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. Epithelial tissue is polar, meaning that it is sided. This means that one side or one surface of the epithelial tissue is structurally and functionally different from the other side or surface. This leads us to two notable surfaces of epithelial tissue. The first notable surface is the apical surface, which is usually shown above in most diagrams of epithelial tissue. More importantly, the apical surface faces the open space immediately adjacent to the epithelial tissue.

The second notable surface is the basal surface, which is typically shown below in diagrams. More importantly, the basal surface faces an extracellular structure known as the basement membrane. To better understand these ideas, let's take a look at our diagram below. Notice that toward the top, we're showing you epithelial tissue and labeling an epithelial cell in the diagram. The darker structures you see in these regions represent the nuclei of the epithelial cells coming together to form the tissue. Below, in the layer shown in blue, some connective tissue is presented, separated from the epithelial tissue by the basement membrane.

In our next lesson video, we'll discuss more details about the basement membrane. For now, we want to focus on the polarity of epithelial tissue. Once again, the polarity means that one side or surface of the epithelial tissue is structurally and functionally different from the other. The side facing the open space, usually shown above, is the apical surface. The side usually shown below but facing the basement membrane is the basal surface. Note that the apical surface usually contains cilia or microvilli. In this diagram, we're showing some cilia, which are tiny hair-like structures that move like oars to help move material in the open space. Microvilli are structures important for maximizing absorption ability and nutrient uptake.

The basal surface does not have cilia or microvilli but is important for anchoring the epithelial tissue to the basement membrane, ensuring it is set in place. This concludes our discussion on the first commonly recognized characteristic of epithelial tissue, its polarity. We'll talk about the second commonly recognized characteristic in our next video. 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. And so one of the key features of epithelial tissue is that the cells are held really tightly together through cell junctions. And because these epithelial tissue cells are held really tightly together, that leaves very little room between the cells, and that leaves very little room for the extracellular matrix, or the ECM. And so epithelial tissue is going to have really tightly packed cells with very little ECM. And so the epithelial tissue is pretty much analogous to a brick wall, and that's why we're showing you a brick wall right here. And so what you'll notice is that these red bricks that are so tightly packed together represent the cells of the epithelial tissue. And the cement that's holding these red bricks together here represents the extracellular matrix. And so what you'll notice is that 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.

And so if we take a look at our diagram down below, what you'll notice is that we're showing you all of these different cell junctions right here. And so right here, we're showing you a tight junction. And so 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. And so what you'll notice is that there are plenty of tight junctions throughout this epithelial tissue holding the epithelial tissue cells really tightly together creating a leak-proof barrier. And so what that means is 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.

Now, what you'll also notice is that 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. And so we have a few desmosomes throughout, and that helps the epithelial tissue to remain really tightly and anchored firmly. Now, down below right here, we're showing you some gap junctions, which you might recall creates a gap here, essentially linking of the neighboring epithelial tissue cells to allow them to exchange nutrients with each other, for example.

And so 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. And so 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. And the second layer of the basement membrane is going to be the reticular lamina. Now, 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.

And so what 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. And so let's take a look at our diagram to visualize this a bit better. And once again, in our diagram, we have the epithelial tissue up above shown here. And then down below, we have the connective tissue in a bluish color. And separating the epithelial and connective tissue is this structure here that we are calling the basement membrane. And so you can see the basement membrane right here in our diagram. And 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. And so the thin extracellular layers are going to be the basal lamina and the reticular lamina.

Now, 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 that you can notice we've used these 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. And so you can see the B B B here can hopefully help remind you that the basal lamina is going to be produced by the epithelial tissue, and then, of course, that means that the reticular lamina must be produced by the underlying connective tissue. And so this 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 third commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is avascular but innervated. The term "avascular" means without blood vessels or no blood vessels. The opposite of avascular is just "vascular" without the "a," and 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. 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. Notice that the nerves are highlighted in yellow here and that the nerves that are highlighted in yellow are actually extending into the epithelial tissue. This is a big reason why epithelial tissue is closely connected with the function of allowing sensation.

Epithelial tissue in most cases is going to serve as a boundary, adjacent to an open space. It connects us to the outside world essentially, and it can detect the initial stimulus from the outside world, such as touch, pressure, or temperature. Upon detecting the initial stimulus, it can go on to activate these nerve endings that it is in close connection with. 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, not in the epithelial tissue. 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. This here concludes our third characteristic of epithelial tissue, and we will be able to talk about the fourth commonly recognized characteristic of epithelial tissue in our next video. 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. And so recall that connective tissue is one of the 4 primary types of tissue found in the human body. And 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. And so recall that the epithelial tissue is avascular, which means that it does not have any blood vessels. And 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 recall means that it does contain blood vessels. And so 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. And so the nutrients that can be supplied through the blood vessels of the connective tissue are things such as glucose and oxygen, for example. And the removal of waste could include removing carbon dioxide gas, for example. And so if we take a look at our diagram up above here, notice once again that the epithelial tissue is avascular but innervated with nerves. And because it is avascular, this allows for the vascular connective tissue that's underneath to support the epithelial tissue. And so 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 these nerves. And again, because the connective tissue is vascular with these blood vessels, it is going to allow for nutrients to diffuse from the blood and into the epithelial tissue, supporting that epithelial tissue. And so 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 fifth and final commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is highly regenerative. Meaning that epithelia have 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. 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.

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 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 will 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 consist of tightly pressed cells. And so notice that we have these tightly pressed cells forming a boundary against this open space right here in this image. Here we have 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. Recall that the apical surface of the epithelial tissue is going to be facing toward the open space. So, 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. 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 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. 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 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 and 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, it would be right here in this region. So we could label that as C right there, 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. What you'll notice is that over here in this micrograph, you'll see these tiny little hair-like structures that are right over here. Those tiny little hair-like structures that you see there, represent the cilia. The cilia are these tiny hair-like structures that move like oars that can help to move material through the open space there. 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.