so we've been talking about how bone tissue is formed by two mechanisms by endochondral ossification where we start with hyaline cartilage and replace the hyaline cartilage after it dies with bone tissue and intramembranous ossification where we start with a sheet of fibrous connective tissue called mesenchyme and the fibroblasts or the mesenchymal cells they're transformed into osteoblasts and begin to secrete matrix and this ultimately becomes spongy bone this is happening in the fetus but what about bone growth in children and adolescents because certainly their bones continue to grow so this is what we want to look at here in bone growth and remodeling so from the learning objectives we're still going to be referencing intramembranous and endochondral ossification we're going to describe how bones grow in links and how they grow in thickness and we're going to compare and contrast bone growth and bone remodeling and saladin the important pages are pages 211 - 215 alright so this is the picture that we left off with when we were talking about endochondral ossification so at this point our cartilage or hyaline cartilage template has been almost completely replaced by bone we've formed a shaft we've got a nutrient artery that's providing a blood supply to the bony tissue and then we had our secondary ossification Center here in the epiphysis we've replaced most of the cartilage in the epiphysis here with spongy bone tissue here but we do have a thin layer of articular cartilage on the most superficial surface and then we have the epiphyseal plate which is a line of cartilage that separates the shaft of the bone where the diaphysis and metaphysis from the epiphysis here in both cases in children and really in adolescence the cartilage is actually viable so we have living chondrocytes that are present in the cartilage here that can continue to grow now I've got a another picture here we go this is a less detailed picture so this is still the diaphysis of the bone here and the epiphysis is here and I'm trying to show you the bony tissue that's right here and there's actually spongy bone tissue that's right here and all of this is actually cartilage so the articular cartilage would be here and epiphyseal cartilage would be here and everywhere there's cartilage we can have the chondrocytes grow and where the chondrocytes grow in and the chondral ossification we can have that cartilage tissue replaced with bony tissue and so this is actually what we see when bones grow in links is that the cartilage proliferate here at the epiphyseal plate and underneath the articular surface here so as we grow more cartilage here the bone will actually follow and our bone overall grows and whoops grows in length here I have two images this is I'm gonna call this old knee and I actually got this image from a place knee clinic and this is a site that's dedicated to knee replacements and so this is actually an x-ray of an osteoarthritic knee now just to orient you a little bit this is femur up here and this is tibia unmistakeably timi tibia and this is going to be the joint cavity or the space in between the two bones where they come together the reason they are showing this particular x-ray is they want to show you the degradation of the joint cavity and how close together these two bones come when you have a degradation or a loss of this joint cavity you lose the lubrication there when you've got bone on bone contact that can be very painful and that's when you've got to have your knee replaced right so this is clearly an older adult over here we've got this is femur and this is tibia again but look here this is the epiphyseal plate right here and the epiphyseal plate right here so we've got cartilage here instead of having complete bone tissue like we have over here and so wherever we have cartilage this means we can still have bone growth you may be able to see it looks like this might be the remnant of the epiphyseal plate but eventually this cartilage will be replaced with bone and when that happens we say that the epiphyseal plate has closed and when the epiphyseal plate has closed you can have no more linear growth in bones or your bones can't get any longer and that would mean when your long bones have reached their maximum length you have reached your maximum height now this is a close-up image that I like a lot of the cartilage at the epiphyseal plate let me orient you to this so the epiphyseal side is here and the diaphysis side is here so you have to imagine that this bone is actually upside down from the bones that we had seen in the previous slide so this would be the shaft of the bone here and then the epiphyseal plate is going to be here now what I want to show you here are the different appearance or the different zones I guess of the cartilage that we find at the epiphyseal plate so if we're looking here on the epiphyseal side this looks very much like hyaline cartilage so you can see it's kind of clear and glassy you can see the chondrocytes dispersed throughout that here as we get closer as we move a little bit closer to the shaft what's happening then is that the chondrocytes are proliferating in fact the chondrocytes are reproducing and dividing so quickly that they don't have enough time to secrete matrix or really create much more cartilage so they end up getting showing this very distinct appearance they look like pennies and a wrapper here we've got just stacks and stacks of these converse that are here and this is where the cartilage is rapidly proliferating now after the cartilage after the chondrocytes have proliferated then remember the hypertrophy which means that they get big and they begin to die and as the chondrocytes die the leftover matrix becomes calcified on that calcified matrix this is where we have osteoblasts that will come in they dissolve the calcified cartilage and then they begin to secrete matrix as well and so we have bony tissue that's being formed here and the bone then is going to be growing in this direction because this is the epiphysis or the epiphyseal side all right now what about grown grouse bone growth in width what we see in this situation is that right underneath the periosteum remember there is a layer of osteoblasts that are associated with that as the bone is responding to some kind of stimulus the osteoblasts become active and they begin to secrete more matrix as they secrete more and more matrix eventually it could actually surround a blood vessel and as they secrete more and more matrix we would actually form another osteon now why would bones increase in width obviously bones would increase in length if you had someone who is growing taller in width this might be a bone that is developing more density because it is being exposed to greater forces so in this case your bone would becoming stronger or thicker or bigger in order to resist the forces that muscles are placing on it when the muscles pull on it and this actually happens you know all the time so your bones are responding to we think it's electrical signals that are generated when the muscles are used to when the muscles pull on the bones and that keeps everything sort of healthy and and make sure that your bones are strong enough to stand up to the stresses that you are experiencing now bone remodeling is this sort of constant turnover of bone tissue so just like you would remodel your kitchen say to get rid of appliances that didn't work or things that were just not very functional anymore bone remodels for the same reasons so we see that the bone tissue will old bone tissue will be broken down by osteoclasts and then replaced by new bone tissue and this can happen in response to hormonal changes this can happen in response to mechanical changes or mechanical stresses as we talked about when bones increase in wood and bone turnover is constant constant so your bones are replacing the osteoblasts and osteoclasts are replacing this tissue very you know frequently to be sure that you've got healthy functional bone tissue now I like this picture a lot I got this from an article that I found out in ResearchGate actually and I liked it because I think it's very descriptive it kind of shows you what's happening so here is our quiescent bone tissue and these are osteoblasts I'm sorry our osteocytes that are distributed in the matrix and these are our osteoblasts here okay we have some stimulus that causes an activation and this in turn causes osteoclast precursors to be activated they go to the site they become osteoclasts osteoclasts remember are those big roughly bordered cells with many nuclei and what they do is literally they secrete acid that dissolves the bone tissue here so these osteoclasts have dissolved this bone tissue and they are going to be followed right behind by new osteoblasts that are going to be laying down a new matrix and eventually all of the tissue that had been dissolved by the osteoclast will be replaced by the osteoblasts so in this particular image then we see that we've got some old tissue and some new tissue and the new tissue results from the coordinated action of osteoclasts and osteoblasts and so this is important as I said in keeping bones healthy now I'll give you a little preview of what's coming up right if osteoclasts break down bone tissue and osteoblasts rebuild bone tissue if we have an imbalance between these two things we can have either too much bone breakdown and that can lead to osteoporosis or we can have an inappropriate buildup of bone and that can lead to a disease may be called Paget's disease all right now an example of why you would have bone remodeling can be seen here so this is a weightlifter he I believe if I understand correctly he is an Iraqi soldier this was taken a while ago 2004 and he is working out in a gym I don't know how many pounds he is lifting but it looks like a lot so if we were to actually look you know obviously his muscles have become quite big right he looks pretty strong here and so because his muscles have become big they are placing more stresses on the bones we would also expect that his bones would also become much bigger if we could look at them so one reason that bones remodel is again because of the mechanical stresses that are placed on them so we actually would see in this case that his bones would get bigger but in a person who had been exposed to fewer forces saying astronauts who had been away from gravity we actually see that their bones become thinner because they're not being exposed to those stresses okay whoops all right and so I've got another question for you in the discussion board how could x-rays of the femur be used to determine if someone has reached their full height so think about that