Hello students. We're going to be going over the how bones grow. So the bone growth in appositional growth and longitudinal growth. When we talk about longitudinal growth, remember this is occurring at the epiphosis. So the ends of the bone. So between the metaphysis and the epiphosis. And um it's occurring on the side closest to the diaphosis. So the oification piece is closest to the diaphosis and the reserve cartilage whenever we're looking at at images of these where the cartilage is is at rest that is going to be closest to the epiphosis. Okay? Just to orient yourself on some of these pictures because sometimes it feels like it's upside down. But always make sure that you're you're picturing where the where we are at on the bone. So remember the epiphysial plate is that cartilage zone between the epiphosis and the diaphosis or metaphysis to be more correct. And there's five different zones during in in this space. The zone of reserve cartilage closest to the epiphosis. Okay. So furthest away from the diaphosis is um not involved in growth. This is where we're just cartilage hanging out condraittes just sitting in their lacuna watching TV doing their own thing. Um they're not doing anything. Okay. Eventually they may be dividing but then they'll become the zone of the next zone which would be the zone of proliferation. The next zone down or closer to the diaphosis is the zone of proliferation. This is the zone that is dividing. So we're making more conondrictes. So more condraittes. So we're more cells. Okay. And more cells will then grow. So this is the zone of hypertrophy. So those cells grow. And as they grow, they get bigger. And so they push the epiphosis away from the diaphosis as they grow because you made a whole bunch more cells. So now they're bigger and they're pushing that diaph um epiphosis away from the diaphosis. So this is that zone of hypertrophy. So these are your mature condondraites, but there's many more of them because we divided. And then the next zone that's closer to the um diaphosis or these are the cells that already grew and they had they had matured and now they're calcifying. Okay, so they're closest to the zone of oification. So they're the they're they're next basically next on the list. So these are the ones that may be calcifying or may have already died in their space. And then the last zone is the zone of ocification. So this is the zone that is currently under the process of remodeling. We're taking cartilage and the dead condraittes are gone. We have open spaces and now osteoblasts are now laying down bone in those spaces, taking over that space and laying down matrix and surrounding themselves and becoming osteoccytes. And so this is our actually zone of of bone deposition. Okay. So if you see here, so picture this picture orientation. So this is upside down. This is closest to um the the medularary cavity. This is closest to the epiphosis. So this is they're not the these cells down here are doing nothing. These cells are dividing. So we're getting more of them. These cells are getting bigger. They're maturing. and they're pushing this lower um or the down here the epiphosis away from the diaphosis. So they're lengthening the bone and then the the cells up here are calcifying and some may actually be dying. Okay. And then we've got this is our cell this is our zone of oification. So now we're replacing those dead condraittes with actual osteoblasts coming in and laying down bone around them. They lay down the organic matrix. It becomes mineralized and they become they differentiate to osteoccytes and maintain that matrix around them. Okay. So each zone is involved in this growth remember except that reserve zone the one closest. So condraittes have to divide first then they grow and as they grow they push away the epiphosis. Okay. And then and that makes the space larger and then they become calcified and then replaced by bone. Okay. Um longitudinal growth continues until about um it well cell division slows at about 12 to 15 years can continue up to depending on the growth plate up to 21. Um the epiphysal plate will shrink so that the width of that plate will shrink and shrink. So the zone of calcification will be catching up to that zone of division the proliferation. So um at some point when the calc when when the when the oification and calcification catch up to the por proliferation and the the dividing cells become calcified, we're done. We're over. they're going to become um bony. We take over the whole growth plate and we no longer can divide. We're done. That's the end of the growth. And so anything that speeds up that rate is going to affect the closure of those growth plates. That is very important to understand. Okay. So the epiphysal plates shrink the older the older we get. So about 18 to 21, 18 to 21 is when the growth plates close. It depends on it depends on the sex. It depends on the person. Um and again this is for longitudinal growth and that is when that those cells become calcified. So it's when that zone of proliferation gets caught up by the calcification. Once they become calcified, they can't divide anymore. Okay? And so these are just some different growth plates that you can see on this particular um long bone which is the fe femur. Um oppositional growth oppositional growth is growing to um in width. So growing to to the side and so we cannot just grow in length if we don't grow in width as well. Can you imagine a tree that grew only in height and never grew its trunk in width? It would just fall over and break, right? because of the weight of the tree and the height. So, same thing with our bones. They have to they have to grow in in width. And so what happens is the perryioium osteoblasts lay under the perryioium and they will lay down bone underneath the perryioium and we add to the width. It's like slapping on boards on the outside, right? We add to the width, but osteoclast on the inside will degrade the inside. hollow out the inside and and create that medularary cavity to prevent the weight of the bones being too much and they create that space. And when we add this bone on the outside, we add circumferential lamea because remember the compact bone is all on the outside. Okay? So we add circumferential lamea all the way around and as we keep adding and we keep adding and these the vessels that come in and a vessel will start to divide and go up and then we'll surround that vessel. So the further away we get from that vessel that was coming in if you have a vessel that's extending up then you can start laying bone around that and you can remodel that into an oion. So first we add the circumferential lamea. We remodel the old oons and then lay down new oons. And again this is dependent on different hormones um and growth factors and nutrition and um forces. Like if you're not using your bones, you're not going to lay down as much bone. This is so important. Children and kids and humans that are growing need to be doing weight bearing exercise because this is the time that they lay down the bone. And if they're not doing that, they're going to lay down less bone. They will have less bone density as adults. And that is going to set them up for less bone density when they get into their senior years and they start increasing their bone resorption, which is the bone breakdown. They're not going to have as much. they're not going to have as much to survive on or to support them when they start that bone breakdown. So, it's very very important that we we stimulate bone deposition as much as possible. So, which hormones do this? We're going to talk about hormones in later chapters. You have had some exposure to hormones already in Bio5 and anatomy. If you can't remember what these hormones do, take a quick peek at your hormones. go back to those chapters and and take a look at them. Growth hormone remember is from the anterior pituitary and this stimulates cell division in all tissue. It stimulates protein synthesis. Um so this is going to be very important in stimulating growth and stimulating longitudinal and appositional growth. So it increases the the zone of proliferation. So we're going to increase division of the condraittes, it increases the activity of the osteoggenic cells. So getting them to um lay down bone, right? To differentiate into osteoblast and and um and lay down bone in the in the actual medularary cavity and that spongy bone. And then of course the osteoblast under the perryioium for appositional growth to lay down bone appositionally to widen the bone. So we want to longitudinally and appositionally grow the bone. Testosterone also important for appositional and longitudinal growth. And testosterone has a has a has a stronger effect on this than than estrogen when it comes to the density of bones. And so men have have thicker bones than women. They have stronger bones and this is why they're more resistant to um getting osteoporosis than women because of the effects of testosterone. So you get this increase in appositional growth. You get thicker bones. It increases mitosis at the epiphysical plate. But it also increases the rate of closure. So this means that that the rate that the calcification catches up to the zone of proliferation speeds up and so that's going to promote that that closure of that growth plate. So the more testosterone we get, we're going to close that growth plate. But interestingly enough, estrogen closes it sooner. And so um women typically do not grow as tall as men in general because that estrogen will close it sooner. Now there are genetics obviously based on that but um as a whole estrogen will close the growth plate sooner and faster than testosterone. Estrogen has protective effects for women in that it inhibits osteoclass. So it's it inhibits that bone resorption and so that gives an added benefit in the if you're you know talking about bone resorption and bone um deposition and they're supposed to be equal. If you're inhibiting the resorption, this helps you deposit bone and so it's very protective. But when we lose estrogen during menopause, this is this is detrimental because we now lose that protective effect. And so we now actually have osteoclasts doing their thing and we have increased bone breakdown during menopause and this can cause excessive bone loss and can lead to osteoporosis in women. Um ex what happens if you have excessive growth hormone? So if I have excessive growth hormone before puberty, before the growth plates close, I can actually um this can result in gigantism. Typically this is from a pituitary tumor. So um excessive growth hormone secretion, not usually too much hormone administration because it's tightly regulated regulated, but this is from a tumor. And so this will result in gigantism. So you will have a very large person because the growth plates haven't closed yet. So you will get excessive growth during um if it if it occurs before growth plates have closed. If you if excessive growth hormone occurs after growth plates have closed. So after the end of puberty, we get something called acromegaly. So you will still have an increase in appositional growth in some bones, but you don't necessarily get the um the longitudinal you're not going to get the longitudinal growth because the growth plates already closed. So you're not going to get the height, but you can get the the thickening of the bones. And you actually get thickening of specific bones and enlargement of specific tissues like the tongue and the heart and um um certain bones of the face. um you'll get thickened like the jaw and the skull and the hands and the feet. And this also growth hormone we'll talk about later affects glucose regulation. So this can set a person up for type 2 diabetes. And so it is it's not something to play with and not something you should be taking without um strict supervision because excessive amounts of growth hormone can be detrimental. What if um let's go back um real quick. What if we didn't have growth hormone during development? What if we didn't have enough um before before growth plates close? that would result in dorphism. Yes, we would not be able to grow properly, right? So, we wouldn't have the the stimulus that we needed to grow properly and that's very important. So, you do need these hormones for actual normal growth. So, our next lecture is going to be on bone remodeling and repair. So, this is going to be all about the processes after puberty. So what are we doing on a regular basis to um remodel and repair? And then what happens when we have trauma like a fracture? Um how do we repair that? But the remodel and repair also includes like everyday little micro fracture stuff when we're just walking around what's going on as far as bone deposition and um bone resorption. And this will include our calcium regulation. So stay tuned for that. I'll see you there.