The Epidermis: Layers - Online Tutor, Practice Problems & Exam Prep

The epidermis consists of five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale contains stem cells that proliferate and differentiate into keratinocytes, while the stratum spinosum features spiny keratinocytes and immune cells. In the stratum granulosum, keratinocytes begin to harden and die through keratinization. The stratum lucidum, found only in thick skin, consists of flattened, dead cells. Finally, the stratum corneum serves as a waterproof barrier, shedding dead keratinocytes regularly, crucial for skin protection and homeostasis.

Introduction to Layers of the Epidermis

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. And so 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. And notice that zooming into the epidermis, we can clearly see that there are several distinct layers of cells.

And so, it's important to note that the term stratum is a term that means layer in Latin. And the plural form of this word is actually strata, which means layers. Now notice that down below we have listed and numbered the layers of the epidermis. And notice that 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. And so 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.

And 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. Now, 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. Now, it's important to note that the number of layers of the epidermis can actually vary depending on the type of skin. And so there is what is known as thin skin, and there's also what's known as thick skin. And 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. And 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. And so that's something that's very important to note. Now, 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 of the epidermis.

And 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. And so, 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.

Thin vs. Thick Skin

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 two 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 second 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.

The Epidermis: Layers Example 1

So here we have an example problem that says, imagine you're a pathogen trying to enter a human body via the skin on a person's sole of the feet. Which layers of the epidermis do you have to get through and in what order? And so, to solve this problem we need to recall the memory tool for remembering the layers of the epidermis in order from deep to superficial. And recall that memory tool is beautiful skin glows like crystals. And so, notice here, I've drawn a sketch of the memory tool.

Beautiful skin glows like crystals, which again, organizes these layers from deep to superficial. And notice that the first letter of each of these words is going to represent the layers of the epidermis. And so, you can see the layers are stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. Now, what we also need to realize is that the stratum lucidum is only going to be present in thick skin, and it's absent in thin skin. And so, we need to realize that we're entering the skin on a person's sole of the feet.

And recall from our last lesson video that the sole of the feet is going to have thick skin. And so that means that the stratum lucidum is going to be present in this journey that the pathogen is taking. And so, let's imagine again that we are this pathogen and let's represent the pathogen as this red dot. We know that we're going to start outside of the body, and that means that we're going to start on the superficial side. And we want to enter into the body which means that we're going to need to cross through all of these layers to get deeper and deeper into the body.

And so that means that in terms of the layers of the epidermis that we have to get through and the order that we need to go through them, it's going to be all 5 of the layers in this particular order from superficial to deep. Starting with the stratum corneum, then the stratum lucidum, then the stratum granulosum, then the stratum spinosum, and then finally entering through the stratum basale. And that is what's going to allow us to penetrate through the epidermis into deeper layers of the skin. And so, this here concludes this example problem. Hopefully, you can see the value in this memory tool.

Beautiful skin glows like crystals, and hopefully that can help you solve some of the problems. So, I'll see you all in our next video.

Keratinocyte Development in Epidermal Layers

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. And so 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. And so the keratinocytes in the layers of the epidermis are in various stages of development.

And so what's really important for you to know is 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 upwards, or pushed up, into more superficial layers. And so 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. And so if we take a look at the image down below on the left hand side, again, this is just a zoom into the epidermis.

And so notice that the light pink that you see down below, highlighted here, represents the dermis that lies underneath the epidermis. And so the first epidermal layer, or the deepest epidermal layer, is shown highlighted right here. And this is the stratum vesale. 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. 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 in the stratum corneum.

And so, again, the cells that are deeper in the stratum basale, they are originating in the stratum basale. And so the deeper cells are going to be newer cells, as indicated here in our image. And then as the cells divide, the keratinocytes are pushed upwards into more and more superficial layers. And so the superficial cells are going to be older cells. Now, recall that keratinocytes produce the protein keratin.

And so 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. And so the cells that are more superficial are going to have more keratin than the cells that are deeper. Now, what you'll notice is that we have this really interesting analogy over here on the right to help you remember the layers of the epidermis. And so this analogy entails a character called Cary the keratinocyte.

And so notice that Cary the keratinocyte here is just a little baby. And so Cary the keratinocyte is actually born in the stratum basale. And so 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. Now again, as the cells divide, they move up into more and more superficial layers. And so the next layer here is the stratum spinosum.

And so notice that Cary is sprouting in the stratum spinosum. And so, Cary is now an adult. Now, again, the cells are going to continuously age and get pushed up over time into more superficial layers. And so notice that Cary the keratinocyte is now a granny in the granulosum. And so, again, that is a helpful memory tool that can help you remember the layers in order of development.

Now, in the stratum lucidum, what you'll notice is that unfortunately, Cary is now lifeless. And so, in fact, the keratinocytes are actually dead in this layer of the stratum lucidum. And so notice that we're showing you a coffin here, and you can see some images of Cary the keratinocyte in other more deep layers. And so lifeless Cary is actually locked in a thick coffin. And that thick coffin reminds you that this layer, the stratum lucidum, is only found in thick skin.

And so Cary is locked in a thick coffin in the stratum lucidum. And then finally, what we have at the top is the stratum corneum. And notice that Cary the keratinocyte is a cloudtop Casper casting off from the corneum. And in fact, in this layer, the stratum corneum, the keratinocytes can actually cast off from the surface and actually be rubbed off. And so what that means is that the stratum corneum is constantly shedding cells, and so the cells in the stratum corneum need to be replaced by deeper cells.

And so again, 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. And then again, 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. And so the dermis is going to be vascular and supply nutrients through the blood. And so the cells in the stratum basale have more access to more nutrients, and so they have more tendency to divide.

And 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 so they lose their capacity to divide as they move up into more superficial layers. This is something 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.

The Epidermis: Layers Example 2

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 carry 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, carry 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 many layers of cells and that is the stratum spinosum. Now, the next layer here what you can see is that carry the keratinocyte is aging into a granny in the stratum granulosum. And so, notice that carry 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.

Now, 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 carry the keratinocyte is lifeless in this layer. And so, it's actually a layer of dead cells. And what you'll also notice is that carry 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 carry 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, these 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.

Which of the epidermal layers is likely to have the greatest regenerative capacity (ability to divide)?

• A. Stratum basale.
• B. Stratum spinosum.
• C. Stratum granulosum.
• D. Stratum lucidum.
• E. Stratum corneum.

Epidermal Layers: Stratum Basale (Basal Layer)

This video we're going to begin talking about key features of each of the epidermal layers starting with the deepest epidermal layer of the Stratum basale, which is also sometimes referred to as the basal layer since it's found on the basal surface of the tissue. Now the Stratum basale or the basal layer is usually shown as the bottom layer in diagrams of the epidermis. And so you can think that the b in basale or the b in basal layer is for the b in the bottom layer. Now, the Stratum basale or Basal Layer is actually made up of just one single row of cells, which is a unique feature of the Stratum basale that does not apply to any of the other layers of the epidermis. And so notice over here on the right, we're showing you a diagram of all of the layers of the epidermis.

And notice that they're all grayed out except for the stratum basale, or the basal layer, which you can see is highlighted right here. And indeed, the stratum basale is just one single row of cells. And in the stratum basale are what are known as stem cells. And these stem cells have the ability to proliferate or divide to create more stem cells, and they also have the ability to differentiate or change into other types of cells such as keratinocytes, melanocytes, or tactile epithelial cells, for example. And so they are able to do this and maintain the epidermis.

And so recall from our last lesson video that the cells on the superficial side of the epidermis actually cast off, and so they can be rubbed off, and these cells that are rubbed off of the epidermis, or flake off of the epidermis, need to be replaced by deeper cells. And so, the stem cells help to maintain the epidermis. Now, in the Stratum basal or basal layer, of course, it's going to contain keratinocytes, which recall are the most abundant cell type in the epidermis and make up about 90% of all of the epidermal cells. And the keratinocytes are shown highlighted here. But the stratum basal is also going to contain melanocytes, which recall from our previous lesson videos are shown as these purple cells, and they produce the pigment melanin, which protects our skin from UV light, or ultraviolet light from the sun, which can cause damage.

And so the melanocytes protect from UV damage by producing melanin. And again, that melanin can be transferred to keratinocytes that are neighboring the melanocyte. And so those keratinocytes can carry melanin that was originally produced by the melanocytes. And then the Stratum basale will also contain tactile epithelial cells or Merkel cells, which recall from our previous lesson videos the term tactile is a term that means touch. And so these tactile epithelial cells work really closely with nervous tissue, as you can see here in yellow, in order to allow for sensations of touch.

And so, again over here in this diagram on the right, you can see the melanocytes and the tactile epithelial cells along with these keratinocytes as well. Now, here what we have is an image from the analogy that we talked about in our previous lesson video. So this is Kary the keratinocyte as a baby. And notice that Kary the keratinocyte, this baby, is born in the stratum basale. And so this can remind you that the keratinocytes originate in the stratum basale.

And so these cells are going to be relatively new in the stratum basale. And over time, as cells divide, the keratinocytes are going to get pushed up into more superficial layers. And as they get pushed up, over time, they have more and more time to produce more and more keratin. So, the cells that are in more and more superficial layers will have more and more keratin. And one thing to note is that the melanocytes and the tactile epithelial cells have functions that allow them to essentially remain localized in the stratum basale.

So the melanocytes and tactile epithelial cells are not going to get pushed up into more superficial layers, It's really just the keratinocytes that are pushed up into more superficial layers. And so this here concludes our lesson on the key features of the stratum basale, the deepest layer of the epidermis. And so I'll see you all in our next video where we'll get to talk about the next superficial layer which is the stratum spinosum.

Epidermal Layers: Stratum Spinosum (Spiny Layer)

In this video, we're going to talk about key features of the second epidermal layer in our lesson which is the stratum spinosum. And so 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. Now, the stratum spinosum or the spiny layer is the second deepest layer of the epidermis right behind the stratum basale. And so you can think that the s in spinosum or the s in spiny layer is for the s in the word second to remind you of the second deepest layer.

Now, 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. Now, 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 nutrients or 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.

Whereas the cells that are further away are going to have less access to nutrients and therefore less capacity to divide. And so, in fact, as the cells divide and get pushed up into more and more superficial layers, what we'll notice is that the next layer, the cells have lost their capacity to divide entirely because they are so far away and have very limited access to any nutrients from underlying connective tissues. Now, it is worthy of noting that the stratum spinosum or the spiny layer is usually the thickest epidermal layer, especially in thin skin. But in thick skin, usually the stratum corneum is the thickest layer, and the stratum corneum is the most superficial layer of the epidermis. Now keratinocytes in the stratum spinosum or spiny layer will actually begin to flatten at the top of this layer.

And so what you'll notice is that here in this diagram, the 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. So 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, and so that's why they start to flatten due to that keratin production. Now, it's also worthy of noting that in the stratum spinosum is where we can find dendritic cells.

Specifically, the Langerhans cells. And recall that these dendritic cells, or Langerhans cells, are immune cells that help to provide immunity by activating the immune system. And so later in our course, we'll get to talk more details about the exact functions of dendritic cells when we're focusing on the immune system. But really these dendritic cells are just scavenging through the stratum spinosum of the epidermis looking for signs of infection or invasion. And 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.

And so what you'll notice here is that we've got this image of our analogy. And so notice that Cari the keratinocyte in the stratum spinosum is starting to sprout. And so now Cari the keratinocyte is this young adult. And so again, as cells divide, they will get pushed up into more superficial layers. And so the next superficial layer here is the stratum granulosum.

And so we'll talk more about those key features in our next video. So I'll see you all there.

Epidermal Layers: Stratum Granulosum (Granular Layer)

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. And so keratinocytes in this layer actually stop dividing. And so notice looking at this diagram on the right, the stratum granulosum or the granular layer is colored. And what you'll notice is that the cells in this layer are quite far from the underlying vascular connective tissue of the dermis. Meaning that they have very limited access to nutrients from those blood vessels.

And ultimately, this is the reason for why the cells in this layer stop dividing. Now, the keratinocytes in this layer are also going to begin to harden and begin to die. Now, 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. And ultimately, that helps to allow those keratinocytes to harden.

Now recall from our previous lesson videos that keratin is a very tough, fibrous, and water-resistant protein produced by keratinocytes. And it helps to give the epidermis its protective properties and helps to allow the epidermis to serve as an effective barrier. Now as its name implies the stratum granulosum, or the granular layer, is going to have keratinocytes that are filled with granules. And so these granules, or containers within cells, are going to help promote the hardening of the keratinocytes and the waterproofing of the stratum granulosum. And so under the microscope, the cells in the stratum granulosum do take on a noticeably different appearance with these granules.

And so notice that the granules are all of these little dots packed within these keratinocytes. And they can be visualized especially with the proper staining techniques. Now, in terms of the types of granules, really there are 2 main types of granules used in, the granulosum, the stratum granulosum. And 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 then there are also lamellar granules, which are packed with a water-resistant glycolipid.

And ultimately that glycolipid can be secreted into the environment, which helps to create a very waterproof layer in the stratum granulosum. Now, in the stratum granulosum or the granular layer, the keratinocytes, what you'll notice is that the nuclei and the organelles within those keratinocytes are going to start to disintegrate in this layer. And ultimately, this is what causes these cells to die. And again, this is because they have limited access to nutrients and things of that nature. Now notice here that what we have is an image of the analogy from our previous lesson videos.

And so notice here we can we have Carrie the keratinocyte is a granny in the granulosum. And so, hopefully, that's 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 also they're packed with lots and lots of granules. And so this here concludes our brief lesson on the stratum granulosum. And again, as the cells continuously divide they'll get pushed up into more and more superficial layers. And the next superficial layer in the epidermis is the stratum lucidum, which is only found in thick skin and so we'll talk about key features of that layer in our next video.

So, I'll see you all there.

Epidermal Layers: Stratum Lucidum (Clear Layer)

In this video, we're going to talk about the key features of the 4th epidermal cell layer in our lesson, which is the stratum lucidum, also known as the clear layer. This is a protective epidermal layer that is only present in thick skin, such as the skin on the palms of our hands and the soles of our feet, which have to encounter a lot of friction since we're constantly grabbing things, and our feet are constantly walking on floors. Evolutionarily, the thick skin has evolved to have this extra layer, this stratum lucidum. However, the stratum lucidum, or the clear layer, is not present in thin skin, which makes up most of the skin on our body.

Most of the skin on our body does not have the stratum lucidum. The stratum lucidum, or the clear layer of thick skin, consists of these flattened, dead, densely packed, and transparent keratinocyte cells that actually lack organelles. Recall from our previous lesson video that the organelles are degraded in the previous epidermal cell layer, the stratum granulosum. Notice over here in this diagram, you can see the stratum lucidum, or the clear layer, as this relatively small layer of cells that are transparent. The lack of organelles in the cells of this layer contributes to its transparency, which is why it's also known as the clear layer.

Now, notice here we have an image from the analogy from our previous lesson videos. We know that the keratinocyte, unfortunately, is going to be dead here, and lifeless Kerrie is locked in this thick coffin that you can see here, which is going to remind us of the stratum lucidum. You can see that there are these photographs of the previous epidermal cell layers, including the stratum granulosum, stratum spinosum, and the stratum basale. This here concludes our brief lesson on the stratum lucidum, or the clear layer. As cells divide, these cells are going to continuously get pushed up into more superficial layers.

The final superficial layer is the stratum corneum, and we'll talk about that layer in our next video. So, I'll see you all there.

Epidermal Layers: Stratum Corneum

In 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. This layer is the most superficial epidermal cell layer and 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. This allows 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 helps to prevent our body from losing too much water to the environment. Although these cells are dead, they are still 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 off, or washed off of the surface of the epidermis.

Notice over here 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, or 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 flake off of 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, and as they divide, they push older cells into more superficial layers.

Ultimately, those cells are going to be shed or cast off of 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. Eventually, those keratinocyte cells are going to be shed or cast off. In fact, in 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 cycle 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 of the corneum. Hopefully, this can help you remember some of the features of the stratum corneum. This here 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.

The Epidermis: Layers Example 3

So here we have an example problem that says, imagine you're a pathogen trying to enter a human body via the skin on a person's arm. Now, if you look at the skin on your arm, you'll notice that it has some hairs on it, and that's an indication that this is going to be thin skin. And so we can go ahead and label the skin on a person's arm here as thin skin. Now, the problem here asks, which layers of the epidermis do you have to get through and in what order, and what might be some challenges you encounter along the way, and it ends with asking how would this journey differ if you were entering via the palm of their hand. Now, if you look at the skin on the palm of your hand, you'll notice that it does not have any hairs on it and that's an indication that it is thick skin.

And so recall that the skin on the palm of your hand and the skin on the soles of your feet is going to have thick skin. Now, we're going to approach this problem by breaking it up into 2 parts. The first part will focus on the epidermal layers and the second part will focus on the challenges that the pathogen can encounter. And so, of course we know that the epidermal layers in thin skin are going to be different than the epidermal layers in thick skin. And of course, because thick skin is thicker, we know it's going to have more epidermal layers than thin skin.

And so, recall the memory tool that helps us remember the epidermal layers in order from the deepest layer to the most superficial layer is "beautiful skin glows like crystals." And so, that reminds us that it is the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum. And of course, thick skin is going to have all of these layers. And recall that the stratum lucidum is unique to thick skin. It is only found in thick skin.

And so, in terms of the layers in thin skin, really it's going to have the exact same layers. The only difference is that once again, it's not going to have the stratum lucidum, so we can cross that one off. And so, in terms of the pathogen, of course the pathogen is going to start on the outside of the body. And that means it's going to start on the most superficial side of the epidermis. And so, that means that the pathogen is going to start at the top and make its way down.

And so, in thin skin, such as the skin on a person's arm, the pathogen will need to go from, starting from the stratum corneum to the stratum granulosum, then the stratum spinosum, and ending with the stratum basale. And then in thick skin, it'll make the same journey except again it has to go through the stratum lucidum as well. And so really that's the biggest difference between the journey the pathogen would need to take in thin skin versus thick skin. Now, in terms of some challenges that the pathogen will encounter along the way, of course, it's going to encounter these really tightly and firmly connected keratin-filled keratinocytes, which recall make up the vast majority of the epidermis. Something like 90% of the epidermal cells are keratinocytes.

And those really, really tightly packed keratinocytes are going to create a very difficult barrier or create a barrier that is very difficult for the pathogen to penetrate through. And so, the keratinocytes are found in all of these layers and so, that will create a challenge for the pathogen throughout its entire journey. Now, another challenge that the pathogen is going to encounter along the way are dendritic cells, which recall dendritic cells are cells that basically scout the epidermis for signs of pathogens. And then they can actually leave the epidermis and alert the immune system and activate components of the immune system.

And so, a pathogen would need to be able to evade these dendritic cells in order to be successful at invading the body. Now, recall that these dendritic cells are predominantly found in the stratum spinosum layer. And so, that is a challenge that the pathogen would encounter mainly when it gets to the stratum spinosum. And so, this here concludes this example problem and I'll see you all in our next video.

Which of the following layers of the epidermis is responsible for generating new epidermal cells?

• A. Stratum basale.
• B. Stratum spinosum.
• C. Stratum granulosum.
• D. Stratum corneum.

Which of the following is true about thick skin?

• A. Hair grows on thick skin.
• B. There is no stratum corneum in thick skin.
• C. Stratum lucidum is only found in thick skin.
• D. Thick skin is only composed of stratum lucidum.

Which epidermal layers contain dead, flattened keratinocytes?

• A. Stratum basale and stratum corneum.
• B. Stratum lucidum and stratum corneum.
• C. Stratum granulosum and stratum spinosum.
• D. Stratum granulosum and stratum lucidum.

Why do the cells in the more superficial layers of the epidermis die?

• A. They experience a great deal of mechanical stress.
• B. They're exposed to oxygen in the air, causing them to age faster.
• C. They're exposed to harsh chemicals contained in soaps, lotions, and other products.
• D. They are far away from any blood supply, causing a lack of nutrients.

Here’s what students ask on this topic:

What are the five layers of the epidermis and their functions?

The epidermis consists of five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale contains stem cells that proliferate and differentiate into keratinocytes, providing new cells for the epidermis. The stratum spinosum features spiny keratinocytes and immune cells, offering structural support and immune defense. In the stratum granulosum, keratinocytes begin to harden and die through keratinization, forming a barrier. The stratum lucidum, found only in thick skin, consists of flattened, dead cells that provide an additional layer of protection. Finally, the stratum corneum serves as a waterproof barrier, shedding dead keratinocytes regularly, crucial for skin protection and homeostasis.

What is the difference between thin skin and thick skin?

Thin skin and thick skin differ primarily in the number of epidermal layers. Thin skin lacks the stratum lucidum and is found on most of the body, containing hair follicles and oil glands. Thick skin includes the stratum lucidum and is located on the palms of the hands and soles of the feet, areas subject to high friction. Thick skin does not have hair follicles or oil glands but has more sweat glands compared to thin skin. These structural differences allow thick skin to provide extra protection and durability in high-friction areas.

How do keratinocytes develop and move through the layers of the epidermis?

Keratinocytes originate in the stratum basale, the deepest layer of the epidermis, where they are produced by stem cells. As they divide, they are pushed upwards into more superficial layers. In the stratum spinosum, keratinocytes begin to produce keratin and start to flatten. In the stratum granulosum, they stop dividing, harden, and begin to die due to keratinization. In the stratum lucidum (only in thick skin), keratinocytes are dead and densely packed. Finally, in the stratum corneum, they are fully keratinized, dead, and eventually shed off, maintaining the skin's protective barrier.

What is the role of melanocytes in the stratum basale?

Melanocytes in the stratum basale produce the pigment melanin, which protects the skin from ultraviolet (UV) light damage. Melanin absorbs and dissipates UV radiation, reducing the risk of DNA damage that can lead to skin cancer. Melanocytes transfer melanin to neighboring keratinocytes, which helps to distribute the pigment throughout the epidermis, contributing to skin color and providing a protective barrier against UV radiation.

Why is the stratum corneum important for skin protection?

The stratum corneum is the outermost layer of the epidermis and plays a crucial role in skin protection. It consists of dead keratinocyte cells that are fully packed with keratin and have a water-resistant glycolipid membrane. This layer creates a waterproof barrier that prevents water loss from the body and protects against external environmental factors such as microbes, chemicals, and physical abrasions. The stratum corneum also regularly sheds dead cells, which helps to remove potential pathogens and maintain the skin's integrity.