okay this video we're going to talk about the different types of bone development so ossification also called osteogenesis is the process of bone tissue formation and formation of the bony skeleton it begins within the second month of fetal development and we talk about how most bones in your body start out as cartilage so that basically ossification occurs in month two but it's basically you know osteogenesis of cartilage now postnatal bone growth occurs until early adulthood so we're saying that after birth bones continue to grow and we'll talk about and differentiate between those different types of bone growth and bone remodeling and repair are considered lifelong as well now in terms of formation of the bony skeleton there are two major forms of ossification or bone development up until about week eight the fibrous membranes and hyaline cartilage are replaced with bony tissue through two major mechanisms we have endochondral ossification and intramembranous ossification endo means within chondros cartilage so endochondral ossification is actually bone formation within a heiling cartilage model so that bones that are endochondral bones basically cartilage they form most of the skeleton so most of the bones your body start out as cartilage but then are ossified through endochondral ossification now intramembranous ossification is actually where bone develops in fibrous sheets of membrane typically for me forming you know flat bones so endochondral ossification is the first type we'll talk about here and it forms essentially all bones inferior to the base of the skull except your clavicles this begins late in the second month of development and it usually uses previously formed hyaline cartilage to serve as the model for bone tissue formation and it requires the breakdown of the hyaline cartilage prior to ossification so that bone I'm sorry the cartilage is basically ossified into bone and it begins what we call the primary ossification Center now you find this in this the shaft of the cartilage and essentially blood vessels infiltrate the pair can bring into periosteum and then mesenchymal cell specialized into osteoblasts which then can turn into that particular type of connective tissue which is bone now the five major steps of ossification are bone collar formation this is the first step that it begins around the diathesis of the cartilage model the second step here is that cartilage in the diaphysis then calcifies and develops cavities periosteal blood that invades the cavities leading to formation of spongy bone and the bud is made of blood vessels nerves in red marrow as well as different types of cells like osteoclasts and osteoblasts along gates in the medullary cavity forms because of the breakdown of that cartilage and the secondary ossification center also appears in the epiphysis the fifth step here is the epiphysis ossify and then hyaline cartilage remains in the epiphyseal plates were sandwiched between the primary and secondary ossification centers so what this slide is showing is basically the process of endochondral ossification and it gives you one two three four five steps as well as the timeline here we see that up until about the third month you know most your bones are still cartilage here and you see that the first step of you know bone formation is the formation of this bony collar near the diaphysis of this Highland cartilage model and then we get the formation of a primary ossification center here in the center of this diocese now in the second step here we get deteriorating cartilaginous matrix for the primary ossification center begins to expand the bone outward and upward and the bony collar also thickens in elongates the third step here is the periosteum bud invades the primary ossification center this brings in your blood vessels your nerve fibers in osteogenic cells and osteoclasts which effectively then turns this area into technically bone because now it's got blood vessels and then the fourth step here is that the diaphysis begins to elongate all the cartilage is actually eliminated from the diaphysis so what you're left with is just bone and then the secondary ossification centers appear right around the time of birth so we're saying that at birth there really aren't much of an epiphysis to this point right so the second house of Kafka's ossification centers in the epiphysis have just started to form when someone's born which kind of makes sense because you want their bones become flexible and movable in order to be more easily passed the birth canal we find that is that from birth up until childhood as adolescents we see that the secondary ossification centers continue to grow expand until essentially what you're left with is what looks like a long bone so you have your epiphyses at the ends here we have our diaphysis and the only cartilage that's left from the Highland cartilage that it began as is essentially the articular cartilage that you find at the ends of the long bones as well as the epiphyseal plate when the epiphyseal plate is basically this little band of cartilage trapped between the prime secondary and primary ossification centers it's essentially a band of cartilage that just didn't fully ossify and this is the growth plate so this is going to continue to divide and elongate the bone through childhood and adolescence in adult this epiphyseal plate is fused and what you're left with is an empathy Co line which is just a little line of compact bone now this differs from intramembranous ossification because n't remember substitution is a more rare type of bone formation but it effectively forms most of your cranial bones and flat bones so like your frontal parietal occipital temporal in clavicles are all formed by intramembranous ossification and it basically occurs within sheets of mesenchymal fibrous tissue so there are four major steps involved here we have ossification centers that are formed when these mesenchymal cells cluster and become osteoblasts these osteoblasts secrete osteoid which is then calcified woven bone is formed when hostilities laid down around the blood vessels which forms a trabecular appearance of spongy bone and then the lamellar bone replaces woven bone and we find that is that red marrow appears within the woven bone spaces so in terms of these steps you have one two three four and the first step here is that these mesenchymal stem cells within the fibrous tissue can start to aggregate and form clusters these stirrers then can differentiate into osteoblasts which then start to lay down osteoid remember osteoid is the unmanned realized organic Bodhi matrix which is basically calcium binding protein in collagen fibers now after the osteoid is laid down effectively that gets mineralized by hydroxyapatite which with calcium phosphate salts some of these cells get trapped in the osteoid itself in the mineralized osteoid in this actually a lacuna now you see that the cell it's trapped and lacunae well it's now called an osteocyte because it has no choice but to become less active more dormant because it's now trapped within a space in the bone now we got a line of osteoblasts at the outer margin that that continue to lay down new osteoid until effectively what you form is woven bone woven bone is basically it looks like spongy bone but it's basically just you know bony matrix that is wrapped around the blood vessels that traverse through this connective tissue now you know essentially the fourth step here is that osteoblasts continue to line up until they lay down lamellar bone on the outside edge basically like a bony collar that surrounds the outside edges this will later become what we know is compact bone with our spongy bone here trapped in the middle now in the fourth step also what happens is the spongy material here die flow basically it's filled with yellow marrow and that is a source of Hamato Puig stem cells to help regenerate you know lost blood cells so you can see that the process of intramembranous ossification is similar but different than endochondral for one it begins in sheets of mesenchymal stem cell tissue and - it basically has you know one ossification Center that essentially expands outward and eventually becomes our compact with spongy bone in the middle now we're gonna do next is actually differentiate different types of bone growth after birth so the first one we'll talk about here is interstitial bone growth so interstitial bone growth is a lengthening of bone along the longitudinal length of that so this is actually a type of bone growth where bones can increase in thickness and in length and bones stopped growing during adolescence because of the loss of the epiphyseal plate but some facial bones continue to grow slowly throughout life so interstitial bone growth requires the presence of epiphyseal cartilage remember epiphyseal cartilage is what you found in long bones and it's basically a little band of hyaline cartilages trapped between the epiphysis and diathesis and it's essentially that highly cartilage unless left behind from development now the epiphyseal plate maintains a constant thickness but the rate of cartilage growth is balanced by bone replacement so we find that is that this epiphyseal cartilage divides it continues to make new cartilage but this new cartilage is essentially turned into or ossified into new bone and we're saying that is that that cartilage will grow die and then get converted to new bone tissue so we can divide our up if you see you'll play into five zones we got resting zone proliferating zone where the where the cartilage is dividing we have the hypertrophic zone where these cells get larger and begin to die there the calcification zone where the dead cartilage gets calcified and then we have the ossification zone also called osteogenic zone where that dead calcified cartilage is then converted to bony tissue now the resting zone here is an area of cartilage on the epiphyseal side of the plate that's relatively inactive and nearby we have the proliferating zone that's on the diaphysis side of the epiphyseal plate and it consists of chondroblasts that are rapidly dividing these actually help to create new basically extracellular material and essentially making new cartilage which pushes the epiphysis away from the diaphysis causing lengthening now the next zone here is where the older chondrocytes closer to the diaphysis start to enlarge and erode forming these interconnected spaces and by the calcification zone it's basically dead cartilage so the chondrocytes die and deteriorate and in this zone we have basically calcified cartilage that's dead in the v zone that calcified dead cartilage is actually turned into bullying tissue so we're saying here is that chondrocytes deteriorate leaving these spicules and the calcified cartilage of the epiphyseal diaphysis junction these spicules are eroded by osteoclasts and then can cover with new bone tissue by osteoblasts and ultimately replace with spongy bone the medullary cavity will enlarge in as the spicules are eroded so we're saying here are those those five zones of the epiphyseal plate so this should have an epiphyseal plate here if you zoomed in on this you find we have a resting zone then our proliferating zone where the cartilage is dividing our hypertrophic zone where the cartilage cells get larger our calcification zone where the where the chondrocytes die and then you're left with calcified dead cartilage and then we have the ossification zone where spicules are eroded by osteoclasts and then converted to basically osseous or bony tissue by nearby osteoblasts you can imagine then that this is going to divide and push downward and as it gets pushed downward the cartilage is dying and be converted to bone this is effectively what helps bones grow in length now in terms of what determines the growth of bones and length is that near adolescence chondroblasts divide less often the epiphyseal plate becomes thinner it ultimately is replaced by bone completely we call this epiphyseal plate closure when the emphasis and the diathesis fuse together and there's no more epiphyseal cartilage now we have the empathy Co line which is just bony tissue there bone lengthening ceases and females around age 18 and in males around age 21 now in terms of bone growth in width we call this a positional bone growth this is where they widen as they lengthen as well bones become thicker in response to increased stress and osteoblasts painful periosteum secrete bony matrix on external bone osteoclasts remove bone from the endosteal surface and what we see that is that actually as a consequence a widening of bone because osteoblasts are letting down bone on the outside osteoclasts are breaking it down from the inside ultimately causing the whole bone to become wider but portion aliy the same thickness in terms of compact bone now there's usually more building up than breaking down which leads to like thicker stronger bone that's not too heavy so we're seeing here that it is actually bone growth and we're seeing that happening of the bone can grow in length and what we need then is also bone remodeling and bone remodeling includes a positional bone growth a positional bone growth is actually where bones widen as they linked in you can see for one yes the bone lengthen because it's slightly longer then it was in the beginning that's because of this epiphyseal cartilage dividing and then basically laying down new new bony material on the diaphysis side now we also see a corresponding widening of the bone because of a positional bone growth we're essentially osteoblasts lay down new bony matrix on the outside osteo class break it down from the inside that's what you're saying this dotted line means basically where the bone used to be you can see that all this bony material was broken down but then it was expanded outward here by osteoblasts so we end up with it as a longer bone because of the endochondral i'm sorry the interstitial bone growth here and a wider bone because of the oppositional bone growth now there's also lots of hormones that can regulate bone growth and we see that growth hormones one of the most important hormones in stealing epiphyseal plate activity thyroid hormone plays a role as well as well as testosterone estrogen and males and females and accesses or deficits in any hormone can lead to abnormal skeletal growth like if someone has growth hormone excess that can lead to gigantism or if you have a growth hormone deficiency it can lead to patooty to do or fizzing or they actually abnormally short