in this video we're going to be covering bone growth and bone remodeling and we're going to start off with the long bone when we talk about a long bone we're thinking about the way that a long bone is able to grow in length this is called endochondral ossification the word ossification means bone formation when we have a growth in length what happens is that cartilage continually grows and is replaced by bone as shown so we start off with this hyaline cartilage structure which will eventually become this hypothetical bone inside of the body which looks somewhat like a humerus but it's certainly not a humerus so you're basically just showing some hypothetical non-bone that's supposedly found in your body so steps one through five step one the basic idea is that osteoblasts begin to secrete osteoid around the diaphysis producing a bone collar step two the cartilage starts to degrade because of calcification of the bone at step three you start to actually see that primary ossification center directly in the center of the diaphysis or what will eventually become diaphysis and the osteoclasts are degrading this calcified region this is the first time that you actually start to see something that resembles spongy bone and not just um not just that cartilage matrix step number four the osteoclasts form a medullary cavity very clearly shown here you start to see blood vessels entering into here and at this point um i would say at stage number four you're starting to see the secondary acidification centers which you will notice is in the direction middle of what will eventually become your epiphyses and in the step five you basically have the uh the uh a fairly well-formed bone you'll still see areas of hyaline cartilage present throughout childhood and throughout the the end of adolescence hyaline cartilage will remain inside of what we call the growth plate or the epiphyseal plate and that will remain there until an adulthood is reached until the long bone can stops growing this is also called the growth plate so this is where your growth is actually going to be occurring at the end of adolescence then this is going to harden into bone it's going to ossify eventually at about age 20 or so but the very last of the hyaline cartilage will remain throughout your lifetime hopefully this is called articular cartilage and it's a particular type of hyaline cartilage that is on the very bone ends of your epiphyses as the name suggests articular is at a place where one bone will articulate with another bone or touch another bone so this is an example of endochondral ossification and we should say that there's one other type of ossification and that is called intramembranous ossification intra membranous ossification consists of four steps that are very similar to what happens here with the intramembranous ossification this is basically the bones of the skull and instead of starting with hyaline cartilage you start with um you begin with a fibrous connective tissue membrane instead so you'll see the acidification in the center and you start to see the acidification of these fibrous connective tissues in the center and then as you can see you have osteoblasts that begin to grow outward and continue to secrete osteoid you can see that there's an osteocyte being buried inside of the osteoid as the osteoblasts grow outward from that ossification center in the in the center of the bone it basically begins to grow from the inside out eventually you have what is considered to be or appears to be spongy bone and you have the vascularization of this bone and eventually you will have spongy bone in the center and then you will have compact bone around the outside and you can see that osteoblasts are present at every step of the way here lastly let us cover the concept of remodeling remodeling is the idea that yes bones begin bones continue to grow in length throughout your childhood but they stop growing in length after a certain point after that point it's still possible for bones to grow appositionally or grow horizontally rather than lengthwise they can widen or narrow whatever is most appropriate so in this i don't know if it's very easy to tell exactly what's happening here this is bone remodeling here where we have the ability to reshape bones depending on what is what is needed at the time so a good example of bone remodeling is wolf's law w-o-l-f-f apostrophe s wolff's law is that a bone states that a bone grows in response to the demands placed on it so for example long bones are going to be thickest midway along the shaft because that is where the bending stress is the greatest curved bones are thickest where they are most likely to buckle wolf's law has some very interesting implications for how your bones continue to be structured or how they start to lose bone density so try to think in your mind of the implications of bone remodeling the implications of wolf's law and think of how bones can be reshaped either by activities or by lack of activities and we'll talk about examples in class