the process by which bone is formed is called oifc or osteogenesis oifc occurs in four principal occasions Across the Life Span the initial formation of bones in an embryo and fetus during the growth of Bones during infancy childhood and Adolescence during bone remodeling which is a Contin replacement of old bone tissue by new bone tissue and during the repair of fractures or breaks in the bone now the initial formation of bone and bone growth across infancy childhood and Adolescence will be the focus of this video recording the process of bone remodeling and Fracture repair will be discussed shortly so the embryonic skeleton or the skeleton of a baby who has not yet been born is initially made up of mesen in the general shape of bone during the sixth week of development the mesen begins a process of ossification during this time there are two processes by which bone is formed intramembranous ossification where bone forms directly within the mesen So intram membranas within the membrane of the mesen and then endochondral ossification where bone forms within the Highland cartilage that develops from the mezen Kim with endochondral within the cartilage these two processes which both involve the replacement of a pre-existing connective tissue with bone do not lead to any differences in the structure of a mature adult bone but are simply different methods by which bone can develop so taking a look at intrus ossification first and this type of oifc is the development of bone directly from mezen Kim intramembranous oifc occurs in our flat bones such as the bones of our skull sternum ribs and clavicle this process begins with the development of an oifc Center which is simply the site where bone begins to develop specifical uh specific chemical messages cause the cells of the meesen to Cluster together and to differentiate first into osteoprogenitor cells and then into osteoblasts Osteo blasts then secrete osteoid until they're surrounded by it at which point these cells are now called osteocytes Next the secretion of osteoids stops and the osteocytes sitting within their Lacuna extend their little processes or arms out into the con iuli or the tunnels which radi radiate out in all directions over the next few days calcium and phosphate from the blood will be delivered to the extracellular Matrix where they combine to form crystals of hydroxyapatite which are then deposited into the extracellular Matrix hardening or calcifying the tissue as the bone extracellular matrix forms and hardens it develops into tabula that fuse with one another to form spongy bone around the network of blood vessels in the tissue connective tissue associated with the blood vessels in the tracula differentiate into red bone marrow in conjunction with the formation of tracula the mezen Kim condenses at the periphery or the outskirts of the bone and it develops into our periosteum EV a thin layer of compact bone will replace the surface layers of spongy bone although the spongy bone remains in the center over time and once a baby is born and then grows much of this newly formed bone is remodeled so destroyed and then reformed this occurs as the bone is transformed and grows into its normal adult size and shape so give you a bit of a break from all of that text here is that same process but in the form of a figure so the first step in intramembranous oifc is the development of the oifc center where the mezen chimal cells will cluster together they then differentiate into our osteop progenitor cells and then our Osteo blasts our Osteo blasts will then secrete osteoid until it completely surrounds a cell at this point we then call the cell the osteocyte and the secretion of osteoids stops calcium and phosphate are then delivered to the area via the channels that carry the blood vessels and the extracellular fluid in the canicula hydroxyapatite crystals form and then they're deposited into the extracellular Matrix or that network of collagen fibers which then hardens and calcifies the tissue as that extracellular Matrix forms and calcifies it begins to form our tabula and the spongy bone around that network of vessels in the bone tissue the connective tissues around those blood vessels then begin to form the red bone marrow lastly on the surface of the bone our per osteum will begin to develop underneath our periosteum compact bone will replace our spongy bone endochondral ossification is then the replacement of cartilage by bone so our mesenchimal cells first differentiate into Highland cartilage which then develops into bone all of the bones of the body undergo endochondral ossification except for our flat bones which as we just spoke about form via our intramembranous oifc so endochondral ossification begins with the development of a cartilage model at the site where bone is going to form specific chemical messages cause those mesenchimal cells to crowd together in the general shape of the future bone and then develop into condro blasts the condro blasts secrete EX cage extracellular Matrix producing a cartilage model of the bone made up of Highland cartilage a covering called the peric condum which is just an outer layer of cartilage develops around the cartilage model once the condro blasts become deeply buried in the cartilage extracellular Matrix much like our osteocytes We Now call these cells condr sites the cartilage model then begins to grow in length due to the continual cell division of the condra sites and the further secretion of the cartilage extracellular Matrix as the cartilage model continues to grow our conides can also hypertrophy or increase in size and towards the end of this process some of the carage extracellular Matrix begins to calcify as a result some of the conos sites will die because the nutrients can no longer diffuse as easily through the extracellular Matrix as these condra sites die the spaces left behind by the dead condra sites merge into small cavities called Luna primary oifc which is just that first sight of oifc then proceedes inward from the external surface of the bone and nutrient artery which is simply the artery which supplies a bone penetrates the peric condum and the calcifying cartilage model and it stimulates osteoprogenitor cells in the peric condum to differentiate into osteoblasts which then eventually form our periosteum near the middle of the bone capillaries from the periosteum will grow into that disintegrating calcified cartilage inducing the growth of our Primary oifc Center where bone tissue replaces most of the cartilage osteoblasts then begin to deposit that bone cellular matrix or the osteoid over the remnants of the calcified cartilage model forming our spongy bone tacul primary ossification then spreads from the central location towards the ends or the epip fees of that carage model as this primary oifc Center grows towards the ends of a bone osteoclasts break down some of that newly formed spongy bone tissue leaving a hollow space called the meder cavity eventually most of the spongy bone surrounding the meder cavity will be replaced with compact bone blood vessels will then enter the epiphyses or the ends of the bone and secondary oifc centers will develop usually around the time of birth bone formation is similar to what occurs in the primary alific Center however in the epiphyses of the bone no medel cavity is formed so that spongy bone tissue remains and in primary ocation where the oifc moves inward with secondary oifc it moves outwards towards the outsides of those epes lastly the Highland cartilage that covers the epes or the ends of the bone become our articular cartilage and prior to adulthood Highland cage which remains between the diaphyses and the epes becomes our epipal or our growth plate which is the region responsible for the lengthwise growth of the lung bones so once more looking at that process but in figure form and in our first step up here the mezan Kim will differentiate into condra blasts forming this highland cartilage Model A covering called the peric condran will surround that cartilage model the cartilage model then begins to grow through both cell division and hypertrophy of our conas sites or cartilage cells in the center of our cartilage model some of that cartilage extracellular Matrix begins to calcify a nutrient artery or the artery that supplies the bone will then penetrate the perichondrium this stimulates the transformation of our peric condum into our periosteum capillaries from the periosteum will then go in towards the bone and start to replace the cartilage with the bone tissue forming our Primary oifc Center as this Primary oif Center begins to spread towards the end of the epip Theses of the bone osteoclasts start to break down some of that newly formed spongy bone tissue creating our medel cavity blood vessels then enter the epes of the bone and stimulate the development of our secondary oifc Center where spongy bone continues to replace the cartilage the Highland cartilage that remains around the ends of the epis becomes our articular cage which remember is the Cartage that uh is formed in a joint where two bones will meet the Highland cage at the metopes which remers that region between the epes and the diapees becomes our growth Pate or our epip plate and this is where a bone grows in length until a person reaches adulthood so beyond birth into infancy childhood and Adolescence bones will continue to grow in length we call this interstitial growth they can also grow in thickness which we call appositional growth so the growth of a bone in length involves two major events the growth of cartilage on the epipal side of the epipal plate so the side of the epiphysis closest to the end of the bone and then at the same time the replacement of cartilage on the diapal side of the epipal plate by bone through endochondral ossification so this occurs on the side closest to the diaphysis or the shaft of a bone now to understand this process which I will explain in more detail shortly we need to know a bit more about the structure of the epipal plate so the epipal plate also known as the growth plate is a layer of Highland cartilage in the metaphysis of a growing bone that consists of four zones of cartilage looking at this figure here this is an x-ray we've got your feet so lots of different cartilage cells in 1 2 3 four different zones so looking at that zones and first we have the zone of resting cartilage which is closer to the epis or the end of the bone so it would be this section that kind of sits on this side here and it consists of small scattered condra sites or cartilage cells and the term resting is used because the cells in this Zone do not participate in bone growth rather they anchor the epipal plate to the epis of the bone moving towards the diis so now we're move Ming closer towards the shaft of the femur here this is the zone of proliferating cartilage so here slightly larger cartilage cells are arranged a bit like a stack of coins these cartilage cells undergo interstitial growth as they divide and secrete extracellular Matrix which lengthens this region of cage the condra sites produced in this region replace Lo that die at the diapal side of the epipal plate when they eventually turn into bone we then have the zone of hypertrophic carage so again we're still moving towards the diaphysis or the shaft of the bone and here we have larger maturing condra sites which are also arranged in columns and here as a name suggests the conides hypertrophy or increase in size also increasing the this region of cartilage lastly we have the zone of calcified cartilage which is the final zone of the epipal plate and it's closest to the diaphysis or the shaft of the bone and this zone is only a few cells thick and consists mostly of conoides that are dead because the extracellular Matrix around them has been calcified osteoclasts then begin to break down the calcified cartilage an osteoblast will begin to replace the calcified cartilage with bone via endochondral ossification so in terms of what that looks like for growth so interstitial growth at the epipal plate is the only way that a bone can grow in length as a bone grows the condra sites will proliferate or divide and make more on the epipal side of the epipal plate so if we have a look here this is our epiphysis this is our diis this purple line here is our zone of proliferating cartilage where we're making more and more of those cartilage cells the new condra sites replace older ones which have grown in size so that's this green zone here the zone of hypertrophic cartilage where those mature condra sites will increase in size these increased in size cartilage cells and then destroyed in this white zone here by calcification and that cage is then calcified and replaced by bone on this diapal side so in essence this diis of the bone moves out this way to increase the length of the bone we have the zone that anchors the epiphysis to the epipal plate The Zone where we make more cartilage cells The Zone where these C cells increase in size and then the Zone where that cartilage is replaced by bone now when adolesence comes to an end about 18 years of age in females and 21 in males these epipal plates will close that is the epipal cartilage cells will stop dividing and Bone replaces all of the remaining cage cells the epipal plate Fades and it leaves a bonus structure called the epipal line with The Disappearance of the epipal plate B growth in length completely stops so bone can no longer get any longer bone can then grow in thickness or diameter via appositional growth so at the bone surface cells of the periosteum will differentiate into osteoblasts which secrete osteoid the osteoblasts become surrounded by The extracellular Matrix and develop into our osteocytes at the same time as this new bone tissue is being deposited or built on the outside of a bone osteoclasts on the inside of the bone which lie in that meder cavity are eating away the bone tissue so that the size of the medall cavity increases as the bone increases in thickness or diameter so that this thickness of the bone here stays pretty much the same size so on the outside on our periosteum we have Osteo blasts secreting osteoid increasing the thickness of the bone here at the same time we have the osteoclast lining the Maller cavity or the inside of the bone eating away at that bone tissue so this thickness here remains relatively the same