The Epidermis: Layers - Video Tutorials & Practice Problems | Channels for Pearson+ (2024)

This video, we're going to continue to talk about the epidermis as we focus on the layers of the epidermis. And so the epidermis, or the outermost portion of our skin, is actually composed of several distinct layers of cells. Notice that down below, on the left hand side, we're showing you the diagram of the integumentary system. And once again, we're only coloring the epidermis, or the outermost portion of our skin. Notice that zooming into the epidermis, we can clearly see that there are several distinct layers of cells.

It's important to note that the term stratum is a term that means "layer" in Latin. The plural form of this word is actually strata, which means "layers". Notice that down below, we have listed and numbered the layers of the epidermis. The numbering is actually in order from the deepest layer of the epidermis, which is furthest away from the external environment, to the most superficial layer of the epidermis, which is immediately adjacent to the external environment. The reason that we have these layers numbered in this particular order, from deepest to most superficial, is because that is the order that we are going to be covering these layers moving forward in our lesson. Also, numbering the layers in this particular order from deepest to most superficial is going to help us better understand the development of the cells in these layers over time.

The layers of the epidermis, in order from deepest to most superficial, are the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum. It's important to note that the number of layers of the epidermis can actually vary depending on the type of skin. There is what is known as thin skin and there's also what's known as thick skin. In our next lesson video, we're going to distinguish further between thin skin and thick skin. But what's really important to note here in this video, is that the stratum lucidum has an asterisk next to it. This is because the stratum lucidum is a layer of the epidermis that is only found in thick skin and it is not found in thin skin. That's something that's very important to note.

We do have a memory tool to help you remember the layers of the epidermis in order from the deepest layer of the epidermis to the most superficial layer. That memory tool is "beautiful skin glows like crystals". Where the first letter of each of these words is a symbol to represent the first letter of each of the layers of the epidermis, in order from the deepest layer to the most superficial layer. Moving forward in our course, we're going to talk more details about each of these layers of the epidermis, and we'll also get to distinguish between thin skin and thick skin. So, I'll see you all in our next video.

In this video, we're going to briefly distinguish between thin skin and thick skin. And so, although skin all over the external body is very similar in structure, there are some local variations that led to these 2 majorly recognized skin types. The first being thin skin and the second being thick skin. And so notice down below we have a table comparing some of the major features of thin skin and thick skin. And so notice the left-hand side has features of thin skin, whereas the right-hand side has features of thick skin. And so, thin skin, as its name implies, is going to be thinner than thick skin. Because as we briefly mentioned in our last lesson video, thin skin has all of the layers of the epidermis except for the stratum lucidum, which is the 2nd most superficial layer. And so, thin skin does not contain the stratum lucidum in its epidermis. Whereas thick skin is going to be thicker because it contains all of the layers of the epidermis including the stratum lucidum.

Now, thin skin actually makes up most of the skin on our body. Whereas thick skin is really only located in specific regions such as the palms of our hands, for example, and the soles of our feet. And so if you had to take a guess at what type of skin it is, you might as well go with thin skin. Because again, it makes up most of the skin on our body except for the skin in these particular regions. Now, thin skin actually contains hair follicles and oil glands as well. Whereas thick skin, such as the palms of our hands and soles of our feet, do not contain any hair follicles or oil glands. Now, thin skin in general is going to have fewer sweat glands. Whereas, thick skin tends to have more sweat glands than thin skin.

And so there are some other differences between thin skin and thick skin, but these are some of the major differences that we think you should be aware of for your exams. And so, moving forward in our course, we'll be able to apply some of these concepts and learn more about the layers of the epidermis. So, I'll see you all in our next video.

In this video, we're going to talk about keratinocyte development over time in the epidermal layers while also introducing some really interesting analogies and some excellent memory tools that you can only find here on Pearson Plus Channels. And so notice in the top right we have a diagram showing you the integumentary system. And once again, we're only coloring the epidermis, or the outermost portion of the skin since that's the main focus of this video. Notice that the image down below on the left-hand side is really just a zoom into the epidermis where you can see the layers of cells in the epidermis. The keratinocytes in the layers of the epidermis are actually in various stages of development. It's really important for you to know that the keratinocyte cells originate in the stratum basale, which recall is the deepest layer of the epidermis and is usually shown at the bottom of the diagrams of the epidermis. And over time, as cells divide, these keratinocyte cells are pushed up into more superficial layers. What this means is that the cells that are deeper in the stratum basale are going to be newer cells since this is where they originate, whereas the cells that are more superficial are going to be older cells. Notice that the light pink that you see down below, highlighted here, represents the dermis that lies underneath the epidermis. The first epidermal layer, or the deepest epidermal layer, is shown highlighted right here, which is the stratum basale. The next layer of the epidermis is this larger layer that you can see right here that actually consists of multiple layers of cells, as you can see, and this is the stratum spinosum. The next layer of the epidermis is highlighted here, and that is the stratum granulosum. The next layer is highlighted here, and that is the stratum lucidum, only found in thick skin. And then at the very top, what we have is the final most superficial layer of the epidermis, the stratum corneum.

Recall that keratinocytes produce the protein keratin. When they first originate in the stratum basale, they have not yet had enough time to produce a lot of keratin. But as the cells are pushed up over time into more superficial layers, they've had more and more time to produce more and more keratin. The cells that are more superficial are going to have more keratin than the cells that are deeper. We have this really interesting analogy over here on the right to help you remember the layers of the epidermis. This analogy entails a character called Cary the keratinocyte. Notice that Cary the keratinocyte here is just a little baby. Cary the keratinocyte is actually born in the stratum basale. You can think the B in stratum basale is for the B in being born, reminding you that the keratinocytes originate in this bottom layer. As the cells divide, they move up into more and more superficial layers. Notice that Cary is sprouting in the stratum spinosum, and Cary is now an adult. The cells are going to continuously age and get pushed up over time into more superficial layers. Notice that Cary the keratinocyte is now a granny in the granulosum. This is a helpful memory tool that can help you remember the layers in order of development. In the stratum lucidum, you'll notice that unfortunately, Cary is now lifeless. The keratinocytes are actually dead in this layer of the stratum lucidum. Notice that we're showing you a coffin here, and you can see some images of Cary the keratinocyte in other more deep layers. Lifeless Cary is actually locked in a thick coffin, reminding you that this layer, the stratum lucidum, is only found in thick skin. Finally, what we have at the top is the stratum corneum. Notice that Cary the keratinocyte is a Cloud Top Casper casting off of the corneum. In this layer, the stratum corneum, the keratinocytes can actually cast off of the surface and actually be rubbed off. This means that the stratum corneum is constantly shedding cells, and the cells in the stratum corneum need to be replaced by deeper cells. When the cells originate in the stratum basale, they will get pushed up into more and more superficial layers until ultimately they are shed off. They need to be replaced by cells that are dividing in the stratum basale. Recall from our previous lesson videos that the cells that are deepest in the epidermis are closest to the underlying vascular tissue. The dermis is going to be vascular and supply nutrients through the blood. The cells in the stratum basale have more access to more nutrients, and they have more tendency to divide. As the cells get further and further away from the underlying connective tissue in the dermis, they have less and less access to the nutrients from the blood, and they lose their capacity to divide as they move up into more superficial layers. This is an idea that we'll get to talk more about as we move forward in our course, and talk more details about the features of each of these layers of the epidermis. But for now, hopefully, this memory tool and this analogy can be helpful to you to remember keratinocyte development in epidermal layers, and we'll be able to get some practice applying these concepts and learn more as we move forward. So, I'll see you all in our next video.

So, here we have an example problem that says to label the epidermal strata or the epidermal layers in the image below. And so, notice in the top right we have the same diagram from our previous lesson videos where only the epidermis is colored. And so, the epidermis is this outermost layer of the skin, and the epidermis itself is actually made of several layers of cells, and that's really what this larger image is all about. And so, we do have multiple memory tools for helping you remember the layers of the epidermis. And so, the first memory tool that we have is beautiful skin glows like crystals. And so, notice that we've got that memory tool right here for you, and the first letter of each of these words represents one of the epidermal layers, and they are organized from deep to superficial. And so, if we use this memory tool, we can figure out the first word for each of these layers. And so, of course, we can say that beautiful skin glows like crystals. And so that allows us to fill in these blanks.

Now, we also have other memory tools that we talked about in our previous lesson videos, that include carry the keratinocyte. And so, notice in this diagram down below in light pink, this represents the underlying connective tissue that's part of the dermis of the skin. And the epidermis begins right here at this next layer, and so this is going to represent the stratum basale where Cary the keratinocyte is born, and the stratum basale is usually shown as the bottom most layer of the epidermis. And so, you can think that the B in stratum basale is for the B and the keratinocyte being born and also the B in bottom since it's the bottom most layer usually. Now, in the next layer here, Cary the keratinocyte is sprouting in the stratum spinosum. And so the stratum spinosum is going to be this next layer here of cells where you can see it's quite a large layer has many layers of cells and that is the stratum spinosum. Now, the next layer here what you can see is that Cary the keratinocyte is aging into a granny in the stratum granulosum. And so, notice that Cary the keratinocyte is aging as it goes into more and more superficial layers. And so, the stratum granulosum is represented as this layer that you can see highlighted here.

Next, what we have is the stratum lucidum. And so, in the stratum lucidum, it's only going to be found in thick skin. And so, what you'll notice is that we have a coffin here because, Cary the keratinocyte is lifeless in this layer. And so it's actually a layer of dead cells. What you'll also notice is that Cary the keratinocyte is lifeless and locked inside of this thick coffin reminding us that the stratum lucidum is only found in thick skin and it's absent in thin skin. Then, in the final layer what we have is Cary the keratinocyte is a cloud top Casper casting off of this corneum, the stratum corneum. And so, you can see here that in this layer these cells can actually flake off into the environment and they need to be replaced by cells that are dividing in the deeper layers. And so as cells divide, those cells are going to continuously get pushed up into more superficial layers. So, the cells toward the top are going to be older cells and as you go down the cells are going to be newer cells and younger cells. And so now that we've filled in and labeled all of the epidermal strata, we've completed this example so I'll see you all in our next video.

In this video, we're going to talk about key features of the second epidermal layer in our lesson, which is the stratum spinosum. The term spinosum actually means spiky or spiny, which is why this layer is also called the spiny layer. Now, it gets its name from the spiky or spiny artifacts that appear during the preparation of cells in this layer for microscopy, but those spiky or spiny artifacts do not exist in the actual living tissue. The stratum spinosum, or the spiny layer, is the second deepest layer of the epidermis, right behind the stratum basale. You can think that the 's' in spinosum or the 's' in spiny layer stands for the 's' in the word "second" to remind you of the second deepest layer. As you can see over here in this diagram where the stratum spinosum is colored like this, most of the stratum spinosum consists of many rows of these dividing keratinocytes.

It is worthy of noting that the keratinocytes that are deeper in the stratum spinosum are actually closer to the underlying vascular connective tissue, and so they have more direct access to nutrients from those blood vessels. Whereas, the cells that are more superficial in the stratum spinosum are further away from the underlying connective tissue, and so they have less access to nutrients from those vascular tissues. And because that's the case, the cells that are closer to the underlying tissue have more direct access and therefore have more capacity to divide. In contrast, the cells that are further away have less access to nutrients and therefore less capacity to divide. In fact, as the cells divide and get pushed up into more and more superficial layers, what we'll notice is that the cells in the next layer have lost their capacity.

It is also worthy of noting that the stratum spinosum, or the spiny layer, is usually the thickest epidermal layer, especially in thin skin. However, in thick skin, usually the stratum corneum is the thickest layer, and the stratum corneum is the most superficial layer of the epidermis. Keratinocytes in the stratum spinosum or spiny layer will actually begin to flatten at the top of this layer. What you'll notice here in this diagram is that keratinocytes in the stratum spinosum are flattening toward the top of the layer, or the more superficial side of the layer, and the reason that they flatten is because of their keratin production. Recall that the deeper cells are newer cells since they originate in the stratum basale. But the more superficial cells are older cells, and so they've had more time to produce more keratin, which is why they start to flatten due to that keratin production.

In the stratum spinosum, we can find dendritic cells, specifically the Langerhans cells. These dendritic cells or Langerhans cells are immune cells that help provide immunity by activating the immune system. Later in our course, we'll get to talk more about the exact functions of dendritic cells when we're focusing on the immune system. These dendritic cells are just scavenging through the stratum spinosum of the epidermis looking for signs of infection or invasion. If they detect a sign of infection or invasion, they can actually migrate out of the epidermis into the lymphatic system, travel to a nearby lymph node to help activate other immune cells in that lymph node, and those other immune cells can generate a significant immune response to help prevent the infection.

In this image analogy, notice that the keratinocyte in the stratum spinosum is starting to sprout, symbolizing how, as cells divide, they will get pushed up into more superficial layers. Cari, the keratinocyte, is now represented as this young adult. We will discuss more about the key features in the next superficial layer, the stratum granulosum, in our next video. I'll see you all there.

In this video, we're going to talk about key features of the 3rd epidermal cell layer in our lesson, which is the stratum granulosum, also known as the granular layer. Keratinocytes in this layer actually stop dividing. Notice looking at this diagram on the right, the stratum granulosum or the granular layer is colored. You'll notice that the cells in this layer are quite far from the underlying vascular connective tissue of the dermis. This means they have very limited access to nutrients from those blood vessels. Ultimately, this is the reason why the cells in this layer stop dividing. The keratinocytes in this layer are also going to begin to harden and begin to die.

The reason that the keratinocytes begin to harden is because of the process of keratinization, which, as its name implies, is the process where keratinocytes fill up with lots and lots of keratin. Ultimately, that helps to allow those keratinocytes to harden. Recall from our previous lesson videos that keratin is a very tough, fibrous, and water-resistant protein produced by keratinocytes. It helps to give the epidermis its protective properties and allows the epidermis to serve as an effective barrier. As its name implies, the stratum granulosum or the granular layer is going to have keratinocytes that are filled with granules. These granules or containers within cells are going to help promote the hardening of the keratinocytes and the waterproofing of the stratum granulosum.

Under the microscope, the cells in the stratum granulosum take on a noticeably different appearance with these granules. Notice that the granules are all of these little dots packed within these keratinocytes. They can be visualized especially with the proper staining techniques. In terms of the types of granules, there are two main types of granules used in the stratum granulosum. Those two main types of granules are keratohyalin granules, which help with the aggregation of keratin and ultimately help with the hardening of the keratinocytes, and lamellar granules, which are packed with a water-resistant glycolipid. This glycolipid can be secreted into the environment, helping to create a very waterproof layer in the stratum granulosum.

In the stratum granulosum, the keratinocytes' nuclei and organelles start to disintegrate in this layer, which causes these cells to die, again due to their limited access to nutrients. Notice here that we have an image of the analogy from our previous lesson videos. Carrie the keratinocyte is a granny in the granulosum. Hopefully, this is a helpful memory tool to remind you that the stratum granulosum or the granular layer is going to have older cells that are still alive, but their nuclei and organelles are starting to disintegrate and wear down, and they're packed with lots and lots of granules. This concludes our brief lesson on the stratum granulosum. As the cells continuously divide, they'll get pushed up into more and more superficial layers. The next superficial layer in the epidermis is the stratum lucidum, which is only found in thick skin. We'll talk about key features of that layer in our next video. So, I'll see you all there.

This video we're going to talk about key features of the 5th and final epidermal cell layer in our lesson which is the stratum corneum, which is actually the most superficial epidermal cell layer and so it is immediately adjacent to the external environment. The stratum corneum consists of these dead keratinocyte cells that are fully packed with keratin and that have a H₂O or a water-resistant glycolipid membrane. Allowing them to create a waterproof barrier that prevents water from penetrating deeper into our body when we're taking a bath, for example, or swimming in a pool, but also that helps to prevent our body from losing too much water to the environment. Now, although these cells are dead, the cells still are tightly and firmly connected by tight junctions and desmosomes, allowing them to create an effective barrier to microbes and chemicals that may try to penetrate our skin.

Also, the keratinocyte cells of the stratum corneum are regularly shed, cast, or washed off the surface of the epidermis. Notice in this diagram, the most superficial layer that is in color is the stratum corneum. You can see these keratinocyte cells that are flaking off, shedding, or casting off, and being washed or rubbed off. This is a natural process that occurs over time, but also any kind of friction, anytime we touch something, cells can flick off the epidermis, the superficial side of the epidermis from the stratum corneum. Because these cells are regularly shed off, they need to be replaced by cells that are deeper in the epidermis. Recall that the cells that are deepest in the epidermis are actively dividing. As they divide, they push older cells into more superficial layers, and ultimately, those cells are going to be shed or cast off the stratum corneum. Cells can remain in the stratum corneum for maybe about 2 weeks or so, it depends, and it can be different for different people. But ultimately, eventually, those keratinocyte cells are going to be shed or cast off. In fact, an average lifetime, a person could shed about 40 pounds worth of these epidermal skin flakes, which is quite a lot if you think about it.

Again, those cells that are shed off ultimately need to be replaced by underlying cells. This cyclical process brings us back to the original layer. Notice here we have an image of our analogy where Carry the Keratinocyte is a cloud-top Casper that is casting off the corneum. Hopefully, that can help you remember some of the features of the stratum corneum. This concludes this lesson, and I'll see you all in our next videos where we'll be able to apply these concepts and practice problems.