The answer is A, osteoclasts. So to build new bone, it is important that you do weight-bearing exercise.

Weight-bearing exercise stimulates bone remodeling. You're basically telling your body that you're going to have some more weight to withstand, so you better build the bone stronger. Very similar to how the body builds muscle.

When you use your muscles more, you're going to build it. So use it or lose it. The more you use your bones, the more they will be built.

So for example, for astronauts who are in space, they actually lose a lot of their bone density because they don't have the ability to exercise the way that we do. So people in space have to do certain kinds of exercises in order to maintain their bone remodeling. Otherwise, their bones would just decompose.

So now let's talk about ossification, which is the process of bone formation that begins during the sixth or seventh week of embryonic life. And I'm going to talk very briefly about ossification. There are two types of bone formation and development.

There's intramembranous. That's how we get certain flat bones of the skull, parts of the collarbone and part of the mandible. And then we have endochondral ossification. I'll talk about each of those. First, intramembranous ossification forms flat bones, and flat bones forms...

between connective tissue membranes of the embryo. So what this means is that there are connective tissues inside the membrane that become osteoblasts and make spongy bone. Remember that flat bones have spongy bone in the inside.

Osteoblasts outside of the membrane deposit compact bone. So again, we have the inner part making spongy bone. The outer part makes compact bone.

and it's called intramembranous because the bone forms between two membranes. That's what we call intramembranous. It's formed in between membranes.

The next kind of ossification is endochondral ossification. In endochondral ossification, we see that bone forms over a hyaline cartilage model. So it starts off as hyaline cartilage is replaced by bone. And again, this happens around the sixth week of field development and continues into a person's 20s.

And this is how most bones of the body develop. So most of your long bones, your vertebrae, your ribs, sternum, scapula, are all formed by endochondral ossification. Osteoblasts of the periosteum can make a collar around the diaphysis that starts to form compact bone.

So cartilage becomes calcified, which hardens it. And around the diaphysis, we start seeing compact bone developing. And as the cartilage is calcified, it is being killed.

So chondrocytes, which are cartilage cells, are being killed. And then osteoblasts will replace that inner cartilage with spongy bone. As the cartilage is being calcified, we have spongy bone being formed instead of the cartilage.

And the osteoclasts remove spongy bone to form the medullary cavity in the center. And of course, the medullary cavity allows us to store bone marrow. So in this image, you can see endochondral ossification of the long bones forming and intramembranous.

ossification of the flat bones of the skull. So pause here and try to answer this rapid response question. The answer is B, chondrocytes.

Remember, endochondral ossification starts off with cartilage. So the chondrocytes, the cartilage cells, end up being killed by the deposition of calcium salts by osteoblasts. So again, I went through that very, very quickly. You don't need to know all the information contained in the textbook on ossification. So try this one more question before we move on.

So you can pause here. And the answer to this question is D. So ossification does not stop at birth. It continues throughout our life with the growth and remodeling of bones. And bones need to grow in two directions, both in length and in width.

So as we age up until a certain point, about in our mid-20s, our long bones continue to grow in length and we grow taller as a result. This is in response to the growth plate. The growth plate, also called the epiphyseal plate, is a thin band of cartilage between the diaphysis and the epiphysis of long bones. And this allows the shaft of the long bone to grow as well as the head of the long bone to grow. And the way that this happens is at this epiphyseal plate, it's made of cartilage.

So we have chondrocytes growing, so cartilage cells being grown, and as the cartilage cells grow, they are then replaced by bone. So those chondrocytes, those cartilage cells, could then be calcified, and then bone gets deposited as the chondrocytes die. So again, The way that bones grow is first it starts off as cartilage and at this epiphyseal plate we see cartilage grow in length that will help us grow taller.

Also the bones could grow in diameter around the periosteum. Bones can be built by osteoblasts and that gets our diameter of the diaphysis bigger. So again all is because of cartilage. What happens at around age 25 is that the growth plate hardens into bone, and that's when growth permanently ends.

And we can see that in an adult bone as the epiphyseal line. That's where the fusion of diaphysis and epiphysis happens. So this was once the epiphyseal plate where that cartilage was that allowed the bone to grow in length as well as in diameter. There are many factors that can affect bone growth and repair. So just nutrition, what you eat, provides the raw materials to build bone.

So the calcium, phosphorus, and the proteins in your diet have a lot to do with how your skeletal system can be remodeled. Vitamin D promotes calcium absorption in the intestines. So in order for our calcium to be taken from our intestines into our bloodstream, we need vitamin D's help.

That's another... dietary requirement we need and we also need the sunlight to get proper vitamin d we also rely on growth hormones and sex hormones to promote the proper building of bone and like i mentioned before weight-bearing exercise is the stimulus to make bones stronger so bone stores the majority of calcium in our body our body contains about 1 100 grams of calcium and 99 of that is in our bones and Calcium is required for many functions outside of the skeletal system, right? A lot more than just the bone structure.

We need calcium to maintain our heart rhythmicity. We need calcium for muscle contraction. We need calcium for our nerves to function properly. We need calcium also for our blood to clot. And again, this is a small subset.

Calcium is also required in many other physiological processes. And calcium is continuously exchanged between the solid form in bones and the dissolved form in blood. The normal calcium concentration in blood plasma is about 9.2 to 10.4 milligrams per deciliter, and that's a very narrow margin of safety.

If you have hypocalcemia, that means you are deficient in blood calcium levels, and that can cause excessive excitability of the nervous system. and what's called tetany, which are muscle spasms. And if you had tetany of the muscles of the larynx, that could cause death by suffocation.

So if you have a vitamin D deficiency or excessive diarrhea or thyroid tumors, this could impact your ability to store calcium, and that could cause a calcium deficiency. And pregnancy and lactation increase the risk of hypocalcemia. hypercalcemia, a calcium excess, could make ion channels less responsive.

And thus, nerve and muscle cells are less excitable. They're less able to respond to our brain signaling. And that is a lot rarer. So hypercalcemia is rare, but hypocalcemia is a lot more common. And again, like I said, pregnancy and lactation puts women at risk of hypocalcemia.

because they need to have a lot of calcium to make milk and also to ossify the fetal skeleton. So now that we understand just how critical blood calcium level is, let's try to explain how the body controls calcium homeostasis. How do we make sure that we have the right amount of calcium in our blood at all times?

Well, we have three important hormones that help our body regulate calcium homeostasis. The first is calcitriol, also known as active vitamin D. And calcitriol functions to increase calcium in the blood. So active vitamin D increases calcium in the blood.

Vitamin D is produced by the skin, liver, and kidneys. There's actually a multi-step process to get active calcitriol. We have a precursor to vitamin D in our skin. We need UV light to activate it.

Then there's a little step in the liver and a final step in the kidneys to make it fully functional. And once it's fully functional, it can raise our blood calcium by increasing absorption by the kidneys and the small intestines. What this means is absorption is helping keep calcium in the body. So calcitriol can go over to the small intestines and make sure that it's keeping the calcium in.

It's not going to excrete it. So calcitriol goes to the small intestines and say, okay, do not let it. be excreted, keep it inside the blood, keep it in the body.

Same idea at the kidneys. It's telling the kidneys do not let the calcium be eliminated in the urine. Keep it in the body, right?

So absorption means keep it in the body, and that's what calcitriol is doing. Calcitriol can also stimulate osteoclasts to increase calcium resorption from the skeleton. What resorption means is we're going to take the calcium from the bone and put it into the blood. Remember, calcitriol will be released when we need more calcium in the blood. So what that calcitriol can do is it can activate osteoclasts to break down bone so the calcium could be then released into the bloodstream where we need it more.

So this is how calcitriol acts to increase the concentration of calcium in the blood. right? It could act on bone resorption, reduced excretion of calcium, and increased absorption of calcium in the small intestines.

Next, we could talk about calcitonin. Calcitonin is produced by the thyroid gland, and this is when blood calcium levels are high. So this is another way, so when blood levels are too high, then we want to have calcitonin be secreted from the thyroid gland. So calcitonin inhibits osteoclasts from degrading the bone matrix, and it stimulates bone deposition by osteoblasts.

So this is the opposite of calcitriol. Calcitonin inhibits osteoclasts so they can no longer break down bone. So this will make sure that our blood doesn't get any more calcium in it.

right? We don't want any more calcium if it's already high enough. So calcitonin will inhibit osteoclasts, making sure they don't degrade any bone matrix, and they will stimulate osteoblasts.

So the osteoblasts will start taking the calcium from the blood and putting it into bone. So remember, calcitonin we need when blood calcium levels are too high. Calcitriol we needed when the calcium levels were too low. So these are doing opposite things.

And again, calcitonin is released from the thyroid. So he said it inhibits osteoclasts and stimulates osteoblasts. Finally, the third hormone we'll talk about is parathyroid hormone, and this, like calcitriol, functions when blood calcium levels are low. Parathyroid hormone is named so because it is secreted from the parathyroid gland.

I should have shown this before, but right in front of the larynx, we have the thyroid gland. That's where the calcitonin is released from. And then we have these four parathyroid glands where PTH is secreted from.

Again, those are just these little glands on the posterior surface of the thyroid glands. So when blood calcium is low, the parathyroid glands release parathyroid hormone. And parathyroid hormone will stimulate osteoclasts to degrade bone matrix and release calcium into the blood, very similar to what calcitriol did. All right.

Parathyroid hormone also helps with retention of calcium from the intestines and the kidneys. So calcium isn't excreted. So very similar to calcitriol, active vitamin D, you know, is parathyroid hormone. So let's review. If we have too much calcium in our blood, we could have calcitonin be secreted from the thyroid.

And what that will do, well, inhibit osteoclasts. So they'll stop raking down bone. They'll also activate osteoblasts.

So the osteoblasts will take the excess calcium from the blood and put it into the bone. And together, that will return the blood calcium back to normal. We said if there is too little blood, too little calcium in the blood, PTH gets secreted into the bloodstream by the parathyroid glands. And that does several things.

increases osteoclast activity. So they'll continue to break down bone so more calcium can get into the blood. They will stop the osteoblasts from building more bone.

And they will also make sure that calcium is conserved, making sure that calcium does not leave in the urine, does not get excreted and gets absorbed by the small intestine. And altogether, that will make sure that our blood calcium is maintained. We want to make sure that we have enough. So let's answer a few of these rapid response questions. So pause here.

The answer is C, bone deposition is the building of bone. What is the effect of vitamin D on the skeletal system? So pause here. So the answer is A. So vitamin D functions to raise the blood calcium concentration, and it does so by making sure the small intestine does not get rid of any of that calcium.

What is the effect of calcitonin on the skeletal system? So you can pause here, and the answer is D. Calcitonin inhibits the activity of osteoclasts because calcitonin is secreted when there's already enough calcium in the blood. So we don't want osteoclasts to break down bone and put even more calcium in the blood. So calcitonin inhibits the activity of osteoclasts.

In response to low blood calcium, which would not occur? So pause here and try to use process of elimination for this one. So the answer is A.

In response to low blood calcium, osteoclasts would not build new bone. There's not enough calcium in the blood to begin with to even build new bone. But in response to low blood calcium, parathyroid hormone would be active, and that would actually activate osteoclasts to break down bone so we could get more calcium inside the blood.

And calcitonin would not be active because calcitonin inhibits osteoclasts, and we want osteoclasts to break down bone so we can get more calcium in the blood. So the answer is A. So to conclude, orthopedics is the branch of medicine that deals with the prevention and correction of injuries and disorders of bones, joints, and muscles.

Osteoporosis is a disorder you've probably heard of already, and that's a decrease in bone density. And that leads to bones that are very weak and can be more easily fractured. So osteoporosis really looks like pores within the bone.

And the risk of osteoporosis increases with age, and women are at greater risk than men. And osteoporosis is not just a calcium deficiency that can be fixed by taking calcium pills or having more milk. It has a lot to do with the endocrine system. And in women, estrogen levels decrease after menopause. And estrogen would normally help stimulate osteoblasts to build bone and would usually help to absorb more calcium.

So when you have less estrogen, women have less osteoblast functioning to build bone and less ability to absorb calcium. So there's always a dietary approach to make sure that there's adequate intake of calcium, protein, vitamins, and minerals, but there's also the exercise approach. And weight-bearing exercise is known to promote stronger bones.

And that's often the suggestion a doctor might give to somebody with osteoporosis. So you can watch a video contained in the ebook. on Connect about osteoporosis. So finally, I just want to talk about types of fractures.

So a fracture is a break in the bone that can be caused by one of two things. You can have a stress fracture. That's when you kind of you fall or something happens in an accident.

So it's an abnormal trauma to the bone. It's a stress fracture. You can also have a pathological fracture that's due to disease, and that's more of a break that's due to bone cancer osteoporosis.

something that would not have normally broken a healthy bone. So if the fracture breaks, if a bone breaks when it normally would not have, it's pathological because it was probably due to the weakened bone itself. Fractures can be classified by their structural characteristics.

For example, the direction of the fraction line, whether the skin is broken or not, and how many pieces. the fracture is broken into. So you can look in the book a little bit more about different types of fractures and they can be repaired either closed or open.

So closed reduction is a procedure in which the bone fragments are manipulated without surgery, so with a cast for example. Open reduction involves surgical exposure of the bone and you can pin the bones back into position. So you realign the fragments using plates or screws surgically. So there's closed reduction versus open reduction.

So an uncomplicated fracture can heal in about 8 to 12 weeks, but more complex fractures take longer, and in older people, all fractures heal more slowly. An uncomplicated fracture heals in about 8 to 12 weeks, but complex fractures take a lot longer, and in In older people, all fractures heal a lot more slowly. But fractures can be healed in a basic four steps. So first we have hematoma formation, which is a blood clot.

So a bone fracture would have severed a lot of those blood vessels of the bone and the periosteum, which caused the bleeding, and that formed a blood clot. Then we start having some osteogenic cells becoming very abundant within 48 hours of the injury. So osteogenic cells start coming in and those osteogenic cells start to become chondroblasts and they start making a fibrocartilage and fibroblasts start making collagen.

So this is called soft callus formation. We have collagen being made by fibroblasts and chondrocytes are making this fibrocartilage. So we have this soft callus being formed.

Then we have the formation of a hard callus. So the soft callus is converted to a hard callus as osteogenic cells differentiate into osteoblasts, which deposit a bony collar around the fracture to unite all the broken pieces. And that hard callus can be there for about three to four months.

And during that time, we have bone remodeling, where small bone fragments are removed by osteoclasts, where osteoblasts continue to deposit spongy bone. and then convert it into compact bone. So again, this is the four basic steps to fractures being healed. And that is the end of chapter seven. We will continue with talking about the skeletal system in the next chapters, and then followed by that we'll talk about joints.

So that is it for now. I'll see you next time.

Transcript for:Lecture on Bone Tissue Overview

Transcript for:
Lecture on Bone Tissue Overview