Hi everyone! We're going to be talking about the integumentary system and I'd like to start with the most superficial layer, the epidermis. The epidermis itself is also composed of a few different layers and cell types and we're going to go over those in this PowerPoint or I guess it'll be a video by the time you guys get it. The integument, or basically the skin of the body, the largest organ of the body, is made up of two basic layers, the epidermis and the dermis.
The epidermis is what we're going to be focusing on primarily today. Also, we have the hypodermis, which is another layer which is essential for skin function, but it's not exactly part of skin. It's just that skin's so big and has so many different jobs, especially in humans, many unique jobs in humans, which we'll talk about at a later time. It needs a lot of support. Anyway, let's go ahead and start talking about the superficial layer or the epidermis.
This is what I would like to call the birthday cake slice layer of the skin, this little diagram. Basically, it helps to know all of the names of the parts that are on it. And we'll go through those as well as their function. As you can see for today, the epidermis is just this like top. part, right?
It's kind of, for lack of a better analogy, the buttercream frosting part of the integumentary system or the skin. The epidermis has four or five, depending on where you're looking, what part of the body you're looking at, distinct layers. And those layers are, of course, a keratinized stratified squamous epithelial layer.
epithelial it's on the outside squamous it's all kind of squished together and irregularly shaped which we'll look at later on it's stratified meaning that your skin has many deep layers of cells to it and keratinized because keratin is the most broadly abundant protein uh unique to skin or the epidermis i should say so the layers we'll look at starting from the most deep to the most superficial are the stratum basale strata meaning that layer of the the layers essentially. Basal layer. Stratum spinosum because there's a lot of spines on those which you'll see when we look at the cells themselves.
Stratum granulosum because when you look at these cells that layer looks like there's all these little granules inside of it like little pieces of sand inside. Stratum lucidum which is a tricky one. It's not always present in all of the parts of the body. It's only in thick skin, so basically skin that's places like the palm of your hand or the bottom of your feet.
Also, finally, the stratum corneum. So this is the horny layer because it has lots of little horns to it. It's primarily layers of very dead skin, the most external layers, which actually these dead skin cells perform a very essential function, which we'll look at in a bit.
So there's four types of cells that you need to just be aware of, know these top players. And they are going to be the keratinocytes, the melanocytes, the immune cells or the longer hand cells. Oops, there it is. And then, of course, the tactile or Merkel cells, a type of sensory organ. So here it is.
This is the buttercream frosting of the integumentary system, that top layer, the epidermis, right? Just above, outside of the dermis. So, shown here on this little list are only four of the layers, but we'll also talk about the stratum lucidum as well.
It's indicated in this one. It's just that there are a whole lot of different textbooks, and then also you need to kind of think about what part of the body are we looking at for analyzing layers of the skin right some have stratum lucidum and some don't okay so let's actually start from the inside going out right um so the point of contact is with the dermis right hence this is the epidermis so the cells of the dermis are relatively well different from the outer side or the more superficial layers and that they're very alive and very active. You'll notice that nowhere within any part of this model will you really see a lot of vascularization.
You don't see blood vessels. So basically when cells don't have access to the circulatory system They starve over time and that starvation process has become something that we've selected for. It's intentional in order to make the outer layer of the skin something that's durable and something that's resistant and something that's okay to lose.
All right, so the first layer that we're going to be looking at or the most deep layer is the stratum basale, right? The base layer, the foundational layer. So this is the most deep epidermal layer. What happens here is this is the cell nursery. So these are where mitotically active cells are.
Most of the stem cells of the epidermis are. So they're dividing and they're making new skin cells because as you've probably noticed, you sometimes can get a little sheddy, a little ashy, a little dusty, all of that. Those are dead cells found within this layer of the epidermis.
We shed cells constantly. constantly need to be making brand new replacement cells. That happens here in the stratum basale.
We see a diversity of cells in the stratum basale. Primarily, we're going to see these mitotic skin cells, these soon-to-be keratinocytes, as it were. They're also tied together very closely because we don't need the skin to fall apart. It's kind of hard to show you all here. but if you can see the little links between the cells those are called the desmosomes.
Those just kind of hold everything together like duct tape right so they just don't fall off all together. They're nice tight junctions. They're cell to cell adhesion points. Other cells we see in this area are things like the melanocytes.
Melanocytes cells that carry the pigment that we have that we are able to tint our skin in response either to UV or our natural abundance of pigmentation. Let's see here. Oh, stratum spinosum. So stratum spinosum is going to have more of these keratinocytes than anything else.
This is when we start losing things like the keratinocytes that are metodically active and we start seeing mature keratinocytes, right? So now they're growing up a little and one of the main things they're going to start doing is producing lots and lots of this keratin protein, this hard protein. And so they're going to start looking a little spiny at the edges.
Those are going to migrate upwards into the stratum granulosum. So by now keratin is very visible, right? And so instead of looking like plain kind of semi-translucent cells, they actually start looking a little spotty, which is why we call them granulosum.
because it looks like grains of something are on the inside. These granulosome cells have now been away from things like a capillary and food supply and water and all that for a while. And now they've also started really accumulating a lot, a lot, a lot of this keratin protein.
It chokes them a little bit out in their cell bodies and also their cell bodies are dehydrating. So they're starting to die. Those desmosomes become very important because now these dead cells, these true keratinocytes.
are starting to layer up towards the top so as you can see the final layer the stratum corneum right here is very thick like 20 symptoms 30 layers of just dead cells thick so we have this very um rough exterior other things that are in there things like glycol lipids oils are used to produce a smooth layer preserve what we have of this protective keratinocyte stratum so right The other one I want to come up to and just kind of revisit real quick is the idea of the stratum lucidum right here. So remember that's palms of the hands, so is the feet, and it's a clear layer of cells. They're very thick, they are protein filled, and they're thought to add resistance, but more research is being done to truly find some of the activity of these cells in particular. So here are the cells of the epidermis.
These are the ones you need to know, by name at least. and a little bit about what their jobs are in the epidermis. So we have four of these cells, which we've talked about a little bit already.
The keratinocytes, right? The cells with keratin, essentially, is what their name means. Now we also have melanocytes, or the cells that produce lots of melanin, or little, but do produce it. And then finally, we have two other classes of cells.
Those that work kind of in an immunogenic component, such as the dendritic or the longer hand cells. And then we also have sensory type cells, ones that are reporting back to the nervous system. And we call those the tactile Merkel type cells.
Right. So keratinocytes, as we've been looking at the skin, the skin has a lot of keratin. Keratin is actively synthesized in the stratum basale, right, with the metodically active cells, which means those cells are having lots of cell babies because you're doing something all the time.
And that is you're shedding skin cells. That's right. You're making dust.
You're a little ashy. You probably need to exfoliate. You're covered in dead skin cells. It's an important part of how we develop the epidermis. The outermost layer is full of these keratinocytes that are dead, that are held together by those desmosomes.
Remember those tight adhesions, those cell-cell junctions, right? And it's an important thing that they are also dead cells. And we'll look at why that is in Anyway, so there's about 20 to 30 rows of those that we keep producing because we keep losing that topmost layer, the outermost layer, just in day-to-day activities. One of the other things is in humans, we have the ability to do things like tan. And we have this range of not really skin colors because mainly it's just the sh...
shade of the same basic color that is either really light or really dark, right? We couldn't be like peacocks, I guess, and have greens and blues, which would have been really cool. But that's, let's just get back to human stuff.
Okay, humans have this relationship with the sun. We need to make vitamin D in order to have babies, but if we are exposed to too much sunlight, we could have too much sun exposure, essentially UV rays. touching and mutating our DNA in the cells that are making cell babies in stratum basale and that can result in skin cancer a couple of different kinds of skin cancer actually so The melanocytes do this thing where they produce melanin which is the pigments of humans right again No, blues no greens no pinks. Well, I guess some people but that's not that's not really what we're talking about here anyway, so they add enough of a sun shield to be protective while also adding or alleviating enough of the sun resistance so that we can synthesize vitamin D. So they're photosensitive.
Cool things, melanocytes. You can see them here. Let's see.
Let's get the green ones. Here's a melanocyte right here. And it's making all this melanin. It's depositing that protein right up here.
So when we talk about a corneocyte, it's essentially the same thing. as this keratinocyte. So this is because it's in the stratum corneum, and that's why we named it that.
Also cornea means horny, like little horns sticking out. And so that's one of the reasons they were named that. Let's see, where are we? Ah, dendritic longer hand cells.
So these are kind of the external frontline patrol, if you will, of the epidermis. So these are essentially immune cells. And you can see, let me change color. How about red?
Alright dendritic cell right here and it's dendritic because it has dendrites like little fingers that are kind of patrolling the area and all these other cells and it's looking for stuff primarily it's looking for invaders bacteria fungi anything that shouldn't be there it actually works in a macrophage capacity so if it is able to come up it's able to sense and sometimes begin the protective I guess steps which I will get into a little bit later in the semester by preventing the spread of these different types of pathogens within the body. So let's move on to Merkel cells. These are tactile cells.
So these are sensory, these are the touch type receptors and these are a little different from the others. So Merkel cells like you can see kind of here are actually part of the nervous system technically, literally. And so what they do is they're sensing touch, they're sensing motion, pressure, stuff like that, and they're allowing us to sense our environment from the outside.
So anytime there's a, if you were to touch part of your skin, as you notice, like if you touch your arm it kind of pushes down a little bit, hopefully it's pushing down a little bit, and it's springing back up. So those changes in the conformations of cells kind of send back some biofeedback to the nervous system and it allows us to just say, okay, well, this is happening on the outside of the body in this area. And so that's how that works.
So going back to the layers. So I wanted to start at stratum basale a little bit more. So again, let's just, you know, kind of get a little our fingers into this epidermal concept a little bit more.
But We'll start with layer basale, so the basal layer, the layer that's actually touching the dermis proper. So it's the deepest epidermal layer. It's down here on our grid. As you can see here, this little cell with all those little Xs, that's like it's in metaphase. It's about to divide, right?
It's going to make some more baby cells. Here we got some tactile Merkel cell action right here, sensing the environment, what's going on. So essentially it's the stratum basale is just one row thick of mitotically active cells.
So they are going to be needing access to things like the capillaries and you can see down here these are this little like blue and red business that's supposed to be the capillary loops. So they need a lot of feedings and they need a lot of water and they need to be waste have waste taken away because That mitotically active or that cell dividing process by which we make baby cells, brand new cells, replacement cells, takes a lot of energy and generates a lot of waste. So they're the closest and they're right there on the dermis.
So it takes basically about a month, sometimes a little more, sometimes a little less, from one of the cells that results from these cell divisions to actually go through the process of making all kinds of protein and then finally being out. on the stratum corneum at the most superficial part of that layer right on the furthest part of the outside the thing is is it it actually dies slowly dies as it completes this process and what it's doing after it you know is basically born after cell division on that 25 to 45 day tour is it's making a ton of keratin right and some other types of proteins so that by the time it gets up here to the stratum corneum it's nice and it's thick and it's dense and it's strong it's a shield it's like a scale right and it's really tightly attached to all of the other skin cells so it's protective right and the other thing is um it doesn't need to be maintenanced because it's just basically formed what it's supposed to form a ton of protein a little scale a little shield and we don't need to feed it with any kind of blood supply or whatnot so that protects the blood supply because we're able to layer it out a little bit right The other thing is melanocytes are important in this layer. They can compose between 10 to 25% of it.
Melanocytes are abundant in all humans. We all basically have the same number of these melanin producing cells. It just depends on how much melanin we're producing and maybe how reactive we are to UV.
So stratum spinosum is named spinosum because of the spines that we saw when we looked at it under the microscope. Right. So it's it looks very prickly and instead of one layer thick we start looking at now these stratified layers right so this is several actually quite a few layers thick and we start to to really connect strongly our cells here right so part of the prickly looking phenotype are those spines are those desmosomes those like tight junctions of the cells uh showing up in the microscope slides right holding those cells together keeping all of the skin cells together so we don't just fall apart Alright, so we have lots of these dendritic cells patrolling the area as well as these melanosomes.
The next one is stranulatum, or stranulatum, stratum granulosum. Wow, I just invented a new biology word. It sounded like a real word too, huh? But it's not. It's not a real word.
Don't write that one down. Stratum granulosum, the granular layer, and it looks like this. Because again, these cells, when you take a peek at them under the microscope, they look like they have so much keratin that the true keratinization process has begun.
And so all of these little keratin does the thing where it forms like little plaques inside the cell. And it looks like little granules of sand. And so that's why we named them that.
We, not like I was there, you know, scientists of the olden days. We as humans. Anyway.
So they're going to accumulate these granules and they're going to kind of like, they do this thing called... resulting in a lamellar phenotype. So a lamella is basically like just another layer. So think of a tree, right, as it grows. It's a tiny little tree and there's the thin part of the trunk and then every year it gets rings and it adds it on.
So that's a lamella, like lamellae is what it also is known as. So keratin lays down as here's the first little protein of keratin and then another one slaps on and then another one slaps on and then it becomes lamellar, right? So that's what that refers to. So the nice thing about these particular types of cells and the fact that they have all of this keratin in it is they become very water resistant and we want that because most of us don't live in an ocean and the ocean is the perfect salinity of our cells.
Right, there's nothing like that. So we need to keep the water we have on the inside of our body. That's kind of our A plan at this point.
And so these cells kind of act as like this nice saran wrap in a way to keep water on the inside of our body, right? So anything above this layer, stratum granulosum, dies. And the reason is because they're basically like twofold.
One, they're clogged. with keratin. Just imagine if you were a cell and you had, you know, you were making all this protein and it just was all over your body, it clogs you up. Like you can't perform normal cell function, right? Which is, you know, what you're supposed to do, but it's also a little sad, you know, if you're that cell.
But that's your fate. That's what you're supposed to do in the end. The other thing is, uh, got cold.
um anyway so the second thing is that there's again no capillaries right so if you look over here we have all the capillaries super far down on the bottom but but when we're up here at stratum granulosum they've been away from food and water and waste removal for a long time so they're dying they're starving they're toxic with their own waste this is this is a layer of dying cells but yay right So this is the stratum lucidum. It's not everywhere. It's some places and some of those places are going to be the palms of your hands and the bottom of your feet, right? So like where your paws are, basically the bottom of your paws. So it's funny because if you look at the literature, officially a lot of folks say that we still don't know why it exists and why it's there.
I guess from like an evolutionary standpoint, I would say because we walked on those areas for a long time, we do know some stuff about them. We do know that they are protein filled. And we do know that this like this extra padded layer of protein filled cells, at least in humans, is very thin.
But it's still very responsive, right? So it can become a little bit thicker. We know that they are also not nucleated, right?
There's no nuclei in them, which is a good thing because they're not really protected as much by melanin, and they're very clear, right? They are clear. So if we had nuclei in stratum lucidum, we might be worried about things like UV damage, maybe mutations being introduced, stuff like that.
Right, so these are essentially, they're a type of keratinocyte, and they're still kind of learning a little bit more about this one as it goes. But yeah, that's that, that extra thick skin feeling, that's stratum lucidum. Pretty tough stuff for just a few rows.
So finally, the one we're all most concerned with most superficially, because it is the most superficial layer, and that's the one we see. is our stratum corneum right the horny layer because it has little horns because it just went through this process and now they're if you look at them under a microscope they look like they're just covered in horns they look like goat heads when they're detached from each other quite frankly so they're 20 to 30 rows they um also have lost their their nuclei as well right we don't need any reproduction they're dead dead dead they're essentially not really even cells at this point they're uh what we call membrane enclosed sacs right so membranous sacs of keratin and a handful of other proteins it's a lot it's it's a good 75 percent of the thickness of the epidermis itself which is good it's a healthy protective layer they're very they're very dead there's no I know here we just show like a little bit but you got to imagine this being like really crazy thick And it's good because it means that if you scratch yourself or whatnot, then we just hopefully you'll just be losing these dead skin cell layers. Right.
And they're they're keeping us shaded from UV. They're doing all kinds of stuff. One of the most important things they're doing and nobody really talks about it is the stratum corneum plays an incredibly, incredibly vital role in your immune system.
Maybe not the components that we properly classify as the immune system, but in the role of protecting the body as a barrier against the outside world. So everything from toxins and stuff we could come into contact with and absorb to just keeping the bacteria on the outside of the body and the fungus and all of that stuff and parasites, for heaven's sakes. So it does a great job of that, making it very difficult to get to the nutrient-rich meaty bits.
of the human body. So right, right, right. That's the epidermis.
Those are the players, and that is how we stratify out all of those layers in their role. So I hope that was helpful. If you have any questions, just feel free to reach out and email me. All right, have a great day.