our first topic in the skeletal system will be osseous tissue specifically the bone cells found in your body and we'll be looking at the difference between compact bone and spongy bone but first a little introduction starting with the functions of the skeletal system there are a bunch and some are really obvious like the skeletal system supports your body and gives it structure protects your internal organs and provides leverage for locomotion meaning muscles pull on your bones to move your body but there are some functions that are a little bit less obvious for example storage of minerals now 98 percent of the calcium in your body is stored in your bones and your bones will actually give out a little bit of calcium when your body needs it or if there is plenty of calcium the bones will take in some of that calcium to store it and this is important because you really need just the right amount of calcium being provided to the cells of your body at all times you don't want too much and you don't want too little we'll talk more about that later on the skeletal system also stores fat in the form of yellow bone marrow and red bone marrow is where all the different blood cells are made and those two will talk about more detail later on one thing to keep in mind as we explore the skeletal system is that bones are not these static structures that aren't changing they are changing a lot is happening in your bones and on your bones this is especially true if you're in a lab situation and you're studying these plastic bones it really starts to make you feel like that's how they look or behave in the body but I want to make sure we know that's not true in fact the whole last video in this osseous tissue series will be all about how bones are dynamic how they are innervated with nerves and their vascularized with blood vessels they go through ossification processes they grow and they remodel themselves bones are pretty cool just think about braces you know when you have braces on those aren't changing your teeth they're pulling on your teeth to change your bone to remodel your facial bones so bones can change okay now let's think about osseous tissue so this is a type of connective tissue and so like all types of connective tissue osseous tissue has what you remember all connective tissue has specialized cells and an extracellular matrix and when you look at the matrix there is always except maybe for blood and lymph there's just about always fibers found there and some kind of ground substance so in the case of bone our specialized cells are osteocytes literally meaning bone cells and then some other osteo cells that we'll talk about next the matrix has fibers in this case collagen fibers they help give a little bit of flexibility to your bones and tensile strength just helps resist stretching and twisting of bones think about gymnasts or something they're putting a lot of tension on their bones a lot of twisting motions and for the most part our bones are pretty good at not snapping even from all that twisting in that force ground substance in this case is hydroxyapatite which are calcium salts that's what's really making the bones hard so that they can resist compression you can look up some of the facts about bones I think they say that you know pound for pound they're stronger than steel but then have the added benefit of being able to repair themselves so the extra cellular cellular matrix is really key to having both that strength but also that flexibility all right so let's look at some bone cells osteocytes we already mentioned osteocytes are these mature bone cells that when the bones not changing much it's just kind of sitting there doing its thing that's the osteocytes area these other types of cells are when bones are growing or shrinking or changing so you have stem cells called osteogenesis you have some Builder cells called osteoblasts and you have some cells that dissolve bone called osteo class so we'll look at each of those a little bit more closely so those osteogenic cells are the stem cells those will be active when your skeleton is growing you know as embryo and into your young years but also as an adult if you fracture a bone and it needs to be repaired the osteogenic cells are going to help create new bone to repair it osteocytes those are those mature bone cells and they are monitoring the matrix meaning they're making sure the fibers are all good the ground substance remember that hydroxyapatite those calcium salts make sure that's all good at the appropriate levels and so on those osteocytes are arranged in layers called lamellae so in this case these concentric rings like a tree stump those are the lamellae the osteocytes sit in these little depressions these are like little cavities called lacunae so those are like little pools little lakes and then connecting these lacunae are these little canals called canaliculi two other types of bone cells are osteoblasts and osteoclasts osteoblast build new bone through a process called osteogenesis osteo class cut away old bone through a process called osteo lysis last build class cut really what osteoblasts are are these immature bone cells they start as the osteogenic cells those stem cells and they turn into osteoblasts and then for a while they'll build new bone but as they make bone they will surround themselves in bone until they're stuck there they can't really make anymore then they become osteocytes osteoclasts are kind of a whole different thing they're sort of like these immune cells they have a bunch of nuclei they're really big and they basically eat away bone to release the calcium and phosphate and other minerals into the blood these two have opposite effects and so as we'll talk about later they are the key to calcium homeostasis here's a picture of them let's say you need some calcium in the blood because your cells are crying out for more calcium so the big multinucleate osteo class will cut away some bone and free up those minerals let's say at another time you want to build up some bone then you've got these osteoprogenitor or osteogenic cells that will turn into osteoblasts you can see here they're building some new bone but eventually they'll be surrounded by that bone and then they become osteo sites okay so now let's look at compact and spongy bone the first thing to know is that compact bone is a dense type of bone that forms the outer cortex or the hard outer shell of most of the bones in your body so here's a long bone maybe this is the humerus and you can see that outer compact bone or if we want to look at a flat bone like the bones in your skull here's two layers of compact bone spongy bone is a more porous type of bone that contains bone marrow and usually it's surrounded by the compact bone so there's a bunch of spongy bone in this long bone there's a bunch of spongy bone in this flat bow let's look at the use in a little bit more detail compact bone we said is a very dense type of bone and it is dense relative to spongy bone but it's still full of passages like we have these central canals and perforating canals and little canaliculi and then little cavities so all types of little holes and little passages through this bone we'll look at these on a osseous tissue model that looks like this but to really understand where this bone is coming from look at this first here's this banana now I'm gonna cut a disc out of this banana okay so now I have this disc but the next thing I'll do is take a slice out of it like a slice of pizza or a slice of pie now I've got is the peel here forming this very durable outer part of the banana and then the inside soft part of the bananas softer and smushy r that's what we're looking at in this model or this diagram we've taken a long bone like pretend this was maybe the the humerus or femur or something they've taken a slice out of it and then cut a slice of that slice so we can see the outer compact bone we're like the peel of the banana is and then the softer spongy bone inside that's what we're looking at here now let's find some of these structures on this figure first is osteons osteons are the structural unit of compact bone meaning all that compact bone is is mostly just a whole bunch of osteons there are these long cylinders that have a little central canal inside which is full of blood vessels and nerves and other stuff on this model here's one osteon if this whole thing is an osteon that's like a circle right but if we look from the side here we can see how it's actually a cylinder that runs down the length of the bone so your humerus has osteons running all the way down through the compact bone with the central canal in the middle and you can see some blood vessels inside oh and why our Aussie Anse like a bunch of straws well if you have one straw it's relatively weak but these cylinder shapes actually provide quite a bit of support if you have a whole bunch of straw shaped structures together they can hold a pretty good amount of weight and so the osteons are like straws in that one isn't very strong but a bunch of them provide a lot of structural support okay let's look for these lamellae these are rings around the central canal and other areas so here's our concentric lamellae again but we also have circumferential lamellae going around the whole circumference of the bone and we have interstitial lamellae these in-between lamellae tucked in and amongst the osteons are the interstitial lamellae just different names for what's pretty much the same thing here lacunae again these cavities so we can't quite make them out but inside these little lacunae are osteocytes canaliculi are the little canals these are gonna make sure that as the nutrients arrive inside this little artery that they can make their way to all of the lacunae to make sure that all these osteocytes that are kind of isolated inside their little cavities make sure they all have access to nutrients and last are those Volks mins canals or they're called perforating canals you can see those here these are just running perpendicular to the central canals but their goal is the same get blood into the bone and then take the ewes blood and all the wastes and co2 away from the bone spongy bone is also called cancellous bone there's no osteons here there are osteocytes bone cells of course and there are lamellae so there is kind of an organization to the spongy bone but mostly what we care about are that trabecular these are the branching plates little thin fibers that you can see criss crossing through spongy bone and Temecula is what makes the spongy bone so light now one thing about trabecula is that when it's healthy it's these branches are fairly thick but osteoporosis is one type of disease that leads to a reduction in bone mass causing the bone to be less dense so you can really see that in spongy bone when these trabeculae get very thin and there are lots of risk factors for osteoporosis pretty much your highest risk factors come if your woman who is of postmenopausal and smokes and doesn't exercise much and so on but also genetics plays a role one last note about trabeculae though are the these are healthy trabeculae and these are kind of thin trabecula but these are trabeculae - in your heart and these are trabeculae - in your spleen what for you youngsters I put a emoji in what's what here's the thing these trabeculae are boom this is just another case of where the anatomist they like a term and so they reuse it so in your heart there are little branches of cardiac muscle called trabeculae and in the spleen there are little branches of connective tissue called trabeculae so we'll see this term a lot it just means like a bridge or a branch of tissue