Under Your Skin

Dr. Stephen Flanagan

Welcome to the Flanatomy podcast. I’m your host, Dr. Stephen Flanagan. We’re diving into the wild world of anatomy with a side of crazy stories from my bizarre life. Let’s explore what makes you, you!

Keshia Rayna

And I’m Keshia Rayna, your co-host, keeping it one hundred and making sure Doc doesn’t get lost in his wild tales. I’m locked and loaded with my phone to fact-check and break down the nerdy stuff so y’all can vibe with the science. Back in my physiology lab days, I spent hours slicing skin samples under the scope, staining with H&E, and trying not to sneeze on the slides. I’ve seen more keratinocytes than most people have skin cells—so yeah, I’m ready to get under the skin. What’s under the microscope today, Doc?

Dr. Stephen Flanagan

Oh, Keshia, we’re getting into the integumentary system. So grab your dermoscopes, your tattoo guns, and your sunscreen, ‘cause we’re diving deep into the skin—the body’s ultimate multi-tool, fashion statement, and protective wrap. And trust me, this episode is going to be skin deep—we’re talking full layers, cells, functions, appendages, burns, cancer, aging, and even a wild story about my first tattoo. Let’s start with the big picture. The skin is our largest organ, accounting for about 7% of total body weight—that’s like 10-11 pounds on an average adult. It varies in thickness from 1.4 mm on your eyelids (thinner than a dime) to 4.4 mm on your heels (thicker than a stack of three pennies). It’s not just one sheet—it’s a dynamic, layered system with two main regions: the epidermis (the covering) and the dermis (the support). Deep to the dermis is the hypodermis, made of areolar and adipose tissue, but—and this is key—it’s not part of the integumentary system. Think of it as the skin’s structural undergarment, not the outfit itself.

Keshia Rayna

Hold up, Doc—7% of body weight? That’s wild. And the hypodermis isn’t official skin? My lab prof always said it was the “subcutaneous layer,” but you’re saying it’s separate?

Dr. Stephen Flanagan

Exactly, Keshia! The integumentary system is epidermis + dermis only. The hypodermis is like the foundation the house sits on—crucial, but not part of the house. It’s also called the superficial fascia, and we’ll get to it later. But first, let’s talk functions—the skin does five major jobs: Protection — from physical bumps, chemicals, water loss, and ultraviolet (UV) radiation. Body temperature regulation — through sweat and blood vessel changes. Excretion — urea, salts, and water lost via sweat. Production of vitamin D — epidermal cells use UV to make it. Sensory reception — packed with nerve endings for touch, pain, temperature. This isn’t just a barrier—it’s a smart barrier. It’s like a high-tech spacesuit with built-in AC, sunscreen, and Wi-Fi.

Keshia Rayna

Spacesuit with Wi-Fi? Yo, I’m stealing that. So, five functions—protection, temp control, excretion, vitamin D, sensation. Let’s break down the epidermis—what cells are in there?

Dr. Stephen Flanagan

The epidermis is a keratinized stratified squamous epithelium, meaning it’s made of layered, flat cells full of tough keratin. It has four main cell types: Keratinocytes — the MVPs, 90% of cells. Melanocytes — pigment producers. Tactile epithelial cells (Merkel cells) — touch sensors. Dendritic cells (Langerhans cells) — immune patrol. Let’s start with keratinocytes. They’re born in the deepest layer, produce keratin—a tough, fibrous protein that waterproofs and strengthens the skin—and also make antibiotics and enzymes to fight invaders. As they rise, they die and flatten, forming the surface barrier. By the time they reach the top, they’re dead, keratinized scales—like zombie armor. We shed about 40,000 of them per minute. That dust in your room? Mostly dead you.

Keshia Rayna

40,000 per minute? That’s a full-body exfoliation every few weeks! So, keratinocytes are the builders and the dead soldiers. What about the others?

Dr. Stephen Flanagan

Melanocytes — from Greek melas, black — are spider-shaped cells in the stratum basale. They produce melanin, the pigment that gives skin color and UV protection. They don’t divide much, but they inject melanin into keratinocytes via melanosomes—like little pigment grenades. More sun = more melanosomes = tan. Tactile epithelial cells, or Merkel cells, are oval, attached to sensory nerve endings in the epidermis. They detect fine touch and texture—like reading Braille or feeling a guitar string. Rare, but critical. Dendritic cells, or Langerhans cells, are star-shaped immune cells from bone marrow. They patrol for pathogens, grab antigens, and present them to T-cells—like the skin’s security team. In H&E stains at 400x, they look like pale cells with long processes, nuclei dark, cytoplasm light.

Keshia Rayna

Melanin grenades? Merkel touch sensors? Langerhans security? Doc, you’re on fire. So, keratinocytes dominate, melanocytes pigment, Merkel touch, Langerhans defend. Now, the layers—stratum basale?

Dr. Stephen Flanagan

Yes—let’s walk through the epidermal strata, from deep to superficial. Think of it as a keratinization assembly line. Stratum basale (ba-SAW-lay or ba-SAY-lee?)Pronunciation debate: basale from Latin basis, base. I say ba-SAW-lay, but some say ba-SAY-lee. Fight me in the comments on X. Single layer of cuboidal/columnar keratinocytes on the basement membrane. High mitosis—new cells every 30 minutes. Contains melanocytes, Merkel cells, and stem cells. At 400x H&E: dark pink, nuclei round and crowded, like a busy factory floor. Stratum spinosum (“spiny layer”)8–10 cell layers thick. Cells look prickly due to shrinkage during fixation—desmosomes everywhere, pulling cell membranes into spines. Tonofilaments (keratin precursors) thicken. Contains Langerhans cells. At 400x H&E: pink, spiky cells, desmosomes like tiny bridges, nuclei dark. Stratum granulosum (“grainy layer”)3–5 layers. Cells flatten, nuclei fade. Keratohyalin granules (for keratin cross-linking) and lamellated bodies (lipid waterproofing) appear. At 400x H&E: darker pink, black granules like pepper, nuclei degenerating. Stratum lucidum (only in thick skin—palms, soles)Thin, clear layer. Cells full of eleidin, a keratin precursor. At 400x: translucent, no nuclei, like glass. Stratum corneum (“horny layer”)20–30 layers of dead, flattened, keratinized cells. Full of keratin, sloughing off. At 400x H&E: flaky pink scales, no nuclei—like cornflakes or fish scales. Name from Latin cornu, horn—tough as horn.

Keshia Rayna

Cornflake skin? Yo, that’s breakfast and anatomy! So, basale is the factory, spinosum is spiky with desmosomes, granulosum is grainy, lucidum is clear in thick skin, corneum is dead and horny. Dermis next?

Dr. Stephen Flanagan

The dermis—strong, flexible connective tissue, richly vascularized and innervated. Two layers: Papillary dermis (top 20%)Loose areolar connective tissue. Contains dermal papillae—finger-like projections into epidermis. Increase surface area for nutrient exchange. Lie atop dermal ridges → form epidermal ridges = fingerprints. At 400x H&E: loose pink collagen, papillae like hills, blood vessels visible. Reticular dermis (80% of dermal thickness)Dense irregular connective tissue. Collagen bundles in all directions—strength in every pull. Cleavage lines (Langer’s lines)—gaps between collagen bundles. Surgeons cut parallel to cleavage lines → less tension → better healing, less scarring. Flexure lines—deep creases (palms, wrists, soles).At 400x H&E: thick, wavy pink collagen, elastic fibers, fibroblasts. I’ve never been in a surgery, Keshia—but I’ve watched hundreds on YouTube, and every derm doc maps those Langer’s lines like a pro. It’s like following a GPS for better scars.

Keshia Rayna

YouTube surgery? Yo, that’s my late-night binge too! So, cleavage lines are the scar-minimizing path. Hypodermis?

Dr. Stephen Flanagan

Hypodermis—deep to dermis, also superficial fascia. Areolar + adipose tissue. Anchors skin to muscles/bones. Insulation, shock absorption, energy reserve. Distribution differs: Females: more in hips, thighs, breasts. Males: more in abdomen. At 400x H&E: fat lobules (empty bubbles), loose pink areolar, blood vessels.

Keshia Rayna

Fat distribution—explains love handles! Pigments?

Dr. Stephen Flanagan

Three pigments: Melanin — from tyrosine, in melanocytes, UV protection. Carotene — yellow-orange from carrots, tomatoes, accumulates in stratum corneum. Hemoglobin — red in blood, shows through in light skin.

Keshia Rayna

So, melanin dominates, carotene adds yellow, hemoglobin adds pink. Appendages—hair?

Dr. Stephen Flanagan

Hair—flexible strand of dead, keratinized cells. Root: embedded in skin. Shaft: projects above. Three layers: Medulla: central core, air/loose cells. Cortex: pigment, strength. Cuticle: overlapping scales (like roof shingles). Curly vs. straight: Straight hair: round shaft, symmetric cortex. Curly hair: flat/oval shaft, asymmetric cortex → uneven keratin → curl. Grey hair: melanin gone, air in medulla/cortex scatters light → white/silver. I’m getting greys, Keshia—teaching stress!

Keshia Rayna

Grey from air bubbles? Explains my silver streak! Sebaceous glands?

Dr. Stephen Flanagan

Sebaceous glands—simple alveolar, secrete sebum (oil). Soften/lubricate hair/skin, collect dirt. Most open into hair follicles (holocrine secretion).Absent on palms/soles.At 400x H&E: pink, lobulated, cells disintegrating.

Keshia Rayna

Sebum’s the skin’s natural moisturizer. What about Sweat glands? that is water on the skin, right

Dr. Stephen Flanagan

Sort of, these little factories turn blood filtrate into sweat — 99% water, plus salts, urea, and a dash of metabolic waste. Two main types: Eccrine glands — the workhorses. Most common, found everywhere (palms, soles, forehead). Coiled deep in the dermis, duct straight to the surface. This is your true sweat — clear, odorless, keeps you cool. Apocrine glands — the drama queens. Only in axillary (armpits), anal, and genital areas. Bigger, coiled in the dermis, but dump into hair follicles, not the surface. Produce a thicker, milky fluid — musky when bacteria break it down. That’s your BO.

Keshia Rayna

Eccrine = cooling system. Apocrine = stink factory. Got it. What about ear wax and milk?

Dr. Stephen Flanagan

Modified apocrine! Ceruminous glands in your ear canal → ear wax (cerumen). Keeps ears clean, traps dust. Mammary glands → milk. Same structure, different job.

Keshia Rayna

So ear wax and breast milk are apocrine cousins? Wild!

Dr. Stephen Flanagan

Totally. But here’s the real mind-blower: X-inactivation in females. Every woman has two X chromosomes — but only one works per cell. Early in development, one X gets randomly silenced — like a genetic coin flip. Some cells use Mom’s X, some use Dad’s. The pattern is locked in for life.

Keshia Rayna

So it’s like… half the cells are Team Mom, half Team Dad?

Dr. Stephen Flanagan

Exactly! And because sweat gland genes are on the X, some patches of skin have active sweat glands, others are duds. Result? Patchy sweating — one armpit might sweat more, or random dry spots. It’s why calico cats have orange, black, and white patches — same X-inactivation, but for fur color genes.

Keshia Rayna

Wait — so women have calico sweat maps? Like, one arm sweats, the other chills?

Dr. Stephen Flanagan

100%! Not every woman notices, but it’s there. a Woman might say her left armpit is an overachiever — sweats like a sprinkler. Right side? Lazy. That’s X-inactivation in action.

Keshia Rayna

Calico sweat? Genetic patchwork? Doc, this is peak nerd gold. I’m never looking at deodorant the same way. What's next?

Dr. Stephen Flanagan

Let’s talk nails — your body’s built-in multitools.

Keshia Rayna

Nails? Like, the things I paint when I’m bored?

Dr. Stephen Flanagan

Exactly — but way cooler than glitter polish. Nails are a scalelike modification of the epidermis, made of hard keratin — same protein as hair, but denser, like upgraded armor. Each nail has: Nail plate — the hard part you see. Nail bed — skin underneath, rich in blood vessels (why it’s pink). Lunula — that white half-moon at the base, where new keratin is made. Cuticle — seals the gap, keeps out bacteria. They grow from the nail matrix — a pocket of dividing cells under the lunula. Fingernails grow ~3 mm/month, toenails ~1 mm/month — slower ‘cause less blood flow. Fun fact: the middle finger nail grows fastest, pinky slowest.

Keshia Rayna

Middle finger fastest? Of course it is — always flipping the bird in a hurry!

Dr. Stephen Flanagan

Ha! And that lunula “moon phase” myth? Total bunk. It’s not a growth marker — just where the nail’s thicker. Nails protect fingertips, enhance fine touch (like picking up a coin), and even act as tools — ever open a soda can with one?

Keshia Rayna

Guilty. So nails are body armor + Swiss Army knife. What’s next — burns?

Dr. Stephen Flanagan

You read my mind. Skin burns can be categorized a few ways. the old way was like this... 1st-degree: epidermis only, and redness. 2nd-degree: upper dermis, blisters. 3rd-degree: full thickness, white/red/black, no pain (nerves gone). Now it is more like 1st and 2nd degree burns are just called partial thickness burns where 3rd degree burns are called full thickness burns.

Keshia Rayna

Okay, Doc, we just went through burns — first, second, third degree. Since we’re talking sun damage, isn’t this a good time to hit skin cancer?

Dr. Stephen Flanagan

Perfect segue, Keshia. And yeah — this one hits close to home. A few years back, my wife and I were obsessed with beach days. She’d drag me out to the Malibu beaches every weekend — no sunscreen just wasn't a priority. She’d laugh and say, “Flan, you’re gonna be a lobster!” Well, one day, she found a weird pearly bump on her back. Dermatologist took one look and said: basal cell carcinoma. Not aggressive, but still cancer. They cut it off and left a huge keloid scar, the nurse that did the cutting wasn't trained to follow those cleavage lines I mentioned earlier and the scar is pretty wild— the whole process scared the heck out of us. That was our wake-up call. SPF 50 every morning, every day since.

Keshia Rayna

Whoa — your wife had basal cell? That’s the most common one, right?

Dr. Stephen Flanagan

Exactly. Basal cell carcinoma — arises from the stratum basale cells. It’s the least malignant, grows slow, rarely metastasizes. Looks like a shiny pearl or sore that won’t heal. Caught early? Easy fix. Then there’s squamous cell carcinoma — comes from the keratinocytes in the stratum spinosum. More aggressive than basal cell, can spread if ignored. Shows up as a scaly red patch or ulcer. And the big bad wolf: melanoma — from melanocytes. The deadliest. Grows fast, spreads early. Look for the ABCDEs: Asymmetry, Border irregular, Color varied, Diameter greater than 6mm, Evolving. One bad mole can change everything.

Keshia Rayna

ABCDEs — I’m writing that down. But how does the sun actually cause this? I know UV, but what’s happening at the DNA level?

Dr. Stephen Flanagan

Great question. UV-B rays hit your skin and smash into DNA in keratinocytes and melanocytes. Specifically, they cause thymine dimers — two thymine bases on the same DNA strand bond together, forming a kink. Normally, your cells have repair enzymes — excision repair — that snip out the damage and patch it. But if you get too much UV, the repair system gets overwhelmed. Mutations pile up. One of those mutations hits a growth-control gene? Boom — cancer.

Keshia Rayna

So thymine dimers are like DNA speed bumps — and if the repair crew can’t keep up, you’re on the road to cancer city.

Dr. Stephen Flanagan

Nailed it. That’s why my wife and I now live in the shade. For us, sunscreen isn’t optional. It’s anatomy armor. And speaking of armor — you know Freddie Freeman, the MLB first baseman? He wears long sleeves under his jersey every single game, even in 100-degree heat. Why? His mom, Rosemary, died of melanoma when he was 10. He started wearing the sleeves in the minors, and now it’s his tribute. Every pitch, every swing — he’s playing for her. And yeah, he’s got fair skin like me, so it’s protection too. That’s what this is about: respecting the skin you’re in.

Keshia Rayna

Freddie Freeman in long sleeves for his mom? That’s powerful. From your wife’s basal cell to his mom’s melanoma — this hits hard. Sunscreen’s not just armor, it’s legacy protection. Alright, Doc — life cycle next?

Dr. Stephen Flanagan

Alright, Kesh, let’s talk about where all this skin comes from in development — the life cycle of the integumentary system.

Keshia Rayna

Lay it on me, Doc — embryonic origins?

Dr. Stephen Flanagan

You got it. The epidermis develops from ectoderm — that’s the outer layer of the embryo, the one that also gives us the nervous system and hair. The dermis and hypodermis, on the other hand, come from mesoderm — the middle layer, which builds muscle, bone, and connective tissue. And the melanocytes? They’re special — they migrate from the neural crest, a unique population of cells that split off from the ectoderm early on. So your pigment cells are basically neural rebels that went full goth.

Keshia Rayna

Neural rebels? I love it. So ectoderm for the top layer, mesoderm for the support, neural crest for the pigment crew. What about aging?

Dr. Stephen Flanagan

As we age, the skin thins, becomes less elastic, and inflammations get more common. Collagen and elastin break down, cell turnover slows, and UV damage piles up. That’s why older skin bruises easier, wrinkles deeper, and heals slower. It’s not just cosmetic — it’s structural wear and tear.

Keshia Rayna

So the skin’s like a well-loved leather jacket — tough at first, but over time it cracks, fades, and tells your whole story.

Dr. Stephen Flanagan

Exactly. And speaking of stories on skin Speaking of skin art, my brother’s a famous tattoo artist. At 18, he did his first tattoo on me in a random apartment kitchen—undisclosed location. He was nervous, hands shaky. Ink went into the dermis, where it stays ‘cause macrophages trap it. about 45 min in I hear him say "oops". Not exactly a sound you wanna hear from a tattoo artist. It wasn't a big deal, he just went back over a line he didn't mean to. It’s faded now, but tattoos are permanent and the story will stay in my memory forever.

Keshia Rayna

“Oops” at 45 minutes? Doc, I’d have run! Macrophages lock the ink—permanent art. Wrap it up!

Dr. Stephen Flanagan

Skin’s our shield, factory, sensor, and canvas. From stratum basale to hypodermis, it’s the ultimate multitasker. Tattoo or not—you’re wearing a masterpiece!!

Keshia Rayna

Thanks for sticking with us—no pun intended—on Flanatomy. Next time we might explore something even stranger under the surface. Take care of your skin, folks.

Dr. Stephen Flanagan

You too! Stay weird, stay curious, and catch us next time. Bye!