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Week 6 · Chapter 5

The Integumentary System

Skin is the body's largest organ, and it's built almost entirely from the tissue types you studied last week — a direct, practical application of Chapter 4. This week covers skin's layers, its accessory structures, and the critical functions it performs.

By the end of this week, you will be able to:

1Describe the layers of the epidermis and dermis
2Identify skin's accessory structures — hair, nails, and glands
3Explain the main functions of the integumentary system
4Explain how skin participates in thermoregulation
5Outline the four stages of wound healing
6Classify burns by depth

Big Picture

Skin: an organ built from last week's tissue types

Skin isn't a single tissue — it's an organ, meaning it's built from multiple tissue types working together. Its outer layer (epidermis) is stratified squamous epithelial tissue — exactly the kind you learned about last week for protection. Its inner layer (dermis) is dense connective tissue. This is Chapter 4's content, applied directly.

Skin covers roughly 1.5–2 square meters of surface area in an adult and makes up about 15% of total body weight — by both measures, it's the largest organ in the body.

Vocabulary & Word Parts

Integumentary system roots

derm/o-, -dermskin

Epidermis = "upon the skin." Hypodermis = "below the skin."

cutane-skin

Subcutaneous = beneath the skin (another name for the hypodermis).

kerat/o-hard, horny

Keratin is the tough, waterproofing protein that fills dead skin cells at the surface.

seb/o-oil, sebum

Sebaceous glands secrete sebum (oil) to lubricate skin and hair.

sudor/o-, hidr/o-sweat

Sudoriferous glands = sweat glands. Hyperhidrosis = excessive sweating.

melan/o-black, dark pigment

Melanocytes produce melanin, the pigment responsible for skin color and UV protection.

Structure & Function

The layers of skin

The epidermis and dermis are the two true layers of skin. The hypodermis (subcutaneous layer) lies beneath skin and anchors it to underlying muscle, but it's technically NOT part of the skin itself — a detail exam questions love to test.

Structure & Function

Epidermis: from deep to surface

The epidermis is stratified squamous epithelium — the exact classification pattern from last week — organized in layers, with the deepest layer constantly producing new cells that migrate upward and eventually die.

1 · Stratum basale (deepest)

The single layer of living, actively dividing cells that produces every new epidermal cell.

2 · Middle layers

Cells migrate upward, gradually filling with keratin and flattening out as they move away from their blood supply.

3 · Stratum corneum (surface)

Dead, flattened, keratin-filled cells that form a tough, waterproof outer barrier — constantly shed and replaced from below.

The pattern to remember: cells are born deep, die as they rise, and are shed at the surface — the entire epidermis renews itself roughly every 3–4 weeks.

Structure & Function

Accessory structures: hair follicles

The hair bulb at the base of the follicle is where active cell division produces the growing hair shaft. The arrector pili muscle can pull the follicle upright ("goosebumps"), and the nearby sebaceous gland keeps both the hair and surrounding skin lubricated.

Structure & Function

Two kinds of skin glands

Both gland types are exocrine glands (they secrete onto a surface rather than into the bloodstream), but they serve very different jobs — one lubricates, the other cools.

Structure & Function

What skin actually does

Protection — a physical barrier against pathogens, UV radiation (via melanin), and water loss.
Thermoregulation — sweat glands and blood vessel diameter work together to release or conserve heat.
Vitamin D synthesis — skin cells convert a cholesterol-based precursor into active vitamin D when exposed to UV light.
Sensation — nerve endings in the dermis detect touch, pressure, temperature, and pain.
Excretion — small amounts of waste (urea, salts) leave the body through sweat.

Structure & Function

Thermoregulation: negative feedback in action

This is the exact negative feedback pattern from Exam Block 1's Chapter 1 — a stimulus (rising temperature) triggers a response (sweating, vessel dilation) that reverses the original change. Skin is one of the body's primary tools for executing this loop.

Structure & Function

Wound healing: four overlapping stages

Hemostasis stops the bleeding (platelets, clotting) within minutes. Inflammation clears debris and pathogens (hours to days). Proliferation rebuilds tissue with new blood vessels and collagen (days to weeks). Remodeling strengthens and reorganizes that new tissue (weeks to over a year).

Your Turn — Guided Practice

Apply it before the quiz

A patient comes in with a sunburn that is red, painful, and limited to the outermost layer of skin — no blistering.

Question 1 — Which layer of skin is primarily affected?

A burn limited to redness and pain with no blistering and no deeper damage is confined to the epidermis — this matches the definition of a first-degree burn. Blistering is a sign the dermis has also been affected.

Question 2 — Which skin function is most immediately compromised by this kind of surface damage?

Surface-level epidermal damage directly compromises the skin's protective barrier function first and most obviously. Vitamin D synthesis still occurs in surviving skin cells and isn't the primary concern with a superficial burn.

Clinical Correlation

Classifying burns by depth

Burn severity is classified almost entirely by which skin layers are affected — a direct clinical application of the layered structure covered earlier this week. First-degree burns affect only the epidermis; second-degree burns extend into the dermis; third-degree burns destroy the full thickness of skin, often requiring skin grafts since the regenerative stratum basale is gone.

Clinical Correlation

Melanoma: when epithelial regeneration goes wrong

This connects directly back to last week's discussion of why epithelial tissue is a common cancer site.

🩺 In practice

Melanoma is a cancer of melanocytes, the pigment-producing cells in the epidermis. Because epithelial tissue already regenerates rapidly (Chapter 4), a mutation that disrupts that normal regulated growth can lead to dangerously fast, uncontrolled cell division — which is part of why UV exposure (a direct DNA-damaging trigger) is such a significant risk factor, and why dermatologists watch for changes in mole shape, color, and size as early warning signs.

Mini Knowledge Test

Show what you know — 7 questions

Question 1

Which layer of skin contains the actively dividing cells that produce new epidermal cells?

The stratum basale is the deepest epidermal layer and the only one with actively dividing cells. The stratum corneum is the dead, surface layer; the hypodermis isn't technically part of the skin at all.

Question 2

The hypodermis is best described as:

The hypodermis sits beneath the dermis and anchors skin to muscle, but by strict anatomical definition it is not part of the skin itself — only the epidermis and dermis are. Hair follicles and most glands are found in the dermis, not the hypodermis.

Question 3

Sebaceous glands primarily secrete:

Sebaceous glands secrete sebum, an oily substance that lubricates skin and hair. Sweat comes from sudoriferous glands instead; keratin is a structural protein made within epidermal cells, not a gland secretion.

Question 4

Which skin function directly demonstrates negative feedback from Chapter 1?

Thermoregulation is a textbook negative feedback loop — rising temperature triggers sweating and vessel dilation that reverse the change. Vitamin D synthesis and sensation are important skin functions, but neither is itself a feedback loop.

Question 5

Which wound healing stage happens first?

Hemostasis (stopping the bleeding) happens within minutes and comes first. Proliferation (rebuilding tissue) and remodeling (long-term strengthening) both come later, in that order.

Question 6

A burn that extends into the dermis but not through its full thickness is classified as:

Second-degree burns extend into the dermis (often causing blistering) without destroying it completely. First-degree is epidermis-only; third-degree destroys the full thickness of skin.

Question 7

Melanocytes are found primarily in which skin structure?

Melanocytes reside in the epidermis, where they produce melanin for pigmentation and UV protection. They are not found in the hypodermis or within sweat gland structures themselves.

Score: 0 / 7

Debrief

Week 6 Quick-Reference Recap

One page to glance at before Exam 2 — everything from this week, condensed.

Skin Layers

Epidermis — stratified squamous epithelium
Dermis — dense connective tissue
Hypodermis — NOT technically part of skin

Epidermal Growth

Stratum basale (divides) → migrates up → stratum corneum (dead, shed).

Accessory Structures

Hair follicle + hair bulb
Sebaceous gland — oil
Sudoriferous gland — sweat

Skin Functions

Protection, thermoregulation, vitamin D synthesis, sensation, excretion.

Wound Healing

Hemostasis → Inflammation → Proliferation → Remodeling.

Burn Classification

1st degree — epidermis only
2nd degree — into dermis
3rd degree — full thickness

Key Terms This Week

EpidermisDermisHypodermisKeratinMelanocyteSebaceous GlandSudoriferous GlandThermoregulationWound Healing

Next — Week 7: Osseous Tissue & Bone Structure. You'll meet the most rigid connective tissue type from Chapter 4 in full detail, including how bones grow, remodel, and regulate calcium.

Next: Week 7 →