 All right, we already know that we have the dermis and the epidermis, but I hinted, or didn't hint, I just straight up Stone Cold told you that the epidermis was made of five layers and the dermis is made of two layers. And so let's draw a picture because drawings are way cooler than word lists. All right, nobody knows how I'm going to draw this. This is going to be my space. So I'm going to draw my epidermis on top and my dermis on bottom. And I'm going to try and draw it, I don't know, it's going to look very lovely. So don't ever forget that fact that this looks very lovely. Hi, what's the word? That was on purpose, the waves are on purpose. Even though I was like earlier, like dude, what is happening? Why, what happened with my pen? Why can't I draw a straight line? I wasn't trying to draw a straight line there. All right, I'm going to draw my first layer. What kind of tissue am I drawing right now? The answer is not, oh good God. Oh, you guys are so patient with me as I draw my crazy lines. What kind of tissue is that? Don't look at that edge. Look at the apical edge, right, to identify the tissue. And we know there's a space there, so we know this is an epithelial tissue. And I already told you the answer to this I've probably, hopefully, will tell you a million times. I just drew square cuboidal-looking cells. Those are not cuboidal epithelium. That is the basal layer of stratified squamous epithelium. So you know at the top of this thing that I'm going to end up with these super flat dead cells. Aw, poor little buddies. Okay, all of my layers are called stratums. And for the heck of it, I'm going to start my stratums over here. This layer, this one layer that is attached to the basement membrane. Look, there's my basement membrane. Oh, I wish I hadn't made this quite so wide because now I have to draw really long in front of you. That's my basement membrane. And this guy right here that I just pointed my arrow to, this is the deepest layer of the epidermis and it is stratum basale. Stratum basale is the most basal layer of the epidermis. So it attaches to the dermis via the basement membrane. Stratum basale is mostly made of keratinocytes. I mean, I mean keratinocytes, mostly keratinocytes. What do you think keratinocytes are going to do? Their whole game is to produce keratin. I feel like I've said this, but I honestly can't remember. But keratin is that protein that fills the epidermal cells and ultimately they die because they get so filled with keratin. Most of the cells in the epidermis are keratinocytes. Some of the cells are melanocytes, melanocytes. What do you think those guys are producing? They're producing melanin. And in fact, you can imagine that like this cell like sprinkled in here are these melanocytes. And melanocytes are super cool. They've got like these little extensions. Dude, I just had a whole lesson on skin color that I'm going to share with you in the next section. Melanocytes are my new favorite cell ever. They're amazing. Okay, so some of the cells in stratum basale are melanocytes and they're going to contribute to the pigmentation of our skin. We'll talk about that in the next section. Okay, all of our layers of skin are generated. All of our layers of epidermis are generated in stratum basale. And all that means is that the cells in stratum basale are alive, they're well, and they're going through mitosis, which means they're just dividing. And awesome, they're going to go through mitosis and produce another layer of cells that sort of gets pushed up closer to the space. The next layer of cells that is the stratum basale becomes this next layer of cells. Does that kind of make sense? This second layer is stratum spinosum. Stratum spinosum is one of the thicker layers of the epidermis. And the cells, the keratinocytes, they're starting to think about producing keratin. And so they're probably starting to not feel quite so hot. Like, they're reaching the point where they're going to start. It's going to start getting ugly here. The cells, again, this happens sort of organically because stratum basale is still going to be producing more cells and they get pushed up into stratum spinosum. And then stratum spinosum, those cells eventually sort of mature and they turn into another layer. And this layer is stratum granulosum. And the cells in stratum granulosum fill with granules. And those granules are like pre-keratin granules. And they're also pre-lippid granules. And tell me what you know about lipids. Give me a lipid, halahala. Lipids are waterproof. This is one of the ways that we end up becoming waterproof. The cells in the layer stratum granulosum are starting to, they're definitely feeling sick. The more keratin that's produced, the more of this lipid that's produced, there's granules all over the place, starting, they're going to like start barfing out their granules. And basically the granules are going to be filling up with all these granules and they're pretty much going to choke on their granules. This is a sad story. Guess what happens next? They choked. In fact, this next layer, all those layers were alive. This next layer, they choked. The stratum lucidum is a layer of the epidermis where truly the cells have started to die. In fact, that's, it's a clear layer. It's like super light layer. Granulosum is super dark. You get this super light layer and it's because the cells are dying. The last layer of the epidermis is the thickest. And this is stratum corneum. And I think of stratum corneum, corny, I think stratum corneum means horny, like not like that, like, like horn material, like that. So it just means that it's like, I'm just going to stop, I'm going to tell you that it's a dead layer. So when you like touch somebody's skin, you're like touching deadness, like that's all dead. If you like scratch it, you might be able to scratch down to some living layers. And if you scratch it such that you can draw blood, then what do you know? Are you an epidermis if you drew blood? No, that would be a great external brain question. You know, you scratch someone's face and they start bleeding. How deep did your scratch go and how do you know? Ah, stop that, I say. It didn't like that question. You know that you made it into the dermis because the only thing that you're going to find, I mean, the dermis has all sorts of stuff in it, but you know that in connective tissue you are going to have blood. So look, that's a blood vessel. Holy creative blood vessel, that might be a problem. We might not want that to be the blood vessel going through my connective tissue layer. Okay, let's, two layers. The whole thing is the dermis, but I got something to show you here. Do you see these little, those aren't mistakes. They're like little, what, nipples that poke up into the epidermis and they actually increase the surface area. They increase the surface area of contact between dermis and epidermis. So they increase the amount of basement membrane that can connect them and then they make it so that if you like smear around your epidermis it doesn't slide off of your dermis. It has like these prongs that are reaching up into the epidermis and sort of holding it in place. Thank you, because wouldn't that be disgusting if you could just like go like that and off would slough your entire epidermis? Yes. So this layer right here, this light blue layer, it's made of aryler or loose connective tissue. Aryler connective tissue and it's called the papillary layer. It's not called stratum papillary. I think I told you that all of them are stratums, but only the epidermis are stratums. The dermis is our layers. So the papillary layer is made of aryler connective tissue and papilla means nipple. That's why I always think of nipple-y things when we're talking about papillas. All right. Deep to the papillary layer and this is going to make you sad and you might not like me anymore. That will make me sad. Don't stop liking me. This is the reticular layer of the dermis. The reason why you might not like me anymore is because the reticular layer is dense irregular connective tissue. You remember, I hope I told you this like five billion times already, now that we're in lecture three. Don't abbreviate anything on any quiz or any exam unless there's like a note on the board that says you can. Otherwise, don't do it. So that dense irregular connective tissue, don't do that. That's what the reticular layer is made of. You may want it to be made of reticular connective tissue? No. You may want it to have reticular fibers in it? No. It's dense irregular connective tissue and it's called the reticular layer because reticular is a word that means something like mess and it is a mess because dense irregular connective tissue is a mess and the fact that reticular describes so many different things is a mess. Don't be confused. I will ask you about the reticular layer of the dermis so stick that in your brain and make it stay there. Deep to the dermis is one more layer. Okay, and this is Pictionary with my favorite color which means you can't see it. This is how I know that I am in this layer because the reticular layer is a mess. But as soon as I see this tissue, I know that I am now in the layer called the hypodermis. Hypodermis, it has another name. Of course it does because tons of structures have multiple names. What is it? Oh, the subcutaneous layer, subbelow cutaneous skin. The hypodermis is not actually considered a layer of skin but it is associated with skin so I'm going to throw it in here and how do I know I made it to the hypodermis? Dude, those look like nipples too. What is happening in my world? I don't even know, like what is this? That is adipose tissue. If you see adipose tissue, you know you are no longer in the dermis. You could literally draw a line separating the adipose tissue layer from what's above it and you identified where hypodermis is and where dermis is. Those are your layers. Put them in your brain. We actually are going to have slides in lab where you are going to be able to identify all those layers and it will be a piece of cake. But I want to spend just a little bit of time talking about skin color because these melanocytes seriously are my new heroes in physiology land so I have to just tell you a little bit about them.