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 good for being giant, for 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 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.
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.
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.
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
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.
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 heavier 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,
1 weird one 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!
Now, you'd think that we'd have nailed down the basics
of the human skeleton a long time ago,
because our teeth and bones are the biggest, 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?
Must 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 to 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' 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, y'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 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 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 the 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 had 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.
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 of your other 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 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.
They first secrete a gelatinous goo that's a combination of collagen
and a polysaccharide that acts like a kind of organic glue.
Then they start absorbing minerals and salts from the blood
in the capillaries all around them, and unsurprisingly,
they are 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.
Now, even though a bone can take all kinds of forms,
from big, flat plates protecting your 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 bones.
The marrow, of course, makes not only new red blood cells
but almost all of your different blood cells
by a process called hematopoeisis.
I'd need like about 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 1 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 your biggest bone in your body.
The main shaft is called the diaphysis,
and each rounded end is an epiphysis.
When 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,
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 pea nestled at 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 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 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:
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 and 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 carbonate into calcium ions
phosphate, 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 levels of calcium in your blood.
The glands that call the plays during the bone-breaking
part of 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 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 osteoclasts work
to break down the bone matrix, so that it can be re-formed.
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 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 Philip L. Wright Zoological Museum
at the University of Montana.
Sorry, I just hit you.
Check out their tumblr at 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.
And if you have any questions for us, of course,
we will be in the comments below,
as are 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.