so in the previous short video we talked about the features of bones we talked about the micro anatomy of bone tissue and we talked about the differences between spongy bone and compact bone and we talked about bone cells here I want to talk about how bone is formed by one of two mechanisms and the mechanism that I want to talk about here is called intra membranous ossification and the name tells you something about how the bone is being formed so intra means from the inside and membrane like from a membrane means from a membrane so we start with a flat sheet of tissue and from this flat sheet of tissue we make bone tissue right so the learning objectives then are going to be describing intramembranous and endochondral ossification processes at some point the pages and saladin that are important are pages 208 to 209 and I want to point out here that different authors will have slightly different perspective I don't want you to get hung up in the steps I want you to be thinking about this as an overall process so that if you had to describe this and a couple of sentences you could do that all right different authors are kind of dividing it up into different steps because it makes sense when you're when you're trying to explain it to anybody but you can sort of tell it you know on a on a continuum alright so what this author is showing you here and this figure is over here he's showing you the bones that are formed from this type of ossification intramembranous ossification and essentially this is going to be the flat bones of the skull parts of mandible and also parts of clavicle as well now here we're actually going to start here and the following slides that I have will show each of these steps individually but what I want you to see is that we start with an ossification Center in a flat sheet of tissue and we end up with a flat bone like in the skull alright so the first thing that happens is that we have this embryological tissue that's called mesenchyme it's kind of flat it's kind of like connective tissue and what we see is that the cells the mesenchymal cells actually start to aggregate in this flat sheet of connective tissue and as they aggregate these cells they form they transform into osteoblasts now remember that osteoblasts are the cells that build bone these are the bone cells that secrete matrix so what these cells start to do after they aggregate is they start to secrete matrix and notice where they are right they're kind of in this flat sheet of tissue so they secrete the the matrix around themselves here kind of in between these blood vessels now as we go forward right these osteoblasts they secrete matrix and the matrix becomes calcified so the hydroxyapatite or that calcium hydroxide calcium phosphate salt complex deposits here and makes the matrix hard because the cells these osteoblasts are now kind of trapped in this matrix which is now hard they transform again they become osteocytes and the places where they are living then are going to become lacunae additionally we have other osteoblasts that we see here kind of around the perimeter of this ossification center these osteoblasts are going to continue to secrete more matrix and eventually what we see is that this new matrix it lays down and lays down and lays down so our ossification Center kind of grows and grows and grows and it kind of pushes the other features like the blood vessels you know into a smaller and smaller space as our ossification site actually gets bigger and bigger here so it kind of expands kind of from the inside out here our ossification Center now in this image you have to imagine that you've sumed out a bit alright so we can see our blood vessels here the cells we can't see the individual cells but we can't see the matrix that they've lied down and they've kind of laid down this matrix around these blood vessels we still have these osteoblasts they're gonna form this kind of layer around the edges of our tissue of our ossifying tissue here and this really is starting to look kind of like spongy bone and tissue right so we've got ossification places that are going to merge with other ossification centers and that's going to form the trabeculae those spicules of bony tissue the sort of honeycomb within the within the center of the of the spongy bone now as this continues as this process continues this is exactly what we see the blood vessels here that were originally sort of the places that the bone you know deposited around have degenerated and the blood vessels are going to be replaced with red bone marrow in each of these spaces around the periphery we can see that some of this bone tissue then has been remodeled or broken down and rebuilt so that we've got a layer a thin layer of compact bone now that surround on either side our spongy bone here in the middle and we still have this layer of osteoblasts this is now affiliated with a true layer of connective tissue this is now our periosteum so this means then that we could have further growth in width of these bones by having these osteoblasts laid down more matrix and this would make the bone thicker now I've got a couple of histology images that I want to show you this comes from a and P revealed let me orient you to this figure a bit the bone tissue that's been laid down is here you can see that we've got there are some small these little specks are actually going to be the osteocytes and they're lacuna they've been laid down and then the bone marrow cavities the remaining cavities are going to be here in between you can see the remnants of some looks like blood vessels that were there and you can also see the layer of osteoblasts right here and right here these are very nice okay so these are going to be the cells that are going to be producing more matrix as those as the bone continues to develop and you can clearly see that the bony tissue here are forms these kind of spicules these kind of trabeculae kind of honeycomb-like structures they kind of weave around the bone marrow places there at the bone marrow spaces alright now I said earlier that through intramembranous ossification this really is a mechanism that allows bones in adults to grow in width because the matrix is being secreted by the osteoblasts now what happens when this process goes awry these are two images of a patient who has a disease called fibro dysplasia ossificans progressiva and in this disorder what happens is that connective tissue for some unknown reason transforms into bone tissue so in the places where you would have connective tissue or even muscle tissue those cells transform into osteoblasts they begin to secrete bony matrix and then you've got bone tissue that is laid down in very odd and non-functional patterns you can clearly see that this is outside of the skeleton and so this bone is not really functional this disease is not curable there is no treatment for it and it's also very painful right because you've got all this extra bone that really serves no purpose on this next image this is actually an x-ray of a person with fibro dysplasia ossificans progressiva and you can see there are some things that look like bones for example here's a humerus right here you can see some ribs right there but then you can also see clearly that there are structures that don't look like any normal bone here right and they're clearly sort of outside or extra skeletal furthermore as you look at the features of these bones here you can clearly see some of the features are exaggerated and you can just see that it's not it's just not right you know these bones are just out right so even the bones that are regular bones are not are just not functional looking there's a lot of extra bone tissue and certainly there's a lot of other evidence that there's bone tissue that has been formed where it really shouldn't be this is an x-ray of a child's feet and it shows a particular abnormality that appears to be associated with the development of fibro dysplasia ossificans progressiva all right so from here I want to talk about the second mechanism of bone formation which is endochondral ossification so stay tuned