The Skeletal System: Crash Course A&P #19


In March of 2015, American astronaut Scott
Kelly and his Russian colleague Mikhail Kornienko, began an unprecedented mission in space. They began a one-year term of service aboard
the International Space Station, the longest tour of duty ever served on the ISS. Now, I imagine there’s all sorts of stuff
to worry about when you’re packing for a year-long space voyage, like, say, “How
many books should I bring? How many pairs of underwear? Am I really okay with pooping
into a suctioned plastic bag every day for a year? Will I come upon a derelict ship haunted
by some stranded and insane astronaut from a forgotten mission, like in pretty much every
space horror movie ever? Will there be coffee?” Reasonable questions, all, but in reality,
another one you might want to ask is: “Will I be able to walk when I get back home?” We know micro-gravity is hard on a body, and
this mission is largely about testing the physical effects of being weightless for so long. Astronauts often experience things like trouble
sleeping, puffy faces, and loss of muscle mass, but perhaps the most serious damage a
microgravity environment causes is to the bones. And bones, well, they’re pretty clutch. Though they may look all dried up and austere,
don’t be fooled — your bones are alive. ALIVE I tell you! They’re actually as dynamic as any of your
organs, and are made of active connective tissue that’s constantly breaking down, regenerating,
and repairing itself throughout your lifetime. In fact, you basically get a whole new skeleton
every 7 to 10 years! In short, your bones do way more than just
providing your squishy sack of flesh with support and scaffolding and the ability to
move around. Your bones are basically how you store the
calcium, phosphate, and other minerals you need to keep neurons firing and muscles contracting. They’re also crucial to hematopoiesis, or
blood cell production. All of your new blood — and we’re talking like, a trillion blood
cells a day! — is generated in your bone marrow, which also helps store energy as fat. Bones even help maintain homeostasis by regulating
blood calcium levels and producing the hormone osteocalcin, which regulates bone formation
and protects against glucose intolerance and diabetes. So, the big buzzkill about life in space is
that, up there, a person suffers one to two percent bone loss EVERY MONTH. By comparison, your average elderly person
experiences 1-2 percent bone loss every YEAR. So for Kelly and Kornienko, that could mean
losing up to 20 percent over a year in orbit. Given everything your bones do for you, that’s
really serious. And while most of that loss is reversible
once they’re back on earth, it’s not as easy as chugging some of Madame Pomfrey’s
Skel-E-Gro potion. Rehabilitation can take years of hard work,
and that’s just after a few months in orbit… Which is why Kelly and Kornienko are heroes
of science, and not just for scholars of anatomy and physiology everywhere, but for anybody
who has bones. An average human body contains 206 bones,
ranging in shape and size from the tiny stapes of the inner ear to the huge femur of the
thigh. That’s a lot of bones to keep tabs on, so
anatomists often divide these structures first by location, into either axial or appendicular
groups. As you might guess, your axial bones are found
along your body’s vertical axis — in your skull, vertebral column, and rib cage. They’re kind of like your foundation, the
stuff you can’t really live without — they carry your other body parts, provide skeletal
support, and organ protection. Your appendicular bones are pretty much everything
else, the bones that make up your limbs, and the things that attach those limbs to your
axial skeleton, like your pelvis and shoulder blades. These are the bones that help us move
around. From there, bones are generally classified by their
shape, and luckily those names are pretty obvious. Long bones are your classic-looking, dog-bone-shaped
bones — the limb bones that are longer than they are wide, like tibia and fibula of your lower legs,
but also the trio of bones that make up your fingers. Follow some of those long bones to your foot
or hand, and you’ll hit a cube-shaped short bone, like your foot’s talus and cuboid,
or your wrist’s lacunate or scaphoid. Your flat bones are the thinner ones, like
your sternum and scapulae, and also the bones that make up your brain case. And your irregular bones are all the weirdly-shaped
things like your vertebrae and pelvis, which tend to be more specialized and unique. But despite their variations in size, shape,
and finer function, all bones have a similar internal structure. They all have a dense, smooth-looking external
layer of compact, or cortical bone around a porous, honeycomb-looking area of spongy
bone. This spongy bone tissue is made up of tiny
cross-hatching supports called trabeculae that help the bone resist stress. And it’s
also where you typically find your bone marrow, which comes in two colors, red and yellow. Red marrow is the stuff that makes blood cells,
so you should be glad that you have some of that. And yellow marrow stores energy as fat — if
you happen to be a predatory animal, yellow bone marrow can be one of the best sources
of calories you can find. The arrangement of these bone tissues, though, can be
slightly different, from one type of bone to the next. In flat, short, and irregular bones, for example,
these tissues kinda look like a spongy bone sandwich on compact-bone bread. But in some of your classic long bones, like
the femur and humerus, the spongy bone and its red marrow are concentrated at the tips. These flared ends, or epiphyses bookend the
bone’s shaft, or diaphysis, which — instead of having spongy bone in the center — surrounds a hollow
medullary cavity that’s full of that yellow marrow. Now, although bone can look rock-solid, grab
a microscope and you’ll see that it’s actually loaded with layered plates and laced with
little tunnels. It’s intricate and kinda confusing in there,
but the more you zoom into the microanatomy of bones, the better you can see how they’re built
and how they function, right down to the cellular level. Let’s start with the basic structural units
of bone, called osteons. These are cylindrical, weight-bearing structures
that run parallel to the bone’s axis. Look inside one and you’ll see that they’re composed
of tubes inside of tubes, so that a cross-section of an osteon looks like the rings of a tree
trunk. Each one of these concentric tubes, or lamellae, is
filled with collagen fibers that run in the same direction But if you inspect the fibers of a neighboring
lamella — either on the inside or outside of the first one — you’ll see that they run in a
different direction, creating an alternating pattern. This reinforced structure helps your bone resist
torsion stress, which is like twisting of your bones, which they experience a lot, and
I encourage you not to imagine what a torsion fracture of one of your bones might feel like. Now, bone needs nourishment like any other
tissue, so running along the length of each osteon are central canals, which hold nerves
and blood vessels. And then, tucked away between the layers of
lamellae are tiny oblong spaces called lacunae. As tiny as they are, these little gaps are
where the real work of your skeletal system gets done, because they house your osteocytes. These are mature bone cells that monitor and
maintain your bone matrix. They’re like the construction foremen of your bones, passing
along commands to your skeleton’s two main workhorses: the osteoblasts and the osteoclasts. Osteoblasts — from the Greek words for “bone”
and “germ” or “sprout” — are the bone-building cells, and they’re actually
what construct your bones in the first place. In the embryonic phase, bone tissue generally
starts off as cartilage, which provides a framework for your bones to grow on. When
osteoblasts come in, they secrete a glue-like cocktail of collagen, as well as enzymes that absorb
calcium, phosphate, and other minerals from the blood. These minerals form calcium phosphate, which
crystallize on the cartilage framework, ultimately forming a bone matrix that’s about one-third
mineral, two-thirds protein. From your time in the womb until you’re
about 25, your osteoblasts keep laying down more collagen and more calcium phosphate, until
your bones are fully grown and completely hardened. So while your osteoblasts are the bone-makers,
your osteoclasts are the bone-breakers — which is a kind of violent image. Maybe think of
them as like a bone-breaker-downer. Although the two kinds of cells do exact
opposite jobs, they’re not mortal enemies. In fact, I’m happy to report that they get
along fabulously, and create a perfect balance that allows your bones to regenerate. It’s like if you want to renovate your house,
you’ve gotta rip out all those busted cabinets and the musty carpeting before you can bring
in the nice hardwood floors and custom countertops. These cells work in a kinda similar way, in
a process that I’d argue is less stressful than home improvement — it’s called bone
remodeling. The supervisors of this process are those
osteocytes, which kick things off when they sense stress and strain, or respond to mechanical
stimuli, like the weightlessness of space, or the impact of running on pavement. So, say you’re out running and something
happens — nothing to be alarmed about! — but suddenly the osteocytes in your femur detect
a tiny, microscopic fracture, and initiate the remodeling process to fix it up. First, the osteocytes release chemical signals
that direct osteoclasts to the site of the damage. When they get there, they secrete
both a collagen-digesting enzyme, and an acidic hydrogen-ion mixture that dissolves the calcium
phosphate, releasing its components back into the blood. This tear-down process is called
resorption. When the old bone tissue is cleaned out, the
osteoclasts then undergo apoptosis, where they basically self-destruct before they can do any
more damage. But before they auto-terminate, they use the hormone hotline to call over the osteoblasts,
who come in and begin rebuilding the bone. The ratio of active osteoclasts to osteoblasts
can vary greatly, and if you stress your bones a lot, through injury, by carrying extra weight,
or just normal exercise, those osteoclasts are going to be swinging their little wrecking balls
non-stop, breaking down bone so it can be remade. In this way, exercising stimulates bone remodeling
— and ultimately bone strength — so when you’re working out, you’re building bone
as well as muscle. Which brings us back to our two space-heroes-slash-
guinea-pigs, Scott Kelly and Mikhail Kornienko. Space crews generally need to exercise at
least 15 hours a week to slow down the process of bone degradation, but even that can’t
fully stave loss of bone density. In microgravity, osteocytes aren’t getting much
loading stimuli, because less gravity means less weight. But, for reasons that we don’t understand
yet, the osteoclasts actually increase their rate of bone resorption in low gravity, while
the osteoblasts dial back on the bone formation. Because there’s more bone breaking than
bone making going on, everything is out of balance, and suddenly people start experiencing
1 to 2 percent monthly loss in bone mass. So, in addition to providing astronauts with
oxygen and water and food and protection from radiation and an environment that will keep
them mentally stable, it turns out that we also have to figure out how to keep their
bodies from consuming their own skeletons. But at least today we learned about the anatomy
of the skeletal system, including the flat, short, and irregular bones, and their individual
arrangements of compact and spongy bone. We also went over the microanatomy of bones,
particularly the osteons and their inner lamella. And finally we got an introduction to the
process of bone remodeling, which is carried out by crews of osteocytes, osteoblasts, and
osteoclasts. Special thanks to our Headmaster of Learning
Thomas Frank for his support for Crash Course and free education. And thank you to all of
our Patreon patrons who make Crash Course possible through their monthly contributions.
If you like Crash Course and you want to help us keep making cool new videos like this
one, you can check out patreon.com/crashcourse This episode was co-sponsored by The Midnight
House Elves, Fatima Iqbal, and Roger C. Rocha Crash Course is filmed in the Doctor Cheryl
C. Kinney Crash Course Studio. This episode was written by Kathleen Yale, edited by Blake
de Pastino, and our consultant, is Dr. Brandon Jackson. Our director is Nicholas Jenkins,
the editor and script supervisor is Nicole Sweeney, our sound designer is Michael Aranda,
and the graphics team is Thought Café.

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