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 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 want to have lunch together. At the same time. On Taco Tuesday. If you crammed everyone into one giant lunchroom, you’d have lots 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 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 on 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.

[Opening music]

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 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. 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 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 multi-layered, 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 through 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 largely made up 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 thyroxine, 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 body 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 Cafe.

[Theme music]