What is skin? (Epidermis)

- They say that beauty is only skin deep, but did you know that the outermost layer of your skin is actually composed of entirely dead cells? I have no idea what's beautiful about dead tissue, but in this video, we'll seek to figure that out. We'll talk about the different layers of skin. As you can see here, there are many layers to the skin, and there are three different sets of layers that we can talk about. There's the epidermis, which is the first five layers, the dermis, which is the next two layers, and then the last part, the subcutaneous tissue or the hypodermis. In this video, we'll start by talking about the most superficial part of your skin, and that is the epidermis, and I'm sure your friends have told you before that your epidermis is showing. The epidermis is the topmost layer of skin, and itself is comprised of five layers or as we call them, strata. So, five layers or strata, and each strata or stratum has its own important characteristics. So, we'll start from the bottom and talk about the deepest or bottommost layer of the epidermis, and that's known as the stratum basale or the basal layer. The stratum basale sits right above the dermis and is the place where we first generate what are called keratinocytes. And so, I'm drawing a whole bunch of keratinocytes right here. These are keratinocytes, and, in fact, the stratum basale has multiple layers of keratinocytes. And as you might recall, cyte just means cell. So, what does keratino mean? Well, it comes from a protein or an intermediate filament that's called cytokeratin, cytokeratin, which is something we first start making in the stratum basale. Sometimes it's just called keratin, and it helps give our skin it's tough outer layer when we get to the top, as we'll talk about in a minute, and in other animals, keratin actually is the main ingredient for things like horns, so horns or even hooves. So, it's definitely the tough stuff that help protect our skin. And another very important thing to know about the stratum basale is this is where we have very rapid cell division. Rapid cell division because this is the bottom layer of our epidermis. So, cells are made here, and slowly they move upwards. Another thing that's important about the stratum basale is this is where we get our skin color from. So, there are components of the stratum basale that determine what our skin color is going to be, and the cells that confer skin color for us sit here as well, and these guys, you may have heard about them before, are called melanocytes, melanocytes. So, melanocytes, and as you know, cytes again just means cell, but melano just indicates that there's a special pigment that's made in these cells that helps determine our skin color, and that pigment, as you may have guessed, is called melanin, melanin, and the interesting thing about these cells or this pigment is that it's not the number of melanocytes that determine how dark your skin is. In fact, people with dark skin color and light skin color have the same number of melanocytes. It's actually melanin, the amount of melanin that you have, that determines how dark your skin color is. Meaning that darker skin people have more melanin. All right, so as we move up to our next layer over here, we see that our keratinocytes make it up here too, and we might start to notice some of the cytokeratin that's being produced here a little bit, and one of the unique features about this next layer is that we have this little connection that's starting to be generated in between our keratinocytes. This guy is called a desmosome, a desmosome, and this is involved in what gives this layer its name. So, this layer that sits above the stratum basale is called the stratum spinosum, the stratum spinosum, which, as you might guess from the fact that we call this our basal layer, this stratum is our spiny layer, and the reason why we call it that is because of the desmosome. When we look at this layer of skin, sometimes underneath some microscopes, where we have lost moisture or the water that is in our cells, we'll have these shrunken cells that actually look like stars or kind of like spiny cells, very pointy shapes, and the reason why they're pointy is because they're still connected by these desmosomes, but they've lost the water. So, that's why sometimes these layers look shriveled, so shriveled because they've lost water, as well as spiny. So, that's how we remember the name of the stratum spinosum. Another type of cell that sits in this layer is an immune cell that is looking for foreign bodies or pathogens or things that can be destructive to our body like bacteria and fungi and eat them. These are called Langerhans cells, Langerhans cells, and as you may have guessed, these are part of our immune system. So, they hang out here but can definitely travel to other layers, and we talk about them more in our dedicated immune system tutorial. All right, and so, it's onward and upward to our next layer that's here. This more superficial layer is called the stratum granulosum, the stratum granulosum, which just means the granular layer, and the reason why we call it this is because the keratinocytes that sit here now, in addition to the desmosomes that we talked about from the spiny layer, have a tremendous amount of granules that are being produced here, and these granules are very apparent when we look at them under a microscope. These granules are called kerato, keratohyalin granules, so keratin hyalin granules, and I know what you might be thinking. So, does this mean we're making a whole bunch more keratin in this layer? Well, we are, but these granules don't actually house keratin or the cytokeratin we talked about. They hold a whole bunch of other proteins that help handle our keratin. Just know that these granules make proteins that are best remembered as your keratin-handling proteins. So, your keratohyalin granules make your keratin-handling proteins, because they move your cytokeratin around in the cell. The other thing the stratum granulosum does is that it releases these things that are called lamellar, lamellar bodies, and these lamellar bodies contain a whole bunch of lipids that then come together and form a strong lipid layer that sits at the top of our skin. This lipid layer is impermeable. It gives the skin its water-tight capabilities that prevent foreign pathogens from making their way down deeper into our skin and eventually into our bloodstream. And so, from there we go to our next layer up, to the stratum lucidum, the stratum lucidum, and sure enough, this is our lucid layer or our clear, I'll write that here, this is our clear layer. And the way I think about it, is that in the stratum granulosum, our keratinocytes worked so hard to make these keratohyalin granules and release these lamellar bodies that they actually died by the time they make it up to the stratum lucidum. So, these cells that we have here, these keratinocytes, are actually dead. These are zombie keratinocytes, and they look that way on microscope because they've lost their nuclei or other organelles that usually gave them the color we saw at our earlier, more alive layers. And so, as a result, these layers will not have these granules or nuclei or organelles that we see in the previous layers. Instead, they're going to be far clearer or more see-through than what we saw earlier, and that's an important note to make off to the side. The stratum granulosum has lost their nuclei and organelles, which are usually supposed to give our cells color when we look at them under a microscope, and this trend continues when we move on to our topmost layer here. This is the stratum corneum, the stratum corneum, our topmost layer of the epidermis of our skin entirely, and the way I think about this is that this is the coroner's layer, the coroner, and as you know, the coroner's office is where dead bodies go for autopsies, and so this is also a layer of dead skin. And one of the key characteristics of the stratum corneum is it will have stacked layers of our keratinocytes, our dead keratinocytes, and we can have up to 15 to 20 stacked layers of these flat, simple, squamous epithelial cells. So, I'll write that off over here. We can have about 15 to 20 layers of our keratinocytes. Again, they're still dead, because they died in the stratum lucidum, and because this is our topmost layer, we'll see that these cells will randomly, as well as continuously, slough off. So, they will randomly and continuously slough off or leave, they'll fall off, and that makes way for newer cells underneath or newer keratinocytes to make their way up to the top. And another interesting thing about this layer is that reptiles, I'll write this in like a reptile green, so reptiles will molt this layer or shed this layer off in one piece, and so that's why you'll see like snakeskin that comes off as a single, intact covering to the snake. So, they molt off the entire stratum corneum, which is very interesting and also very gross. All right, so those are all your layers of your epidermis, all five of them, the top two being dead, and these bottom three, or deeper three layers of the epidermis, being alive, and as we move deeper down and start to talk about the dermis, those cells are also alive and have vessels in them to nourish them. We don't have vessels anywhere in the epidermis because our nutrients and our oxygen come from below and make their way upwards.