Under the Surface: Anatomy of the Skin

Claire Brooks

Let's start with a number. Between three and five kilograms. That's what your skin weighs, roughly seven percent of your total body mass, spread over about 1.5 to 2 square meters in an adult. It's the largest and heaviest organ you own, and most of us treat it like wrapping paper.

Marcus Hayes

Wrapping paper that's working the whole time. And here's the part that gets awkward. The outer layer of that wrapping gets completely replaced every 30 to 40 days, so about four to six weeks. You shed millions of dead flakes every day. Dander. You are, in a very literal sense, slowly leaving yourself everywhere.

Claire Brooks

Which is a good reason to understand how it's built. This is Chapter 5 of Human Anatomy, the integumentary system. That's the skin, hair, nails, and the glands in the skin. The skin by itself is called the integument, or the cutaneous membrane, and the branch of medicine that treats it is dermatology.

Marcus Hayes

So, the architecture. Think of a three story building. Strictly speaking, the skin has two true layers. The top floor is the epidermis, a keratinized stratified squamous epithelium. Under it is the dermis, which is connective tissue. Then below both of those, you've got the hypodermis, also called the subcutis, made of fat and areolar tissue. It isn't technically skin, but it anchors the skin to the muscle and bone underneath.

Claire Brooks

And the thickness isn't the same everywhere. Skin gets classified as thick or thin, and the classification looks at the epidermis alone. Thick skin has an epidermis about 0.5 millimeters thick. That's the palms, the soles, and the palm side of your fingers and toes. It has no hair and no sebaceous oil glands, but it does have sweat glands, and it's built for friction.

Marcus Hayes

Thin skin is everything else, with an epidermis around 0.1 millimeters. That's the stuff with hair follicles, oil glands, and sweat glands. Five times thinner at the top layer. And the overall thickness of skin across your body varies mostly because of the dermis, not the epidermis. So the classification is a little quirky.

Claire Brooks

Now, here's a detail that makes the layers click. Why does a tattoo hurt and bleed?

Marcus Hayes

Oh, that's a good one. Because the epidermis is constantly being replaced. If you put ink only in the epidermis, it would go out with the flakes within weeks. So the needle has to go down into the dermis, where the ink stays put. But the dermis is where the capillaries and the sensory nerves live. So the very depth that makes a tattoo permanent is the depth that makes it bleed and hurt. The skin is basically telling you which floor you've reached.

Claire Brooks

And the epidermis upstairs has no blood vessels at all. It's avascular. That single fact explains the next chapter of the story.

Marcus Hayes

Right, so picture an elevator. The bottom floor is the stratum basale, a single layer of cuboidal cells, many of them stem cells, dividing rapidly by mitosis. They sit right on top of the dermis, which has the capillaries. Oxygen and nutrients diffuse up from there, because the epidermis has no blood supply of its own.

Claire Brooks

And mitosis needs plenty of oxygen and nutrients. So every new keratinocyte gets born next to the supply, and then it gets pushed up by the next generation underneath. The farther it travels from the dermal capillaries, the less support it gets. Eventually it basically suffocates.

Marcus Hayes

But on the way up, it's doing real work. It's flattening out and filling itself with keratin, that tough fibrous protein. By the time it reaches the top, it's dead, flat, and has no nucleus. It's a tiny armor plate.

Claire Brooks

Let's walk the floors, from deep to superficial. After the basale comes the stratum spinosum, a thick layer of cells held together by little filament connections called desmosomes. When the tissue gets dehydrated for a microscope slide, the cells shrink and those connections stick out, so it looks spiny. Hence the name.

Marcus Hayes

Next is the stratum granulosum, three to five layers of cells that are flattening and filling with dark keratohyalin granules, plus vesicles carrying a lipid coating. That coating helps build the waterproof barrier. Then the stratum lucidum, a thin, clear band of flat dead cells. But it only exists in thick skin, the palms and soles. And at the top, the stratum corneum, 20 to 30 layers of dead, scaly keratinocytes. That's your barrier against trauma, drying out, and infection.

Claire Brooks

Quick memory trick from one of our sources, the Crash Course series. From the top down: Corneum, Lucidum, Granulosum, Spinosum, Basale. Come, Let's Get Sunburned.

Marcus Hayes

Which is rude, because that's the opposite of what you want. Also worth saying, friction speeds this whole elevator up. Rub or pressure on a spot, and the skin responds by building more dead keratinocytes there. That's a callus or a corn. Your skin is basically defending itself with extra armor.

Claire Brooks

Now the cast beyond keratinocytes, which make up the vast majority of epidermal cells. There are three other specialists. Melanocytes live in the stratum basale and make melanin, a pigment in two main flavors: dark eumelanin and reddish yellow pheomelanin. And they package it into little compartments called melanosomes.

Marcus Hayes

The part I love is how they deliver it. Melanocytes have spidery extensions, and they hand melanosomes off to neighboring keratinocytes. Inside those cells, the melanin clusters over the nucleus like a little umbrella, shielding the DNA from ultraviolet radiation.

Claire Brooks

And this is the point people get wrong all the time. Humans of every skin tone have roughly the same number of melanocytes. The difference in color comes from the amount, the type, and the distribution of melanin, not from how many melanocytes you have. On a cellular level, as Hank Green puts it in the Crash Course episode, we're all the same.

Marcus Hayes

Then two more. Dendritic cells, also called Langerhans cells, come from bone marrow and settle into the spinosum and granulosum layers. They're star shaped sentinels, macrophages that ingest pathogens and toxins that get past the surface. And tactile cells, or Merkel cells, sit in the basale and connect to nerve endings in the dermis, so you can feel light touch.

Claire Brooks

Under the epidermis is what I'd call the engine room. The dermis gives the skin its strength, and it holds the blood vessels, sensory nerves, hair roots, and glands. It has two zones. The papillary layer is the top 20 percent, loose areolar tissue full of capillary loops, with little peg shaped bumps called dermal papillae pushing up into the epidermis.

Marcus Hayes

Those pegs are why you have fingerprints. On the hands and feet, they shape the surface into friction ridges that help you grip. They form during fetal development, and they stay the same your whole life. Not even identical twins match.

Claire Brooks

The deeper zone is the reticular layer, the other 80 percent. Dense irregular connective tissue, packed with collagen and elastin fibers. That's what gives skin strength and snap. And if you stretch it too far, say in pregnancy or major weight gain, collagen fibers can tear, and you get striae. Stretch marks.

Marcus Hayes

And a blister is what happens when fluid collects at the boundary between the dermis and the epidermis, usually from friction or a burn. Below all of that is the hypodermis. Fat and areolar tissue. It's a shock absorber, insulation, and an energy reserve. And because it's so well supplied with blood vessels, it's an ideal spot for hypodermic injections. The name is literally the instructions.

Claire Brooks

So that's the structure. But the skin does a lot more than hold you together. It's a thermostat. When you're hot, vessels in the dermis dilate, releasing heat, and you sweat. Even when you aren't noticing, you lose about half a liter a day as insensible perspiration. During hard exertion, sensible perspiration can reach up to 12 liters a day.

Marcus Hayes

And in the cold, the opposite. Dermal vessels constrict and shunt blood toward the core organs. Which is why your fingers go pale and numb before your chest does. And that connects to something I didn't expect. The skin is a blood reservoir. About five percent of your total blood volume sits in your skin at any time.

Claire Brooks

So when you sprint, the dermal vessels constrict and push that blood into general circulation where your muscles need it. Your skin lends blood to your legs.

Marcus Hayes

It also feels. Tactile corpuscles, also called Meissner corpuscles, detect light touch. Lamellar corpuscles, or Pacinian corpuscles, sense deep pressure and vibration. And receptors around hair follicles pick up slight air movement. That's how you notice a breeze, or a mosquito walking, on your arm.

Claire Brooks

Then there's vitamin D, which is a team effort. UV radiation hits cholesterol derived precursors in epidermal keratinocytes, producing cholecalciferol. The liver and then the kidneys convert it into calcitriol, the active form, which regulates calcium uptake and bone health. And that creates an evolutionary tension. You need enough UV to make vitamin D, but UV also degrades folate and damages DNA. Skin tone variation balances those two pressures.

Marcus Hayes

And one more function that comes with a myth attached. Skin does excrete small amounts of nitrogenous wastes in sweat, urea, uric acid, ammonia.

Claire Brooks

Which is where the wellness industry jumps in and says sweating detoxifies you. It doesn't, not in any meaningful way. The liver and kidneys do the overwhelming majority of detoxification. Sweating cools you down. That's its job.

Marcus Hayes

Okay, the awkward chapter. Glands. There are five kinds in the skin. Eccrine sweat glands are the main sweaters, three to four million simple coiled tubes opening at pores on the surface. Watery sweat, and it carries an acid mantle with a pH of 4 to 6 that slows microbial growth.

Claire Brooks

Then apocrine glands, about 2,000 of them, in the armpits and the anogenital region. They switch on at puberty and release a thicker, protein and lipid rich sweat into hair follicles. And here's the twist. That secretion has little odor on its own. Bacteria break it down, and that decomposition is what produces body odor.

Marcus Hayes

So deodorants go after the bacteria, and antiperspirants physically plug the ducts. Different strategies. I just like that the smell is basically your microbes eating lunch.

Claire Brooks

Sebaceous glands secrete sebum, an oily, lipid rich secretion into hair follicles across thin skin. It moisturizes and waterproofs, and when the ducts get blocked, you get acne. Then two modified apocrine glands. Ceruminous glands in the ear canal make earwax. And mammary glands produce milk during pregnancy and lactation.

Marcus Hayes

Hair and nails are built of hard keratin, which is tougher and more compact than the soft keratin in the stratum corneum. Each hair shaft has overlapping, shingle like cuticle cells. Conditioner temporarily smooths those down. And hair follicles cycle. Anagen is the growth phase, six to eight years. Catagen is a short degenerative transition, two to three weeks. Telogen is resting, one to three months. Then it starts again.

Claire Brooks

Now the clinical side, because skin color is a diagnostic window. Cyanosis is a blue tint from low oxygen. Jaundice is yellowing from bilirubin buildup, tied to liver dysfunction. Erythema is redness from vasodilation, as in fever, inflammation, or allergy. Pallor, pale skin, means reduced blood flow, as in shock or anemia. And a hematoma, a bruise, is clotted blood from dermal vessels showing through.

Marcus Hayes

And the cancers map right onto the layers we just walked through. Basal cell carcinoma comes from the stratum basale. It's the most common and the least deadly, a shiny bump with beaded edges that rarely metastasizes. Squamous cell carcinoma comes from keratinocytes in the stratum spinosum. Scaly, can ulcerate, and it can spread to lymph nodes.

Claire Brooks

Malignant melanoma comes from melanocytes. It's less than five percent of skin cancers, but it's the aggressive one. It spreads rapidly and has a high mortality if it isn't removed early. Early excision is the difference. Which is why moles are worth watching for changes in color, diameter, or contour.

Marcus Hayes

And burns follow the layers too. First degree burns affect only the epidermis. Red, painful, and they heal without scarring. Second degree burns go partway into the dermis, with blisters and pain. Third degree burns go through the entire dermis, into the subcutaneous tissue.

Claire Brooks

And this is the counterintuitive one. A third degree burn can be initially painless, because it destroys the nerve endings. It's also the most dangerous. The risks are catastrophic fluid loss, hypovolemic shock, and infection from toxic eschar, the dead burned tissue. Treatment means aggressive fluid resuscitation, debridement, and skin grafts.

Marcus Hayes

So the through line is layers. Which is a good place to end, because every function, from armor to thermostat to blood reservoir to warning light, depends on which floor of the building you're standing on.

Claire Brooks

And on the fact that the most visible organ you have is mostly dead cells at the top, with a living, busy engine room underneath. Not wrapping paper at all.