Anatomy and Physiology 1, Chapter 6, Osseous Tissue. Okay, so in this chapter, we're moving forward into talking about the skeletal system. And in order to really understand the skeletal system, we need to first understand bone as a tissue.
So in chapter 4, we talked about how there is a type of tissue called... supporting connective tissue and that included cartilage and also bone. So this chapter is going to focus in on bone tissue.
So we're not really looking at the individual bones yet, we're just looking at what makes the actual bone up. So the skeletal system is going to include bones of the skeleton, cartilage, ligaments, and other connective tissues. And the main reason to have a skeleton is to give you support, storage of minerals and lipids.
So we're mainly going to be storing minerals in the form of calcium. There's some others, but calcium is the biggie. And then lipids in the form of fat found in the yellow bone. marrow.
We're going to be producing blood cells in the red bone marrow. We'll use bone for protection. Think about all your major organs and tissues. Think about the heart and the lungs and how they're enclosed within the rib cage. If we think about the brain which is enclosed within the skull and also the spinal cord which is enclosed within the vertebral column.
So all of these things are very well protected by the skeleton. We also have bones for leverage. Also leverage muscles use bones.
as levers. Bones are classified by their shape and also their structure. So there are a few bone shapes to be familiar with and most of these are pretty straightforward.
So first we have sutural bones and sutural bones are not a thing that everyone has. Some people have them, some people don't. But what these are are bones that form within a suture. So if you've begun looking at the skull for one of your labs, then you know that there are areas where the skull bones come together that are called sutures. And sometimes sutural bones, also known as wormian bones, will form between the flat bones of the skull.
They can be tiny like the size of a grain of sand or up to the size of a quarter. and again they are found within those sutures. Then we have irregular bones and irregular bones are bones like the vertebrae. So they have complex shapes and they can be ridged or notched or flat and vertebrae again is a great example.
So we can look at one of those here. Then we've got short bones, which are what they sound like. Those are going to be small, short, and boxy. So great examples of short bones would be the carpal bones, which look like a bunch of rocks in your wrist. We've then got flat bones, which are what they sound like, very flat.
Good examples of flat bones, which are nice and thin, would be bones of the skull. Also the sternum or breastbone. Long bones are again as they sound long and those are going to include bones like the ones found in the arms and legs like this humerus for example, which is your upper arm bone and then we've got finally the sesamoid bones. And sesamoid bones are going to be small and round and flat and they're found on the knee, is one of the ones that everyone's going to have, which we refer to as the patella. Some people might have extra sesamoid bones or form them within the joints of the hands and feet, but everyone should have kneecaps, so bones that are small, round, and flat like the patella.
So a little bit more detail in your notes. So we've got, again, sutural or wormian bones. Those are small, flat, irregular, and form between the bones of the skull in the sutures and some people have them and some people you can't see them irregular bones are complex and good examples would be the spinal vertebrae and the pelvic bones so here's our sutural bone up close formed within a suture here's our vertebrae Again, complex shapes with lots of spines, notches, ridges, and openings.
Short bones, again, are boxy, and examples are carpal bones and tarsal bones, so that would be wrist and ankle. Flat bones are thin, with parallel surfaces like the bones of the skull, the sternum, ribs, and scapulae, which would be your shoulder blades. So here are short bones, which again are boxy, look like little rocks. See those in the carpals and the tarsals.
Flat bones, like the bones of the skull, nice and thin. Long bones are long and slender, like the ones in the arms, legs, palms, and soles of feet. And sesamoid bones, small, round, and flat.
develop within tendons. We all have them in the patella. The location and number of these vary between individuals. So here's our long bone, humerus. Another example would be the femur.
There's our sesamoid patella. So we're going to look now at the structure of a long bone. So now we're focusing in specifically on the long bone and we can divide it up into areas. We're going to make a drawing on this right after this explanation here in the slide. So the structure of a long bone includes three major parts.
We've got the diaphysis, which is the shaft of the bone. It has a wall of compact bone. So compact bone can also be referred to as dense bone and it is hard and rigid And you can see it.
It looks very solid when you look at it in a picture Inside the central space inside of the shaft of the bone is called the marrow cavity And this is where bone marrow would be kept Sometimes it's referred to referred to as the medullary cavity Then we have the epiphysis or epiphyses if we're talking about more than one. These are the ends of the bones and they're going to contain mostly spongy bone. Spongy bone is also known as trabecular bone and we'll take a look at that a little bit later. But spongy bone looks more porous, so not solid like compact bone looks.
And the metathesis or metaphyses is where the diaphysis and epiphysis meet. We will also see in that area what is known as the epiphyseal cartilage or growth plate if we have someone still growing. Or the epiphyseal line or growth line if the person has stopped growing.
So again, we'll come back and revisit that too. So we're going to use a femur as our example bone in this drawing and what we've got on the ends of the bone we would call these the epiphyses. And that would be an es for plural so that's both the ends of the bone, epiphyses. And then we have this area which is known as the shaft of the bone and that is called the diaphysis.
It's the shaft of the bone and then on either end in this area where we would find the growth plate or in adults though growth line which we'll talk about a little later that transitional area is going to be known as the metaphyses so metaphysis for singular and metaphyses is plural. So metaphysis, singular, metaphyses, plural. And then in the center of the bone, we're going to find an area that is going to contain marrow, and this will be called the marrow cavity.
Now the marrow cavity, you can see we've drawn lines to two things in this area, so diaphysis is the whole shaft of the bone. bone and the marrow cavity is within the diaphysis here in this drawing but it's what contains red and yellow bone marrow. Finally on the ends of the bone wherever there's going to be a joint we have some special thin smooth shiny cartilage.
This again is wherever there's going to be a joint and this is called the articular cartilage and it's called that because an articulation is a joint so this is articular cartilage for the joint and we talked about where there's a joint so what joint do we think would go at the top of this femur? We should expect for that to be the hip joint and then down below we have the knee joint. Okay, so those are some articulations, but this is the main, are the main areas of a long bone.
Okay, so we made our drawing and we went over those parts again, and this is picture from the book just to further drive that point home. Again you can see epiphyses on the end, diaphysis in the middle which is the shaft, there's the cavity that contains the bone marrow, and then we've got the metaphysis where the epiphysis and diaphysis meet. So in flat bones, like the bones of the skull, we will see a mixture of spongy bone and compact bone. The spongy bone will be between two layers of compact bone. of like sort of like a sandwich so we've got the the two pieces of bread or compact bone and then the sandwich filling the the meat or the cheese in the middle that's going to be the spongy bone within the cranium the layer of spongy bone is called the diploe So here is a picture of a flat bone or a bone of the skull that has been cut and you can see the the sort of sandwich arrangement that we talked about.
So we've got the cortex which is the compact bone on top and bottom that's our bread and then in the middle of the sandwich we have the spongy bone so this is a great representation of the structure of a flat bone so bone tissue remember this is a supporting connective tissue it contains specialized cells in the form of bone cells And then remember that all connective tissues must also have a matrix which is going to hold those specialized cells. And the matrix of bone is going to be made up of collagen as its protein. So we talked about how matrix is going to have a ground substance and also protein fibers. And the protein fibers of bone are collagen. So bone has a very dense matrix due to deposits of calcium salt and the calcium salt is going to make bone very very rigid.
We're going to find in the matrix some specialized cells called osteocytes. Osteocytes are mature bone cells and they live in little pockets of the matrix called lacunae. Lacunae are their little houses and they're very organized. These houses are going to set up shop around blood vessels where they can receive oxygen and nutrients.
So we will draw all this out so we can get a good visual of what this looks like. Candeliculi are narrow passageways that will allow for nutrients, waste, and gases to exchange between the osteocytes and also the central blood vessel in the bone tissue. So again, we're going to take a look at that in a drawing very soon.
The periosteum is a membranous structure that covers the outer surface of bones and consists of an outer fibrous and inner cellular layer. So this is going to be a good protectant. bones. So bone matrix has calcium phosphate, which is about two-thirds of your bone mass, and it will interact with calcium hydroxide to form crystals of what is called hydroxyapatite. Hydroxyapatite is a calcium salt.
We'll also have other calcium salts such as calcium carbonate, and ions like magnesium to help make up the matrix. So again, the matrix of bone is primarily calcium, salt, and also collagen. On its own, calcium is strong, but it's very brittle.
On its own, collagen is very rubbery, but would not be good at supporting a body. So if we combine the two, calcium and collagen, bone becomes very strong. Not only is it tough because of the calcium, but it also has a teeny bit of flexibility because of the collagen. A bone lacking a calcified matrix looks normal, but would be very flexible, too flexible.
So here's an example. On the left, you can see a normal bone. which has calcium and collagen.
On the right, the calcium has been degraded and we're left with mostly collagen. So you can see what would happen if you did not have that calcium in place. Your bones would be extremely flexible, which would be no good at protecting or supporting your body. So again, the matrix protein of bone, we talked about how it's about two-thirds calcium salt. The other one-third of your bone mass is collagen fibers.
And again, that collagen is going to give us a little bit of flexibility to make bone even tougher than if it just had the calcium. So we talked about the matrix. The matrix, again, calcium and collagen, but we also need specialized cells.
The specialized cells in bone are going to include four primary bone cells. They make up about two percent of your bone mass. So we're going to take a look at all four.
So here are the names. We have osteogenic cells. We have osteoblast. osteocytes, and osteoclasts. So let's look at them individually.
So starting with osteogenic cells, also known as osteoprogenitor cells, these are mesenchymal cells, and again we're throwing back to chapter 4, it seems like we're always doing that. But mesenchymal cells are embryonic cells. And you could look at them as stem cells that divide to produce osteoblasts, which we will talk about next. They are located in the inner layer of the periosteum.
So remember that the periosteum is covering the outside of the bone. And we'll look at a picture of this. And they're also found in the endosteum, which lines the marrow cavity. These guys are going to help assist in repairing fractures.
So here is the shaft or diaphysis of a bone and we started with the osteogenic cells. So we're going to find them in the periosteum, which would be out here, and we're also going to find them in the endosteum, which is the lining of the marrow cavity. So these again are stem cells that produce osteoblasts.
So what is an osteoblast? So an osteoblast is an immature cell that makes new bone matrix during osteogenesis. So osteogenesis is the creation of bone also known as ossification and these osteoblasts, once they are surrounded by matrix, become osteocytes which we will talk about next.
Okay, so here's what an osteoblast would look like. Again, it's an immature bone cell that secretes components of the matrix. Okay, so osteocytes, these are mature bone cells. They do not divide and they live in little houses called lacunae between layers of matrix. They have cytoplasmic extensions that pass through canaliculi.
So I am going to draw all of that out for you again shortly when we get to the osteon. There are two major functions for osteocytes. Their job is to maintain the protein and mineral content of the matrix.
So in other words, keep the matrix strong, which will help to repair damaged bone. So here's what an osteocyte would look like up close and you can see that we find it in dense or compact bone. So kind of in the middle in this figure. So here is the osteocyte and around it is matrix, which would be this tan area.
And you can see that it has these little tentacles coming off of it in the form of cytoplasmic extensions. and those cytoplasmic extensions move through little canals called canaliculi. Finally we have osteoclasts.
Osteoclasts, their job is to absorb and remove bone matrix. They are large and they have many nuclei. That's because they're going to create acid and protein digesting enzymes to dissolve bone to release the minerals from the bone out into areas like the bloodstream. This is called osteolysis when we dissolve bone which seems like it would be a bad thing but it is not always a bad thing. We'll talk about some reasons why later but this is called osteolysis when we dissolve bone and it's very important to homeostasis.
So here is a picture of an osteoclast and it is a cell that has lots of nuclei. You can see bunches of nuclei in there and again it's going to secrete acid and enzymes to help dissolve bone matrix. Okay so at this time we're going to take a look at the osteon.
The osteon is a unit of dense or compact bone. So let's draw that out now. Okay, so next we're going to go over the bone tissue, the tissue of dense or compact bone. And dense or compact bone is made up of osteons.
So osteon is a unit of dense or compact bone. We're going to draw one osteon here, but we should be comfortable with the fact that dense or compact bone is made of tons and tons and tons of these osteons. And we'll look at some pictures that will show where the osteons are kind of growing into each other to form the whole bone.
So if we could imagine that we were looking under a microscope, we would see these individual units. So let's start with one of the main center points, which is called the central canal. And the central canal is going to contain blood vessels. So I'm going to put a cross section of some blood vessels in there.
So we've got the red and the blue to represent the blood vessels inside of this canal, which is known as the central canal. So we'll go ahead and write that off to the side just so we don't forget central canal We will also be going over it or we have gone over it in our notes And we'll go over it a few more times just to make sure we're comfortable So the central canal is where the blood vessels are remember that blood vessels carry blood Which is going to be rich with oxygen and nutrients all the things that a healthy cell or tissue would want so around this central canal, this source of nutrients and oxygen, we're going to have some bone cells. And I'm going to make these bone cells just little round circles, just to keep it simple.
And these bone cells, there are a few types of bone cells, but these bone cells are called osteocytes. And osteocytes are mature bone cells. They do not divide so their job is to maintain the protein and mineral content of the tissue. So they're going to keep the bone strong but they're not going to actually reproduce anymore.
So this again is an osteocyte and we've represented them in blue. Now the osteocytes need a place to live. They need a home. So we're going to put them each in their own little house, and I know that sounds really cute, but it's legit.
So these little houses have a name. They're called lacunae for plural, and even though thinking of it as a little house is way cuter, the official definition of a lacunae or lacunae for many is a pocket. in the matrix, a pocket in the matrix. Now remember that matrix is what cells are suspended in, and we talked about in chapter 4 that connective tissue like bone, bone is a supporting connective tissue, that we're going to have specialized cells, which we do, these are osteocytes, and we are also going to have a matrix, which is what the cells are suspended in. And so the matrix would be the area here in between these cells and lacunae.
And the matrix of bone is going to be primarily made of calcium, salt, and collagen, which we touched on before we got to this point of the video. So calcium and collagen all in here. So that brings us back to the function of the osteocyte, which is to maintain the protein and mineral content of the bone matrix. So they live in these little houses, these little pockets in the matrix called lacunae. Now, this is just one.
So I'm writing it in a singular form, just one. And these lacunae need to be able to communicate with each other and also with the central canal. So we're going to draw some little lines and you'll notice in your book that they're not as exact as what I'm drawing here.
But again, we're just trying to keep it simple to get the point across. So these little passageways are, I like to think of them as little highways that lead to the neighboring osteocytes and then lead them all to the central canal. So this way they can actually get nutrients from the central canal and oxygen and then pass waste across as well. So this connects them to each other and also connects them to the central canal. This way they're not working as individuals, they are working as a team, which is what we want to see in tissue.
Now these little canals have an official name. They are the canaliculi. And the canaliculi, the official definition, is not little highways, even though I like to think of them like that.
The official definition is cytoplasmic extensions of the osteocytes. And that sounds a lot more complicated, but the osteocytes have extensions coming out of them, which are cytoplasmic of the cytoplasm that connect each other and also connect them to the central canal where they can receive oxygen and nutrients. Okay, so in addition to this, you will also notice, I hope, that there is kind of a pattern to this osteon.
And the pattern looks a bit like rings. So if you think about, like, if you cut a tree down and you think about how you look at the stump of the tree and you can see rings in the tree, well, you should be able to see a similar thing here. So I'm going to highlight that just to make it more visible.
And I've only got two rings in my osteon. I've kept my osteon really simple. But this ring pattern is kind of by design.
If you think about, again, osteocytes want to be close to where they can get nutrients and oxygen. So they build their homes around this central ring. And then bone expands by adding rings.
So these rings, these highlighted rings, also have names. Those are called the lamellae. So lamellae, again, that would be plural.
Lamella would be just one. And these are the layers of a matrix. That's how we would define those as the layers of matrix. And there are different kinds of lamellae, which we will make sure and mention as well.
Okay, so now that we have seen the basic drawing of an osteon, let's review the main components and then take a look at a more detailed picture. So an osteon is a functional unit of compact or dense bone, as we said. We looked at the central canal, which has blood vessels in it. And then we saw some perforating canals, or we are going to see. We didn't draw those, but we will see.
the perforating canals in another drawing, which are perpendicular to the surface of bone and carry blood vessels into the deep bone and the marrow because all of that needs nutrients and oxygen as well. We saw lamellae, which are layers of bone matrix. We drew the concentric lamellae, which are the ones that surround the central canal. What we did not draw were the interstitial and circumferential lamellae. So interstitial lamellae is going to fill the spaces between the osteons.
So as a kind of a filler between the adjacent osteons, we'll take a look at. And circumferential lamellae are at the outer and inner bone surfaces. So we're going to find those on the very outside of bone and close to the center of the bone. So here is an electron microscope view of what osteons actually look like. So this would be on the surface of dense or compact bone and we can see some of our major points that we drew out.
So this would be a central canal there and also here. You can definitely see the blood vessels in that central canal and then we can see these little pockets in the matrix. Those pockets are our lacunae, our little houses, and we can also see the very subtle rings.
These rings are the concentric lamellae. This whole unit here and here and here, those are osteons, which again, unit of compact or dense bone. So here's a figure from the text and it gives us a good view of everything.
So again, starting here we have the central canal with the blood vessels. Surrounding that in kind of a pinkish color would be the osteocytes in lacunae. The little tiny canals we can see every which way in brown.
Those would be the canaliculi, which contains cytoplasmic extensions of the osteocytes. We've got the rings, which are concentric lamellae. The whole thing, here, here, here, here, osteons.
And then notice in between. the osteons there is some filler bone so those would be interstitial lamellae like here and here and then going around the whole entire bone we have circumferential lamellae which goes around if you see the word there goes around the circumference of the bone spongy bone lacks osteons The matrix instead forms an open network of trabeculae. There are no capillaries or venules.
Red bone marrow fills the space between the trabeculae, forms blood cells, and contain blood vessels that supply nutrients to the osteocytes by diffusion. Yellow bone marrow is found in other sites of spongy bone and stores fat. So in this picture you can see this is the spongy bone, trabeculae. It does not have the osteon pattern.
So looking up really close we have the femur here that has been cut mid-sagittally and we can see the dense or compact bone on the outsides of the shaft or diaphysis. And then inside the bone here we can see very clearly spongy bone which has more of an open strut like appearance. This is the trabeculae.
So trabeculae up close as well. So weight bearing bones, trabeculae in the epiphysis of the femur will transfer force from the pelvis to the compact bone. the femoral shaft.
So again spongy and compact bone both have really important functions. Compact or dense bone is really good at handling the force of your weight and spongy bone is really good at handling forces from multiple directions. So together they make bone really really tough. Back to the periosteum. So the periosteum is a membrane, again, that covers the outside of bone except within a joint cavity.
It has an outer fibrous layer and an inner layer that we call the cellular layer. The fibers of the periosteum are interwoven with those of the tendons that are going to connect muscle to bone. The periosteum works to isolate the bone from surrounding tissues. gives a route for blood vessels and nerves.
It's also really helpful in bone growth and repair which we will look at both of those. So here is a close-up view of the endosteum, excuse me, the periosteum that's being pulled back and in this view you can see that there is an outer fibrous layer which is very protective and then the innermost layer is made up of cells which is called the cellular layer. The endosteum is an incomplete cellular layer that is in the medullary cavity or the marrow cavity.
It's very active during bone growth, repair, and remodeling. and will cover the trabeculae of spongy bone. It lines the central canals of compact bone and is made up of those cells we called osteoprogenitor or osteogenic cells. So here's a view of the endosteum taken from the marrow cavity. If we look up close you can see that it is made of cells.
We've got an osteoclast and we've also got osteogenic cells as well. So bones develop and there are two terms we use to talk about this. The first one would be ossification, which is going to be responsible for osteogenesis. That's bone formation. And calcification, which is the deposition of calcium salt.
And this occurs during ossification. There are two forms we need to go over, endochondral ossification and intramembranous ossification. So let's start with endochondral ossification.
Endochondral ossification is how most of our bones are going to form and it begins at the primary ossification center which will develop inside Hyaline cartilage model. Hyaline is a very common cartilage and in this type of ossification it will be gradually replaced by bone in seven main steps. So step one we have a cartilage model so of course all of this is happening this endochondral ossification is happening beginning in the womb.
So when you are developing and the cartilage model is in place and as it enlarges Cartilage cells which are called chondrocytes near the middle of the shaft will start to increase in size the matrix will be reduced to small struts that will begin to calcify the chondrocytes die and disintegrate leaving cavities within the cartilage. So this sounds like a good place to form a marrow cavity. Step two, blood vessels will grow near the edge of the cartilage and cells of the perichondrium, which is the outside covering of the cartilage model, will begin to convert to osteoblasts. The cartilage shaft becomes in sheathed or covered in a layer of superficial or surface bone.
You can see that on either side. Step three, blood vessels will then penetrate into the shaft of the cartilage. Fibroblast will differentiate into osteoblast and start producing spongy bone here in the core of the cartilage. We call this the primary ossification center because this is where bone production really picks up, really gets going. The bone formation will spread along the shaft towards both ends of the cartilage model.
Step four, we continue to remodel creating a marrow or medullary cavity which is very visible here. The osseous tissue of the diaphysis or shaft gets thicker and the cartilage near the epiphysis is replaced by shafts of bone. Further growth increases in length and diameter of this forming bone. Step 5. Capillaries and osteoblasts then migrate into the epiphysis, which are the ends of the bones, creating secondary ossification centers, which we can see here and also here. This is the second location.
Bone really picks up. Okay, step six. The epiphyses eventually become filled with spongy bone. The metathesis, which is here, we talked about that before, is now called the epiphyseal cartilage or growth plate and this separates the epiphysis from the diaphysis. Osteoblasts continue to invade the cartilage and replace it with bone and new cartilage is produced at the same rate on the epiphyseal side.
So we're going to keep this epiphyseal cartilage or growth plate because we'd like to grow after we're born, right? We don't want to stay the same length as when we're first born. At puberty the rate of epiphyseal cartilage production slows. and the rate of osteoblast activity accelerates.
So as a result, the epiphyseal cartilage gets narrower and narrower until it ultimately disappears. This is called epiphyseal closure and will be left with an epiphyseal line after growth is completed. So when we're born, this epiphyseal cartilage or growth plate, which is again made of cartilage, will remain so that we can continue to lengthen and grow our bones because these cartilage cells will convert into bone, lengthening the bones.
But once we go through puberty and growth is completed, that growth plate will seal shut and fully ossify, will be left with an epiphyseal line or a trace of where the growth plate used to be, but we will no longer grow in height at this point. So here is a summary view taken from the text of the seven steps that we just went over. Okay, so interstitial growth is growth in length. Secondary ossification centers develop. Epiphyseal closure.
Completion of epiphyseal growth. This is what we just talked about. The width of the epiphyseal cartilage reveals timing of endochondral ossification. Former location of epiphyseal cartilage is visible on the x-ray as an epiphyseal line, and that will stay after epiphyseal closure. So let's take a look at some x-rays to compare.
So this is an x-ray of a growing individual. and we can see for sure all the epiphyseal cartilage areas. So remember in an x-ray, bone shows up but cartilage appears dark.
So anywhere there is cartilage or soft tissue, it should appear much darker than the bone. So if we look here, we can see in the radius, there's a clear growth plate there. In the bones of the metacarpals, which is the palm, there is a clear growth plate.
plate there and also in all of these metacarpals growth plates. Growth plates in the fingers or phalanges very clear and this is an individual that is past the phase of growth and you can no longer see any growth plates but you can see a very faint shadow of where the growth plate was which is known as the epiphyseal lines. Okay so next is intramembranous ossification. That's our second type of ossification and it's also known as dermal ossification because it happens in the dermis. We're going to produce bones like the mandible which is your lower jaw and the clavicles which are your collarbones.
There are five main steps. to intramembranous ossification. So here are some areas that form from intramembranous ossification highlighted in the drawing.
So again we're talking about in utero or in embryonic development. This is going to start about the eighth week of embryonic development and what will happen is we'll have mesenchymal cells, remember those are embryonic cells, they will cluster together differentiate into osteoblasts which help to form the matrix of bone and they will start to secrete the organic compounds of the matrix. This osteoid which is formed becomes mineralized with calcium, salt, forming bone matrix.
So what you can see here in this picture are here the little mesenchymal cells in purple, those are the embryonic cells. They cluster together and begin to differentiate into osteoblasts which look like this. Then they begin secreting.
the mineral content of bone matrix which forms these pockets of bone matrix called osteoids. Ossification will continue and some of these osteoblasts will be trapped inside pockets where they then differentiate into osteocytes. The developing bone will grow outward from the ossification center in little tentacles that are called spicules.
So here are the osteocytes that are now trapped in bone. And osteocytes are considered mature bone cells, so these guys are going to hang out here and continue to make sure that the matrix of bone remains hard. We're sending out these little tentacles of bone. called spicules. Blood vessels will branch within the region and grow between the spicules.
Bone growth continues because now we've got oxygen and nutrients from the blood vessels. The spicules will interconnect and trap blood vessels inside of the bone. So we can see all this tan tissue here. This is bone tissue and it's spreading, spreading, spreading.
And the spicules are reaching out, trapping blood vessels. So there's a blood vessel that has been trapped and there's another blood vessel that has been trapped in the bone. So these trapped vessels, this is a good thing because these vessels are supplying oxygen and nutrients to further increase bone production.
The continued deposition of bone by these osteoblasts close to blood vessels results in spongy bone with blood vessels weaving throughout. So here's our bone all around and you can see we've now trapped lots of blood vessels. Subsequent remodeling around these blood vessels will produce osteons that we find in compact or dense bone.
an osteoblast on the bone surface become the periosteum. So again we can see all this bone. Here's our blood vessels that are trapped. We've got spongy bone inside and then close to the outside we have compact bone and a periosteum which is the covering of bone. So this looks like bone from a skull bone.
We've got the compact on the outsides. And inside we've got the spongy bone, the sandwich that we talked about. So bone remodeling is going to occur throughout your life, so we never stop remodeling our bone. We're always maintaining it.
And by recycling and renewing bone matrix, we keep up the maintenance in our bones that's needed. This remodeling is going to involve those osteocytes, the mature bone cells. osteoblasts that make new matrix and osteoclasts that dissolve matrix.
These activities are usually balanced. So if we're removing or dissolving bone with the osteoclast, we are usually building it just as quickly in the osteoblast to keep the amount of bone balanced. If bone removal begins to happen quicker than bone replacement, the bones will weaken. If depositing bone predominates, then bones will strengthen.
Exercise actually makes bones stronger. Mineral recycling allows the bones to adapt to stress and bones that are stressed heavily become thicker and stronger. So weight-bearing exercise will stimulate the osteoblast. Bone Degeneration.
Bone degenerates quickly. Up to one-third of your bone mass can be lost in a few weeks of being inactive. That's kind of scary. So nutrition and hormones can affect your bone.
Calcium and phosphorus are minerals that are required in your diet for strong bones. And also small amounts of magnesium. fluoride, iron, and manganese are helpful to the health of your bones.
Calcitriol and vitamin D3 are also very integral to bone strength. Calcitriol is made in your kidneys and is for normal calcium and phosphate ion absorption in the digestive tract. So we need it so that we can absorb calcium and phosphates normally in your digestive tract. We make calcitriol from vitamin D3. We talked about this when we were talking about skin.
So once we convert vitamin D3 or we synthesize calcitriol from vitamin D3, this helps our intestines absorb calcium better. Growth hormone and thyroxine will stimulate bone growth. Sex hormones like estrogen and testosterone will stimulate osteoblasts. Parathyroid hormone and calcitonin will maintain calcium ion homeostasis in your blood.
So let's look at that a little bit more closely in just a second. The skeleton is your body's calcium reserve. The bones store 99% of the body's calcium in the skeleton. In addition to other minerals, calcium is the most abundant mineral in our body. and is vital to lots of things in our physiology, so not just the strength of the bones.
So here's a breakdown of the main composition of bone. So bone is made up of 39% calcium, and then the next largest thing would be collagen, which we talked about, 33%, followed by phosphate at 17%. Alright, so going back to the hormones that we just referenced quickly, calcium ion concentrations in your body fluids must be closely regulated. Parathyroid hormone and calcitonin are going to affect the storage, absorption, and excretion of calcium in the bones, digestive tract, and your kidneys.
So let's start with parathyroid hormone, or PTH. Parathyroid hormone is produced by the parathyroid glands in the neck. So these are on the back of the thyroid and they are going to, or parathyroid hormone is going to increase your blood calcium by doing three things. Stimulating osteoclast activity, increasing the absorption of calcium in your intestines. and decreasing calcium excretion by the kidneys.
So let's take a closer look at that in this figure from the book. So if your calcium levels in the blood are low, because we need healthy levels of calcium in the blood, because as we said, calcium is not just for bones. We need it in other areas.
So we need our levels in the blood to be normal. And if they are not, if they're low, then parathyroid hormone will be released and it will help in three ways. It will cause osteoclasts to release calcium from the bone.
So osteoclasts are going to release enzymes to dissolve bone and when we dissolve that's going to release calcium into the blood which makes the blood level go up. That's good. PTH is also going to help us to absorb more calcium from our diet. Blood levels go up.
That's good. And finally, kidneys are going to absorb more calcium ions. Calcium is conserved, so there's going to be less calcium going out in the urine and more of it reabsorbed.
blood levels go up so that's good as well so those three things that happened are going to heighten blood calcium levels when they are low now the opposite is true calcitonin is secreted by c cells in the thyroid and it's going to decrease blood calcium by doing three things inhibiting osteoclast activity, increasing calcium excretion, and reducing calcitriol secretion by the kidneys. decreasing intestinal absorption of calcium. So let's take a look at that in a similar figure.
So if our calcium ion levels in the blood are too high, then the thyroid gland will release calcitonin. Calcitonin tones down blood calcium in three ways. One, it's going to stop those osteoclasts from dissolving as much.
So calcium will not be released into the blood as readily. Calcium levels will go down. Calcitonin will also help to reduce the amount of calcium absorbed by the small intestine.
Calcium levels in the blood will go down. Kidneys will excrete calcium ions, so more calcium will leave in the urine instead of getting absorbed into the body. calcium levels will go down. Okay, so finally in the chapter we are going to end with fractures. Fractures are cracks or breaks in the bone due to physical stress.
They can be open, compound, or closed. Simple. Fractures are repaired in four main steps.
First we form a fracture hematoma, then a callus, spongy bone formation, followed by compact bone formation. So the fracture hematoma is first. This is a large blood clot that establishes a network of fibers.
Bone cells in the fractured area will die and then we will begin to form a callus. In the callus, cells of the endosteum and the periosteum will divide and migrate into the area where the fracture is, the fracture zone. Calluses will stabilize the break as we heal. There will be two kinds.
We have an internal callus and an external callus. The internal callus develops in the medullary cavity or the marrow cavity, and the external callus will develop in the medullary cavity. on the outside of the bone.
So this is the will first be cartilage and then will convert into bone. So let's look at that in a couple of pictures. So here we have a broken tibia, which is a bone of your shin. You can see the fracture there and the bone is bleeding. So the first thing we're going to do is form a fracture hematoma.
So that again is a large blood clot. You can see some broken bone fragments. This is dead bone. And we're next forming an internal and external callus. So the spongy bone is reforming in the internal callus.
And the external callus is a bridge of cartilage, which will later convert into bone. There's our periosteum, which is the outer fibrous covering of the bone. Spongy bone formation.
So osteoblasts will replace the central cartilage of the external callus with spongy bone. And the repaired bone may be slightly thicker and stronger than normal. So the compact bone, the repaired bone, can be thicker and stronger than normal.
So here's the internal callus where we have reunited the spongy bone and the external callus which is spongy bone and compact bone so there may be a swelling a thicker area in the compact bone may be thicker and stronger but over time if the fracture was not too bad, osteoclasts can dissolve some of this bump and make it look a little bit more smooth and remodeled. So bones become thinner and weaker with age. Osteopenia is a condition where there is inadequate ossification or reduction of bone mass.
It begins between the ages of 30 and 40. Women lose about 8% of the bone mass per decade. Men lose 3%. The epiphyses, vertebrae, and jaws are most affected.
This can result in fragile limbs, reduction in your height, and tooth loss. Osteoporosis is severe loss of bone mass, which can compromise the bone's ability to function normally. Over the age of 45, it occurs in 29% of women and 18% of men. Hormones can be involved. Sex hormones can help maintain bone mass.
And in women... Osteoporosis accelerates after menopause. This is when hormones can decline and get a little bit crazy for a while before they level out and this can have an effect on your bone mass. So at the top we see a view of normal spongy bone nice and healthy and firm and in the bottom this is spongy bone in osteoporosis, which you can see is extremely porous looking.
very prone to fracture. This concludes chapter 6, osseous tissue. We'll begin next in chapter 7, which is going to go over the axial skeleton.