We're going to talk about bone growth and remodeling. In WileyPlus, there's a great animation and interaction on bone growth and remodeling. I strongly suggest using it to help you understand how bone works, how you grow, both in length and thickness, how it remodels when damaged or just when rebuilding, and you can interact and assess your knowledge as you go through. So, bone growth and length. is going to be called interstitial growth.
Interstitial growth just basically means within cartilage and growth in length is going to occur in one area. The area it occurs in is going to be within the epiphyseal plate and the epiphyseal plate is located within the metathesis region. So how many epiphyseal plates are you going to have within your bones?
If you said two, you're correct. You're going to have a proximal and a distal one. What we can see right now is we're looking at a femur and a tibia.
Here on the femur, this is your distal epithelial plate, but here on the tibia, that's your proximal epithelial plate. The epithelial plates are made out of what type of cartilage? If you said hyaline, you're correct. When we look at an x-ray, we're not going to actually see the cartilage.
You can see that on this x-ray right here, we can tell that that is this gray area. That's where the cartilage is. And then all the white is the bone. Now, your epiphyseal plates are going to be made out of four different zones. So your first zone is going to be called the zone of resting cartilage.
And the zone of resting cartilage is on the epiphyseal side. So the zone of resting cartilage will attach to the epiphysis. And the name is a little bit misleading because when you think of the word resting, you don't think of much action going on.
But the zone of resting cartilage is when the cells are actually preparing for work later on. So in this zone, the cells are inactive in the sense of division. But as they move into the next zone, the zone of proliferating cartilage, the word proliferation means to divide. So the zone of proliferating cartilage is when the chondrocytes will start to divide rapidly.
As the cells divide, they're going to move into the next zone, which is called the zone of hypertrophic. Hyper means a lot. Trophy means growth.
So in this zone, what we see is that the cells... they start to get larger in size. The hypertrophic zone, the cells will lose the ability to divide, but they do increase in their size.
And this leads us to the last zone, the zone of calcified cartilage. You can tell what's happening in this zone based off of the name. The chondrocytes are dying. And when they die, osteoblasts are going to move in and lay down new bones.
So the cartilage is actually becoming hardened. This zone is always going to be next to the diaphysis. So when we talk about these four zones and their actions, we have to remember how many epiphyseal plates do we have?
We do, we have two, you're right. And so when we talk about these two epiphyseal plates, What we're going to see is that everything's happening twice. So if I'm drawing a long bone, we have four zones. The zone closest to the epiphysis is going to be our resting zone. So notice that the resting zones touch the epiphysis.
The next zone is going to be the proliferating zone. This is where the chondrocytes divide rapidly. The next zone is going to be the hypertrophic zone.
This is the zone where the chondrocytes lose the ability to divide, but they do get larger in size. And then our last zone is the calcified zone. This is where the chondrocytes die and osteoblasts begin to lay down new bone, so it's hardening. This calcified zone is always going to be touching the diathesis.
So what we have is we have new bone being laid down at both ends of the diathesis, so when bone grows in length, it grows. both directions pushing away from the diaphysis size. So when we look at this, what we have to imagine is if chondrocytes are dying, that means we need to have chondrocytes being produced to keep up basically supply and demand homeostasis. So as we see, Right here in the zone of proliferating cartilage, new chondrocytes are formed at about the same rate that chondrocytes die in the calcified cartilage.
So homeostasis has to be maintained. Now for this to occur, there's lots of factors and some of the main factors are going to be hormones. So think about some of your biggest growth spurts.
When do some of your biggest growth spurts occur? If you said puberty, you're right. And what happens at puberty?
What do you start producing higher amounts of? You said sex hormones, you're right. So at puberty, we have one of our first major growth spurts, and that's because estrogen and testosterone help with bone growth. Estrogen is going to promote osteoblast activity.
And it helps slow down osteoclast activity. So we're promoting bone building while inhibiting bones being broken down. Other things that affect around puberty, you're going to have an increase in human growth hormone.
Human growth hormone is released from the pituitary gland in your brain. And human growth hormone is going to communicate with your liver. and your liver will release a group of hormones called insulin-like growth factors igfs insulin like growth factors the igfs will actually target the proliferating zone when they target the proliferating zone what i mean what cell do these hormones say to divide rapidly so the igfs are actually targeting chondrocytes to divide.
Now IGFs also will affect osteoblasts and they're going to tell osteoblasts to lay down bone faster. So there's a lot of different factors that go into it. It's not a simple answer but this is a very brief explanation of how your bone growth can be affected in length by hormones.
So estrogen affects osteoblast activity, increases it while inhibiting osteoclasts. IGFs are going to be told to be released once HGH targets the liver. HGH comes from the pituitary gland. It's released into the blood, so it's a hormone. When it gets to the liver, your liver recognizes the signal.
It releases IGFs, insulin-like growth factors. IGFs are then going to target the proliferating zone, so the chondrocytes, and they're going to start to divide rapidly, and IGFs will also target. the osteoblast and tell them to lay down bone quickly so that affects bone growth and length now as this occurs we're going to continue to grow up until again 18 to 21 that's just kind of an average and at that point all of these zones have actually calcified when these zones have all become calcified we no longer call them the epiphyseal plate we now call them the epiphyseal line Again, epiphyseal lines occur at different ages for every individual, but the average is 18 to 21. Now, this only talks about growth in length.
We also grow in width, and this is called appositional growth. Now, appositional growth is going to occur beneath the periosteum. And this is going to occur throughout life.
So interstitial growth is only occurring within the epiphyseal plate until you're around 18 to 21. Appositional growth is going to be beneath the periosteum, and it occurs throughout life. It's basically a four-step process. The first step is we have our periosteum. And we can see that our periosteum has two main layers. The first layer is the fibrous outer layer.
The second layer is that osteogenic layer, the one that has the cells, the mesenchymal cells, that basically turn into osteoblasts, and that's important. So the periosteum is going to form what we call periosteal ridges, and you'll notice that the ridges that are forming are forming around a blood vessel. This is really important. because we have to remember that bone is living so we need lots of nutrients. Well those ridges are basically forming, I like to call them mountain peaks, and the blood vessel is going to be in the valley.
As those ridges form they're going to end up forming around the blood vessel and they're going to fuse together. So we basically get what I call the tunnel. Now when we look at that, the periosteum is still here on the outside. But now that this periosteum has enclosed in a circle, we now call it the endosteum.
Now can anyone kind of think what shape they're starting to see now? Is anyone starting to see the shape of an osteon? So...
After we have our ridges fused together and we form that tunnel, we're going to start to see layers of hydroxyapatite, basically the matrix, be laid down. And they're going to be laid down in a circular pattern. So we have to remember that right here, the endosteum, which is the periosteum, has this layer of osteogenic cells.
Those osteogenic cells are going to lay down new bone. As they lay down new bone, they're going to create layers towards the inside. These layers are going to be the lamella, and they're going to close in on that blood vessel. As they close in on the blood vessel, that tunnel is now going to be called the central canal. And we can see that we now have multiple lamella, one, two, three, that forms a new osteon.
As you can see, what's happening on the edge is we have two new periosteal ridges that have fused around this blood vessel, forming a new tunnel. and the whole process starts again so we can get a new osteon. Now we have to remember osteons are only formed in what type of bone? Compact.
So when we grow in width it's always occurring beneath the periosteum and it's always going to be forming osteons to form compact bone. So what I want to do is go show you a quick video. that summarizes all of this in a really fast manner.
So that just helps us understand the four steps of appositional growth. First step, ridges from the periosteum begin to form. Those ridges touch and fuse together. When they fuse together, we now have a tunnel, and the tunnel is lined by the old periosteum, which we now call the endosteum. The osteogenic cells in the endosteum now begin to lay down new rings of salt, the lamella.
As the lamella close in on that blood vessel in the tunnel, we're going to form our new osteon. So we can tell that bone is laid down right beneath the periosteum on the outside of older bone or older cartilage.