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Today on Crash Course Anatomy & Physiology, Hank breaks down the parts and functions of one of your body’s unsung heroes: your epithelial tissue.
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Transcript Provided by YouTube:
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As any teacher will tell you, when you’re dealing with certain elements that are being
00:03
feisty and fidgety and basically not cooperating, there’s pretty much only one thing you can
00:07
do: you gotta keep ‘em separated.
00:09
And there’s a whole system of biological tissue that’s dedicated to doing just that
00:12
— creating order where there would otherwise be total mayhem.
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Because you and pretty much every other animal is made up of incredibly complex, feisty, fidgety
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systems that need to be kept apart to some extent if they’re going to get anything done.
00:25
Think of it this way: Say all the middle-schoolers in your town wanted to have lunch together.
00:30
At the same time. On Taco Tuesday.
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If you crammed everyone into one giant lunchroom, you’d have a lot of interesting and talented
00:36
people in one place, yes, but you’d also never get a handle on them with everyone shoved
00:42
and talking, and jostling, and flirting, and farting, and and stepping on toes, and haggling over tater tots.
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It’d be like a John Hughes movie gone horribly wrong.
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So what you need is a solid system of organization — like, separate lunch lines for separate
00:53
groups of kids, or tables that arrange students in alphabetical order. Your body is like that
00:57
crowded lunchroom — it needs order for it to function. It can’t have your liver all
01:02
up in your brain, or squished between your kidneys. Your organs and their systems need
01:07
their personal space.
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And that is where your unsung epithelial tissue steps in, like a burly gym teacher with a
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whistle and plan.
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This is the tissue that lines, and covers, and generally organizes your body, creating
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order from what would be chaos. Without epithelial tissue, you’d essentially be
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a mushy pile of unarticulated goo.
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When we talk about your epithelial tissue we’re really talking about two things. There’s
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the “proper” epithelium, which covers and lines your outer and inner body. And then
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there is the glandular epithelium, which forms glands and secretes hormones and other substances.
01:50
Your primary epithelium protects your whole body, inside and out. It’s a great organizer,
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partitioning everything into separate but connected units. It covers the surface of
01:59
your body when it combines with connective tissue to create skin, but it also lines your
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body cavities, and coats the internal and external walls of many of your organs.
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Because, your body doesn’t just interact with the outside world through your skin.
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We — and all animals from the simplest worms on up the Tree of Life — are really just
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tubes, corridors of tissue running from a mouth to an anus. Epithelial tissue covers
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both the inside and the outside of that you-tube.
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To get a better sense of what I mean, take a look at this balloon. The latex is like
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the outer covering of your body, in part made up of epithelial tissue. It separates what’s
02:34
inside the balloon from the rest of the world. Now if I stick my hand in there, you can see
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how, while the tissue still forms an outer layer, it also folds in on itself, creating
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a continuous barrier that lines all of the cavities.
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In a very similar way, the membranes covering your lungs for example, are actually invaginations
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of your epithelium — where the tissue that makes up your you-tube folds to form a cavity
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— just like this balloon when I push my fingers into it.
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The epithelium does all this to protect your deeper layers of tissue from injury or infection
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— like for example, by lining your stomach with epithelial cells that produce mucus,
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so that you don’t digest yourself along with your lunch.
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And all of your epithelial tissues are avascular — meaning they don’t have a blood supply.
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Instead they rely on the blood supply in the supporting connective tissues around them
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for the materials they need.
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But these tissues come in different varieties that serve different purposes.
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And a lot of what classifies the different types of epithelium boils down to their shape
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and layering — that is, the shape of the individual cells, and the number of layers that they form in.
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And there are three basic shapes — squamous, cuboidal, and columnar — and they’re pretty
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easy to tell apart because (unlike most terminology you’ll be exposed to in this course) their
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names actually describe what they look like!
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Squamous cells are flat. Their name means “scale,” and they look kind of squished,
03:50
like fish scales. Even the cell’s nucleus, which gets darkly stained and is usually easy to see, is flattened.
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Cuboidal cells are — you guessed it — cube-ish shaped, about as tall as they are wide. They
04:00
absorb nutrients and produce secretions, like sweat. Their nucleus is pretty circular.
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Columnar cells are tall and thick and look like columns, and they cushion underlying tissues.
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And as if they were cuboidal cells that got stretched tall, their nuclei also are stretched into an ellipse.
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And here’s yet another instance where the form of a structure relates to it purpose.
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In this case, the shape of each kind of epithelial cell correlates with its function.
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For example, squamous cells are flat, which makes it easy for materials like oxygen to
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move across them to the other side. So we see these kinds of cells where absorption
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or transportation is most important, like in say, the air sacs of your lungs, or in your blood vessels.
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But if the cells that make up a tissue need to, say, brew up hormones or mucus, they’ll
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need the internal machinery it takes to make that stuff, and that takes up a lot of space.
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So those cells can’t be flat — they’ve got to be cuboidal or columnar to accommodate
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more room for taking care of business.
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So that stomach lining that I mentioned, for example, is made up of big columnar cells,
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because they have to make and secrete mucus.
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But when it comes to what kind of cells are found where, an important thing to keep in
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mind is the fact that cells are, biologically speaking, expensive — they take a lot of
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time and energy and raw materials to make.
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So in places where you lose a lot of cells, like your outer skin, or in your mouth, you
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have more of squamous cells — because they’re smaller, and flatter, and therefore cheaper,
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practically disposable — rather than big, expensive cuboidal or columnar ones.
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Which brings me to the other trait that we use to classify epithelial tissue — its layering.
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A simple epithelium has only one layer of cells.
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A stratified type has multiple layers set on top of each other, like the bricks and
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mortar of a wall.
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And pseudostratified epithelium is mostly just one layer, but the cells can be different
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shapes and sizes, and the nuclei can be at lots of different levels, so it looks sort
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of messy and multilayered, even though it really isn’t.
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And when we describe a type of epithelial tissue, like in a lab setting, we cite both
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its shape and its layering. You can think of a tissue’s first name as its number of
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layers, and its last name as the shape of its cells.
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For example, a simple squamous epithelium refers to a single layer of flat, scale-like
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cells, like the lining of the air sacs deep in your lungs.
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A stratified cuboidal tissue, meanwhile, would have layers of cube-shaped cells, like the
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linings of the ducts that leak sweat and spit.
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When you put the shape of a cell together with its type of layering, you can begin to
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see how both traits inform the function of your epithelial tissue.
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Let’s go back to those squamous cells. Because they’re thin, like scales, it takes many
06:24
layers of them to form a tissue that’s thick enough to offer protection. So you end up
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with a really dense stack of cells that, on an individual basis, are small and cheap to make.
06:34
That’s why when I, like, scratch my hand or hit the inside of my mouth with a toothbrush,
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I can lose a couple of layers, no big deal. Those squamous cells are a dime a dozen. There’s
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still lots of layers left. Plus, epithelial tissue regenerates really quickly.
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But if you, say, get tossed off a moving motorcycle, you’ll lose a lot more layers. And if your
06:53
road rash is really bad, you could scrape all the way through all of those squamous
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cells, down to the nerves and the blood and all of the underlying connective tissue, plowing
07:00
through a lot more expensive cells, and wind up with a real, like,
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can-you-please-get-me-to-the-hospital-I-need-to-get-to-the-hospital kind of problem.
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Of course, when we talk about epithelial tissue protecting you, it’s not always protecting
07:10
you from the outside world. It also creates order among all of those rambunctious seventh
07:14
graders that are your organs. And here it’s important to note that all of your epithelial
07:19
cells are polar, meaning they have distinct sides. The apical or upper side, is exposed
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to either the outside of your body, or whatever internal cavity it’s lining. The basal side,
07:29
or inner surface, is tightly attached to the basement membrane, a thin layer of mostly
07:34
collagen fibers that helps hold the epithelium together, and anchors it to the next-deeper
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layer — your connective tissue.
07:41
Many of these boundaries that the cells form aren’t absolute — instead, they’re selectively
07:45
permeable, allowing for some level of absorption, filtration, and excretion of substances.
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The tissue lining your small intestines, for instance, is what allows you to absorb nutrients
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through diffusion and active transport, so that’s pretty important. And all of your urinary
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waste gets filtered through a different epithelial lining in your kidneys.
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So by now you’re probably starting to get it: Every interaction that your body has with
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the rest of the physical universe is made possible somehow by your epithelium.
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But that is not all!
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Remember: Your glands are also made up largely of epithelial tissue, so it ALSO plays
08:15
a big role in facilitating all of your secretions — from sweat and mucus, to hormones and enzymes.
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This glandular epithelium forms two different kinds of glands — your endocrine glands,
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the ones that secrete hormones right into your bloodstream or to nearby cells,
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and your exocrine glands, the type that secrete their juices into tubes or ducts that lead
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to the outside of the body, or the inside of your tube, rather than right into the blood.
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The hormone thyroxin, for example, is secreted by an endocrine gland — your thyroid — and it
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needs to be distributed throughout the entire body so that it can stimulate the metabolism in all of your cells.
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Some examples of exocrine secretions would be sweat, saliva, mucus, stomach acid, and
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milk, if you’re lactating.
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All those secretions go right into ducts where they’re ferried to an epithelial surface
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— which could be your outer layer of skin, in the case of your sweat, or the edge of
09:04
your stomach lining if it’s your stomach acid.
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So, hey the system works. And it’s due in large part to the humorless gym teacher that
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is your epithelial tissue. It may not always be a ton of fun, but darn
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it, it gets results.
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Today you learned how your unsung epithelial tissue creates the inner and outer boundaries
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that keep you alive. We looked at how proper epithelial tissue is classified by both layering
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— simple or stratified — and shape — squamous, cuboidal, or columnar — and how the structure
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of these tissue types match their function. We also talked about how epithelial cells
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are polar, having both apical and basal sides, and are selectively permeable, and lastly
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we took a brief look at how our glandular epithelial tissue forms both out endocrine
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and exocrine glands.
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Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course
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possible. To find out how you can become a supporter, just go to subbable.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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is Sarah Mesimer, the 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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