Anatomy & Physiology · Chapter 5

The Integumentary System

Spread flat, your skin would cover about two square meters. It weighs more than any other organ you have. Yet most of us never think of it as an organ at all. This chapter will change that.

You will build the skin from the top down, one layer at a time. Then you will follow a single cell on its month-long trip. It starts as a busy, living cell and ends as a dead flake on the surface. You will learn where skin color comes from. You will meet the hair, nails, and glands that grow out of the skin. You will also see the five quiet jobs your skin does every second. We end with burns and skin cancer, the two main ways this amazing barrier can fail.

By the end of this chapter you can

Name the two true layers of the skin, the epidermis and the dermis. Name the hypodermis below them. Say what tissue each is made of and what job it does.
List the five epidermal strata (layers) in order. Follow a skin cell through keratinization, from its birth to the day it flakes off.
Explain how melanocytes and melanin give skin its color and shield DNA from ultraviolet light.
Describe how hair, nails, and the sebaceous and sweat glands are built. Say what each one adds.
Sum up the skin's five functions: protection, thermoregulation (heat control), sensation, vitamin D synthesis, and excretion.
Sort burns by how deep they go. Estimate how much skin is burned with the rule of nines. Tell apart the three skin cancers.
Use the ABCDE warning signs to tell a harmless mole from a possible melanoma.

Section 1Layers of the skin

The three layers of the skin, seen from the side — **Reference.** The layers of the skin: epidermis, dermis, and hypodermis — OpenStax, CC BY 3.0 (via Wikimedia Commons)

An organ you can see

The integumentary system is your skin plus everything that grows out of it. That means hair, nails, and glands. The word integument just means "a covering." That is the simplest way to picture skin. It is a wrapper. It keeps the wet, carefully controlled world inside you apart from the dry, risky world outside.

But "wrapper" sells it short. The skin is a true organ, a body part built from several tissue types working together. In fact, it uses all four types. It is the largest organ in the body, both by weight and by surface area. One fact holds this whole chapter together. The skin is two layers stacked on top of a third, supporting layer. And that third layer is not really skin at all.

Two layers of skin, one layer beneath

Read the skin from the surface down, and you pass through three named layers. The top two are the skin itself. The third one holds the skin to the body.

Layer	Tissue	What is there and what it does
Epidermis	Keratinized stratified squamous epithelium (many layers of flat, tough cells)	The tough, waterproof outer sheet. It has no blood vessels. Food and oxygen seep up to it from below.
Dermis	Dense irregular connective tissue	The living core. It holds blood vessels, nerves, hair follicles, glands, and the collagen that makes skin strong.
Hypodermis (subcutaneous)	Areolar (loose) and adipose (fat) tissue	Not part of the skin. It holds skin to muscle and bone, keeps in heat, and stores energy.

The epidermis is the top layer. It is made only of epithelium, a sheet of tightly packed cells. It is also avascular, which means it has no blood vessels of its own. Its living cells get oxygen and food that seep up from the dermis below. The outer part of the epidermis is too far from this supply. So those outer cells are all dead.

The dermis is where the action is. It is a thick mesh of collagen and stretchy fibers. It is packed with blood vessels, nerves, hair follicles, and glands. These are the parts that let skin do its jobs. The dermis has two smaller layers. On top is a thin, bumpy papillary layer. Its bumps, the dermal papillae, make your fingerprints. Below it is a thick reticular layer that holds most of the collagen.

Under the dermis lies the hypodermis. It is also called the subcutaneous layer or superficial fascia. It is mostly fat. It pads and warms the body. It also ties the skin loosely to the parts underneath.

The epidermis is fed by diffusion. Diffusion means things slowly spread from where there is a lot to where there is less. The epidermis has no blood supply. So every bit of food it uses must travel up from tiny blood vessels in the dermis. This one fact explains two things. A shallow scrape does not bleed, and the surface cells are dead. But "no blood vessels" does not mean "no nerves." Free nerve endings reach well up into the epidermis. So an injury to the epidermis alone still hurts. Think of a scrape or a first-degree sunburn. Bleeding needs the dermis. Pain does not.

Thick skin and thin skin

Not all skin is the same thickness. Thick skin covers the palms of your hands and the soles of your feet. These are the spots that get rubbed the most. Thick skin has all five layers of epidermis, including an extra clear one.

Thin skin covers the rest of your body. It is more delicate and has only four layers of epidermis. It is also the only skin that grows hair. Watch out for one common mix-up. The names "thick" and "thin" describe the epidermis only. They do not describe how thick the whole skin is.

Interactive Tap a layer to explore the tough epidermis, the living dermis, and the fatty hypodermis that holds it all in place. Open full ↑

Key points

The integumentary system is the skin plus hair, nails, and glands. The skin is the body's largest organ.
The skin has two true layers. The epidermis is made of epithelium and has no blood vessels. It sits on the dermis, a connective tissue layer that has blood vessels and holds the skin's structures.
The hypodermis below is fat and loose tissue. It is not skin. It holds the skin in place and keeps in heat.
Thick skin (palms and soles) has five layers of epidermis. Thin skin has four and grows hair.

Section 2The five epidermal strata

The five layers of the epidermis, stacked from deep to surface — **Reference.** The layers (strata) of the epidermis — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A conveyor belt of cells

The epidermis is like a factory that never stops. Its main cell is the keratinocyte. It is born at the very bottom. New cells form below it and push it upward. As it rises, it fills with a tough protein called keratin. It dies along the way. At last it flakes off the top.

The whole trip takes about four weeks, or roughly 25 to 45 days. So the outer surface of your body is fully replaced about once a month. This slow climb is called keratinization. It is the most important process in the epidermis.

The five layers, bottom to top

In thick skin, the keratinocyte passes through five named layers, or strata, on its way up. Learn them in order. Start at the deepest layer, where cells are born. End at the surface, where they are shed:

Stratum (deep → surface)	Nickname	What happens here
Stratum basale	The nursery	One row of dividing stem cells. New keratinocytes are made here by cell division (mitosis). Melanocytes and touch-sensing Merkel cells live here too.
Stratum spinosum	The spiny layer	Several rows of keratinocytes held together by strong desmosomes, tiny "rivets" between cells. They give the layer a spiny look. Immune dendritic cells patrol here.
Stratum granulosum	The dying layer	Cells fill with grains of keratin and a waterproof fat. Their nuclei break down, and the cells start to die.
Stratum lucidum	The clear layer	A thin, see-through band of dead cells. It is found only in thick skin (palms and soles).
Stratum corneum	The horny layer	15–30 layers of flat, dead cells packed with keratin. This is the tough, waterproof barrier you actually touch.

A memory trick helps. Read from the base up: Basale, Spinosum, Granulosum, Lucidum, Corneum. Think "Bad Students Get Lousy Classes." The bottom two layers, basale and spinosum, are alive. They are sometimes grouped together as the stratum germinativum, the growth zone. From the granulosum up, the cells are dying or dead.

By the time a cell reaches the stratum corneum, it is just a flat bag of keratin. It is glued to its neighbors and coated in fat. That dead, waterproof crust is what makes skin such a good barrier. You shed about a million of these cells every day.

Dead cells do the hard work. It sounds backward. But the layer that protects you best is made entirely of dead cells. In the stratum corneum, these dead cells are packed with keratin and sealed with fat. They keep water in and germs out. Much of the dust in your home is shed stratum corneum.

Interactive New cells are born at the bottom and pushed up through five layers. They fill with keratin, die, and flake off the top. Watch one cell make the month-long trip. Open full ↑

Key points

Keratinocytes are born in the stratum basale. They are pushed upward and shed from the surface in about a month. This process is called keratinization.
The order from deep to surface is basale → spinosum → granulosum → lucidum → corneum ("Bad Students Get Lousy Classes").
The stratum lucidum is found only in thick skin. The stratum corneum is the dead, keratin-filled barrier you touch.

Section 3Skin color & melanin

A melanocyte passing melanin to nearby skin cells — **Reference.** Melanocytes and the transfer of melanin to keratinocytes — OpenStax College, CC BY 3.0 (via Wikimedia Commons)

Three pigments, one main player

Skin color comes from three pigments mixed together. A pigment is a substance that gives color. Melanin is the main one. It is a brown-to-black (or red-to-yellow) pigment made in the epidermis. Carotene is a yellow-orange pigment that comes from food, such as carrots. It collects in the stratum corneum and in the fat of the hypodermis.

Hemoglobin is the red pigment in blood. It shows through where the epidermis is thin. It gives lighter skin its pink glow. Of the three, melanin matters most. It is also the one that varies most from person to person.

Melanocytes hand off their pigment

Melanin is made by a special cell called the melanocyte. It sits down in the stratum basale. Here is the surprising part. Melanocytes do not keep their pigment. They pack melanin into tiny sacs called melanosomes. Then they hand these sacs to the keratinocytes around them.

Inside a keratinocyte, the melanin moves to the sunny side of the cell. It forms a small cap over the top of the nucleus. It is like a tiny umbrella held over the cell's DNA. There it soaks up incoming ultraviolet (UV) radiation, the invisible rays in sunlight. It catches the UV before it can reach and harm the DNA. That is the whole point of skin pigment. Melanin is sunscreen your cells make for themselves.

Everyone has about the same number of melanocytes. Differences in skin color come almost entirely from how much melanin those cells make. They also depend on what kind of melanin is made. They do not come from how many melanocytes you have. A tan is just melanocytes making more melanin after UV exposure. It is a defense against light that damages DNA.

When pigment goes wrong, and reading skin color in the clinic

Two conditions show what happens at the extremes. In albinism, an inherited condition, melanocytes are present but cannot make melanin. So the skin, hair, and eyes have little or no color, and they burn easily. In vitiligo, the immune system destroys melanocytes in patches. This leaves pale blotches on the skin.

Skin color also gives some of the fastest clues to illness. Health workers can see it right at the bedside. That is because skin color reflects what is happening in the blood underneath.

Color change	What it looks like	What it usually means
Cyanosis	Bluish skin, lips, and nail beds	Blood that is low in oxygen (low O₂)
Pallor	Pale, washed-out skin	Low blood flow or anemia (too few red blood cells). Seen with shock, fear, or cold.
Erythema	Redness	More blood flow. Seen with fever, swelling and irritation (inflammation), blushing, or sunburn.
Jaundice	Yellow skin and yellow whites of the eyes (sclerae)	A buildup of bilirubin, a yellow waste. Usually a liver problem.
Bronzing	A golden-bronze tone	Adrenal glands that do not make enough hormone (Addison disease)

Interactive Melanocytes make melanin and hand it to nearby skin cells. There it caps each nucleus like a tiny umbrella. Turn up the sun and watch it protect the DNA. Open full ↑

Key points

Skin color mixes three pigments: melanin (the main one), carotene, and hemoglobin from blood.
Melanocytes in the stratum basale make melanin. They hand it to keratinocytes. There it caps the nucleus and soaks up UV to protect DNA.
Everyone has about the same number of melanocytes. Color differences come from the amount and type of melanin. A tan is a defense against UV.
Skin color gives clues about the blood. Cyanosis, pallor, erythema, jaundice, and bronzing each point to a certain problem.

Section 4Hair, nails & glands

Hair, nails, and glands in the skin — **Reference.** Accessory structures of the skin: hair, nails, and glands — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Hair: keratin threads with a living root

The skin's extra parts, like hair and glands, are called accessory structures. Each one grows from the epidermis. But each one is tucked down into the dermis, where the blood supply is.

Hair is a bendy strand of dead, hard keratin. The part you see above the skin is the shaft. The part below is the root. The root sits inside a tube called the hair follicle. At the very bottom, the follicle swells into a bulb. Inside the bulb is a small knot of connective tissue called the papilla. It brings in the blood vessels that feed the growing hair.

Just above the papilla is the matrix. It is the only living, dividing part of a hair. That is why a haircut does not hurt but plucking a hair does. A tiny muscle called the arrector pili is attached to each follicle. When you are cold or scared, it tightens. It pulls the hair upright and dimples the skin into "goosebumps."

Nails: a window over the fingertip

A nail is a plate of hard keratin. It protects the tip of each finger and toe. It also gives you a tool for scratching and gripping. The part you see is the nail body. It sits on the nail bed. The pale half-moon at its base is the lunula. It marks the nail matrix underneath, where the nail actually grows. The cuticle (eponychium) is the fold of skin that seals the base against dirt and germs.

Nails are a useful window on health. They turn blue when the blood is low in oxygen. They become curved and rounded, called clubbed, in long-term heart or lung disease. They curve inward like a spoon in iron-deficiency anemia, when the body is short on iron.

Glands: oil and sweat

The skin holds two big families of glands. Sebaceous (oil) glands usually empty into a hair follicle. They release an oily mix called sebum. Sebum keeps the hair and skin soft and slows the growth of bacteria. When a follicle clogs with sebum and dead cells, the result is acne.

Sweat (sudoriferous) glands come in two types. It is worth keeping them straight:

Gland	Where it is and what it releases	Job
Sebaceous (oil)	Into hair follicles. Releases oily sebum.	Keeps skin and hair soft. Mildly fights bacteria.
Eccrine (merocrine) sweat	All over the body, onto the skin surface. Releases watery sweat.	Cools the body as sweat dries (evaporates). The main gland for thermoregulation, or heat control.
Apocrine sweat	Armpits and groin, into hair follicles. Releases thicker sweat with protein in it.	Turns on at puberty. Feeds skin bacteria that cause body odor.
Ceruminous & mammary	Ear canal / breast	Changed sweat glands that make earwax and milk

Eccrine cools, apocrine smells. The sweat that cools you on a hot day is watery eccrine sweat. It is found almost everywhere on the body. The sweat behind body odor is thicker apocrine sweat from the armpits and groin. And it does not smell at all until skin bacteria break it down.

Interactive The skin's built-in extras are hair, nails, and glands. All grow from the epidermis and are tucked into the dermis. Tap each one to see what it is and what it does. Open full ↑

Key points

Hair is dead keratin. It grows only from the living matrix at its base, which is fed by the papilla. It stands up when the arrector pili muscle tightens.
Nails are keratin plates. They grow from the nail matrix under the lunula. Their color and shape can reveal disease in the whole body.
Sebaceous glands make oily sebum. Eccrine sweat glands cool the body. Apocrine glands (armpit, groin) cause body odor once bacteria act on their sweat.

Section 5What the skin does

The five main jobs of the skin — **Reference.** The five major functions of the integument — OpenStax, CC BY 3.0 (via Wikimedia Commons)

One organ, five jobs

It is easy to think the skin just covers you. But covering you is only the first of five real jobs it does every moment. Hold on to all five. They come up again and again in later chapters.

Function	How the skin does it
1. Protection	A physical barrier (keratin and tightly joined cells). A chemical barrier (the slightly acidic "acid mantle" made by sweat and oil). A biological barrier (immune dendritic cells that catch invaders).
2. Thermoregulation	To cool down, sweat dries off the skin and blood vessels in the dermis widen (dilate) to let heat out. To warm up, those vessels narrow (constrict) to keep heat in.
3. Sensation	Packed with receptors, sensors for touch, pressure, vibration, temperature, and pain. It is the body's largest sense organ.
4. Vitamin D synthesis	UV light turns a relative of cholesterol in the skin into vitamin D₃. You need it to absorb calcium from food.
5. Excretion & storage	Sweat gets rid of small amounts of water, salts, and wastes. The dermis stores a blood reservoir (up to about 5% of the body's blood).

The barrier you rebuild every month

Protection is the star job. The dead, keratin-packed stratum corneum keeps water in. Without it, you would dry out in hours. It also keeps germs, chemicals, and UV out.

Sweat and oil coat the surface in a slightly acidic film called the acid mantle. This film makes life hard for bacteria and fungi. Dendritic immune cells are woven through the epidermis. They grab any invader that slips past. The whole barrier is rebuilt about once a month. So small damage heals with barely a trace.

Temperature, sensation, and a vitamin the skin makes

The skin is also your main thermostat, the part that controls your body heat. When you get too hot, eccrine glands pour out sweat. As the sweat dries, it carries heat away. Blood vessels in the dermis also widen to give off heat at the surface. That is why you turn pink when you are hot. When you are cold, those same vessels squeeze down to trap warmth deep inside.

The dermis is also crowded with sensory receptors. These make the skin the largest sense organ you have. They send your nervous system a steady stream of touch, pressure, temperature, and pain signals.

The skin also makes vitamin D, a job no other organ can start. UV light strikes a molecule like cholesterol in the epidermis. This begins its change into vitamin D₃. Without vitamin D, you cannot absorb the calcium your bones and nerves depend on.

Sunlight is a nutrient, in small doses. The same UV that damages DNA also starts vitamin D production. That is the tension at the heart of this chapter. You want enough sun to make vitamin D and lift your mood. But you do not want so much that it overwhelms melanin's protection and raises your risk of skin cancer.

Interactive Your largest organ does a lot. Besides covering you, it does five big jobs. Tap each one. Open full ↑

Key points

The skin's five jobs are protection, thermoregulation, sensation, vitamin D synthesis, and excretion/storage.
Protection is physical (keratin), chemical (the acidic acid mantle), and biological (immune dendritic cells).
Cooling uses sweat plus wider blood vessels. Warming uses narrower blood vessels. Vitamin D₃ is made in the skin with the help of UV. You need it to absorb calcium.

Section 6Burns & skin cancer

How deep each type of burn goes, and the types of skin cancer — **Reference.** Burn depth and the classification of skin cancers — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Burns: it's all about depth

A burn destroys skin. It can be caused by heat, electricity, chemicals, or radiation. How bad a burn is depends first on how deep it goes. The skin's barrier, water control, and heat control all live in these layers. So the deeper the burn, the more dangerous it is. This is not because a deep burn hurts more. In fact, a deep burn may stop hurting at all.

Burn (depth)	Layers destroyed	How it looks and feels
First-degree (superficial)	Epidermis only	Red, dry, and painful, with no blisters. A typical sunburn. Heals in days.
Second-degree (partial thickness)	Epidermis plus part of the dermis	Blisters, redness, and a lot of pain. Heals but may scar.
Third-degree (full thickness)	All of the epidermis and dermis	White, brown, or blackened, and leathery. Often painless because the nerves are destroyed. Needs skin grafting (moving healthy skin onto the wound).

A serious burn has two big dangers: fluid loss and infection. Once the barrier is gone, water pours out and germs pour in. The body can no longer hold its temperature steady either.

To judge how much skin is burned, doctors and nurses use the rule of nines. It splits the adult body surface into regions worth about 9% each, or a multiple of nine. Each arm is 9%. Each leg is 18%. The front of the trunk is 18%, and so on. A burn is usually called critical in three cases. It covers more than 25% of the body. Or it involves the hands, face, or feet. Or it is third-degree over more than 10% of the body.

A painless burn is a worse burn. It sounds odd, but a full-thickness (third-degree) burn is often numb. The nerves in the dermis have been destroyed along with everything else. Never think a burn is minor just because it does not hurt.

Skin cancer: three kinds, one that kills

Skin cancer is the most common cancer of all. Its leading cause is years of UV exposure. There are three main types. Telling them apart matters a lot. Some are just a nuisance, but one can kill. Notice that each one starts in a cell you already met in this chapter.

Cancer	Cell it starts in	How it behaves
Basal cell carcinoma	Cells of the stratum basale	The most common and least dangerous. Rarely spreads. Usually cured by cutting it out.
Squamous cell carcinoma	Keratinocytes of the stratum spinosum	Less common. Can spread if ignored, but usually curable if caught early.
Melanoma	Melanocytes	The least common but most deadly. Spreads (metastasizes) early and fast. Catching it early saves lives.

The dangerous one is melanoma, a cancer of the melanocytes that make pigment. It often starts in or near a mole. It metastasizes, or spreads to other parts of the body, early. So spotting it early is everything.

The ABCDE rule helps you check a mole that looks suspicious. Asymmetry: one half does not match the other. Border: the edges are ragged or blurred. Color: the color is uneven, with several shades. Diameter: it is wider than about 6 mm, the size of a pencil eraser. Evolving: it changes in size, shape, or color over time.

Any mole that shows these signs should be checked by a doctor.

Interactive Explore how deep a burn goes and how much of the body it covers. Then meet the three skin cancers, including the deadly one. Open full ↑

Key points

Burns are graded by depth. First-degree burns hurt only the epidermis. Second-degree burns reach into the dermis and blister. Third-degree burns go all the way through, are often painless, and need skin grafts.
Serious burns kill through fluid loss and infection. The rule of nines estimates what percent of the body surface is burned.
There are three skin cancers. Basal cell is common and mild. Squamous cell can spread. Melanoma is rare but deadly. Check moles with ABCDE.

The chapter in one breath

The integumentary system is the skin plus hair, nails, and glands. The skin is the body's largest organ.
Skin has two true layers. The epidermis is epithelium with no blood vessels. It sits on the dermis, a connective tissue layer with blood vessels. Both rest on the fatty hypodermis.
Keratinocytes ride upward through five strata: basale → spinosum → granulosum → lucidum → corneum. They fill with keratin, die, and flake off in about a month.
Melanocytes make melanin and pass it to keratinocytes. There it caps the nucleus and soaks up UV to protect DNA. Color differences come from the amount of melanin, not the number of melanocytes.
Hair and nails are keratin structures that grow from living matrices. Sebaceous glands make oil. Eccrine glands cool you. Apocrine glands cause body odor.
The skin does five jobs: protection, thermoregulation, sensation, vitamin D synthesis, and excretion/storage.
Burns are graded by depth (first- to third-degree) and by area (rule of nines). The three skin cancers are basal cell, squamous cell, and the deadly melanoma. Moles are checked with ABCDE.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Apocrine sweat gland: A sweat gland in the armpit and groin that empties into a hair follicle. It makes a thick sweat with protein in it, turns on at puberty, and causes body odor once bacteria break the sweat down.
Arrector pili: A tiny smooth muscle attached to a hair follicle. When it tightens, it pulls the hair upright and makes "goosebumps."
Basal cell carcinoma: The most common and least dangerous skin cancer. It starts in cells of the stratum basale and rarely spreads.
Carotene: A yellow-orange pigment from food. It collects in the stratum corneum and hypodermis and adds to skin color.
Dermis: The deep, living layer of skin, made of dense irregular connective tissue. It holds blood vessels, nerves, hair follicles, and glands.
Eccrine (merocrine) sweat gland: The most common sweat gland, found all over the body. It opens right onto the skin, and its watery sweat cools you as it dries.
Epidermis: The top layer of skin. It is a keratinized stratified squamous epithelium (many layers of flat, tough cells) with no blood vessels of its own.
Hair follicle: The tube of tissue around the hair root. The hair grows out of it.
Hypodermis (subcutaneous layer): The layer of fat and loose tissue under the skin. It is not part of the skin itself. It holds the skin in place, keeps in heat, and stores energy.
Integument: A covering. The integumentary system is the skin plus hair, nails, and glands.
Keratin: A tough, water-resistant protein. It fills skin cells, hair, and nails, and makes them strong and waterproof.
Keratinocyte: The main cell of the epidermis. It is born in the stratum basale, fills with keratin as it is pushed up, dies, and is shed.
Melanin: The brown-to-black (or red-to-yellow) pigment that gives skin most of its color. It soaks up ultraviolet light to protect DNA.
Melanocyte: The pigment-making cell in the stratum basale. It makes melanin and passes it to nearby keratinocytes.
Melanoma: The least common but most dangerous skin cancer. It starts in melanocytes, spreads early, and is checked for with the ABCDE rule.
Rule of nines: A way to estimate what percent of the body's surface is burned. It splits the adult body into regions of about 9% or multiples of nine.
Sebaceous gland: An oil gland that usually empties into a hair follicle. Its sebum keeps skin and hair soft and holds back bacteria.
Sebum: The oil made by sebaceous glands. When a follicle clogs with sebum and dead cells, acne results.
Squamous cell carcinoma: A skin cancer that starts in keratinocytes of the stratum spinosum. It can spread if not treated but is usually curable if caught early.
Stratum basale: The deepest layer of the epidermis. It is one row of dividing stem cells and also holds melanocytes and Merkel cells.
Stratum corneum: The outermost layer of the epidermis. It is 15–30 rows of dead, keratin-filled cells that form the tough, waterproof barrier.
Vitamin D: A vitamin the skin starts to make when ultraviolet light hits a relative of cholesterol in the epidermis. You need it to absorb calcium from food.

Check yourself

1. A patient has a shallow scrape that does not bleed at all. Which skin layer was hurt? Why is it normal that it does not bleed?

Only the epidermis was scraped. The epidermis is avascular, meaning it has no blood vessels. Blood vessels are in the dermis below. A wound must reach the dermis before it bleeds. The scrape will still hurt, though. Free nerve endings reach up into the epidermis. That is why a first-degree sunburn hurts but does not bleed. The epidermis lives on food that seeps up from the dermis.

2. Which layer sits just under the skin? What tissue is it made of? Why is it not counted as part of the skin?

The hypodermis (subcutaneous layer). It is made mostly of adipose (fat) and areolar (loose) tissue. It holds the skin in place, keeps in heat, and stores fat. But it is not epidermis or dermis. So it counts as support tissue under the skin, not skin itself.

3. Name the five epidermal strata from deepest to closest to the surface. Which one is found only in thick skin?

Stratum basale → spinosum → granulosum → lucidum → corneum. The stratum lucidum is found only in thick skin (palms and soles). Remember "Bad Students Get Lousy Classes." Cells are born in the basale and shed from the corneum.

4. In which stratum are keratinocytes born? About how long does the trip to the surface take? What is the outer layer made of?

They are born in the stratum basale. The trip takes about a month (about 25–45 days). The stratum corneum is made of dead cells filled with keratin. This upward march to death is called keratinization. The dead corneum is what makes skin a waterproof barrier.

5. Two people have very different skin colors. Is this because one has far more melanocytes than the other? Explain.

No. Everyone has about the same number of melanocytes. The difference is in how much melanin those cells make, and what type. More pigment means darker skin. A tan is the same cells making more melanin after UV exposure.

6. A melanocyte makes melanin but hands it off. Where does the pigment end up inside a keratinocyte? What does it protect there?

The melanin gathers as a cap over the top of the keratinocyte's nucleus, like a tiny umbrella. There it soaks up UV rays before they can harm the cell's DNA. Melanin is really sunscreen the cell makes for itself.

7. Why does a haircut not hurt, but plucking a hair does? Name the part that makes hair grow.

The shaft you see is dead keratin, so cutting it does not hurt. The living, growing part is the matrix at the base of the follicle, fed by the papilla. Plucking disturbs that living root and its nerves. That is why it hurts.

8. How do eccrine and apocrine sweat glands differ? Where is each found? Which one cools you, and which one is linked to body odor?

Eccrine glands are all over the body. Their watery sweat cools you as it dries. Apocrine glands are in the armpits and groin. Their thick sweat is linked to body odor. Apocrine sweat has no smell until skin bacteria break it down.

9. List the five functions of the skin. Which one is a nutrient job that no other organ shares?

Protection, thermoregulation, sensation, vitamin D synthesis, and excretion/storage. Making vitamin D is the special nutrient job. UV light starts turning a relative of cholesterol into vitamin D₃. The body needs it to absorb calcium.

10. On a hot day, the skin does two things to get rid of heat. Name both. What do the same blood vessels do when you are cold?

To cool you, eccrine glands make sweat that dries off, and blood vessels in the dermis widen (dilate) to give off heat. When you are cold, those vessels narrow (constrict) to keep heat in your core. This makes the skin the body's main organ for controlling temperature.

11. A burn is blackened, leathery, and oddly painless. What degree of burn is it? Which layers are destroyed? Why does it not hurt?

It is a third-degree (full-thickness) burn. It destroys all of the epidermis and dermis. It does not hurt because the nerves in the dermis are destroyed. These burns usually need skin grafts. A lack of pain never means a burn is minor.

12. Name the three skin cancers from least to most dangerous, and the cell each starts in. Then give the ABCDE warning signs for the deadliest one.

Basal cell carcinoma (stratum basale, mildest), squamous cell carcinoma (keratinocytes of the stratum spinosum), and melanoma (melanocytes, deadliest). Check a mole for melanoma with ABCDE: Asymmetry, Border that is uneven, Color that varies, Diameter over about 6 mm, and Evolving (changing over time).

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