Februari 01, 2017
this, my friends,is a walrus baculum it's basically a penis bonefound in most placental mammals. interestingly, not in humans. and this is a polar bearskull, which as you can see is more streamlined for swimming in the water than a grizzly bear skull. and over here we have mygiant friend, the rhino head, which is good for being giant,for fighting off predators and fighting for- i don't know, why dorhinos have big heads?
and this is the skullof a pronghorn antelope. it has these horns that comeoff, that are covered in these keratin sheaths thatfall off once a year. these are all bones. parts of skeletons. and they're all prettyfreaking awesome. and i am surrounded by them here at the philip l. wrightzoological museum, at the university of montana.
and all of these bones haveadapted to help animals survive, the horns on the pronghorn formating displays and self-defense, the streamlined skull of a polarbear for swimming in the water, and the walrus baculum,for, longevity, i guess. we're used to thinking of ourskeletons as being the dead parts of us because that's what'sleft over after all of our, like, stuff that lookslike us has rotted away. but the fact is, our bonesmake up a vital organ system. and i don't just mean vitalin that, without them you would be
a sort of disgustingdead pile of lumpy mush, but also in the traditionalmeaning 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 careof its own repair and maintenance. your skeleton is alive people. and walrus penises arejust the beginning.
so you know what bones are,but maybe you didn't know that you don't have to be avertebrate or even a chordate to have a skeleton. jellies and worms, for instance,have hydrostatic skeletons, made up of fluid-filledbody cavities. by squeezing muscles around thecavities, they change their shapes, and that can be usedto produce movement. insects have exoskeletons,of course
made of the nitrogenouscarbohydrate chitin, and most mollusks haveexoskeletons, too, in the form of calciumcarbonate shells. but when it comes to skeletons,the winningest formula has been the endoskeleton. even though we'd probably feela lot safer if we were covered with armored plates likesome race of iron men, having skeletons inside ofour bodies has allowed us to grow larger and have muchmore freedom of movement.
it's good stuff. one of the many reasonsyou don't see ants the size of horseswalking around is well, one, it wouldn'tbe able to breathe, but also, a body withsuch a huge volume would require an exoskeletonthat was exponentially thicker and therefore heavier andclumsier, to support it. so, endoskeletons allowanimals to grow larger by supporting more mass,plus, you don't have to worry
about the embarrassment thatcomes with unsightly molting! as adults, humans have 206 bonesof all kinds of shapes and sizes, including 3 tiny ones in each ear, 1 weird one shaped one likea horseshoe in your throat, 27 in your hands,and 26 in each foot. you also have at least 32 teeth,unless you play too much hockey, and even though they'reincluded 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 ofthe skull as one big bone, but it actually consistsof many separate bones, including 8 plates that cover yourbrain 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, because it's timefor biolo-graphy! now, you'd think that we'dhave nailed down the basics
of the human skeletona long time ago, because our teeth and bonesare the biggest, hardest parts of our bodies, and afterwe leave this mortal coil, they're what stickaround the longest. it's not like they're superhard to find and study. surely all of thoseancient physicians who basically invented medicalscience would have inventoried all of our bones prettysoon after they figured out that we had bones. right?
if the answer was yes, doyou think i'd be sitting here? must of what we know aboutthe human skeletal system is thanks to andries van wesel, who was born in what'snow belgium in 1514. and in those days, if you werelike, a kung fu master of science, you pretty much gotyour own latin name, so today he's knownas andreas vesalius. vesalius came from a long line ofphysicians to kings and emperors, and while studying in paris,he began dorking around
in cemeteries and becameinterested in what's now known as osteology, the study of bones. perhaps vesalius' greatestcontribution was showing the world that everything we thought weknew about osteology was wrong. see, back in those days, if youwanted to become a doctor, you didn't study bodies orsee patients, you read stuff written by ancient romans, whosework was considered indisputable. because, y'know, those guys hadlong beards and they wore robes! but in his research, vesaliusdiscovered that roman texts
about the skeleton,especially the teachings of the philosopher-doctorgalen, were way, way off. see, roman law prohibited thedissection of human bodies, so none of those guys everactually studied human innards. instead, they dissectedapes and pigs and donkeys, and used that to makeassumptions about the human body, and so for 15 centuries,young doctors were taught those assumptions. but vesalius revolutionizedosteology, and all of medicine,
by introducing a new practice,every pre-med student's favorite! human dissection! he instructed studentsby dismembering corpses in front of them andcataloging their parts, giving students thefirst opportunity ever to directly observethe inside of the human body. these new methods drewa lot of attention, particularly from a local judge,who began donating bodies of the criminals heexecuted to vesalius.
suddenly, the dude was up tohis codpiece in pig thieves and murderers, and by thetime he was 28, he had done enough research that he publishedde humani corporis fabrica. on the fabric of the human body, a seven-volume texton human anatomy, including the firstcomprehensive description ever made of the human skeleton. its beautifully detailedillustrations are thought to have been created in the studioof the renaissance artist titian,
featuring pictures offlayed 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 eventhough bones are big and hard, the science behind themis far from obvious. even though we tend to think ofour bones as rigid and fixed, your skeleton is as dynamic asany of your other organ systems. it's built from scratch withingredients in your blood, it's grown according toglands in your head,
and probably coolest of all,it's constantly breaking itself down and rebuildingitself, over and over again, for as long as you live. most new bone tissuestarts out as cartilage, which you may know fromyour nose and ears. it's made of specializedcells called chondrocytes, and in newly forming bones,these cells start dividing like crazy and secretecollagen and other proteins to form a cartilage model, orframework, for the bones to form on
soon, blood vessels worktheir way into the cartilage and bring plump littlecells called osteoblasts "oste," which you'll be hearing a lotof today, just means bone and"blast" means germ or bud. the bone-building that they do iscalled, fittingly, ossification. they first secrete a gelatinous goothat's a combination of collagen and a polysaccharide that actslike a kind of organic glue. then they start absorbing mineralsand salts from the blood in the capillaries all aroundthem, and unsurprisingly,
they are especially absorbingcalcium and phosphate, and they begin depositing thoseminerals onto the matrix. with the help of enzymes secretedby the osteoblasts, these chemicals bond toform calcium phosphate, which crystallizes tomake your bone matrix. in the end, about two-thirdsof your bone matrix is proteins like collagen, and theother third is calcium phosphate. kinda surprising, right? most of your boneisn't even mineral,
and even the part that is,is living tissue. because it's all honeycombedwith blood vessels that allow osteoblasts and othercells to do their jobs. unlike an insect's exoskeleton, even the hardest parts ofyour bones are alive. now, even though a bone cantake all kinds of forms, from big, flat platesprotecting your brain to the tiny stirrup in your ear, inside they all tend to havethe same basic structure.
if you cut one in half,you'd see that the matrix actually forms in two layers. the outer layer, called thecompact or cortical bone is hard and dense and makes upabout 80% of the bone's mass. in the middle the spongyor trabecular bone, is softer and moreporous and contains the marrow and fattytissues in larger bones. the marrow, of course, makesnot only new red blood cells but almost all of yourdifferent blood cells
by a process called hematopoeisis. i'd need like about aweek of your time and a greek dictionary toexplain how it does this, but suffice it to say thatevolution has wisely chosen the innards of our largest bonesto house the blood stem cells that together can produce1 trillion blood cells in you every day. that's 10 to the freakin' 12th. on the outside, the largerbones of your body
have a similar structure. have a look here at this femur, that's your biggestbone in your body. the main shaft iscalled the diaphysis, and each roundedend is an epiphysis. when bones grow, as a child grows, the new tissue forms at theborder between the two, a place called theepiphyseal plate. as they did when they formedthe original bone tissue,
chondrocytes start toproduce new cartilage here, and the osteoblastscome in and lay down more collagen andcalcium phosphate. so as you grow, the ends ofyour bones are actually growing away from each otheruntil, by the time you're about 25, the last of these platesin your bones hardens. by the way, this wholeprocess is stimulated by growth hormones secretedfrom glands all over your body. but the head honcho righthere is the pituitary gland,
about the size of pea nestledat the base of your brain. as adults, this and other glandsproduce less growth hormone, which slows downour bone lengthening. but even though lengthening isa limited-time-only process, the thickness and strengthof bone must continually be maintained by the body. because, of course,like all of your cells, bone cells go through alot of wear and tear and need to be be able toadjust to changing conditions.
so over the course of eachyear of your adult life, about 10% of your skeletonis completely broken down, and then rebuilt from scratch ina process called bone remodeling. here, the main playersare the osteoblasts, again, and another kind of cell that'skind of their complete opposite: osteoclasts, or bone breakers. you'd think maybe that thecells that form bone tissue and the ones that destroy itwould be in some kind of constant battle in yourbody, but during remodeling,
they work closely together andactually communicate nicely. it's like, they'rebasically frenemies. remodeling begins whenosteoclasts are sent, by way of hormone signals,through the capillaries to the sites of microscopicfractures in the bone matrix. once they're in place, theysecrete an acidic cocktail of hydrogen ions to dissolve thecalcium carbonate into calcium ions phosphate, water, and othermaterial that they carry back to nearby capillaries.
then they secrete enzymes thatspecialize in digesting collagen. this whole processis called resorption, and when the old bonetissue has been cleaned up, the osteoclasts send out a hormoneshout-out to the osteoblasts, who come in and dotheir ossification thing. bone remodeling isreally pretty amazing, and it's all ultimatelyregulated by hormones that maintain the levelsof calcium in your blood. the glands that call theplays during the bone-breaking
part of remodeling are theparathyroids in your neck. when the calciumin your blood plasma falls below thelevel of homeostasis, the parathyroid triggersosteoclasts to take calcium out of your bonesand release it back into the blood. likewise, when blood calciumlevels are too high, the parathyroid's cousin,the thyroid gland, signals osteoblasts to takecalcium out of the blood and lay it down on the bonecollagen through more ossification.
and remember last weekwhen we talked about how the kidneys reabsorbsalts and minerals? well, the thyroid alsoregulates how much calcium is reabsorbed in that process,as well as the amount of vitamin d, because vitamin d helpsyour body absorb calcium through the small intestine. and that is why vitamin d isgood for your bones and stuff. now, the relation of activeosteoblasts to active osteoclasts can change dramaticallyunder different conditions.
the more you stress your bones,the more osteoclasts work to break down the bone matrix,so that it can be re-formed. bone stress can include stufflike fractures, of course, but it can also be lesstraumatic and more sustained: exercise causes stress on theskeleton that helps stimulate bone remodeling, so whenyou're working out, you're not only building muscle,you're also building bone. so, as you can tell, it'skind of hard to talk about bones without alsotalking about muscles,
and that's whatwe're going to do on the next episode ofcrash course biology. thank you so much to the philipl. wright zoological museum sorry, i just hit you. check out their tumblr atumzoology.tumblr.com. it's awesome! if you want to reviewanything: table of contents! just click on it, or justre-watch the whole episode, because you know you liked it. and if you have anyquestions for us, of course,
we will be inthe comments below, as are all of the superhelpful people who are always answering questions who are not us. thank you to thosepeople by the way. and we will see you nexttime on crash course biology.
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