Where do our nails and hair come from?

- So to build on our knowledge of the integumentary system, we understand that integument is made up of layers of our skin, as well as things that are called appendages. I'll write that up here, appendages. An appendage is kind of a loaded term. There are a lot of things that fall under this classification. Your nail, for instance, is an appendage. So, how does your nail even grow? Well, why don't we just blow up this interface right there, to get a better sense of how our nail is structured. So, let me just draw your finger right here, and it's going along this way, and then I'm going to draw without the nail for right now, so you can sort of see how it grows, and this is kind of where your finger is coming off. The first thing to realize is that this part right here, which is called the nail root, is attached to your epidermis. So, I'll color it blue, because that's how we've been labeling our epidermis so far right here, and I'll even write it out. So, this is your epidermis. That's your topmost layer of skin that we talked about in a separate video. And this is attached right to your nail root right there. So, that's your nail root. And the interesting thing about this interaction point right here is that when you have cells grow in your epidermis, remember that you have cells ascend from your deeper layers up to the top, but here where the nail root is, you'll also have cells that grow out that way. So, you're going to have cells that come from the stratum basale, if you remember that term for the bottommost layer of your epidermis, and you'll have some of the keratinocytes grow, die, and then eventually extend into the nail, and what that means is that your nail is essentially part of the epidermis. So, I'll draw your nail right there in blue, because it's made up of thick keratin. So, keratin from your keratinocytes or the cells that made up your epidermis, and this keratin is just packed into a whole bunch of dead cells. So, the dead cells that sit at the very top of your epidermis hold or are packed with keratin actually. They are packed with keratin, and they move to the top of the epidermis, and then they kind of shift this way. And recall the keratin is the reason why your skin is so tough, and, in fact, in the nail the keratin is what keeps the nail so stiff, and this is true for your fingernails and your toenails. The difference is your fingernails actually grow about four times faster than your toenails, and I guess just for reference, because we're going to be talking about it in our other appendages, remember that below your epidermis you're going to have your dermis here, and below the dermis there's the hypodermis or the subcutaneous fat or tissue. Great. So, that's how our nail is structured and grows. Let's talk about another appendage that might be back here, for instance. So, hair. Let's say we're talking about this backhanded hirsute right here, which just means somebody that's got a lot of hair on the back of their hand, and we'll do the same trick and blow this up to get a better look at what's going on, where our hair grows. So, the big difference between our hair and our nails is that our hair grows from the dermis. So, I'll draw our skin right here, sort of the same orientation we had down here, and recall that our epidermis is our topmost layer of skin. I'll make it small here, because it's not going to be most of the business we're talking about, and then below it we've got our dermis right here, and I'll label that, because it's important. That's our dermis. This is where we're going to be talking about most of our stuff right now. And then below the dermis, remember there's our subcutaneous fat or the hypodermis. And so, the thing about the dermis to recall is that there are two main parts to it, and I'll kind of just draw it like that. There's the papillary dermis, so I'll write dermis papillary, which is the top layer of the dermis, and then there's the reticular layer, and if you recall, the papillary layer is the thinner looser connective tissue layer, and the reticular layer is the thicker, denser connective tissue layer. So, the way our hair is set up is that there's a follicle that originates here. It's this bulb that sits in your reticular dermis. So, I'll label that off here. This is your hair follicle. You may have heard of that term before. Your hair follicle sits in the reticular dermis, and then from this follicle you've got this hair. We call it the shaft of your hair, that extends upward and out, and realize that just like the skin and with the nail, the hair itself has a whole bunch of flattened or stratified squamous epithelial cells that are filled with keratin. And so, the keratin is inside of the cells that are stacked up here in the hair that I'm coloring in, and it also is surrounded by keratin. So, there's your hair, and actually your hair grows at about a rate of 0.5 inches or 1.25 centimeters a month. So, let me label this right here. This is our hair shaft that you can see protruding through your reticular dermis, into the papillary dermis, into the epidermis, and even to the external environment right here. And the other thing I need to draw now, aside from the hair follicle and the shaft, is this band of muscle that I'm going to be putting in right here. It's important that I mention that it sits here in the papillary dermis, so it's in this top layer of the dermis right here. This band of muscle, I'll write it all the way here, is called the arrector, with an A, arrector pili muscle. The arrector pili muscle. And if you'll recall your types of muscle, this is smooth muscle. This is not under our control. We can't cause our arrector pili muscle to contract. It's something that happens involuntarily, and when this muscle contracts, there are two things that happen. One, you'll have your skin sort of bunch up together, and so you'll have what's commonly referred to as goose bumps, and you can see that here, because if this band of muscle gets shorter or skinnier, you'll just bunch your skin together, and you'll have little lumps that come up here. So, you'll have a lump that shows up, and that's your goose bump. And the other thing that'll happen is that it'll make your hair stand up. It'll make your hair stand up. So, I'll just write hair stand, because I don't have as much space here, and your hair will stand up, and this occurs either from strong emotion or exposure to a cold environment. So, one thing you might be asking is, why do we even have this arrector pili muscle? What's the point of having our hair stand on end? And actually this is something that's more useful in animals than it is for us. So, imagine if we're talking about a polar bear, and I'll give it my best shot at drawing a paw right here, to represent a little polar bear. But we've got this polar bear right here, and this guy's in the cold. So, poor guy's out in the cold, okay. And when a cold breeze hits this guy's little paw right here, what's going to happen is that the hair on this polar bear is going to stand on end like that, just like I talked about in the case of a human being. Now, the hair in a polar bear is a lot longer than the hair in a human being, and by making this furry coat stand up like this, what happens is you create a warm insulating layer. The layer of air that's embedded within the hair right here is significantly warmer than the external cold environment. And so, what that does is that allows the warmth to stay within the paw and also extend to the rest of the polar bear's body, so the polar bear stays warm. We don't have that much hair, and so the arrector pili muscle, to some extent, is actually a vestigial structure, vestigial meaning currently useless or not functional anymore. But it's still a pretty interesting reminder of what other animals would do to deal with the cold and how we sort of try to manage without their abundance of hair.