Characteristics of Epithelial Tissue - Online Tutor, Practice Problems & Exam Prep

Epithelial tissue exhibits five key characteristics: polarity, tight junctions, avascularity with innervation, support from connective tissue, and high regenerative capacity. Polarity refers to the distinct apical and basal surfaces, with the apical surface often featuring microvilli or cilia. Tight junctions create a leak-proof barrier, while the avascular nature means it relies on vascular connective tissue for nutrient supply. The regenerative ability is crucial for replacing cells lost due to environmental stress, making epithelial tissue susceptible to cancer due to rapid cell division.

5 Characteristics of Epithelia

In this video, we're going to begin our lesson on the characteristics of epithelial tissue. And so it turns out that there are 5 commonly recognized characteristics of epithelial tissue. Number 1 is the polarity of the epithelial tissue. Number 2 is tightly pressed epithelial tissue anchored to a basement membrane. Number 3 is avascular but innervated epithelial tissue.

Number 4 is supported by connective tissue. And number 5, last but not least, is highly regenerative. And so moving forward, we're going to talk about each of these 5 commonly recognized characteristics of epithelial tissue in their own separate videos starting with polarity. So I'll see you all in that video.

Characteristic 1: Polarity

In this video, we're going to talk about the first commonly recognized characteristic of epithelial tissue, which is polarity. And so epithelial tissue is polar, which means that the epithelial tissue is sited, if you will. This means that one side or one surface of the epithelial tissue is going to be structurally and functionally different than the other side or surface of the epithelial tissue. And so this leads us to 2 notable surfaces of epithelial tissue. The first notable surface of epithelial tissue is going to be the apical surface.

And so the apical surface, usually in most diagrams of epithelial tissue, is going to be shown above. But more importantly, the apical surface of epithelial tissue is going to be facing the open space that's immediately adjacent to epithelial tissue. Now, the second notable surface of epithelial tissue is going to be the basal surface. And usually, in most diagrams of epithelial tissue, the basal surface is going to be shown below. But more importantly, the basal surface is going to be facing an extracellular structure known as the basement membrane.

And so in order to better understand these ideas, let's take a look at our diagram down below. And so to orient you on this diagram, notice that toward the top here, we're showing you epithelial tissue. And in fact, we're actually labeling an epithelial cell right here in the diagram. And notice that these darker structures that you see here in these regions represent the nuclei of the epithelial cells that are coming together to form the epithelial tissue. And what you should also notice is that down below in this layer in blue, we're showing you some connective tissue, which you might recall is one of the primary types of tissue that we'll talk about later in our course.

But for now, what I'd like you to notice is that the connective tissue is being separated from the epithelial tissue by this extracellular structure known as the basement membrane. And the basement membrane, you can see in the diagram, is right here. And so the basement membrane is separating the epithelial tissue that's shown above from the connective tissue that's being shown below. Now, in our next lesson video, we'll talk more details about the basement membrane. But for now, we want to focus on the polarity of epithelial tissue.

And so once again, the polarity of epithelial tissue means that the tissue is sided. One side or surface of the epithelial tissue is going to be structurally and functionally different than the other side or surface of the epithelial tissue. And so we know that the side that is facing the open space, usually shown above, is going to be the apical surface. And the side that is usually shown below but facing the basement membrane is going to be the basal surface. And so one thing that you should note here is that the apical surface is usually going to contain cilia or microvilli.

Here in this diagram, we're showing you some cilia. And you might recall from our previous lesson videos that cilia are little tiny hair-like structures that move like oars and can help to move material in the open space. And microvilli, you might recall, are structures that are going to be really important to maximize absorption ability, the uptake of nutrients. Now the basal surface is not going to have cilia or microvilli. Instead, the basal surface is going to be important for anchoring the epithelial tissue to the basement membrane to make sure that the epithelial tissue is set in place.

And so this here concludes our first commonly recognized characteristic of epithelial tissue, the fact that epithelial tissue is polar. And we'll be able to talk about the second commonly recognized characteristic in our next video. So I'll see you all there.

Characteristic 2: Tightly Pressed Tissue Anchored to Basement Membrane

In this video, we're going to talk about the second commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is tightly pressed tissue that is anchored to a basement membrane. One of the key features of epithelial tissue is that the cells are held really tightly together through cell junctions. Because these epithelial tissue cells are held really tightly together, that leaves very little room between the cell, and that leaves very little room for the extracellular matrix for the ECM. Epithelial tissue is going to have really tightly packed cells with very little ECM. Epithelial tissue is pretty much analogous to a brick wall, and that's why we're showing you a brick wall right here.

What you'll notice is that these red bricks that are so tightly packed together represent the cells of the epithelial tissue. The cement that's holding these red bricks together here represents the extracellular matrix. Like a brick wall, the bricks or the cells are going to be really tightly packed together with very little ECM, very little extracellular matrix. If we take a look at our diagram down below, you'll notice that we're showing you all of these different cell junctions right here. Right here, we're showing you a tight junction.

You might recall from some of our previous lesson videos that tight junctions are going to hold 2 neighboring cells really tightly together to create a leak-proof barrier. There are plenty of tight junctions throughout this epithelial tissue holding the epithelial tissue cells really tightly together creating a leak-proof barrier. That means that there can be some liquid here in the open space above the epithelial tissue here, and that liquid will not be able to leak through the epithelial tissue as effectively because of these tight junctions, creating that leak-proof barrier. Over here, we're showing you desmosomes, which you might recall from our previous lesson videos, are more structurally complex cell junctions that are going to really firmly anchor these neighboring epithelial tissue cells together. We have a few desmosomes throughout, and that helps the epithelial tissue to remain really tightly and, anchored firmly.

Down below right here, we're showing you some gap junctions, which you might recall creates a gap here, essentially linking the cytoplasm of the neighboring epithelial tissue cells to allow them to exchange nutrients with each other, for example. You might recall from our previous lesson video that we briefly mentioned that this tightly pressed epithelial tissue is going to be anchored to an extracellular structure called the basement membrane. The basement membrane is really going to consist of 2 thin extracellular layers. The first thin extracellular layer of the basement membrane is going to be the basal lamina. The second layer of the basement membrane is going to be the reticular lamina.

The basal lamina is actually going to be produced by the epithelial tissue or by the epithelia for short. Whereas the reticular lamina is going to be produced by the underlying connective tissue. You'll notice here is that in our brick wall analogy, our brick wall is anchored to a basement membrane that you can see down below that has those 2 thin extracellular layers, the basal lamina and the reticular lamina. Let's take a look at our diagram to visualize this a little bit better. Once again, in our diagram, we have the epithelial tissue up above shown here.

Down below, we have the connective tissue in a bluish color. Separating the epithelial and connective tissue is this structure here that we are calling the basement membrane. You can see the basement membrane right here in our diagram. What you'll notice is that this basement membrane actually consists of 2 thin extracellular layers. We have this pink extracellular layer right here and then we have this semi-purplish extracellular layer immediately beneath. The thin extracellular layers are going to be the basal lamina and the reticular lamina. One thing that can help you remember that the basal lamina is going to be produced by the epithelial tissue and the reticular lamina is going to be produced by the connective tissue is the B's here to help remind you of something important and that is that the basal surface of the epithelial tissue is going to be facing the basement membrane and you can think that the basal surface of the epithelial tissue is going to be producing that basal lamina layer of the basement membrane. Of course, that means that the reticular lamina must be produced by the underlying connective tissue. This here concludes our lesson on the second commonly recognized characteristic of epithelial tissue, and we'll get to talk about the third one in our next video.

So I'll see you all there.

Characteristic 3: Avascular but Innervated

In this video, we're going to talk about the 3rd commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is avascular but innervated. And so, the term avascular is a term that means without blood vessels or no blood vessels. And so, the opposite of avascular is just vascular without the "a." And so the term vascular means with blood vessels. But again, epithelial tissue is avascular, meaning it has no blood vessels.

Now, the term innervated means that it contains nerves or nervous tissue. And so, once again, the epithelial tissue that we're highlighting here in our diagram is going to be avascular but innervated, meaning it has no blood vessels, but it does have nerves. And so, notice that the nerves are highlighted in yellow here and notice that the nerves that are highlighted in yellow are actually extending into the epithelial tissue. And this is a big reason why epithelial tissue is closely connected with the function of allowing sensation. Again, epithelial tissue, in most cases, is going to serve as a boundary, adjacent to open space.

And so it is going to connect us to the outside world essentially. It can detect the initial stimulus from the outside world such as touch, pressure, or temperature. And upon detecting the initial stimulus, it can go on to activate these nerve endings that it is in close connection with. And so, once these nerve endings are activated, they can transmit an electrical signal for processing, allowing for the full sensation. Now, another thing to notice here in this diagram is that the underlying connective tissue here is actually vascular and innervated.

Notice that it does actually have blood vessels. But again, these blood vessels are in the connective tissue, the underlying connective tissue. They're not actually in the epithelia. And also notice that it does have nerves, which is why it is innervated. But again, the nerves extend into the epithelial tissue, which is why the epithelial tissue is innervated.

And so this here concludes our third characteristic of epithelial tissue, and we'll be able to talk about the 4th commonly recognized characteristic of epithelial tissue in our next video. So I'll see you all there.

Characteristic 4: Supported by Connective Tissue

In this video, we're going to talk about the 4th commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is supported by connective tissue. Recall that connective tissue is one of the four primary types of tissue found in the human body. Later in our course, we're going to talk more about connective tissue. But for now, what we're saying is that epithelial tissue is supported by connective tissue. Recall that the epithelial tissue is avascular, which means that it does not have any blood vessels.

This is what allows connective tissue to support the epithelial tissue, because the connective tissue that is underneath the epithelial tissue is going to be vascular, which means that it does contain blood vessels. The vascular connective tissue with blood vessels is going to support the epithelial tissue by supplying nutrients to the epithelia and helping to remove wastes as well. The nutrients that can be supplied through the blood vessels of the connective tissue include substances such as glucose and oxygen. The removal of waste could include removing carbon dioxide gas, for example. If we take a look at our diagram up above here, notice once again that the epithelial tissue is avascular but innervated with nerves.

Because it is avascular, this allows for the vascular connective tissue that's underneath to support the epithelial tissue. Notice that the connective tissue here is going to be vascular in nature because it does have blood vessels, and it's also going to be innervated with nerves. Again, because the connective tissue is vascular with these blood vessels, it is going to allow for nutrients to diffuse from the blood into the epithelial tissue, supporting that epithelial tissue. This here concludes our 4th commonly recognized characteristic of epithelial tissue, and we'll be able to talk about the 5th and final characteristic in our next video. So I'll see you all there.

Characteristic 5: Highly Regenerative

In this video, we're going to discuss the 5th and final commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is highly regenerative, meaning that epithelia has the capacity to divide rapidly. And so if we take a moment to think about this, this actually makes a lot of sense. Recall from our previous lesson videos that a key feature of most epithelial tissue is that it forms a boundary immediately adjacent to open space. We know that epithelial tissue is important for covering body surfaces and organs and lining internal body cavities. Because epithelial tissue forms this boundary, it's going to be subject to the environment, and it's going to be subject to physical stresses and pressures from the environment, such as friction, for example.

And so this can cause damage to epithelial tissue cells and it can cause epithelial tissue cells to rub off and be lost. These damaged or lost epithelial tissue cells need to be replaced by dividing epithelia. This is why epithelial tissue has the capacity to divide rapidly. Now it turns out that the epithelial tissue cells that are closest to the connective tissue that underlies it have the highest capacity to divide. The reason for that is pretty simple.

It's because the underlying connective tissue is vascular, and so it has blood vessels that contain nutrients. The epithelial tissue cells that are closest to the underlying connective tissue are going to have more direct access to the nutrients, and that is going to allow them to have a greater capacity to divide. As the epithelial tissue cells get further and further away from the underlying connective tissue, usually their capacity to divide also decreases. Now because epithelial tissue cells divide rapidly, this is what makes them subject to cancer. In fact, most cancers actually develop from epithelial tissue.

Recall that cancer is a disease characterized by uncontrollable cell growth, and usually mutations cause those cells to become cancers. Every single time a cell divides, there's a small chance of mutations occurring. The more that a cell divides, the greater the chances that a mutation can occur that will lead to cancer. This is why most cancers develop from epithelial tissue because they have to have this ability to divide rapidly. This here concludes our lesson on the commonly recognized characteristic of epithelial tissue, and we'll be able to get some practice applying these concepts as we move forward.

So I'll see you all in our next video.

Characteristics of Epithelial Tissue Example 1

So here we have an example problem that says, in the following images of epithelial tissue that we can see down below right here, identify the following features if present. And the features are apical surface, basal surface, basement membrane, connective tissue, and cilia. And so, one thing that we need to recall from our previous lesson videos is that one of the key defining features of most epithelial tissue is that it's going to consist of a sheet or multiple sheets of tightly pressed cells forming a boundary adjacent to open space. In fact, when we look at each of these micrographs, we can identify the open space, which can be really helpful for us to identify all of these features. And so, notice in the far left micrograph, the open space is going to be right over here in this highlighted region.

In the next micrograph, the open space is going to be right here. And then in the third and final micrograph, notice that there are multiple pockets of open space that you can see in these highlighted regions right there. And again, we know that the epithelial tissue is going to have tightly pressed cells. And so, notice that we have these tightly pressed cells that are surrounding and forming a boundary against this open space right here in this image. Here we have some more tightly pressed cells forming a boundary against the open space right there.

And then over here in this last micrograph, we have tightly pressed cells forming a boundary around each of these open spaces that we can see in this image. And so, once again, this is going to be really helpful for us to begin to identify these features. And so, recall that epithelial tissue is polar, meaning that the tissue is sided. And that just means that one side or surface of the tissue is going to be structurally and functionally different than the other side or the other surface of the tissue. And so, recall that the apical surface of the epithelial tissue is going to be facing toward the open space.

What we can do is we can use the letter A here to label the apical surface in each of these micrographs. And again, the epithelial tissue here, the surface that's facing the open space is going to be the apical surface. So we can put A here and just say, hey, this surface all along here is going to be the apical surface of the epithelial tissue. Over here, again, we have epithelial tissue and again, the apical surface is going to be facing toward the open space. And then over here in each of these, again, the surface that's facing toward the open space is going to be the apical surface. So we can just go ahead and put an A in each of these regions to identify the apical surface.

Now, recall that the basal surface, on the other hand, is going to be facing the basement membrane. Now recall from our previous lesson videos that the basement membrane consists of two thin extracellular layers, the basal lamina, which is produced by the epithelial tissue, and the reticular lamina, which is produced by the underlying connective tissue. Now, although the basement membrane is really important, under a standard light microscope, usually the basement membrane is way too thin to actually be visualized. However, there can be some special staining techniques that allow for better visualization of the basement membrane, and the basement membrane can also be visualized using more advanced microscopes, like electron microscopes, for example. But in most of these standard light microscope images that we see here, visualizing the basement membrane is going to be very difficult.

In fact, it's really difficult to visualize the basement membrane here. However, we do know that the basement membrane is going to be separating the epithelial tissue from the underlying connective tissue. And so, what we can do is we can try to identify the epithelial tissue and the underlying connective tissue, and then we know that the basement membrane will be in between the two. And so, when we take a look at this micrograph over here on the far left, what you'll notice is that the epithelial tissue, let's use yellow here, is going to be basically all of this tightly pressed tissue that you see highlighted right here. And, underneath this epithelial tissue, which I'll highlight in blue, you'll notice that the tissue changes its style.

And so, this tissue that you see here is going to be the connective tissue. And so, what I'll do is I'll highlight this in this blue color and label this as the connective tissue. We'll put a D here to label it as the connective tissue. And then, of course, the basement membrane is going to be separating the epithelial tissue from the connective tissue. And so if the basement membrane were going to be included here, we know that it would be in this particular region, something like that, and then we could label this as C, the basement membrane.

Now, moving on to this next micrograph, the epithelial tissue, the tightly pressed epithelial tissue, is going to be right here highlighted in yellow. But then notice once again that down below, we have a change in the style of that tissue. And so, here we're going to say that this is going to be the connective tissue. So once again, we'll label this as D here, to label the connected tissue. Put this D right over here, put an arrow in.

And then, of course, we know that the basement membrane is going to be separating the epithelial tissue from the connective tissue. So the basement membrane, if we were to be able to see it, would be right here in this region. So we could label that as C right there, the basement for the basement membrane. Moving on to this final one over here, what you'll notice is that the epithelial tissue is going to be this tissue that is immediately surrounding the open space as you see right here, highlighted in these regions. This is the epithelial tissue and the connective tissue is actually all of this other tissue that you see all around it, like so.

And so, we can label the connective tissue D here right over here as so and again, the basement membrane would be separating the epithelial tissue from the connective tissue. So, there we would expect there to be a basement membrane around each of these, as we see right here. And so we could label these as the basement membrane, like so. And so, what you'll notice is that over here in this micrograph, it's, you'll see these tiny little hair like structures that are right over here. And those tiny hair like structures that you see there, that represents the cilia.

Those are the cilia. And recall that the cilia are these tiny hair like structures that move like oars that can help to move material through the open space, there. And so, what we can do is we can take the cilia here and we can label the cilia here. We'll put an E and we'll label the cilia like so. And then, what we'll do is we'll just color in the cilia like so.

Yeah. And so this was a really interesting example problem. Hopefully, this was helpful for you. And, we'll be able to get some practice applying a lot of these concepts as we move forward. So I'll see you all in our next video.

Most cancers develop in epithelial tissues. Which epithelial tissue characteristic makes them prone to cancer?

• A. Epithelial cells have polarity.
• B. Epithelial tissue is avascular but innervated.
• C. Epithelial tissue is supported by connective tissue. 
• D. Epithelial cells divide regularly.

Together the basal lamina and the reticular lamina make up the _____________________. The basal lamina is produced by the _________________, while the reticular lamina is produced by the ____________________.

• A. Basement membrane, epithelial tissue, connective tissue. 
• B. Apical surface, epithelial tissue, connective tissue. 
• C. Basement membrane, connective tissue, epithelial tissue. 
• D. Apical surface, connective tissue, epithelial tissue.

A student scratched herself accidentally. What piece of evidence would suggest the scratch went through the epithelium (outermost layers of her skin) and reached the underlying connective tissue?

• A. She started bleeding.
• B. She felt pain.

The epithelial tissue in the small intestine is specialized to absorb nutrients. Why would tight junctions be important in this function?

• A. Tight junctions allow the cells to transmit ions between cells, allowing the cells of the epithelium to coordinate. 
• B. Tight junctions ensure all nutrients are absorbed through epithelial cells, allowing for selective permeabilty. 
• C. Tight junctions allow specific molecules such as nutrients to enter the cell more easily. 
• D. Tight junctions allow the epithelial cells to sense and recognize specific molecules.

Here’s what students ask on this topic:

What are the five key characteristics of epithelial tissue?

The five key characteristics of epithelial tissue are:

1. Polarity: Epithelial tissue has distinct apical and basal surfaces. The apical surface often features microvilli or cilia, while the basal surface is anchored to the basement membrane.

2. Tight Junctions: Epithelial cells are tightly packed with minimal extracellular matrix, creating a leak-proof barrier through cell junctions like tight junctions and desmosomes.

3. Avascularity with Innervation: Epithelial tissue lacks blood vessels (avascular) but contains nerves (innervated), relying on underlying connective tissue for nutrient supply.

4. Support from Connective Tissue: The underlying vascular connective tissue provides nutrients and removes waste, supporting the avascular epithelial tissue.

5. High Regenerative Capacity: Epithelial tissue can rapidly divide to replace cells lost due to environmental stress, making it susceptible to cancer due to frequent cell division.

Why is epithelial tissue considered polar?

Epithelial tissue is considered polar because it has distinct structural and functional differences between its two surfaces: the apical and basal surfaces. The apical surface faces the open space and often contains cilia or microvilli, which aid in movement and absorption. The basal surface, on the other hand, faces the basement membrane and is crucial for anchoring the epithelial tissue in place. This polarity ensures that each surface can perform specialized functions, contributing to the overall role of epithelial tissue in protection, absorption, and secretion.

How does the avascular nature of epithelial tissue affect its function?

The avascular nature of epithelial tissue means it lacks blood vessels and relies on the underlying vascular connective tissue for nutrient supply and waste removal. This arrangement allows epithelial tissue to form effective barriers and surfaces for absorption and secretion without the interference of blood vessels. However, it also means that epithelial cells must be in close proximity to the connective tissue to receive nutrients and oxygen, which is facilitated through diffusion. This dependency on connective tissue underscores the importance of the supportive role of connective tissue in maintaining epithelial health and function.

What role do tight junctions play in epithelial tissue?

Tight junctions play a crucial role in epithelial tissue by holding adjacent cells tightly together, creating a leak-proof barrier. This barrier prevents the passage of substances between cells, ensuring that materials must pass through the cells rather than between them. This is essential for maintaining selective permeability, protecting underlying tissues, and preventing the leakage of fluids and pathogens. Tight junctions are particularly important in tissues that line organs and cavities, such as the intestines and blood vessels, where they help maintain the integrity and function of these barriers.

Why is epithelial tissue highly regenerative?

Epithelial tissue is highly regenerative because it frequently encounters environmental stress, physical damage, and cell loss due to its role in covering body surfaces and lining cavities. To maintain its protective and functional roles, epithelial tissue must rapidly replace damaged or lost cells. This high regenerative capacity is facilitated by the epithelial cells' ability to divide quickly, especially those closest to the underlying vascular connective tissue, which provides essential nutrients. However, this rapid cell division also makes epithelial tissue more susceptible to cancer, as frequent cell division increases the likelihood of mutations.