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| Anatomy and Physiology |
Study Note
This lecture discusses the importance and role of epithelial tissue in the human body. Epithelial tissue provides structure and order, covering and lining organs to separate systems and maintain bodily organization. Without it, our organs and systems would mix and cause chaos. Epithelial tissue comes in two main forms: proper epithelium (which covers and lines the body, both inside and out) and glandular epithelium (which forms glands and secretes hormones and other substances). The structure of epithelial tissue is specialized based on cell shape and layering to meet various functions such as protection, secretion, absorption, and filtration.
20 Main Ideas
1. Epithelial tissue organizes and protects the body, keeping different systems in order.
2. Without epithelial tissue, the body would lack structural organization.
3. There are two main types of epithelial tissue: proper and glandular.
4. Proper epithelium lines and covers both internal and external parts of the body.
5. Glandular epithelium forms glands that secrete hormones and other substances.
6. Epithelial tissue is essential in maintaining boundaries between different organ systems.
7. Epithelial cells are avascular, relying on surrounding connective tissues for nutrients.
8. Epithelial cells come in three shapes: squamous, cuboidal, and columnar.
9. The shape of epithelial cells corresponds to their function, such as absorption or secretion.
10. Squamous cells are flat and found in areas where diffusion or filtration occurs.
11. Cuboidal and columnar cells are larger and found in tissues involved in secretion or absorption.
12. Simple epithelium consists of one layer, while stratified epithelium has multiple layers.
13. Pseudostratified epithelium appears multilayered but is not.
14. Epithelial tissue regenerates quickly and is classified by both shape and layering.
15. Epithelial cells have two sides: apical (facing the cavity or outside) and basal (anchored to connective tissue).
16. The basement membrane anchors epithelial tissue to the underlying connective tissue.
17. Epithelial tissue is selectively permeable, allowing for absorption, filtration, and excretion.
18. Endocrine glands secrete hormones into the bloodstream, while exocrine glands secrete substances into ducts.
19. Glandular epithelial tissue is essential for various secretions like sweat, mucus, and hormones.
20. Epithelial tissue forms boundaries that separate and protect different parts of the body, ensuring proper function.
20 Key Points
1. Epithelial tissue keeps the body's systems organized and functioning properly.
2. Without epithelial tissue, organs would mix, causing dysfunction.
3. Proper epithelium covers the body’s surface and lines internal cavities.
4. Glandular epithelium forms glands that secrete substances like hormones and enzymes.
5. Epithelial tissue plays a role in protection, secretion, and absorption.
6. Squamous cells are flat and facilitate quick diffusion.
7. Cuboidal cells are cube-shaped and involved in absorption and secretion.
8. Columnar cells are tall and support secretion of substances like mucus.
9. Simple epithelium is one layer thick, allowing easy transport of materials.
10. Stratified epithelium has multiple layers, providing more protection.
11. Pseudostratified epithelium looks multilayered but isn’t.
12. Epithelial tissue regenerates quickly, especially in areas like skin and mouth.
13. Apical and basal sides of epithelial cells help anchor them and interact with surroundings.
14. The basement membrane is crucial for epithelial structure and support.
15. Epithelial tissues are selectively permeable, allowing controlled exchange.
16. Endocrine glands release hormones directly into the bloodstream.
17. Exocrine glands release substances into ducts that lead to the body’s surface or cavities.
18. Epithelial tissue lining the intestines is responsible for absorbing nutrients.
19. Glandular epithelium is involved in producing sweat, saliva, and digestive juices.
20. Epithelial tissue acts as a protective barrier and an organizer for bodily systems.
20 Important Medical Terms and Explanation
1. Epithelial Tissue: A type of tissue that lines and covers the body, providing protection and organization.
2. Proper Epithelium: Covers the body’s surface and lines body cavities and organs.
3. Glandular Epithelium: Forms glands that secrete hormones and other substances.
4. Avascular: A tissue that lacks blood vessels, like epithelial tissue.
5. Squamous Cells: Flat epithelial cells, often involved in diffusion or filtration.
6. Cuboidal Cells: Cube-shaped epithelial cells involved in secretion and absorption.
7. Columnar Cells: Tall epithelial cells that often produce and secrete substances like mucus.
8. Simple Epithelium: A single layer of epithelial cells.
9. Stratified Epithelium: Multiple layers of epithelial cells for protection.
10. Pseudostratified Epithelium: Appears multilayered but is only one layer thick.
11. Apical Side: The side of an epithelial cell that faces the outside or internal cavity.
12. Basal Side: The side of an epithelial cell anchored to the basement membrane.
13. Basement Membrane: A layer of connective tissue that supports epithelial cells.
14. Selective Permeability: The ability of epithelial tissue to regulate what passes through it.
15. Endocrine Glands: Glands that secrete hormones directly into the bloodstream.
16. Exocrine Glands: Glands that secrete substances into ducts leading to the body's exterior or cavities.
17. Mucus: A protective secretion produced by some epithelial cells.
18. Collagen: A protein in connective tissue that supports epithelial cells.
19. Hormones: Chemical messengers secreted by endocrine glands.
20. Diffusion: The movement of substances across epithelial tissue, such as oxygen in lungs.
20 Quotes
1. "When you’re dealing with certain elements that are being feisty and fidgety, you gotta keep ‘em separated."
2. "Your body is like that crowded lunchroom -- it needs order for it to function."
3. "This is the tissue that lines, and covers, and generally organizes your body."
4. "Without epithelial tissue, you’d essentially be a mushy pile of unarticulated goo."
5. "Epithelial tissue protects your whole body, inside and out."
6. "We are really just tubes, corridors of tissue running from a mouth to an anus."
7. "Epithelial tissue covers both the inside and the outside of that you-tube."
8. "Epithelial tissues are avascular -- meaning they don't have a blood supply."
9. "Squamous cells are flat, which makes it easy for materials like oxygen to move across them."
10. "Cuboidal cells absorb nutrients and produce secretions, like sweat."
11. "Columnar cells are tall and thick and look like columns, and they cushion underlying tissues."
12. "Simple epithelium has only one layer of cells."
13. "Stratified epithelium has multiple layers set on top of each other, like the bricks and mortar of a wall."
14. "Squamous cells are smaller, and flatter, and therefore cheaper, practically disposable."
15. "Epithelial tissue regenerates really quickly."
16. "Epithelial cells are polar, meaning they have distinct sides."
17. "The basement membrane helps hold the epithelium together, and anchors it to the next-deeper layer."
18. "Many of these boundaries that the cells form aren’t absolute -- instead, they’re selectively permeable."
19. "Endocrine glands secrete hormones right into your bloodstream."
20. "Exocrine glands secrete their juices into tubes or ducts that lead to the outside of the body."
Transcript
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, are 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 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 a 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.
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 its 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, 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 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 epithelial tissue.
And one thing that enables it to do so much -- other than being basically everywhere -- is its ability to form efficient, continuous sheets. Epithelial cells are tightly packed, and they often form sheets of cells that are connected by either tight junctions, desmosomes, or gap junctions.
Tight junctions keep cells so close together that nothing can pass through, like when you need to create a waterproof seal -- the cells lining your digestive tract, for example, keep digestive enzymes and microorganisms inside your intestines, where they’re supposed to be.
On the other hand, desmosomes are more like Velcro, fastening cells together into strong sheets. While intermediate filaments made of a protein called keratin anchor the junctions in the cytoplasm, some desmosomes are like thimbles that hold the layers of your skin cells together and make them strong and flexible.
And gap junctions provide channels between cells, allowing water and ions to flow between them, and are often found in places like the heart where the flow of ions coordinates contraction.
Ultimately, your epithelial tissue allows your organs to carry out the essential functions that they need to get done -- absorbing and filtering, protecting and sensing. And it does that in large part by creating boundaries, establishing borders between your insides and outsides, and within the insides, too.
So whether it’s holding your organs and body cavities together, or keeping your stomach from digesting itself, you now know where to find epithelium hard at work."