The Integumentary System, Part 1 – Skin Deep: Crash Course A&P #6


Transcript Provided by YouTube:

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When you hear about your “organs,” you probably think of your heart, or your liver,
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or your lungs. Maybe you picture Captain Nemo playing the organ aboard the Nautilus. Why
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do they have an organ on a submarine? That is – that doesn’t make any sense.
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But your first associations with that term probably overlook your biggest organ.
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I’m talking about your skin.
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The glorious fleshy shroud that keeps the world out, and you in.
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Your skin protects your body against infection and extreme temperatures, maintains your balance
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of fluids, and even synthesizes vitamin D for your own personal use.
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Its many nerve endings allow you to sense the outside world, and its sweat glands and
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blood vessels help you maintain a proper temperature and communicate a whole range of stuff — from
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your health to your emotions — through things like blushing, and flushing, and sweating.
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It also accounts for about 3 to 5 kilograms of your body weight, and if you could spread
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it out, it would measure up to two square meters, enough to cover your bed — the most
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disgusting, paper-towel-thin, waterproof, insulating, stretchy, self-repairing, lifetime-lasting
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quilt on the planet!
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It comes in lots of different colors, you can cover it up, or show it off, or tattoo
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the periodic table on it if you want. And of course, without it, you would basically
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shrivel up and die in no time.
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Together with your hair, nails, and sweat and oil glands, your skin forms your integumentary system.
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And if you’ve ever been burned, or had surgery, or stepped on a nail, you know how fast complications
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arise when it gets damaged.
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But it also heals up quite quickly.
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LAYERS.
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Like an everlasting gobstopper, the key to your integumentary system is layers.
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And although you can’t tell by looking at it, your skin actually has three of them,
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each with particular types of cells that have their own skin jobs, to borrow a phrase from
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Blade Runner or BSG… whichever you like!
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The epidermis is the only layer you can actually see, assuming that your skin is intact, which is
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why it’s what you think of, when you think of “skin.” It’s made of stratified squamous epithelial tissue.
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But the dermis just below it is where most of the work that skin does gets done, like
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sweating, and circulating blood, and feeling everything everywhere all the time. And at
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the bottom there’s the subcutis, or hypodermis, composed mostly of adipose or fatty tissue.
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Each of these layers owes its properties — and its ability to do its “skin job” — to
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its unique combination of cells.
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The bulk of your epidermis, for example, is made up of cells called keratinocytes, which
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are the building blocks of that tough, fibrous protein keratin that gives structure, durability,
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and waterproofing to your hair, nails, and outer skin.
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These cells are constantly dying and being replaced — you lose millions of them every
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day, enough to completely replace your epidermis every 4 to 6 weeks.
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That’s why if you want to tell the world you love your mom or commemorate your favorite
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famous physiologist with a tattoo you gotta make sure the ink gets below the epidermis.
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If there’s a cell in the human body that’s been responsible for causing the most pride
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and the most prejudice in human history, it’s another epidermal cell: the melanocyte, the
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spider-shaped cell that synthesizes melanin, the pigment that gives skin its color.
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I’ll spend more time later talking about why skin color differs around the world, but
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one thing to keep in mind is that both the very palest and the very darkest human skins
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on the planet have about the same number of melanocytes.
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Your particular color isn’t about the number of these cells that you have, but instead
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about the breadth of their spidery cellular extensions, which in turn affect the amount
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of melanin that they contain.
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But on a cellular level, we’re all the same.
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Now, your skin, obviously, is also your first line of defense when it comes to protecting
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you from the outside world. So it may not come as a surprise that you have lots of immune
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system cells in your epidermis as well.
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These are your dendritic, or Langerhans cells, which are kinda star-shaped, and like white
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blood cells and platelets, they actually originate in your bone marrow. Once they migrate to
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the epidermis, their long, skinny tendrils run around the keratinocytes and spend much
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of their time ingesting the unwanted invaders that are trying to sneak around your skin.
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Finally, rounding out the quartet of epidermal cells, your tactile, or Merkel cells occur
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deep down at the boundary between the epidermis and the dermis, where they combine with nerve
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endings to create a sensory receptor for touch.
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What’s a little weird, though, is that all these cells are all organized differently
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in the skin that covers your body. In fact, in some places, you have more layers of epidermis than others.
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Your thick skin — and yes, that’s what it’s really called — is the tougher stuff
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on the palms of your hands and the soles your feet, and it consists of five epidermal layers.
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Your thin skin covers everything else, with just four.
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To get to know what’s going on with your thick skin, let’s just imagine you’re
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walking around barefoot in the yard, when suddenly you feel a shooting pain.
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You’ve just stepped on a big ol’ nail, and it’s penetrated all of the layers of
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your epidermis. First it pierced your stratum corneum, which
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means — pardon my Latin — “horny layer.” This is the outermost layer and also the roughest,
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made up of about 20 or 30 sheets of dead keratinocyte cells. This is the layer that you’re always sloughing
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off and feeding to dust mites, but while it’s in place it offers basic protection from environmental threats.
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From there, the nail drives through your stratum lucidum, or “clear layer.” This holds
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two or three rows of clear, flat, dead keratinocytes that are only found in the thick skin of your
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palms and foot soles. So, in places where you only have thin skin, this layer is what’s missing.
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Things start to get more serious in the “granular layer” or stratum granulosum, because this
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contains living keratinocytes that are forming keratin like crazy. This layer looks kind
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of grainy because those cells are getting compressed and flattened as they move up through
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the epidermal layers, maturing as they go.
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The deeper you go through the layers of the epidermis, the younger the cells get. Regeneration
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happens in the lower layers, and new cells move up toward the surface, maturing along
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the way, where they eventually die and slough off from the surface of your skin.
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This whole process is due in part to the fact that the epidermis is epithelial, so it’s
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avascular. That means that all the oxygen and nutrients that its cells need have to
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come from the dermis below it. So, as epidermal cells mature and get bumped up by younger
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cells forming below them, they move further and further from the blood supply, and end
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up essentially suffocating.
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When that nail cuts through the fourth layer — the stratum spinosum, or “spiny layer”
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— it’s getting closer to the point where cell regeneration, or mitosis, is active.
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These cells look prickly when they’re dehydrated for microscope slide preparation — hence
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the name — and that’s because they contain filaments that help them hold to each other.
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And finally, that dang nail touches down on your deepest, thinnest epidermal level — the
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“basal layer” or stratum basale. It’s just a single layer of columnar cells, but
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it’s like a cell factory where most of that new-cell production happens. This stratum
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is also what connects the epidermis to the layer of skin below it, the dermis.
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Feelin’ a little overwhelmed by all the layers? Just remember: “Come Let’s Get
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Sun Burned” — it’s a pneumonic.
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I mean, though, who came up with that, because if you own some skin you know you don’t
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want to get sunburned!
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The ultraviolet radiation in the sun can damage the epidermis, causing elastic fibers to clump
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up, leading to that tell-tale leather-face condition. Plus, getting sunburned temporarily
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depresses your immune system — because, remember, you have immune cells in your epidermis too
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— AND the radiation can actually alter your skin cells’ DNA, leading to skin cancer.
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We’re gonna go into your skin’s love-hate relationship with sunlight more next week,
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but in the meantime, seriously, wear your sunscreen.
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Now, skin damage of any kind can get serious when it affects the dermis, because it’s
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not only got loads of those collagen and elastin fibers, which help make your skin strong and
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elastic, it’s also full of capillaries and blood vessels.
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And it houses the nerve fibers that register sensations like temperature, pressure, and
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pain, as well as parts of your hair follicles and oil and sweat glands with the ducts that
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lead up to the surface of the skin.
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So, the dermis is where most of the skin’s work is done, and it does it in just three
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layers. The upper, papillary layer is composed of
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a thin sheet of areolar connective tissue that’s riddled with little peg-like projections called dermal papillae.
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These papillae are pretty neat because in the thick skin of your hands and feet, these
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tiny protrusions form unique friction ridges that press up through the epidermis to help
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our fingers and feet grip surfaces. Your fingerprints!
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Just below that papillary layer is the deeper, thicker reticular layer that makes up 80 percent
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of your dermis, made up of dense irregular connective tissue. All of the dynamic parts
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contained within the dermis — like the nerve fibers and capillaries — are distributed
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between both its layers.
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So any time you get cut enough to bleed or feel pain, you know that you’ve broken through
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the epidermis and lacerated the dermis. Which, by the way, is the layer that tattoo needles
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have to reach in order to work: It’s the only way to make tattoos permanent, but also
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it means getting tattoos hurts. And bleeds.
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Finally, something of a footnote to your skin is its third and most basal layer — the subcutis,
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or hypodermis. It consists of mostly adipose connective tissue — basically a seam of fat
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— and it provides insulation, energy storage, shock absorption, and helps anchor the skin.
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In short, your hypodermis is where most of your body fat hangs out.
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But there are more skin things to discuss, so in our next lesson we will tackle big
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questions, like — does lotion really do anything? How does deodorant work? And what will make
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my hair soft and shiny and irresistible?
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For now, though, you learned all about skin, the main organ of your integumentary system.
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We looked at the structure, mechanism, and function of your three layers of skin — the
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epidermis, dermis, and hypodermis — and their various sub-layers. We talked about the roles
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of melanin and keratin cells, what happens when you step on a nail, how to ensure you
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get a good tattoo, and why it pays to wear sunscreen.
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Thank you for watching, especially to all of our Subbable subscribers, who make Crash
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Course possible for themselves and for the world. To find out how you can become a supporter,
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just go to subbable.com.
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This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant,
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is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor
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and sound designer is Michael Aranda, and the graphics team is Thought Café.


This post was previously published on YouTube.

Photo credit: Screenshot from video.

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