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this my friends is a walrus baculum it's basically a penis bone found in most placental mammals interestingly not in humans and this is a polar bear skull which as you can see is more streamlined for swimming in the water than a grizzly bear skull and over here we have my giant friend the Rhino head which is a good for being giant for fighting off predators and fighting for I don't know why white rhinos have big heads and this is the skull of the pronghorn antelope it has these horns that come off that are covered in these carrot and sheaths that fall off once a year these are all bones parts of skeletons and they're all pretty freaking awesome and I am surrounded by them here at the Phillip L Wright zoological museum at the University of Montana and all these bones have adapted to help animals survive the horns and the pronghorn for mating displays and self-defense a streamlined skull of a polar bear for swimming in the water and the walrus baculum for longevity I guess we're used to thinking of our skeletons as being the dead parts of us because that's what's left over after all of our like stuff that looks like us has rotted away the fact is our bones make up a vital organ system and I don't just mean vital in that without them you would be a sort of disgusting dead pile of lumpy mush but also in the traditional meaning of Vital meaning it's alive it protects your vital organs it makes locomotion possible it manufactures your blood and on top of it all it takes care of its own repair and maintenance your skeleton is live people and walrus penises are just the beginning so you know what bones are but maybe you didn't know that you don't have to be a vertebrate or even a chordate to have a skeleton jellies and worms for instance have hydrostatic skeletons made up of fluid-filled body cavities by squeezing muscles around the cavities they change their shapes and that can be used to produce movement insects have exoskeletons of course made of the nitrogenous carbohydrate chitin and most mollusks have exoskeletons too in the form of calcium carbonate shells so when it comes to skeletons the winningest formula has been the endo skeleton even though we'd probably feel a lot safer if we were covered with armored plates like some race of iron men having skeletons inside of our bodies has allowed us to grow larger and have much more freedom of movement that's good stuff one of the many reasons you don't see ant the size of horses walking around is well what it wouldn't be able to breathe but also a body with such a huge volume would require an exoskeleton that was exponentially thicker and therefore heavier in clumsier to support it so endoskeletons allow animals to grow larger by supporting more mass plus you don't have to worry about the embarrassment that comes with unsightly molting as adults humans have 206 bones all kinds of shapes and sizes including three tiny ones in each ear one weird shaped one like a horseshoe and your throat 27 in your hands and 26 in each foot you also have at least 32 teeth unless you played too much hockey and even though they're included in the skeletal system they don't count as bones because they're made up of different material namely dentin and enamel the hardest material in your body and you probably think of the skull as one big bone but it actually consists of many separate bones including eight plates that cover your brain and 14 others in your face face bones so simple right well you might want to sit down you probably already are but I'm going to cuz it's time for biography now you'd think that we'd have nailed down the basics of the human skeleton a long time ago because our teeth and our bones are the biggest and hardest parts of our bodies and after we leave this mortal coil there what stick around the longest is not like they're super hard to find and study surely all of those ancient physicians who basically invented medical science would have inventoried all of our bones pretty soon after they figured out that we had bones right if the answer was yes do you think I'd be sitting here most what we know about the human skeletal system is thanks to Andres van vessel who was born in what's now Belgium in 1513 and in those days if you were like kung fu master of science pretty much got your own Latin name so today he's known as Andreas Vesalius say leus came from a long line of physicians two kings and emperors and while studying in Paris he began Dorking around in cemeteries and became interested in what's now known as osteology the study of bones perhaps Vesalius his greatest contribution was showing the world that everything we thought we knew about osteology was wrong see back in those days if you wanted to become a doctor you didn't study bodies or see patients you read stuff written by ancient Romans whose work was considered indisputable because you know those guys had long beards and they wore robes but in his research Vesalius discovered that Roman texts about the skeletons especially the teachings of the philosopher dr. Gailen were way way off see Roman law prohibited the dissection of human bodies so none of those guys ever actually studied human innards instead they dissected apes and pigs and donkeys and used that to make assumptions about the human body and so for 15 centuries young doctors were taught those assumptions but Vesalius revolutionized osteology and all of Medicine by introducing a new practice every pre-med students favorite human dissection he instructed students by dismembering corpses in front of them and cataloguing their parts giving students the first opportunity ever to directly observe the inside of a human body these new methods drew a lot of attention particularly from a local judge who began donating bodies of the criminals that he executed to Vesalius suddenly the dude was up to his codpiece and pig thieves and murderers by the time he was 28 he done enough research that he published de Humana corporis fabrica on the fabric of the human body a seven-volume text on human anatomy including the first comprehensive description ever made of the human skeleton it's beautifully detailed illustrations are thought to have been created in the studio of the Renaissance artist Titian featuring pictures of flayed corpses positioned in symbolic poses and many of the volumes some of which still exist today are bound in human skin so the takeaway here is that even though bones are big and hard the science behind them is far from obvious even though we tend to think of our bones as rigid and fixed your skeleton is as dynamic as any other of your organ systems it's built from scratch with ingredients in your blood it's grown according to glands in your head and probably coolest of all it's constantly breaking itself down and rebuilding itself over and over again for as long as you live most new bone tissue starts out as cartilage what you may know from your nose and your ears it's made of specialized cells called chondrites and in newly forming bones these cells start dividing like crazy and secrete collagen and other proteins to form a cartilage model or framework for the bones to form on soon blood vessels work their way into the cartilage and bring plump little cells called osteoblasts asti which you'll be hearing a lot of today just means bone and blast means germ or bud the bone building that they do was called fittingly ossification first they secrete this gelatinous goo that's a combination of collagen and a polysaccharide that acts kind of like an organic glue then they start absorbing a bunch of minerals and salts from the blood in all the capillaries around them and unsurprisingly there especially absorbing calcium and phosphate and they begin depositing those minerals onto the matrix with the help of enzymes secreted by the osteoblasts these chemicals bond to form calcium phosphate which crystallizes to make your bone matrix in the end about two-thirds of your bone matrix is proteins like collagen and the other third is calcium phosphate kind of surprising right most of your bone isn't even mineral and even the part that is is living tissue because it's all honeycomb with blood vessels that allow osteoblasts and other cells to do their unlike an insect's exoskeleton even the hardest parts of your bones are alive now even though bone can take all kinds of forms from big flat plates protecting the brain to the tiny stirrup in your ear inside they all tend to have the same basic structure if you cut one in half you'd see that the matrix actually forms in two layers the outer layer called the compact or cortical bone is hard and dense and makes up about 80% of the bones mass in the middle the spongy or trabecular bone is softer more porous and contains the marrow and fatty tissues and larger bones the marrow of course makes not only new red blood cells but almost all of your different blood cells by a process called hematopoiesis I'd need like about a week of your time in a Greek dictionary to explain how it does this but suffice it to say that evolution has wisely chosen the innards of our largest bones - how's the blood stem cells that together can produce 1 trillion blood cells in you every day that's 10 - the freakin 12 on the outside the larger bones of your body have a similar structure have a look here at this femur that's the biggest bone in your body the main shaft is called the diaphysis and each rounded end is unopp if assists when the bones grow as a child grows the new tissue forms the border between the two a place called the epiphyseal plate as they did when they form the original bone tissue chondrocytes start to produce new cartilage here and the osteoblasts come in and lay down more collagen and calcium phosphate so as you grow the ends of your bones are actually growing away from each other until by the time you're about 25 the last of these plates in your bones heart by the way this whole process is stimulated by growth hormones secreted from glands all over your body but the head honcho right here is the pituitary gland about the size of a pea nestled the base of your brain as adults this and other glands produce less growth hormone which slows down our bone lengthening but even though lengthening is a limited time only process the thickness and strength of a bone must continually be maintained by the body because of course like all of your cells bone cells go through a lot of wear and tear and need to be able to adjust to changing conditions so over the course of each year of your adult life about 10% of your skeleton is completely bro down and then rebuilt from scratch in a process called bone remodeling here the main players are the osteoblasts again and another kind of cell that's kind of their complete opposite the osteoclasts or bone breakers you think maybe that the cells that form bone tissue and the ones that destroy it would be in some kind of constant battle in your body but during remodeling they work closely together and actually communicate nicely but it's like they're basically frenemies remodeling begins when osteoclasts are sent by way of hormone signals through the capillaries to the sites of microscopic fractures in the bone matrix once they're in place they secrete an acidic cocktail of hydrogen ions to dissolve the calcium phosphate into calcium ions phosphate and water and other material that they carry back to nearby capillaries then they secrete enzymes that specialized in digesting collagen this whole process is called resorption and when the old bone tissue has been cleaned up the osteoclasts send out a hormone shoutout to the osteoblasts who come in and do their ossification thing bone remodeling is really pretty amazing and it's all ultimately regulated by hormones that maintain the levels of calcium in your blood the glands that call all the plays during the bone breaking part of the remodeling are the parathyroids in your neck when the calcium in your blood plasma falls below the level of homeostasis the parathyroid triggers osteo class to take calcium out of your bones and release it back into the blood likewise when blood calcium levels are too high the parathyroids cousin the thyroid gland signals osteoblasts to take calcium out of the blood and lay it down on the bone collagen through more ossification and remember last week when we talked about how the kidneys reabsorb salts and minerals well the thyroid also regulates how much calcium is reabsorbed in that process as well as the amount of vitamin D because vitamin D helps your body absorb calcium through the small intestine and that is why vitamin D is all good for your bones and stuff now the relation of active osteoblasts to active osteoclast can change dramatically under different conditions the more you stress your bones the more osteoclasts work to break down the bone matrix so that it can be reformed bone stress can include stuff like fractures of course but it can also be less traumatic and more sustained exercise causes stress on the skeleton that helps stimulate bone remodeling so when you're working out you're not only building you're also building bone so as you can tell it's kind of hard to talk about bones without also talking about muscles and that's what we're going to do on the next episode of crash course biology thank you so much to the Phillip L Wright zoological museum at the University of Montana sorry I just hit you check out the tumblr at U and zoology tumblr com it's awesome
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