okay there are five major cell types within bone and each of this have their own specialized Form and Function now osteogenic cells are a form of stem cell we find within the periosteum and endosteum osteoblasts of the bone forming cells and osteocytes are the mature bone cells we have bone lining cells also called periosteal cells which basically uh monitor the existing compact bone and the osteoclast so these giant multinucleated immune cells that basically break down the Bony Matrix so osteogenic cells are also called osteoprogenitor cells now these are mitotically active stem cells in the periostium and endosteum when stimulated they can differentiate into osteoblasts or bone lining cells so you might wonder what is the stimulation for osteogenic cells to differentiate well typically damage um where the bone needs to be repaired what we find then is that these osteogenic cells are a nice source of additional stem cells for bone repair and some remain as osteogenic stem cells within the periosteum and endosteum that way you always have some capacity for bone repair now osteoblasts can come from the osteogenic cells and the function of osteoblasts is to secrete a bone uh forming material called osteoid osteoid is made up of collagen and calcium binding proteins and the collagen is actually what makes up 90% of bone protein it's very tough type of protein and calcium binding proteins are necessary for sequestering or holding on to the calcium phosphate minerals that make bone hard now osteoblasts are also actively mitotic so that osteoblasts can divide into more blasts to help make more bone or repair bone now you can see the osteogenic stem cells here can divide and differentiate into osteoblasts which are the bone forming cells so they're responsible for bone growth now osteocytes are the mature bone cells now what makes an osteocyte mature is the fact that it gets trapped in a Lacuna the Lacuna is a space where these cells can no longer divide because they get they trap themselves in a bony cavity called Luna the function of osteocytes then is to just Monitor and maintain the existing bony Matrix they act as stress or strain sensors so that mechanical stimuli like increased force on a bone can help Osteo sites to help to communicate information between other cell types so that bone remodeling or bone uh kind of reformation can occur so what's pretty cool then is these osteocytes can direct the the uh reforming of bone even later on in adulthood now bone lining cells are flat cells on the Bony surfaces that are believed to help maintain the out Outer Matrix and on the external bone surface these cells are called periostal cells and on the internal surfaces they're called endosteal cells again bone lining cells can also come from the osteogenic cells now osteoclasts are very different here osteoclasts are not derived from those osteogenic cells rather the osteoclasts are cells that are derived from homat potic stem cells so instead of having an osteogenic origin Osteo class are these giant multinucleated cells that are uh formed by the uh Fusion of many different macras remember macrophases are an immune cell that can break down material and in this case the osteoclast break down bone so when they're active they actually form little depressions called resorption base and resorption means the breakdown of bone and cells that have a rough old Border and they can actually help increase the surface area for enzyme degradation of bone these also help seal off an area of surrounding Matrix so the remember the osteocytes the mature osteoblast that basically just Monitor and maintain the existing bony mineralized Matrix this differs from the osteoclast because osteoclast are the giant multinucleated cells that are immune in origin and they basically help to resorb or break down the Bony Matrix now compact bone is something we saw earlier and we said that that was what essentially lines the outer surfaces of your bones it's also called Lamela bone because Lamela are these basically layers of bone and compact bone is made of osteons canals canaliculi and interstial and circumferential Lamela now the osteon is what we call a herv verion system and the osteum is the structural unit of compact bone it consists of an elongated cylinder and they have concentric circles that interlock these are made of Lamela and Lamela are basically the interlocking rings of bone that form an osteon now the Lamela contain collagen fibers that run in different directions this helps the the osteons to withstand stress and resist twisting and this eventually makes bone stronger than steel in terms of tensile strength now bone salts are also found between collagen fibers which makes bone hard so looking at the example here of an osteon you see there's these concentric rings of bone called l and the collagen fibers within these Lamela run at opposite angles that way it resists different types of stress here the very center of the osteon contains uh blood vessels and nerve fibers within a space called the central Canal or hersan canal and it turns out that these blood vessels help to nourish the bone from the inside and um these canaliculi are essentially small canals that help to uh transmit nutrients deep within bone now these canaliculi are linked up to a central Canal that runs through the core of each osteon contains your blood vessels and nerve fibers that's what you saw in the last slide and the perating canals are the ones that are actually run perpendicular or right angles to the central canal and they basically just help to connect the blur the blood and nerves of nearby Central canals so um canaliculi are the hairlike canals that connect Lacuna or spaces within bone to each other and they basically are like the irrigation ducts of your bony tissue because they help to transmit nutrients deep within bone Lacuna are the small cavities that contain osteocytes and these osteocytes are nourished by the nutrients that travel through the canaliculi now osteoblasts that secrete bony Matrix maintain contact with each other and um they communicate with via Gap Junctions that way the osteocytes can tell osteoblasts you know how they might want to remodel the bone based on the stresses that are placed on it now when the Matrix hardens these osteoblasts get trapped into into spaces called Luna and that that that way they form osteocytes is also in the canaliculi form and allows for communication of all osteocytes within an osteon now other forms of Lamela or or uh layers of bone consist of interstial and circumferential Lamela the interstitial Lamela are found between osteons and the circumferential Lamela surround the entire outside of the bone and so what this slide shows is a really nice summary of all those terms here so remember out here we see compact bone this is spongy bone only compact bone is made of complete osteons so if you zoom in on the compact bone here you see okay there's an osteon there's an osteon there's one remember the osteons are basically interlocking cylinders or concentric rings of Lamela and each Lamela is is basically a tube of bone and uh between each tube you find little spaces called Lacuna so if you zoom in here on an osteon which you can see down here you find okay here's our Lamela again those rings of bone and between Lamela we have our Lacuna containing osteocytes these osteocytes are all interconnected by little extensions that uh connect the cell to cell through Gap Junctions and these cell extensions connect through canaliculi which these tiny little hairlike projections whose function is to basically transmit nutrients from the central Canal deep within the bone itself and you can see that Illustrated really here in our uh light microscopy image of bone this is one OST on here's our Central canal and you can see our Lacuna with osteocytes kind of interspersed all throughout now these radial lines that are projecting away are all the canaliculi it almost looks like little fractured glass or something these little tiny lines here are the canaliculi which are small canals that help to connect ostey to ostey as well as the central Canal now remember the osteon is a is a whole circular type of structure but what we need then is actually a type of material to fill the space between osteon we call this interstital Lamela so right here we have interstital Lamela spill actually fills the spaces between osteons and then suround the whole outside of bone we got the circumferential Lamela which go around the whole circumference of that bone now on the outside of bone it's lined with periosteum which are Bas which basically serves as the attachment point for tenons and ligaments and has your osteogenic stem cells that can help to replenish the uh broken bone now down each osteon we have a central canal these canals go down the length of the osteon which goes along the length of the bone and these Central canals contain lots of blood vessels and nerve fibers so if bone breaks it will bleed a lot because these blood vessels are very abundant Within These osteons and then there's also a lot of ner fibers in Bones so if bone breaks it will hurt a lot because those nerve fibers are going to be you know irritated now what connects these Central canals together um perpendicularly is are these vulcan's canals or Perforating canals and they essentially allow for blood vessels to intercommunicate between these canals and also allow them to go down deep into bone and then bring blood also more superficial within bone now spongy bone appears more poorly organized but it's actually very organized and what's interesting is that spongy bone lines up along lines of stress so if you place stress on a bone in a particular way you know like if you take up a sport like tennis or basketball um and you start using bones in a certain way it turns out the spongy bone will model and reform along the lines of stress that are placed on those bones so the tracula are basically like cables in the suspension bridge and they help to confer strength to the Bone itself now you don't find any complete osteons in spongy bone but the Tula do contain Lamela and osteocytes that are interconnected by canaliculi the reason why Lamela and osteocytes are here is that spongy bone used to be compact bone but it was broken down by the osteoclasts and turned into kind of a spongy Tor type of appearance now capillaries in the endosteum are essentially what help nourish spongy bone from the inside now looking at spongy bone here you can see compact bone on the outside you have spongy bone sandwich in the middle if you zoom in here this a spongy bone under electron microscope you can see it's got this tcul or honeycomb appearance there would be endosteum lining these spaces and if you if you actually looked at a cross-section of the microscopic organization of spongy bone there would be no complete osteons here only incomplete Lamela but there would be living osteocytes monitoring and maintaining this existing spongy bone now in terms of what bone is made of well it's made of both organic and inorganic components the organic components of bone uh which are basically carbon containing hence name organic include the cells like osteogenic cells blasts and sites and Bone lighting cells and osteoclasts as well as the osteoid now the osteoid is what makes up onethird of the Bony Matrix and osteoid is this material that's secreted by osteoblasts osteoid is made of ground substance and collagen fibers in the ground substance of osteoid you've got calcium binding proteins that help to you know make this mineralized later and it helps to contribute to the high tensile strength and flexibility of bone because of all the collagen fibers as well now the organic components um help to form some resilience of bone because of these sacrificial bonds what we find is that between between the collagen molecules and Bone uh sometimes there's actually little break points here that way if bone receives a lot of stress we have sacrificial bonds that can help absorb some of the shock on that bone just like in newer cars there are these crimple zones where basically if a car gets hit in a certain way the it's designed to Crunch and absorb the force rather than bounce off the car which can damage the person you know inside just like our bones have these sacrificial bonds which are basically there to Crunch and absorb some of the force that's placed on bones but these can reform and eventually helps to dissipate energy and prevents fracture of bone overall now if there's no additional trauma these bonds can reform now the inorganic components of bone are the hydroxy appetite now this makes up 65% of your bone by mass and hydroxyapatites are basically mineral salts now they're mostly calcium phosphate crystals that surround collagen fibers and they're attracted by those calcium binding proteins we talked about in osteoid and it turns out the hydroxyapatites are what are responsible for the harness and resistance to compression that bone has so think of bone as being like a mineralized matrix now in organic components also is uh what contributes to the fact that bone is about as strong as steel and resisting compression and I'm sorry half as strong as Steel in resisting compression but as strong as Steel in terms of resisting tension now uh it turns out that bones last long after death because of the mineral composition they don't Decay as quickly as other tissues and what's pretty cool too is you can look at the mineral composition of of ancient people which can tell you about their diet and lifestyle and other environmental factors that may have been exposed to