This, my friends, is a walrus baculum. It's basically a..a penis bone, found in most placental mammals, interestingly not in humans. And this 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, uh, good for being giant, or fighting off predators, and fighting for...I don't know, why do rhinos have big heads? And this is the skull of a pronghorn antelope, it has these horns that come off that are covered in these keratin 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 Philip L. Wright Zoological Museum, at the University of Montana. And all of these bones have adapted to help animals survive, the horns on the pronghorn for mating displays and self defense, the 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 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. But 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 alive, people. And walrus penises are just the beginning. 

(Crash Course intro)

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 shape, and that can be used to produce movement. Insects have exoskeletons of course, made of the nitrogenous carbohydrate chitin, and mollusks have exoskeletons too, in the form of calcium carbonate shells. But when it comes to skeletons, the winningest formula has been the endoskeleton. Even though we'd probably feel a lot safer if we were covered in armored plates like some race of iron men, having our skeletons inside of our bodies has allowed us to grow larger and have much more freedom of movement. It's good stuff.

One of the many reasons you don't see ants the size of horses walking around is, well, one, 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 heaver and 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 of all kinds of shapes and sizes, including 3 tiny ones in each ear, and 1 weird shaped one like a 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'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 8 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, because it's time for Biolo-graphy!

(theme music)

(03:27) 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, they're what stick around the longest. It's 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 of what we know about the human skeletal system is thanks to Andries Van Wesel, who was born in what's now Belgium in 1514. And in those days if you were like, a Kung-Fu master of science, you pretty much got your own Latin name, so today he's known as Andreas Vesalius. Vesalius came from a long line of physicians, 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's greatest contribution was showing the world that everything we though we knew about osteology, was wrong.

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 skeleton, especially the teachings of the philosopher-doctor Galen, were way, way off.

See, Roman law prohibited the dissection of human bodies, so none of those guys 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 student's favorite, human dissection! He instructed students by dismembering corpses in front of them and cataloging 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 he executed to Vesalius. Suddenly, the dude was up to his codpiece in pig thieves and murderers, and by the time he was 28, he'd done enough research that he published De Humani 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. Its 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.

(06:13) 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, which you may know from your nose and your ears. It's made of specialized cells called chondrocytes, 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. "Oste-," which you'll be hearing a lot of today, just means bone, and "blast" means germ or bud. The bone-building that they do is called, fittingly, ossification. First, they secrete this gelatinous goo that's a combination of collagen and a polysaccharide that act 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, they're 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.

Kinda surprising, right? Most of your bone isn't even mineral, and even the part that is, is living tissue, because it's all honeycombed with blood vessels that allow osteoblasts and other cells to do their jobs. Unlike an insect's exoskeleton, even the hardest parts of your bones are alive.

(08:00) 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 bone's mass. In the middle, the spongy or trabecular bone, is softer, and more porous, and contains the marrow and fatty tissues in larger bone. 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, a week of your time and 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 to house the blood stem cells that, together, can produce one trillion blood cells in you every day. That's 10 to the freakin' 12th.

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 an epiphysis. The bones grow, as a child grows, the new tissue forms at the border between the two, a place called the epiphyseal plate. As they did when they formed 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, and the last of these plates in your bones hardens.

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 at the base of your brain. As adults, this and other glands produce less growth hormone which slows down our bone lengthening.

(09:47) But, even though lengthening is a limited-time-only process, the thickness and strength of the 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 broken 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'd 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 to actually communicate nicely. 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, and the calcium, phosphates, and water, and....other material that they carry back to nearby capillaries. Then, they secrete enzymes that specialize 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 shout-out 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 level 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 osteoclasts to take calcium out of your bones and release it back into the blood. Likewise, when blood calcium levels are too high, the parathyroid's 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 osteoclasts can change dramatically under different conditions. The more you stress your bones, the more your 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 muscle, you're also building bone.

So, as you can tell, it's kinda hard to talk about bones without also talking about muscles. And that's what we're gonna do on the next episode of Crash Course Biology. Thank you so much to the Philip L. Wright Zoological Museum at the University of Montana...sorry I just...hit you. Check out their Tumblr at http://umzoology.tumblr.com. It's awesome! If you want to review anything: table of contents! Just click on it, or just re-watch the whole episode, because you know you liked it. If you have any questions for us, of course, we will be in the comments below, as will all of the super helpful people who are always answering questions who are not us. Thank you to those people, by the way. And we will see you next time on Crash Course Biology!