Tissues, Part 2 – Epithelial Tissue: Crash Course A&P #3

Tissues, Part 2 – Epithelial Tissue: Crash Course A&P #3


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

100 Replies to “Tissues, Part 2 – Epithelial Tissue: Crash Course A&P #3”

  1. Very helpful, but he talks too fast . This video is not for students who are learning english as a second language. Please slow down just a little bit so that we can benefit from your videos.

  2. My ems instructor just sent me a 46 video Playlist
    😂, and said "make sure you learn this". Thanks for making it easy to understand…. My head would explode otherwise

  3. I'm 7th grader sent here by science teacher to study for a test. I'm beginning to wonder if she chose this video for a reason other than science. . .

  4. Very informative, however, you speak too quickly for someone (like me who has little to no knowledge) to fully understand.

  5. I would love to have a teacher like this☺️😍 Classes would full of energy, less boring and not like the typical teachers that while teaching the only thing you can hear 👂 is,,, bla-bla-bla-bla-Blaa science, sciences and that’s set 🙃🙂

  6. Let me tell you i haven't studied nor looked at anything with anatomy for 5 years, since high school. This is an amazing way to catch up! Thanks!

  7. I remember in high school, teachers would show us these videos. I could never really understand them or appreciate them. Now I'm in college and have found that this is my way of studying. The textbook is too boring and I space off every time I open it, but this keeps me engaged and summarizes very well without any unnecessary details. Thank you, Crash Course!

  8. i am new student in vet caurse and its hard to understand this lecture , i try with 2 different drs in my college and both of them are hard to catch the information … BUT i am really glade that i found your channel , here is the subs and like + comment , i hope i get liked on this comment from you sir! thank you from my heart

  9. The one thing I disagree with you on is that I am not an animal as I am made in the image of God who created the Heavens and the Earth. Aside from this, I really like your channel. God Bless..

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