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Transcript Provided by YouTube:
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Check out this amoeba.
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Pretty nice. Kind of a rugged, no-frills life form.
00:04
The thing about amoebas is that they do everything in the same place. They take in and digest
00:08
their food, and reject their waste, and get through everything else they need to do, all
00:11
within a single cell.
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They don’t need trillions of different cells working together to keep them alive. They
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don’t need a bunch of structures to keep their stomachs away from their hearts away
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from their lungs. They’re content to just blob around and live the simple life.
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But we humans, along with the rest of the multicellular animal kingdom, are substantially
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more complex. We’re all about cell specialization, and compartmentalizing our bodies.
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Every cell in your body has its own specific job description related to maintaining your
00:36
homeostasis, that balance of materials and energy that keeps you alive.
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And those cells are the most basic building blocks in the hierarchy of increasingly complex
00:44
structures that make you what you are.
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We covered a lot of cell biology in Crash Course Bio, so if you haven’t taken
00:50
that course with us yet, or if you just want a refresher, you can go over there now.
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I will still be here when you get back.
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But with that ground already covered, we’re going to skip ahead to when groups of similar
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cells come together to perform a common function, in our tissues.
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Tissues are like the fabric of your body. In fact, the term literally means “woven.”
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And when two or more tissues combine, they form our organs. Your kidneys, lungs, and
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your liver, and other organs are all made of different types of tissues.
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But what function a certain part of your organ performs, depends on what kind of tissue it’s
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made of. In other words, the type of tissue defines its function.
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And we have four primary tissues, each with a different job:
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our nervous tissue provides us with control and communication,
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muscle tissues give us movement,
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epithelial tissues line our body cavities and organs, and essentially cover and protect the body,
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while connective tissues provide support.
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If our cells are like words, then our tissues, or our groups of cells, are like sentences,
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the beginning of a language.
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And your journey to becoming fluent in this language of your body — your ability to read,
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understand, and interpret it — begins today.
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Although physicians and artists have been exploring human anatomy for centuries, histology
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— the study of our tissues — is a much younger discipline.
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That’s because, in order to get all up in a body’s tissues, we needed microscopes,
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and they weren’t invented until the 1590’s, when Hans and Zacharias Jansen, a father-son
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pair of Dutch spectacle makers, put some lenses in a tube and changed science forever.
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But as ground-breaking as those first microscopes were then, they were little better than something
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you’d get in a cereal box today — that is to say, low in magnification and pretty blurry.
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So the heyday of microscopes didn’t really get crackin’ until the late 1600s, when
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another Dutchman — Anton van Leeuwenhoek — became the first to make and use truly
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high-power microscopes.
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While other scopes at the time were lucky to get 50-times magnification, Van Leeuwenhoek’s
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had up to 270-times magnifying power, identifying things as small as one thousandth of a millimeter.
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Using his new scope, Leeuwenhoek was the first to observe microorganisms, bacteria, spermatozoa,
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and muscle fibers, earning himself the illustrious title of The Father of Microbiology for his troubles.
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But even then, his amazing new optics weren’t quite enough to launch the study of histology
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as we know it, because most individual cells in a tissue weren’t visible in your average scope.
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It took another breakthrough — the invention of stains and dyes — to make that possible.
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To actually see a specimen under a microscope, you have to first preserve, or fix it, then
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slice it into super-thin, deli-meat-like sections that let the light through, and then stain
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that material to enhance its contrasts.
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Because different stains latch on to different cellular structures, this process lets us
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see what’s going on in any given tissue sample, down to the specific parts of each
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individual cell.
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Some stains let us clearly see cells’ nuclei — and as you learn to identify different
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tissues, the location, shape, size, or even absence of nuclei will be very important.
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Now, Leeuwenhoek was technically the first person to use a dye — one he made from saffron
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— to study biological structures under the scope in 1673, because, the dude was a boss.
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But it really wasn’t until nearly 200 years later, in the 1850s, that the we really got the
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first true histological stain. And for that we can thank German anatomist
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Joseph von Gerlach.
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Back in his day, a few scientists had been tinkering with staining tissues, especially
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with a compound called carmine — a red dye derived from the scales of a crushed-up insects.
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Gerlach and others had some luck using carmine to highlight different kinds of cell structures,
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but where Gerlach got stuck was in exploring the tissues of the brain.
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For some reason, he couldn’t get the dye to stain brain cells, and the more stain he
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used, the worse the results were.
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So one day, he tried making a diluted version of the stain — thinning out the carmine with
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ammonia and gelatin — and wetted a sample of brain tissue with it.
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Alas, still nothing.
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So he closed up his lab for the night, and, as the story goes, in his disappointment,
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he forgot to remove the slice of someone’s cerebellum that he had left sitting in the
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He returned the next morning to find the long, slow soak in diluted carmine had stained all
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kinds of structures inside the tissue — including the nuclei of individual brain cells and what
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he described as “fibers” that seemed to link the cells together.
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It would be another 30 years before we knew what a neuron really looked like, but Gerlach’s
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famous neural stain was a breakthrough in our understanding of nervous tissue.
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AND it showed other anatomists how the combination of the right microscope and the right stain
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could open up our understanding of all of our body’s tissues and how they make life possible.
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Today, we recognize the cells Gerlach studied as a type of nervous tissue, which forms,
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you guessed it, the nervous system — that is, the brain and spinal cord of the central
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nervous system, and the network of nerves in your peripheral nervous system. Combined,
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they regulate and control all of your body’s functions.
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That basic nervous tissue has two big functions — sensing stimuli and sending electrical
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impulses throughout the body, often in response to those stimuli.
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And this tissue also is made up of two different cell types — neurons and glial cells.
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Neurons are the specialized building blocks of the nervous system. Your brain alone contains
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billions of them — they’re what generate and conduct the electrochemical nerve impulses
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that let you think, and dream, and eat nachos, or do anything.
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But they’re also all over your body. If you’re petting a fuzzy puppy, or you touch
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a cold piece of metal, or rough sandpaper, it’s the neurons in your skin’s nervous
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tissue that sense that stimuli, and send the message to your brain to say, like, “cuddly!”
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or “Cold!” or “why am I petting sandpaper?!”
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No matter where they are, though, each neuron has the same anatomy, consisting of the cell
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body, the dendrites, and the axon.
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The cell body, or soma, is the neuron’s life support. It’s got all the necessary
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goods like a nucleus, mitochondria, and DNA.
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The bushy dendrites look like the trees that they’re named after, and collect signals from other
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cells to send back to the soma. They are the listening end.
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The long, rope-like axon is the transmission cable — it carries messages to other neurons,
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and muscles, and glands. Together all of these things combine to form nerves of all different
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sizes laced throughout your body.
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The other type of nervous cells, the glial cells, are like the neuron’s pit crew, providing
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support, insulation, and protection, and tethering them to blood vessels.
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But sensing the world around you isn’t much use if you can’t do anything about it, which
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is why we’ve also got muscle tissues.
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Unlike your nervous tissues, your muscle tissues can contract and move, which is super handy
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if you want to walk or chew or breathe.
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Muscle tissue is well-vascularized, meaning it’s got a lot of blood coming and going,
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and it comes in three flavors: skeletal, cardiac, and smooth.
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Your skeletal muscle tissue is what attaches to all the bones in your skeleton, supporting
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you and keeping your posture in line.
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Skeletal muscle tissues pull on bones or skin as they contract to make your body move.
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You can see how skeletal muscle tissue has long, cylindrical cells. It looks kind of
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clean and smooth, with obvious striations that resemble little pin stripes. Many of
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the actions made possible in this tissue — like your wide range of facial expressions or pantheon
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of dance moves — are voluntary.
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Your cardiac muscle tissue, on the other hand, works involuntarily. Which is great, because
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it forms the walls of your heart, and it would be really distracting to have to remind it
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to contract once every second. This tissue is only found in your heart, and its regular
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contractions are what propel blood through your circulatory system.
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Cardiac muscle tissue is also striped, or striated, but unlike skeletal muscle tissue,
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their cells are generally uninucleate, meaning that they have just one nucleus. You can also
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see that this tissue is made of a series of sort of messy cell shapes that look they divide
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and converge, rather than running parallel to each other.
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But where these cells join end-to-end you can see darker striations, These are the glue
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that hold the muscle cells together when they contract, and they contain pores so that electrical
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and chemical signals can pass from one cell to the next.
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And finally, we’ve got the smooth muscle tissue, which lines the walls of most of your
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blood vessels and hollow organs, like those in your digestive and urinary tracts, and
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your uterus, if you have one.
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It’s called smooth because, as you can see, unlike the other two, it lacks striation.
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Its cells are sort of short and tapered at the ends, and are arranged to form tight-knit sheets.
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This tissue is also involuntary, because like the heart, these organs squeeze substances
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through by alternately contracting and relaxing, without you having to think about it.
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Now, one thing that every A&P; student has to be able to do is identify different types
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of muscle tissue from a stained specimen.
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So Pop Quiz, hot shot!
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See if you can match the following tissue stains with their corresponding muscle tissue
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types. Don’t forget to pay attention to striations and cell-shape!
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Let’s begin with this. Which type of tissue is it?
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The cells are striated. Each cell only has one nucleus. But the giveaway here is probably
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the cells’ branching structure; where their offshoots meet with other nearby cells where
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they form those intercalated discs. It’s cardiac muscle.
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Or these — they’re uninucleate cells, too, and they also are packed together pretty closely
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together. But…no striations. They’re smooth, so this is smooth muscle.
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Leaving us with an easy one — long, and straight cells with obvious striations AND multiple
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nuclei. This could only be skeletal muscle tissue.
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If you got all of them right, congratulations and give yourself a pat on your superior posterior
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medial skeletal muscles — you’re well on your to understanding histology.
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Today you learned that cells combine to form our nervous, muscle, epithelial, and connective
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tissues. We looked into how the history of histology started with microscopes and stains,
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and how our nervous tissue forms our nervous system. You also learned how your skeletal,
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smooth, and cardiac muscle tissue facilitates all your movements, both voluntary and involuntary,
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and how to identify each in a sample.
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Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course
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possible to themselves and also to everyone else in the world. To find out how you can
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become a supporter, just go to subbable dot com.
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This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant
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is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor
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and sound designer is Michael Aranda, and the graphics team is Thought Café.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.
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