Understanding Epithelial Tissue in Biology

Hi, I'm making this as a supplement to the tissues portion of the biology 121 laboratory. I'm going to go basically through the SPOs and help you out with the portions of the SPOs concerning tissues. This video will, I'll be talking about epithelial tissue specifically, and I'm going to make a different video for connective tissues actually may split the epithelial tissue video into two but we'll see if I do then I'll definitely let you know at the end of this video and you'll probably see a link in in the blackboard site so anyway we're gonna start out just talking about what a tissue is specifically but the study of tissues is called histology and so if you're histologist than you're a scientist that studies tissues and probably looks at pathologies of tissues and maybe makes tissue samples. But anyway, histology is basically what we're going to study here. So remember the biological hierarchy that you probably encountered already this semester? We're gonna just review that and show where a tissue is in this hierarchy. So we start out with the cell level right actually we could even start smaller than that and start talk about the chemical level right with molecules and stuff but we're gonna focus here on first talking about the cells which is the smallest structural unit capable of life right and so well what is a tissue well a tissue is when we put cells together and they perform several different functions together right and that is what we call a tissue and so that is really the level that we're going to be dealing with here today tissues there's there's four different kinds we'll see and today we'll talk more specifically in this video about one one specific type but if we take tissues and we combine two or more different kinds of tissues we get organs right so you could take for example cardiac muscle tissue and also connective tissues and those are two different types and combine them into what we call the heart and so we get the organ called the heart and then if we take multiple organs and combine them into a functional unit we get organ systems right so for example the cardiovascular system contains the heart but also contains blood vessels arteries and veins and blood also right and actually blood is known as a type of connective tissue we'll learn if you haven't already so but the main thing here is that the tissue level is what we're going to study and it's you know basically cells working together to perform some type of function so there are four main types of tissues in our body and basically all organs are made up of at least two different types of these tissues. So we have epithelial tissue and this is the type of tissue we're going to be dealing with today. We have connective tissues and we'll have a different video talking about the different types of connective tissues you'll have to identify for your lab practical. Muscle tissue and we'll look at that more when we get to muscles later on this semester. nervous tissue and again we'll get to the nervous system later on this semester and look more closely at that type of tissue. So a tissue I mentioned is a group of cells that work together to perform a specific function or several different functions and again there are four major types of tissues in our body. Epithelial tissues are known for covering surfaces so the surface of your body is covered by an epithelial tissue. They line surfaces and cavities, for example. Your body cavities are lined by epithelial tissues. They line organs. They lie on top of organs. And they also form glands. So really any gland in your body, you know, a sweat gland, for example, a pseudoriferous gland, which is an oil gland, these are all made... of epithelial tissues. So that's what we'll deal with here today. Connective tissues fill in spaces, so adipose tissue, which is fat, is a connective tissue. It fills in spaces in our body. We'll talk about loose connective tissue. Structural support. So for example, bone is a type of connective tissue. And it helps to support our body structurally. Transport. So for example, blood I mentioned is a connective tissue. connective tissue. It helps to transport nutrients and oxygen, dissolve gases and wastes throughout our body and storage. So I mentioned adipose or also known as fat tissue. It fills in spaces in our body but it also stores energy for example. So those are examples of what connective tissues do. And then muscle tissue is the third one and it's involved obviously Obviously movement, your body posture. Right now you're probably keeping your head up in the air, and muscles are helping with your posture of your body right now. Body temperature, when you shiver, right, you're moving your muscles, and that's helping to generate some heat, right? And then finally nervous tissue, which is involved in conducting electrical impulses, and we'll discuss that more later on this semester. Okay, so we're going to deal again with epithelial tissue. So let's get started. There's two groups of epithelial tissue. And we're going to deal mainly with this first one, the epithelial tissue that covers and lines portions of our body. So it has layers of cells that cover internal and external. surfaces and we'll see that as we go along here so you find out you know we'll talk about specific areas where we find specific epithelial tissues but for example the skin you you have epithelial tissue lining your blood vessels inside your blood vessels you have epithelial tissue lining ducts that carry like for example a sweat duct or a tear duct carrying products is lined and covered by epithelial tissue, body cavities I mentioned earlier, and then a whole bunch of tracts, respiratory tract, reproductive tract, digestive tract, urinary. These all have epithelial tissues lining and covering them also. And then there is a second type, which is a glandular epithelial tissue. And like I mentioned before, glands are made up of epithelial tissue, and they secrete certain substances. So sweat glands secrete sweat. Thyroid glands secrete different hormones. We'll find out later this semester. And so anyway, we're mainly going to deal with this first subgroup, which is the covering and lining epithelium. So in your SPOs it says describe the general characteristics of epithelial tissues in the body. And here's some general characteristics of them. They have a characteristic called cellularity, which means that there's a little extra cellular fluid between the cells. So it's mostly cells and hardly any fluid between the cells. So cells that are kind of closely packed together, right? Okay. They have a characteristic called polarity, which basically means there are different surfaces. And you could think of, I'll draw a cell here. Think of like having a couple cells here. I'll draw like three square-shaped or cube-shaped cells, each with a nucleus. And these cells are attached to some other tissue underneath them. and then up here this is an exposed surface maybe this is where we have blood for example maybe this this is a blood vessel and here we have blood we'll say and so this surface right here and I'll put point and arrow to is called the apical surface and you can see that term right here and the apical surface is exposed to in this case blood or maybe it's the surface of your skin exposed to the outside environment of your body. And then the basal surface is a surface attached to underlying tissues. Maybe it's a connective tissue. tissue that it's attached to. Or maybe there's even more epithelial cells under here. And anyway, you have a layer where the cells of the epithelial tissue attach. That's the basal surface. So it can have two different surfaces and that's where the term polarity comes from. Let me erase this. So here's a different picture. Again, here's four cells here. These are more like a cube shape to them rather than a, I'm sorry, rectangular shape or columns rather than cube shapes. the nucleus in them. And this is the exposed apical surface up here. And down here it's connected to what's called the basement membrane. And so this would be the basal surface, right? So that's the polarity characteristic. Epithelial tissues also have a characteristic called attachment to where the cells attach to a basement membrane. And that basement membrane is then attached to underlying connective tissues. And so we just saw in that previous picture this kind of darker purple area here. These kind of thicker line, purple lines called the basement membrane. And it's made up of different structures. structures, connective tissue actually makes up some of this basement membrane, basically protein fibers. But anyway, the basement membrane is connected to the basal surface of epithelial tissues and that's why we have this attachment characteristic that epithelial tissues have. And then they're avascular. Epithelial tissues do not have any vascular tissue. So no blood vessels are in epithelial tissues. So since the cells are alive in most cases, in most epithelial tissues, then we have to get nutrients to them, oxygen and glucose. So that doesn't happen because of blood vessels being directly in the epithelial tissue. It happens because of diffusion from neighboring connective tissues that have... blood vessels so again the other issues are a vascular they do not have any blood vessels you can see that here right no blood vessels here down here in this connective tissue you can see across section of a blood vessel with a red blood cell inside of it. Anyway, the nutrients that are in here need to diffuse up to these living cells in the epithelial tissue because epithelial tissue is avascular. Another characteristic is that epithelial tissues regenerate very easily. So they constantly replace themselves from stem cells at the basal surface of the epithelial tissue. And a lot of cancers originate. from epithelial tissue cells and That's because they're constantly making new cells So you're actually always sloughing off some skin cells and a lot of the dust in your house is from skin cells that have came off of your skin and you're constantly regenerating new cells to replace those skin cells takes about two weeks actually you have a whole new layer of epithelial skin cells because you're constantly getting rid of old ones and replacing them with new ones so kind of interesting to think about that and then I think finally epithelial tissues have a good nerve supply so it's very sensitive and I think we all know that like you know think about reading Braille right and if you're somebody that can read Braille you know that you can feel the little bumps and very fine you know sensitivity in your in our fingertips especially right but other places on our body too so those are all the characteristics of the phthalo tissue and before we get to specific types of epithelial tissue So epithelial tissues are basically named according to two main characteristics. And one is their shape. And you can see here there's three different shapes that epithelial tissues can have. They can have this scale-like squamous shape to them. They can have this cube or box-like shape, right, which is called a cuboidal shape. And they can have this column-like shape. These. He's got a real long, long, long recto. rectangular shapes called columns. So usually epithelial tissues have one of these three shapes to them. And then another characteristic, epithelial tissues are named according to. is the number of cell layers in the tissue. So if there's just one layer of cells, and you can see this one layer here, this is called simple. And what shape does this simple? epithelial tissue have? Well it has a cube shape right? So what do you think we call this type of epithelial tissue? It's called simple cuboidal epithelium. One layer of cuboidal shaped cells. If you have epithelial tissue that has just one layer of cells but it looks like it has more than one layer of cells, then we call it pseudo-stratified. So we can see here that basically it's heart kind of hard to see but basically every single cell is attached to this underlying basement membrane so you know this one doesn't look like it but it is same thing here so every cell we have is attached to this membrane so even though it looks like it's more than one layer it's not pseudo means false and then stratified refers to layered so it's false It's not really more than one layer, but it kind of looks like it, right? And then over here, we have stratified. And you can definitely see there are layers. There's more than one layer of epithelial cells. And so that's where the term stratified comes from. So if you have more than one layer of cells, it's considered a stratified epithelial tissue. So again, looking at this tissue. tissue, what does it look like it might be? Well, it looks like there are cube-shaped cells, so it might be stratified cuboidal epithelium, right? But actually, if it is stratified, it's named. The shape is named according to the shape of the cells on the apical surface. So if these were all cube-shaped, which they look like, right? They look like cube-shaped. cuboidal cells but if up here we had squamish shaped cells on the apical service even though most of it was cube shape we would name it according to the top layer which in our example would be squamous right so if that was a case it would be called stratified squamous epithelium so again if it's stratified you always name it the shape name comes from the shape of the apical surface epithelial cells. So basically, the idea is that all epithelial tissues, all specific ones, get their name basically from two things, the shape and the number of layers of cells. Okay, so now we're going to get into the specific types of epithelial cells. And just for quick reference, when we talk about epithelial tissue, epithelia is plural and epithelium is singular so anyway if you're not aware of that you could use those interchangeably depending on third plural or singular so like I said epithelia is sorted based on the cell shape and the number of layers. So the shapes are squamous, the scale-like thin shape, cuboidal, the cube-like shape, columnar, the rectangular column-like shape, or there's one other that we didn't point out in the previous slides, and that's called transitional. And actually, that shape, the name of that shape, comes about due to the... the cell changing shape due to the tension uh... that the the cells are put on so for example the bladder the urinary bladder the epithelial tissue that makes up part of the bladder uh... will be one shape when there's no urine in the bladder but then when the urine fill the the bladder fills up with the urine and the bladder distends it stretches uh... the cells will change shape because of that so we call that type of epithelial tissue epithelial tissue transitional because the shape is maybe squamous at one point but then may have more of of a cuboidal or columnar shape at a different point if it's not distended, not stretched. And like I said before, if it's stratified, then the epithelial tissue will get its name, the shape name, from the apical, the top surface, the exposed surface layer of cells. So again, if we had a layer of squamous cells up here at the apical surface, it would be stratified squamous epithelium. and the layers right so shape and layers so simple is a single layer stratified as many layers more than one and pseudo stratified is false stratified right appears multi-layered but actually it's it's simple it's a single layer even though it looks like it's more than one layer And each of the different layers have basically different functions. So here's a couple examples of that. So if you have a simple epithelial tissue, it's one layer of cells. and because of that one layer of cells you could have diffusion happening easily happening easily through that one layer oxygen and glucose and different products could diffuse through that one layer easily osmosis so water can move through that one layer easily and then you could secrete substances through that one layer and absorb substances easily so you're gonna have substances kind of moving through that one layer is kind of the idea right if it's a simple epithelial tissue by the way I mentioned this because in your SPOs you will have to identify at least one specific place in the body where each type of tissue is found so you know knowing kind of the functions of these different tissues will help help you do that so that's kind of why I'm pointing this out here stratified if you have many layers of cells helps to protect so for example your skin what do you think what kind of how many layers of epithelial tissue do you think your skin has just one is it simple or is it stratified? Well, you'd probably say it's stratified because if it was just one layer of cells, then it would be easily to get a cut, right? So you actually have many layers of cells that make up the epithelial tissue of the skin, and it helps to protect us, right? So our skin is a type of stratified epithelial tissue. So anywhere where you have more than many layers of epithelial cells, uh, that stratified epithelium protects. And then transitional, like we said before, is all about changing shape. And it changes shape, the cells do, when distension happens, when stretching of that tissue occurs, like in the urinary bladder when it fills up with urine. Okay, so actually I'm going to cut this off, and since it's been going on a little while, and I'm going to pick up actually... The next video will be talking about specific epithelial tissues that you'll have to be able to identify. I've been talking a little while here, so we'll pick up there, and hopefully you find these useful.

But anyway, histology is basically what we're going to study here. So remember the biological hierarchy that you probably encountered already this semester? We're gonna just review that and show where a tissue is in this hierarchy.

So we start out with the cell level right actually we could even start smaller than that and start talk about the chemical level right with molecules and stuff but we're gonna focus here on first talking about the cells which is the smallest structural unit capable of life right and so well what is a tissue well a tissue is when we put cells together and they perform several different functions together right and that is what we call a tissue and so that is really the level that we're going to be dealing with here today tissues there's there's four different kinds we'll see and today we'll talk more specifically in this video about one one specific type but if we take tissues and we combine two or more different kinds of tissues we get organs right so you could take for example cardiac muscle tissue and also connective tissues and those are two different types and combine them into what we call the heart and so we get the organ called the heart and then if we take multiple organs and combine them into a functional unit we get organ systems right so for example the cardiovascular system contains the heart but also contains blood vessels arteries and veins and blood also right and actually blood is known as a type of connective tissue we'll learn if you haven't already so but the main thing here is that the tissue level is what we're going to study and it's you know basically cells working together to perform some type of function so there are four main types of tissues in our body and basically all organs are made up of at least two different types of these tissues. So we have epithelial tissue and this is the type of tissue we're going to be dealing with today. We have connective tissues and we'll have a different video talking about the different types of connective tissues you'll have to identify for your lab practical.

Muscle tissue and we'll look at that more when we get to muscles later on this semester. nervous tissue and again we'll get to the nervous system later on this semester and look more closely at that type of tissue. So a tissue I mentioned is a group of cells that work together to perform a specific function or several different functions and again there are four major types of tissues in our body.

Epithelial tissues are known for covering surfaces so the surface of your body is covered by an epithelial tissue. They line surfaces and cavities, for example. Your body cavities are lined by epithelial tissues.

They line organs. They lie on top of organs. And they also form glands.

So really any gland in your body, you know, a sweat gland, for example, a pseudoriferous gland, which is an oil gland, these are all made... of epithelial tissues. So that's what we'll deal with here today. Connective tissues fill in spaces, so adipose tissue, which is fat, is a connective tissue.

It fills in spaces in our body. We'll talk about loose connective tissue. Structural support.

So for example, bone is a type of connective tissue. And it helps to support our body structurally. Transport. So for example, blood I mentioned is a connective tissue. connective tissue.

It helps to transport nutrients and oxygen, dissolve gases and wastes throughout our body and storage. So I mentioned adipose or also known as fat tissue. It fills in spaces in our body but it also stores energy for example.

So those are examples of what connective tissues do. And then muscle tissue is the third one and it's involved obviously Obviously movement, your body posture. Right now you're probably keeping your head up in the air, and muscles are helping with your posture of your body right now.

Body temperature, when you shiver, right, you're moving your muscles, and that's helping to generate some heat, right? And then finally nervous tissue, which is involved in conducting electrical impulses, and we'll discuss that more later on this semester. Okay, so we're going to deal again with epithelial tissue. So let's get started.

There's two groups of epithelial tissue. And we're going to deal mainly with this first one, the epithelial tissue that covers and lines portions of our body. So it has layers of cells that cover internal and external. surfaces and we'll see that as we go along here so you find out you know we'll talk about specific areas where we find specific epithelial tissues but for example the skin you you have epithelial tissue lining your blood vessels inside your blood vessels you have epithelial tissue lining ducts that carry like for example a sweat duct or a tear duct carrying products is lined and covered by epithelial tissue, body cavities I mentioned earlier, and then a whole bunch of tracts, respiratory tract, reproductive tract, digestive tract, urinary. These all have epithelial tissues lining and covering them also.

And then there is a second type, which is a glandular epithelial tissue. And like I mentioned before, glands are made up of epithelial tissue, and they secrete certain substances. So sweat glands secrete sweat.

Thyroid glands secrete different hormones. We'll find out later this semester. And so anyway, we're mainly going to deal with this first subgroup, which is the covering and lining epithelium. So in your SPOs it says describe the general characteristics of epithelial tissues in the body. And here's some general characteristics of them.

They have a characteristic called cellularity, which means that there's a little extra cellular fluid between the cells. So it's mostly cells and hardly any fluid between the cells. So cells that are kind of closely packed together, right?

Okay. They have a characteristic called polarity, which basically means there are different surfaces. And you could think of, I'll draw a cell here.

Think of like having a couple cells here. I'll draw like three square-shaped or cube-shaped cells, each with a nucleus. And these cells are attached to some other tissue underneath them.

and then up here this is an exposed surface maybe this is where we have blood for example maybe this this is a blood vessel and here we have blood we'll say and so this surface right here and I'll put point and arrow to is called the apical surface and you can see that term right here and the apical surface is exposed to in this case blood or maybe it's the surface of your skin exposed to the outside environment of your body. And then the basal surface is a surface attached to underlying tissues. Maybe it's a connective tissue.

tissue that it's attached to. Or maybe there's even more epithelial cells under here. And anyway, you have a layer where the cells of the epithelial tissue attach. That's the basal surface. So it can have two different surfaces and that's where the term polarity comes from.

Let me erase this. So here's a different picture. Again, here's four cells here.

These are more like a cube shape to them rather than a, I'm sorry, rectangular shape or columns rather than cube shapes. the nucleus in them. And this is the exposed apical surface up here.

And down here it's connected to what's called the basement membrane. And so this would be the basal surface, right? So that's the polarity characteristic. Epithelial tissues also have a characteristic called attachment to where the cells attach to a basement membrane. And that basement membrane is then attached to underlying connective tissues.

And so we just saw in that previous picture this kind of darker purple area here. These kind of thicker line, purple lines called the basement membrane. And it's made up of different structures.

structures, connective tissue actually makes up some of this basement membrane, basically protein fibers. But anyway, the basement membrane is connected to the basal surface of epithelial tissues and that's why we have this attachment characteristic that epithelial tissues have. And then they're avascular.

Epithelial tissues do not have any vascular tissue. So no blood vessels are in epithelial tissues. So since the cells are alive in most cases, in most epithelial tissues, then we have to get nutrients to them, oxygen and glucose.

So that doesn't happen because of blood vessels being directly in the epithelial tissue. It happens because of diffusion from neighboring connective tissues that have... blood vessels so again the other issues are a vascular they do not have any blood vessels you can see that here right no blood vessels here down here in this connective tissue you can see across section of a blood vessel with a red blood cell inside of it. Anyway, the nutrients that are in here need to diffuse up to these living cells in the epithelial tissue because epithelial tissue is avascular. Another characteristic is that epithelial tissues regenerate very easily.

So they constantly replace themselves from stem cells at the basal surface of the epithelial tissue. And a lot of cancers originate. from epithelial tissue cells and That's because they're constantly making new cells So you're actually always sloughing off some skin cells and a lot of the dust in your house is from skin cells that have came off of your skin and you're constantly regenerating new cells to replace those skin cells takes about two weeks actually you have a whole new layer of epithelial skin cells because you're constantly getting rid of old ones and replacing them with new ones so kind of interesting to think about that and then I think finally epithelial tissues have a good nerve supply so it's very sensitive and I think we all know that like you know think about reading Braille right and if you're somebody that can read Braille you know that you can feel the little bumps and very fine you know sensitivity in your in our fingertips especially right but other places on our body too so those are all the characteristics of the phthalo tissue and before we get to specific types of epithelial tissue So epithelial tissues are basically named according to two main characteristics. And one is their shape. And you can see here there's three different shapes that epithelial tissues can have.

They can have this scale-like squamous shape to them. They can have this cube or box-like shape, right, which is called a cuboidal shape. And they can have this column-like shape.

These. He's got a real long, long, long recto. rectangular shapes called columns.

So usually epithelial tissues have one of these three shapes to them. And then another characteristic, epithelial tissues are named according to. is the number of cell layers in the tissue.

So if there's just one layer of cells, and you can see this one layer here, this is called simple. And what shape does this simple? epithelial tissue have? Well it has a cube shape right?

So what do you think we call this type of epithelial tissue? It's called simple cuboidal epithelium. One layer of cuboidal shaped cells.

If you have epithelial tissue that has just one layer of cells but it looks like it has more than one layer of cells, then we call it pseudo-stratified. So we can see here that basically it's heart kind of hard to see but basically every single cell is attached to this underlying basement membrane so you know this one doesn't look like it but it is same thing here so every cell we have is attached to this membrane so even though it looks like it's more than one layer it's not pseudo means false and then stratified refers to layered so it's false It's not really more than one layer, but it kind of looks like it, right? And then over here, we have stratified. And you can definitely see there are layers.

There's more than one layer of epithelial cells. And so that's where the term stratified comes from. So if you have more than one layer of cells, it's considered a stratified epithelial tissue. So again, looking at this tissue.

tissue, what does it look like it might be? Well, it looks like there are cube-shaped cells, so it might be stratified cuboidal epithelium, right? But actually, if it is stratified, it's named. The shape is named according to the shape of the cells on the apical surface.

So if these were all cube-shaped, which they look like, right? They look like cube-shaped. cuboidal cells but if up here we had squamish shaped cells on the apical service even though most of it was cube shape we would name it according to the top layer which in our example would be squamous right so if that was a case it would be called stratified squamous epithelium so again if it's stratified you always name it the shape name comes from the shape of the apical surface epithelial cells. So basically, the idea is that all epithelial tissues, all specific ones, get their name basically from two things, the shape and the number of layers of cells. Okay, so now we're going to get into the specific types of epithelial cells.

And just for quick reference, when we talk about epithelial tissue, epithelia is plural and epithelium is singular so anyway if you're not aware of that you could use those interchangeably depending on third plural or singular so like I said epithelia is sorted based on the cell shape and the number of layers. So the shapes are squamous, the scale-like thin shape, cuboidal, the cube-like shape, columnar, the rectangular column-like shape, or there's one other that we didn't point out in the previous slides, and that's called transitional. And actually, that shape, the name of that shape, comes about due to the... the cell changing shape due to the tension uh... that the the cells are put on so for example the bladder the urinary bladder the epithelial tissue that makes up part of the bladder uh...

will be one shape when there's no urine in the bladder but then when the urine fill the the bladder fills up with the urine and the bladder distends it stretches uh... the cells will change shape because of that so we call that type of epithelial tissue epithelial tissue transitional because the shape is maybe squamous at one point but then may have more of of a cuboidal or columnar shape at a different point if it's not distended, not stretched. And like I said before, if it's stratified, then the epithelial tissue will get its name, the shape name, from the apical, the top surface, the exposed surface layer of cells. So again, if we had a layer of squamous cells up here at the apical surface, it would be stratified squamous epithelium.

and the layers right so shape and layers so simple is a single layer stratified as many layers more than one and pseudo stratified is false stratified right appears multi-layered but actually it's it's simple it's a single layer even though it looks like it's more than one layer And each of the different layers have basically different functions. So here's a couple examples of that. So if you have a simple epithelial tissue, it's one layer of cells.

and because of that one layer of cells you could have diffusion happening easily happening easily through that one layer oxygen and glucose and different products could diffuse through that one layer easily osmosis so water can move through that one layer easily and then you could secrete substances through that one layer and absorb substances easily so you're gonna have substances kind of moving through that one layer is kind of the idea right if it's a simple epithelial tissue by the way I mentioned this because in your SPOs you will have to identify at least one specific place in the body where each type of tissue is found so you know knowing kind of the functions of these different tissues will help help you do that so that's kind of why I'm pointing this out here stratified if you have many layers of cells helps to protect so for example your skin what do you think what kind of how many layers of epithelial tissue do you think your skin has just one is it simple or is it stratified? Well, you'd probably say it's stratified because if it was just one layer of cells, then it would be easily to get a cut, right? So you actually have many layers of cells that make up the epithelial tissue of the skin, and it helps to protect us, right? So our skin is a type of stratified epithelial tissue. So anywhere where you have more than many layers of epithelial cells, uh, that stratified epithelium protects.

And then transitional, like we said before, is all about changing shape. And it changes shape, the cells do, when distension happens, when stretching of that tissue occurs, like in the urinary bladder when it fills up with urine. Okay, so actually I'm going to cut this off, and since it's been going on a little while, and I'm going to pick up actually...

The next video will be talking about specific epithelial tissues that you'll have to be able to identify. I've been talking a little while here, so we'll pick up there, and hopefully you find these useful.

Transcript for:Understanding Epithelial Tissue in Biology

Transcript for:
Understanding Epithelial Tissue in Biology