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