starting up now with our second lecture video on our introduction to bones and from there we're going to go into the microscopic structure of Bones and take a look at what we see under a bit higher magnification um so the idea of Bones as being living tissues is kind of centered around this idea right within the bones there are networks of or what are referred to as canaliculi which connect the locuting to each other and of course inside the lacuni is where the actual osteocytes are going to live it's where they're going to make a living the thing is within each one of those rings of lacuni so each one of these would have like a Lacuna here or a Lacuna here or Laguna here or one over here and inside of that of course there would be bone cells in the center of each one of these Bullseye looking structures there is a dark Center area called the herversion canal aversion system diversion system is we talked about this a while back but it's it's there to contain the blood vessels of bone it's there to contain nerves as well all right so running parallel to the length of the bone there will be nerves and blood vessels within each of these version canals to help Supply the various cells in the various lacuni with nutrients and carry away waste and do those kinds of things right now if we take an entire Bullseye structure with the many lacuni going around in a circle and then the version canal in the center with blood vessels and nerves the entire thing is referred to as an osteon all right so the entire version system one Bullseye is an osteon so again within this there would be lacuni with with bone cells so don't forget about those um okay so we talked about that the the Virgin canals are the ones that run parallel to the long axis of the bone um Bullseye or also like tree rings I've also heard it described that way as well but essentially the space between osteons is referred to as a lamella okay so these spaces which kind of look like tree rings or the outer rings of a bullseye those are referred to as lamella all right lamella in general Anatomy just refers to a layer of something see that serum used quite a bit lamella is a layer of something uh there's also what I referred to as interstitial lamella and circumferential lamella as well circumferential really early in the morning to be saying a word like circumferential um essentially what that means is that those are the ones that are going around the outside of the bone right out here just under the periosteal layer is the circum differential lamella um circumferential romella lamella out here okay and then when we go in within this now we have interstitial lamella which is the ones actually between the cells um and again we we talked about the lacuni the lacuni are actually located on these dark lines and inside the Lacuna is where the bone cells live right okay um we talked about lacuni we talked about conversion canals we have not yet talked too much yet about canaliculi except for just kind of getting the idea across that they're a way for the cells in different lacuni to be able to talk to one another communicate share things right that kind of stuff um the version canals the ones going parallel to the length of the bone so in this picture they would be coming out of the screen at you okay so this aversion Canal is coming out of the screen right at you and it would be filled with a at least a couple of blood vessels and a nerve right now Within let's see if we can take a look at some of our other terminology that we saw in the previous slide um a single osteon there are many lacuni that are oriented in ring the spaces between the Rings are referred to as lamelli and these canaliculi are the ones that connect various lacuni to each other okay these are referred to as volkman's canals so in this picture since this is two-dimensional picture but we do see the canals going out radiating away from the center we know those canals are perpendicular or at right angles to the main axis long axis of the bone that is so really not too much else to say there the lacuni and the cells inside of them more importantly are connected by canaliculi volkman's canals that allow for uh permeation of nutrients and waste and things like that throughout the structure of the bone now if we look at the spongy bone all right so we we just take a quick look here remember the on the the structure of a bone compact bone on the outside spongy bone in the center perhaps there might also be some marrow either red marrow like we see in these spaces in here or yellow marrow which we think of as being kind of in the the long Axis or the shaft of the bone inside um that's where the marrow is there spongy bone sorry does not have a conversion system um it's made up of essentially little thin projections of bone that are referred to as spicules all right so on this picture these are spicules and they're just made of calcium right they're made of the same materials the rest of the bone spicules by that by the way is a term used to describe um other things in biology spicules are a structure that sponges actually produce on the inside of themselves to give themselves a bit of internal structure and uh stability in the case of sponges though they're made out of things like silica which is basically glass calcium carbonate which is the same thing like Limestone but the point is they're very very they're much sharper looking than these are like pointy um in a sponge but it's the same idea they are projections of material um in into a space here just like they are in a sponge but again different very very loose similarity to what sponges have right okay we've talked about the osteocytes before quite a bit so I don't think we need to say too much more about that um and then we also have our canaliculi but instead of linking to um the lamella and the version canals and so forth this bone uh the catalyculi actually link the red marrow to other areas of red marrow so it's it's a different kind of a structure but it's doing basically the same thing okay um osteogenic cells right so you guys know by now that I'm big on trying to break down the word and figure out figuring out what it means without having to Define it makes your life a whole lot easier if you're taking a test and you come across a term that you've never seen or don't remember if you can just take a second to break it down look at the roots of the word um and you'll understand what it means right you don't have to memorize every single word you come across because if you just look at the roots of the word it tells you it tells you what it is especially in biology okay so Osteo is bone and Yannick is producing so these are bone producing cells pretty straightforward um again any time in biology if we see the term or the suffix genic or Genesis it means the beginning of something okay so osteogenic cells are cells that begin the production of bone okay so we are looking at in terms of the overall structure of a bone a group of cells that are responsible for producing it are layered just underneath the periosteal layer um which is on the outer you know outermost surface of the bone okay so we have the periosteum on the outside we have the endosteum on the inside we have aversion canals these are all structures uh that are that are either located within or produced by um these osteogenic cells right um osteogenic cells become osteoblasts there is another step in there between but we don't need to worry about that too much these osteoblasts are the ones that begin to form the Matrix of the bone all right so again bone matrix remember is calcium phosphate that's the material the non-living part of the bone is calcium phosphate you'll also sometimes see it or hear it referred to as hydroxyapatite same thing it's calcium phosphate it's the the hard stuff that makes up your bones so when the osteoplasts do that they begin to produce the Matrix so blasts are building they are going to be able to withdraw calcium from the blood and store that calcium up as calcium phosphate in the bones themselves so a lot of times I'll ask usually not this level of class but like my principles classes I'll ask a question something along the lines of you know explain to me how bones can be considered a calcium bank right so a bank is a place where you can go to deposit things you want to keep safe but you also want to be able to withdraw them at some point you don't want to just give them to the bank so this calcium bank is a place for calcium to be stored in the short term when there's excess calcium available in the blood keeping calcium in the blood at too high of a level has some fairly serious implications everything from kidney stones to hardening of the artery walls calcification of the artery walls so you really don't want to leave a whole lot of calcium in the blood but on the other hand you also need to have some so the bones are a good place to store it such that if you run into a point in time during your life when you are running short of calcium you can go to the bones you can activate the osteoclasts that we talked about before as well right the osteoclasts and they are going to break the bone back down again release calcium phosphate into the blood and now it's available for things like um availability for neurotransmitter release for example which we've already talked about this semester right so these cells are producing a bony Matrix around themselves and when they do that as soon as they're entombed or surrounded by this bony Matrix they transition I guess technically into what are called osteocytes now when we say they transition remember this is a human categorization of a microscopic cell um exactly how and why that transition happens we don't fully understand but um sort of the the cut off functionally for the different types of cells is whether the cell is surrounded by bone or not so if the cell is surrounded by bone it's an osteocyte if it's not surrounded by bone and it's still building bone and things like that and so forth then it's osteoblast osteoblasts though are not mitotic which means they can't divide right so these cells that are producing bone um themselves are relatively short-lived in the body and once you sort of take on your your full size adult frame with ossified bones and so forth when I say ossified it means the um crystals of calcium phosphate have come in and sort of been impregnated into the bone the cartilage model in this case and made it hard all right so that's that's the ossification process so as I was saying these cells don't divide which it probably kind of makes sense I mean if you're an adult and your bones have fully formed you probably don't need a whole lot of like more bone producing cells in your life um most of the cells that you start off with in life as osteoblasts become osteocytes and slow down activity levels remain alive for the bulk of your life but they're not like super super active now you might then ask the question well what happens if a bone breaks or how do bones heal or how do bones remodel in response to various stresses and those are all really good questions all right so let's take a look and see how that how that works so like the osteoblast osteocytes cannot divide they are very long-lived but also very much not very very good at propagating themselves we'll say so these osteocytes are going to help to maintain the bone maintain the composition of the bone make sure it's the right Ratio or proportion of calcium and phosphate and so forth sort of like is keeping track of things essentially they don't have a whole lot of ability to to to build new bone but they can be used to repair New bone now the osteoclasts are a different story they're a very different kind of a cell as it says they're multinucleated which is um something we really haven't seen too much yet this semester but we're going to because we're going to see that skeletal muscle cells in particular are also multi-nucleated cells that have many many nuclei throughout and the idea is the same the the idea is that if you have more nuclei you have more sets of DNA that can be drawn upon to produce proteins in various structures so um fungi do this as well like I said skeletal muscle osteoclast same deal they are going to um be like like it says up to 50 nuclei and each one of those nucleus has a full set of DNA so these relatively large cells are going to have many many sets of DNA to draw from when it comes time for them to produce what they need to produce so it's thought that these osteoclasts are derived from the fusion of monocytes um which again we haven't talked about um you guys haven't covered the circulatory system yet that's an ap2 but monocytes are a type of cell um that has a single nucleus they are typically thought of as being the precursors to macrophages right so macrophages are um I think I've said this before but one of the most important types of immune cells if you were to say top three top two macrophage is definitely on the list all right they're they're important for fighting they're also important for training the rest of the immune system and activating the part that needs to be activated so um macrophages start off their life as a cell called a monocyte these monocytes however don't become macrophages what they're going to do is uh fuse together and send out these long finger like extensions that are able to digest essentially bone so an acid phosphatase which breaks down the collagen breaks down the periosteum so the outermost layer goes away and then the various combination of other acids doesn't really matter specifically what they are for our purposes but they are able to digest or dissolve the bone salts and release calcium and phosphate and so forth into the blood so that's um the function of the osteoclast as far as the Matrix of the bone like we said before most of it is made up of inorganic salts um about two-thirds of it is made up of inorganic salts like hydroxyapatite calcium phosphate um other salts that are not in Crystal form um various things like magnesium and sodium and potassium um CO3 minus carbonate strontium uranium plutonium lead go wow that's a lot of stuff defining bones so it turns out that bones are also really really good and we talked about this in the first lecture section bones are really good at locking up harmful materials that we come into contact with so that they don't hurt the rest of our body okay so bones are kind of like a stable safe long-term place to lock away things like um lead that you've come in contact with throughout your life and everybody is come into contact with lead at this point in their life um sometimes those things could be radioactive so people that have been exposed to nuclear testing or lots of radiation there can be actually uranium and plutonium things like that that are stored in the bone at small levels very small levels we're not we're not talking about glowing here but it's still some so after the um um atomic bombs were drops on Hiroshima and Nagasaki back in World War II um one of the implications of that was that a lot of people were found to have high levels of various Metals in their body some of which were I believe radioactive so long-term effects on the body from that the rest of the Matrix of bone and again Matrix remember is the non-living part right so a lot of people are still kind of struggling with the idea of what a matrix is Matrix is the non-living part outside of these cells and Matrix in this context only occurs in connective tissue okay Matrix is non-living substance around the living part of a tissue and it only occurs in connective tissue now you're going to come across the term Matrix in other areas like for example just off the top of my head the mitochondria has a matrix I lectured on that in my principles class earlier this week um so there isn't just one kind of Matrix and and that's actually a fairly common thing you'll see this similar terms used for similar structures or functions um throughout biology biologists are really good at co-opting terms from other biologists and applying them to their own system so um that is the case here as well and it's the case frequently in biology the organic part of the bone of course is the collagen it's the chondroitin which is the Matrix for cartilage I should remember that um this is what gives the bonus flexibility okay so the inorganic salts are there for strength the organic part is there for flexibility and a very tiny fraction of the organic part consists of excuse me living cells blood vessels nerves those kinds of things that's the organic part of the bone right so we need a strong and stable part protective part and we also need a part which is dynamic and able to respond to uh the changing needs of the body all right so the next thing we're going to take a look at is growth and development and now by this point you probably have noticed a theme on this lecture uh that's this guy right here so I don't know um most of you guys are too too young to remember um He-Man but this is a Skeletor this is he-man's arch nemesis one of my my favorite cartoon growing up and we're talking about bones so throw Skeletor in here as well why not okay so during embryogenesis well that's a new term but again we can break it down because we know that we can do that to learn the meanings of a lot of words so embryo that's pretty straightforward uh but Genesis again is the beginnings of so embryogenesis refers to the period of time after fertilization but we'll say before around eight weeks or so of gestation all right so usually around eight weeks of gestation or development after fertilization um the convention is to stop calling it an embryo and to begin to call it a fetus all right so from week eight up until around week 38 it's a fetus and it's basically just growing at that point embryogenesis the early part of development from you know Day Zero all the way up through week eight primarily is for differentiation and for specialization of a growing body okay so after after week eight you're still specializing but mostly what you're doing is creating adult forms of structures and getting to a size that's going to be sustainably um functional once it's outside of outside of the uterus so this ossification process takes a while what we're going to see is that replacing cartilaginous models that an embryo has in the beginning takes a while we're going to see that even after birth there are some structures that take a couple of decades to fully ossify part of the sternum is one of those it can it can actually continually even a little bit beyond that there's three parts to the sternum or breastbone one of them which we'll talk about here shortly in the lab is the one that is ossified later on in life there's also the ossification of bone that occurs during uh puberty during growth spurts during puberty so we'll see those growth plates as well and how they become ossified over time um that's actually what's what's referred to or what's meant when we say the term endochondral ossification so endochondral is one type of bone development and it's the most common type of bone development in the human body um endochondral development refers to basic processes starting off with a bone that looks like cartilage it's made of cartilage and gradually the cartilage in that bone is is like I said before impregnated with calcium phosphate to form a bone as we think about it when we're born right so a hard bone is is ossified um during development so like we said the body's bone starts off as a variety of cartilaginous models hyaline cartilage so as opposed to like fibro or elastic cartilage this is the hyaline cartilage like we saw in the trachea and so forth um the other form of development that we'll see is intramembranous ossification so this one this actually has a good prefix to learn intra we've talked about before means Within so this particular kind of development refers to a situation where a bone forms between two membranous sheets of material so there's only a few bones in the body that actually that do that most of the bones in the skull develop that way the mandible which is the lower jaw develops that way as well and also the clavicle which if you're not familiar with clavicle you will be but for uh purposes of just having a common reference clavicle is a collarbone all right and that one actually is uh it's a really important structure for reasons that you may not be familiar with but we're gonna see what it is and what it does probably in lab clavicle is a really important thing to um protect yourself when you fall basically uh it's it's a it's a bit of like um it's almost like a fuse right if you think about what a fuse does fuses are designed to break if there's a surge in electricity so whatever is on the other side of the fuse doesn't get damaged by The Surge in electricity basically his audience clavicle is kind of the same deal because if you fall in our tendency when we follow is to try to catch ourselves with our with our hands with our arms our forelimbs um but if you fall in the wrong way or if you fall from high enough uh broken collarbones are very relative well in Falls they're very very common injuries there's a really good reason for that those clavicles are when I uh they're they're set up to break in a very particular way so imagine that you fall and you catch yourself with your arms that is a tremendous amount of force going up through both of your arms through both of your collarbones and really from there into your neck so you can see the danger here if you fall and you get this massive um Force transmitted from your arms through your collarbones into your neck your neck would break so that would be the result of Falls more commonly if we didn't have a clavicle that was fairly easily breakable you fall down you break your neck you die okay so instead what happens is that when you fall and you put your arms out to catch yourself collarbone breaks now it's not fun I've actually never had a broken collarbone but um it's not fun but it heals and it's really not going to be life-threatening as opposed to like say a broken neck so it's it's like a way to limit the damage to a more Expendable structure as opposed to allowing the damage to reach another part of the body which might be a bit more critical like the spinal cord um which if you of course break that especially at that level um it's not going to be good so better to have a broken broken and clavicle than to have a broken spinal cord as a result of a fall okay so anyway back to this um intramembranous development is bone growth that occurs between two sheets of what's referred to as mesenchyme okay so mesenchyme is uh so meso is Middle okay first off and by the way I forgot to say this intro is within something so intramembranous is within two membranes um mesenchyme uh mes or m-e-s-e or m-e-s-o in particular a lot of times means between two things like middle of something like the um the good part of the sandwich not the bread the middle part that's mesenchyme all right that's uh that's what's meant by that term and and this very very dense mesenchyme is able to to form these sheets these membranes within which ossification is going to occur these are also called dermal Bones by the way um which again the ones we talked about the skull the mandible clavicle those are all uh called dermal bones okay um from there the mesenchyme contains cells that become osteogenic cells so this is a good point for me to and I'm really big on doing this because I think it's it's too easy to forget this point um every cell on the body has all the same DNA I know we've talked about this a lot but um it's really important to remember that these cells are doing this as a result of them containing all of these genetic instructions so even cells that have one job if a different set of genetic instructions is activated they can then take on a different job okay so these mesenchymal cells are going to activate a different subset of their DNA different set of genes that's going to cause them to become osteogenic cells osteogenic cells then of course become osteoblasts osteoblasts becomes osteocyte and then we have bones right um so a term that we're going to see a little bit more coming up in the rest of this lecture and we're going to cut the section off here before too long is what's known as a primary ossification Center or POC all right these osteocytes are going to start building bone in Islands I remember islands that are located within the space between these two membraneous sheets of uh amazing kind that we talked about before so that that initial beginning of ossification by these osteocytes in the bones between these membranes is referred to as the primary ossification Center so as these osteocytes continue your osteoblasts continue to function the primary ossification Center is going to get bigger in diameter it's also going to get thicker so these uh these cells are building bone in three dimensions all right they're they're doing it left to right up and down and also front to back I guess is the third dimension there I don't know um you're three-dimensional ossification of this bone the middle part of the bone becomes spongy right so we've seen that before the outer part is compact bone and then eventually once the bone is fully formed the only really remaining um I guess sign of what the bone used to be was a little bit of periosteum um on the actually that's a type of much about that periosters on the outside and Awesome's on the inside real quick sorry about that there we go okay so periosteum on the outside um so if you imagine like um uh one of the bones in your skull one of the bones that makes up your cranium um one of the lar especially one of the larger I want to name them yet but like um the occipital bone in the back your head right that that flat bone on the surface that's just under your skin that is the endosteum I'm sorry that's the periosteum on the outside if you go to the inside of the plate or inside of the bone where the soft tissues are like where the meninges are your brain is that kind of a thing that is the endosteum so periosteum is on the surface of the outer surface of the bone and osteum is on the inner surface of the bone okay so just make sure we remember that make sure you make that change too to the slides I just noticed that okay um okay so I think what we'll do here is actually I am going to wrap up this section here uh we're going to talk about endochondral growth next which is a little bit more complicated so I want to start a fresh set of lectures fresh um video for that that set of growth so we'll start that up here in just a second