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  • Check out this amoeba.

  • Pretty nice. Kind of a rugged, no-frills life form.

  • The thing about amoebas is that they do everything in the same place. They take in and digest

  • their food, and reject their waste, and get through everything else they need to do, all

  • within a single cell.

  • They don’t need trillions of different cells working together to keep them alive. They

  • don’t need a bunch of structures to keep their stomachs away from their hearts away

  • from their lungs. Theyre content to just blob around and live the simple life.

  • But we humans, along with the rest of the multicellular animal kingdom, are substantially

  • more complex. Were all about cell specialization, and compartmentalizing our bodies.

  • Every cell in your body has its own specific job description related to maintaining your

  • homeostasis, that balance of materials and energy that keeps you alive.

  • And those cells are the most basic building blocks in the hierarchy of increasingly complex

  • structures that make you what you are.

  • We covered a lot of cell biology in Crash Course Bio, so if you haven’t taken

  • that course with us yet, or if you just want a refresher, you can go over there now.

  • I will still be here when you get back.

  • But with that ground already covered, were going to skip ahead to when groups of similar

  • cells come together to perform a common function, in our tissues.

  • Tissues are like the fabric of your body. In fact, the term literally meanswoven.”

  • And when two or more tissues combine, they form our organs. Your kidneys, lungs, and

  • your liver, and other organs are all made of different types of tissues.

  • But what function a certain part of your organ performs, depends on what kind of tissue it’s

  • made of. In other words, the type of tissue defines its function.

  • And we have four primary tissues, each with a different job:

  • our nervous tissue provides us with control and communication,

  • muscle tissues give us movement,

  • epithelial tissues line our body cavities and organs, and essentially cover and protect the body,

  • while connective tissues provide support.

  • If our cells are like words, then our tissues, or our groups of cells, are like sentences,

  • the beginning of a language.

  • And your journey to becoming fluent in this language of your body -- your ability to read,

  • understand, and interpret it -- begins today.

  • Although physicians and artists have been exploring human anatomy for centuries, histology

  • -- the study of our tissues -- is a much younger discipline.

  • That’s because, in order to get all up in a body’s tissues, we needed microscopes,

  • and they weren’t invented until the 1590’s, when Hans and Zacharias Jansen, a father-son

  • pair of Dutch spectacle makers, put some lenses in a tube and changed science forever.

  • But as ground-breaking as those first microscopes were then, they were little better than something

  • you’d get in a cereal box today -- that is to say, low in magnification and pretty blurry.

  • So the heyday of microscopes didn’t really get crackinuntil the late 1600s, when

  • another Dutchman -- Anton van Leeuwenhoek -- became the first to make and use truly

  • high-power microscopes.

  • While other scopes at the time were lucky to get 50-times magnification, Van Leeuwenhoek’s

  • had up to 270-times magnifying power, identifying things as small as one thousandth of a millimeter.

  • Using his new scope, Leeuwenhoek was the first to observe microorganisms, bacteria, spermatozoa,

  • and muscle fibers, earning himself the illustrious title of The Father of Microbiology for his troubles.

  • But even then, his amazing new optics weren’t quite enough to launch the study of histology

  • as we know it, because most individual cells in a tissue weren’t visible in your average scope.

  • It took another breakthrough -- the invention of stains and dyes -- to make that possible.

  • To actually see a specimen under a microscope, you have to first preserve, or fix it, then

  • slice it into super-thin, deli-meat-like sections that let the light through, and then stain

  • that material to enhance its contrasts.

  • Because different stains latch on to different cellular structures, this process lets us

  • see what’s going on in any given tissue sample, down to the specific parts of each

  • individual cell.

  • Some stains let us clearly see cellsnuclei -- and as you learn to identify different

  • tissues, the location, shape, size, or even absence of nuclei will be very important.

  • Now, Leeuwenhoek was technically the first person to use a dye -- one he made from saffron

  • -- to study biological structures under the scope in 1673, because, the dude was a boss.

  • But it really wasn’t until nearly 200 years later, in the 1850s, that the we really got the

  • first true histological stain. And for that we can thank German anatomist

  • Joseph von Gerlach.

  • Back in his day, a few scientists had been tinkering with staining tissues, especially

  • with a compound called carmine -- a red dye derived from the scales of a crushed-up insects.

  • Gerlach and others had some luck using carmine to highlight different kinds of cell structures,

  • but where Gerlach got stuck was in exploring the tissues of the brain.

  • For some reason, he couldn’t get the dye to stain brain cells, and the more stain he

  • used, the worse the results were.

  • So one day, he tried making a diluted version of the stain -- thinning out the carmine with

  • ammonia and gelatin -- and wetted a sample of brain tissue with it.

  • Alas, still nothing.

  • So he closed up his lab for the night, and, as the story goes, in his disappointment,

  • he forgot to remove the slice of someone’s cerebellum that he had left sitting in the

  • He returned the next morning to find the long, slow soak in diluted carmine had stained all

  • kinds of structures inside the tissue -- including the nuclei of individual brain cells and what

  • he described asfibersthat seemed to link the cells together.

  • It would be another 30 years before we knew what a neuron really looked like, but Gerlach’s

  • famous neural stain was a breakthrough in our understanding of nervous tissue.

  • AND it showed other anatomists how the combination of the right microscope and the right stain

  • could open up our understanding of all of our body’s tissues and how they make life possible.

  • Today, we recognize the cells Gerlach studied as a type of nervous tissue, which forms,

  • you guessed it, the nervous system -- that is, the brain and spinal cord of the central

  • nervous system, and the network of nerves in your peripheral nervous system. Combined,

  • they regulate and control all of your body’s functions.

  • That basic nervous tissue has two big functions -- sensing stimuli and sending electrical

  • impulses throughout the body, often in response to those stimuli.

  • And this tissue also is made up of two different cell types -- neurons and glial cells.

  • Neurons are the specialized building blocks of the nervous system. Your brain alone contains

  • billions of them -- theyre what generate and conduct the electrochemical nerve impulses

  • that let you think, and dream, and eat nachos, or do anything.

  • But theyre also all over your body. If youre petting a fuzzy puppy, or you touch

  • a cold piece of metal, or rough sandpaper, it’s the neurons in your skin’s nervous

  • tissue that sense that stimuli, and send the message to your brain to say, like, “cuddly!”

  • orCold!” orwhy am I petting sandpaper?!”

  • No matter where they are, though, each neuron has the same anatomy, consisting of the cell

  • body, the dendrites, and the axon.

  • The cell body, or soma, is the neuron’s life support. It’s got all the necessary

  • goods like a nucleus, mitochondria, and DNA.

  • The bushy dendrites look like the trees that theyre named after, and collect signals from other

  • cells to send back to the soma. They are the listening end.

  • The long, rope-like axon is the transmission cable -- it carries messages to other neurons,

  • and muscles, and glands. Together all of these things combine to form nerves of all different

  • sizes laced throughout your body.

  • The other type of nervous cells, the glial cells, are like the neuron’s pit crew, providing

  • support, insulation, and protection, and tethering them to blood vessels.

  • But sensing the world around you isn't much use if you can't do anything about it, which

  • is why we've also got muscle tissues.

  • Unlike your nervous tissues, your muscle tissues can contract and move, which is super handy

  • if you want to walk or chew or breathe.

  • Muscle tissue is well-vascularized, meaning it’s got a lot of blood coming and going,

  • and it comes in three flavors: skeletal, cardiac, and smooth.

  • Your skeletal muscle tissue is what attaches to all the bones in your skeleton, supporting

  • you and keeping your posture in line.

  • Skeletal muscle tissues pull on bones or skin as they contract to make your body move.

  • You can see how skeletal muscle tissue has long, cylindrical cells. It looks kind of

  • clean and smooth, with obvious striations that resemble little pin stripes. Many of

  • the actions made possible in this tissue -- like your wide range of facial expressions or pantheon

  • of dance moves -- are voluntary.

  • Your cardiac muscle tissue, on the other hand, works involuntarily. Which is great, because

  • it forms the walls of your heart, and it would be really distracting to have to remind it

  • to contract once every second. This tissue is only found in your heart, and its regular

  • contractions are what propel blood through your circulatory system.

  • Cardiac muscle tissue is also striped, or striated, but unlike skeletal muscle tissue,

  • their cells are generally uninucleate, meaning that they have just one nucleus. You can also

  • see that this tissue is made of a series of sort of messy cell shapes that look they divide

  • and converge, rather than running parallel to each other.

  • But where these cells join end-to-end you can see darker striations, These are the glue

  • that hold the muscle cells together when they contract, and they contain pores so that electrical

  • and chemical signals can pass from one cell to the next.

  • And finally, weve got the smooth muscle tissue, which lines the walls of most of your

  • blood vessels and hollow organs, like those in your digestive and urinary tracts, and

  • your uterus, if you have one.

  • It’s called smooth because, as you can see, unlike the other two, it lacks striation.

  • Its cells are sort of short and tapered at the ends, and are arranged to form tight-knit sheets.

  • This tissue is also involuntary, because like the heart, these organs squeeze substances

  • through by alternately contracting and relaxing, without you having to think about it.

  • Now, one thing that every A&P student has to be able to do is identify different types

  • of muscle tissue from a stained specimen.

  • So Pop Quiz, hot shot!

  • See if you can match the following tissue stains with their corresponding muscle tissue

  • types. Don’t forget to pay attention to striations and cell-shape!

  • Let’s begin with this. Which type of tissue is it?

  • The cells are striated. Each cell only has one nucleus. But the giveaway here is probably

  • the cellsbranching structure; where their offshoots meet with other nearby cells where

  • they form those intercalated discs. It's cardiac muscle.

  • Or these -- theyre uninucleate cells, too, and they also are packed together pretty closely

  • together. Butno striations. Theyre smooth, so this is smooth muscle.

  • Leaving us with an easy one -- long, and straight cells with obvious striations AND multiple

  • nuclei. This could only be skeletal muscle tissue.

  • If you got all of them right, congratulations and give yourself a pat on your superior posterior

  • medial skeletal muscles -- youre well on your to understanding histology.

  • Today you learned that cells combine to form our nervous, muscle, epithelial, and connective

  • tissues. We looked into how the history of histology started with microscopes and stains,

  • and how our nervous tissue forms our nervous system. You also learned how your skeletal,

  • smooth, and cardiac muscle tissue facilitates all your movements, both voluntary and involuntary,

  • and how to identify each in a sample.

  • Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course

  • possible to themselves and also to everyone else in the world. To find out how you can

  • become a supporter, just go to subbable dot com.

  • This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant

  • is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor

  • and sound designer is Michael Aranda, and the graphics team is Thought Café.

Check out this amoeba.

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ティッシュその1クラッシュコースA&P #2 (Tissues, Part 1: Crash Course A&P #2)

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