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  • When you hear about yourorgans,” you probably think of your heart, or your liver,

  • or your lungs. Maybe you picture Captain Nemo playing the organ aboard the Nautilus. Why

  • do they have an organ on a submarine? That is - that doesn’t make any sense.

  • But your first associations with that term probably overlook your biggest organ.

  • I’m talking about your skin.

  • The glorious fleshy shroud that keeps the world out, and you in.

  • Your skin protects your body against infection and extreme temperatures, maintains your balance

  • of fluids, and even synthesizes vitamin D for your own personal use.

  • Its many nerve endings allow you to sense the outside world, and its sweat glands and

  • blood vessels help you maintain a proper temperature and communicate a whole range of stuff -- from

  • your health to your emotions -- through things like blushing, and flushing, and sweating.

  • It also accounts for about 3 to 5 kilograms of your body weight, and if you could spread

  • it out, it would measure up to two square meters, enough to cover your bed -- the most

  • disgusting, paper-towel-thin, waterproof, insulating, stretchy, self-repairing, lifetime-lasting

  • quilt on the planet!

  • It comes in lots of different colors, you can cover it up, or show it off, or tattoo

  • the periodic table on it if you want. And of course, without it, you would basically

  • shrivel up and die in no time.

  • Together with your hair, nails, and sweat and oil glands, your skin forms your integumentary system.

  • And if youve ever been burned, or had surgery, or stepped on a nail, you know how fast complications

  • arise when it gets damaged.

  • But it also heals up quite quickly.

  • LAYERS.

  • Like an everlasting gobstopper, the key to your integumentary system is layers.

  • And although you can’t tell by looking at it, your skin actually has three of them,

  • each with particular types of cells that have their own skin jobs, to borrow a phrase from

  • Blade Runner or BSGwhichever you like!

  • The epidermis is the only layer you can actually see, assuming that your skin is intact, which is

  • why it’s what you think of, when you think ofskin.” It’s made of stratified squamous epithelial tissue.

  • But the dermis just below it is where most of the work that skin does gets done, like

  • sweating, and circulating blood, and feeling everything everywhere all the time. And at

  • the bottom there’s the subcutis, or hypodermis, composed mostly of adipose or fatty tissue.

  • Each of these layers owes its properties -- and its ability to do itsskin job” -- to

  • its unique combination of cells.

  • The bulk of your epidermis, for example, is made up of cells called keratinocytes, which

  • are the building blocks of that tough, fibrous protein keratin that gives structure, durability,

  • and waterproofing to your hair, nails, and outer skin.

  • These cells are constantly dying and being replaced -- you lose millions of them every

  • day, enough to completely replace your epidermis every 4 to 6 weeks.

  • That’s why if you want to tell the world you love your mom or commemorate your favorite

  • famous physiologist with a tattoo you gotta make sure the ink gets below the epidermis.

  • If there’s a cell in the human body that’s been responsible for causing the most pride

  • and the most prejudice in human history, it’s another epidermal cell: the melanocyte, the

  • spider-shaped cell that synthesizes melanin, the pigment that gives skin its color.

  • I’ll spend more time later talking about why skin color differs around the world, but

  • one thing to keep in mind is that both the very palest and the very darkest human skins

  • on the planet have about the same number of melanocytes.

  • Your particular color isn’t about the number of these cells that you have, but instead

  • about the breadth of their spidery cellular extensions, which in turn affect the amount

  • of melanin that they contain.

  • But on a cellular level, were all the same.

  • Now, your skin, obviously, is also your first line of defense when it comes to protecting

  • you from the outside world. So it may not come as a surprise that you have lots of immune

  • system cells in your epidermis as well.

  • These are your dendritic, or Langerhans cells, which are kinda star-shaped, and like white

  • blood cells and platelets, they actually originate in your bone marrow. Once they migrate to

  • the epidermis, their long, skinny tendrils run around the keratinocytes and spend much

  • of their time ingesting the unwanted invaders that are trying to sneak around your skin.

  • Finally, rounding out the quartet of epidermal cells, your tactile, or Merkel cells occur

  • deep down at the boundary between the epidermis and the dermis, where they combine with nerve

  • endings to create a sensory receptor for touch.

  • What’s a little weird, though, is that all these cells are all organized differently

  • in the skin that covers your body. In fact, in some places, you have more layers of epidermis than others.

  • Your thick skin -- and yes, that’s what it’s really called -- is the tougher stuff

  • on the palms of your hands and the soles your feet, and it consists of five epidermal layers.

  • Your thin skin covers everything else, with just four.

  • To get to know what’s going on with your thick skin, let’s just imagine youre

  • walking around barefoot in the yard, when suddenly you feel a shooting pain.

  • Youve just stepped on a big olnail, and it’s penetrated all of the layers of

  • your epidermis. First it pierced your stratum corneum, which

  • means -- pardon my Latin -- “horny layer.” This is the outermost layer and also the roughest,

  • made up of about 20 or 30 sheets of dead keratinocyte cells. This is the layer that youre always sloughing

  • off and feeding to dust mites, but while it’s in place it offers basic protection from environmental threats.

  • From there, the nail drives through your stratum lucidum, orclear layer.” This holds

  • two or three rows of clear, flat, dead keratinocytes that are only found in the thick skin of your

  • palms and foot soles. So, in places where you only have thin skin, this layer is what’s missing.

  • Things start to get more serious in thegranular layeror stratum granulosum, because this

  • contains living keratinocytes that are forming keratin like crazy. This layer looks kind

  • of grainy because those cells are getting compressed and flattened as they move up through

  • the epidermal layers, maturing as they go.

  • The deeper you go through the layers of the epidermis, the younger the cells get. Regeneration

  • happens in the lower layers, and new cells move up toward the surface, maturing along

  • the way, where they eventually die and slough off from the surface of your skin.

  • This whole process is due in part to the fact that the epidermis is epithelial, so it’s

  • avascular. That means that all the oxygen and nutrients that its cells need have to

  • come from the dermis below it. So, as epidermal cells mature and get bumped up by younger

  • cells forming below them, they move further and further from the blood supply, and end

  • up essentially suffocating.

  • When that nail cuts through the fourth layer -- the stratum spinosum, orspiny layer

  • -- it’s getting closer to the point where cell regeneration, or mitosis, is active.

  • These cells look prickly when theyre dehydrated for microscope slide preparation -- hence

  • the name -- and that’s because they contain filaments that help them hold to each other.

  • And finally, that dang nail touches down on your deepest, thinnest epidermal level -- the

  • basal layeror stratum basale. It’s just a single layer of columnar cells, but

  • it’s like a cell factory where most of that new-cell production happens. This stratum

  • is also what connects the epidermis to the layer of skin below it, the dermis.

  • Feelin’ a little overwhelmed by all the layers? Just remember: “Come Let’s Get

  • Sun Burned” -- it’s a pneumonic.

  • I mean, though, who came up with that, because if you own some skin you know you don’t

  • want to get sunburned!

  • The ultraviolet radiation in the sun can damage the epidermis, causing elastic fibers to clump

  • up, leading to that tell-tale leather-face condition. Plus, getting sunburned temporarily

  • depresses your immune system -- because, remember, you have immune cells in your epidermis too

  • -- AND the radiation can actually alter your skin cellsDNA, leading to skin cancer.

  • Were gonna go into your skin’s love-hate relationship with sunlight more next week,

  • but in the meantime, seriously, wear your sunscreen.

  • Now, skin damage of any kind can get serious when it affects the dermis, because it’s

  • not only got loads of those collagen and elastin fibers, which help make your skin strong and

  • elastic, it’s also full of capillaries and blood vessels.

  • And it houses the nerve fibers that register sensations like temperature, pressure, and

  • pain, as well as parts of your hair follicles and oil and sweat glands with the ducts that

  • lead up to the surface of the skin.

  • So, the dermis is where most of the skin’s work is done, and it does it in just three

  • layers. The upper, papillary layer is composed of

  • a thin sheet of areolar connective tissue that’s riddled with little peg-like projections called dermal papillae.

  • These papillae are pretty neat because in the thick skin of your hands and feet, these

  • tiny protrusions form unique friction ridges that press up through the epidermis to help

  • our fingers and feet grip surfaces. Your fingerprints!

  • Just below that papillary layer is the deeper, thicker reticular layer that makes up 80 percent

  • of your dermis, made up of dense irregular connective tissue. All of the dynamic parts

  • contained within the dermis -- like the nerve fibers and capillaries -- are distributed

  • between both its layers.

  • So any time you get cut enough to bleed or feel pain, you know that youve broken through

  • the epidermis and lacerated the dermis. Which, by the way, is the layer that tattoo needles

  • have to reach in order to work: It’s the only way to make tattoos permanent, but also

  • it means getting tattoos hurts. And bleeds.

  • Finally, something of a footnote to your skin is its third and most basal layer -- the subcutis,

  • or hypodermis. It consists of mostly adipose connective tissue -- basically a seam of fat

  • -- and it provides insulation, energy storage, shock absorption, and helps anchor the skin.

  • In short, your hypodermis is where most of your body fat hangs out.

  • But there are more skin things to discuss, so in our next lesson we will tackle big

  • questions, like -- does lotion really do anything? How does deodorant work? And what will make

  • my hair soft and shiny and irresistible?

  • For now, though, you learned all about skin, the main organ of your integumentary system.

  • We looked at the structure, mechanism, and function of your three layers of skin -- the

  • epidermis, dermis, and hypodermis -- and their various sub-layers. We talked about the roles

  • of melanin and keratin cells, what happens when you step on a nail, how to ensure you

  • get a good tattoo, and why it pays to wear sunscreen.

  • Thank you for watching, especially to all of our Subbable subscribers, who make Crash

  • Course possible for themselves and for the world. To find out how you can become a supporter,

  • just go to subbable.com.

  • This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant,

  • is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor

  • and sound designer is Michael Aranda, and the graphics team is Thought Café.

When you hear about yourorgans,” you probably think of your heart, or your liver,

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