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What lies beneath the epidermis? (Dermis and Hypodermis)

Video transcript
- [Voiceover] So, now that we've talked about the epidermis, or our top most layer of skin, let's descend further down, and talk about the dermis, these next two layers of skin, and then we'll also talk about the hypodermis, which is the bottom most layer of skin here. So as you can see, your dermis has two layers to it, and again, it sits right below your epidermis, so your epidermis, the top most layer of skin sits up here, and if you can recall, the bottom most layer of your epidermis is called the "stratum basale," the basal layer of your epidermis. So below that we have our first layer of the dermis which is called the "papillary dermis." This is the topmost layer of the dermis, and then below the papillary dermis, we have what's called the "reticular dermis," the reticular dermis, right here. The main difference between the dermis versus the epidermis is the type of tissue we have here. Recall that in the epidermis, we had "epithelial tissue." So, I'll write epithelial tissue, up here. And then within the dermis both layers are going to have something that's called "connective tissue." You've probably seen that written out as CT (connective tissue). So connective tissue is different in that it doesn't necessarily involve cells. Epithelial tissue involves things like simple squamous epithelial cells, columnar epithelial cells. Even transitional epithelial cells, that you may have seen in the bladder. And your connective tissue contains a litany of other proteins. Things like actin which is what you must have heard of when we talked about the muscular system, or collagen which is another structural protein. These are some of the most popular proteins you see mentioned when we talk about connective tissue, but they also include things like laminin, elastin, desmin. Connective tissue mainly holds things together. If you remember a tendon, a tendon is connective tissue that holds the muscle onto a bone, and between the papillary dermis, and the reticular dermis, there are two types of connective tissue we see here. In the papillary dermis we have very thin, loose connective tissue, and this allows for all the stuff in the papillary dermis to sort of move around and change shape and position, versus what we have in the reticular dermis which is thicker, more dense, or denser connective tissue. Denser connective tissue, and the purpose of this is to make things stay still. We anchor things down in the reticular dermis, and we'll talk exactly about what we're anchoring down here in a moment, but as we hop back up to the papillary dermis, there are a variety of things that we have here that have to change shape and position and are able to do so because the connective tissue is so loose. One of the main things we find in the papillary dermis are blood vessels. If you recall when you have an arterial from an artery branch off into a capillary, so you've got these purple capillaries right here, and then come back together to make a venule, a venule which goes off to a vein. Realize now that this is the most superficial or the topmost layer of your skin that actually has blood vessels here. So one of the roles is that the capillaries have to deliver oxygen and nutrients that nourish not only your dermis, but actually go up into the epidermis as well because the epidermis doesn't have any blood vessels. It relies entirely on the papillary dermis to nourish it, and thank goodness this is thin, loose connective tissue because there's plenty of room here for the capillaries to engorge with blood, fill up with nutrients, and then these nutrients can diffuse across and go up from the dermis into the epidermis, and even go lower, if necessary. The other thing we have in our dermis are nerve endings. These nerve endings, and I'll write this just because my drawing is not as obvious. These nerve endings that sit in our dermis allow us to perceive things like touch and even pain, and then that will pass on a signal across the nerve going in this direction right here, like that, and this signal is relayed all the way to a cell body, and this cell body can sit somewhere much further away than the skin. Now as we get to the reticular dermis we'll see why it's important to have thick, dense connective tissue down here, and this connective tissue helps anchor things down. One of the things that you can have, and I'll sort of draw it like this ascending to your epidermis and so it leaks out over here. This is a gland. You'll have tons of sweat glands and exocrine glands of other varieties that start in your reticular dermis, and are anchored and held down here that extend out with a duct to secrete their contents to the epidermis to the outside world, kind of at the top layer of your skin. So you've got plenty of glands that sit here. In addition, you'll also have this follicle right here. This follicle will have from it emerging a hair. This hair ... I guess I should label this also. The hair, of course, is going to protrude at the top of your skin. It's not that you just have a hair that sits inside and then this guy right here is just your hair follicle, and so your hair follicle is anchored within your reticular dermis, and the hair itself is allowed to grow and extend out to the top and protrude from your epidermis. One of the other things that we have kind of sitting in your dermis that's anchoring itself onto the hair, or the hair follicle, is this slab of muscle that we've got right here. So this guy, I'll write if off there, this is an "arrector pili muscle." An arrector pili muscle, and this is involved in extending or having your hair stand straight up when you're cold or when you're scared. Finally, now that we've talked about the dermis, we can move to our last layer of skin. Although, it's technically not skin, but it's called the "hypodermis." This is the hypodermis, which is a great name because as you might recall, hypo just means "below" and this layer sits right below the dermis. Another name for it is subcutaneous fat, and I think that that's also a great name for what we have here. Subcutaneous, if you might remember, cutaneous is just another name for skin, so subcutaneous fat, and that's exactly what we have down here. We have tons of layers of fat that could vary from person to person, and although fat gets a lot of flack we have to realize that it's very important because there are two main functions they help us achieve. First of all fat absorbs shock. People that are more insulated with fat are able to withstand traumatic injuries, maybe more effectively than others, and the other thing that it does for us is that it insulates our tissue, because realize that now that we've finished talking about our skin, we've talked about the epidermis, the dermis and now we've even covered the hypodermis. What sits right below this? You're going to find muscle. Muscle sits down here. It's below that and perhaps even bone. You know, if you're talking about your chest wall right above your rib, if you put your finger right kind of where your clavicle is above your rib cage, or even on your ribs, realize that your epidermis, dermis, and your hypodermis are all that separate your finger from your rib bone, or even your muscle if you're pressing on your bicep or your calf. We spent a lot of time kind of talking about the components of what's in our skin, but how does this even help us clinically? How do doctors use this to diagnose diseases? The best one to talk about as an example here would be for the diagnosis of burns. Now a burn is a horrible thing that could happen and one of the things that we do to differentiate the types of burns you can have is to say what degree of a burn you sustained, and interestingly enough the different layers of your skin correspond to the different degrees of burn you can have. You can have a first-degree burn, which is just sort of a reddening that can occur on your epidermis. You can have a second-degree burn, which is characterized by burning into the dermis, so much so that you don't have nerve pain after some time. That's the difference between a first-degree and a second-degree. A first-degree hurts awfully, but if you have a place that's been burned that doesn't hurt so much anymore, it's because you've burned and killed off nerve tissue here, and so they're not able to send a signal up to the cell body. And then finally there's a third-degree burn, which is going past the hypodermis into things like your fat, the muscle that's below, the bone that's below as well, and, of course, the third-degree burn is characterized by not having any pain, and having more of a darker coloration to it, because now you're burning more than just connective tissue. You're actually burning bone or muscle, or fat, as well. It's this exact anatomy, right here, that we've talked about that allow doctors to distinguish between whether a patient has sustained a first, second, or third-degree burn, and that's something you can do now as well.