The Integumentary System: Complete Study Guide (BIOL 2401, Collin College)

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The Integumentary System

Overview and Functions

The integumentary system is the body's largest organ system, serving as the primary barrier between internal tissues and the external environment. It includes the skin, hair, nails, and various glands. This system is essential for protection, sensation, thermoregulation, and more.

Largest organ: Comprises about 16% of total body weight, with a surface area of 1.5–2 m2.
Contains all four tissue types: Epithelial, connective, muscle, and nervous tissues.
Main components: Cutaneous membrane (skin: epidermis and dermis) and accessory structures (hair, nails, glands).

Layers of the Skin

The Three Main Layers

Epidermis: Outermost, avascular, composed of stratified squamous keratinized epithelium. Constantly renews itself; outer cells are dead and flat.
Dermis: Middle, vascular, made of connective tissue (areolar and dense irregular). Contains nerves, blood vessels, hair follicles, and glands.
Hypodermis (Subcutaneous layer): Deepest, not technically part of the skin. Composed of areolar and adipose tissue; provides insulation, cushioning, and anchors skin to underlying structures.

Memory trick: EDH = Epidermis → Dermis → Hypodermis (superficial to deep).

The Epidermis: Five Layers (Strata)

Stratum-by-Stratum Structure

Stratum Basale (Basal Layer): Deepest, single cell layer. Site of mitosis (new cell production), contains melanocytes (pigment) and Merkel cells (touch receptors).
Stratum Spinosum (Spiny Layer): Several layers thick, contains keratinocytes with spiny projections and Langerhans cells (immune defense).
Stratum Granulosum (Granular Layer): 3–5 layers, cells flatten and die, keratohyaline and lamellar granules form (waterproofing).
Stratum Lucidum (Clear Layer): Only in thick skin (palms, soles), thin layer of clear, dead cells.
Stratum Corneum (Horny Layer): Outermost, 20–30 layers of dead, keratin-filled cells; provides waterproof, protective barrier.

Thick skin (palms, soles) has all five layers; thin skin (rest of body) lacks stratum lucidum.

Cells of the Epidermis

Four Major Cell Types

Keratinocytes: Most abundant (90%), produce keratin for protection and waterproofing.
Melanocytes: Located in stratum basale, produce melanin pigment to protect DNA from UV damage.
Langerhans Cells: Immune cells in stratum spinosum, act as macrophages and antigen-presenting cells.
Merkel Cells: Sensory receptors in stratum basale, detect light touch and pressure.

The Dermis

Two Main Layers

Papillary Layer: Superficial 20%, areolar connective tissue, contains dermal papillae (fingerprints), Meissner's corpuscles (light touch), and capillaries.
Reticular Layer: Deep 80%, dense irregular connective tissue, contains collagen and elastin fibers, hair follicles, sebaceous and sweat glands, Pacinian corpuscles (deep pressure).

Sensory Receptors in the Dermis

Meissner's corpuscles: Light touch, in papillary layer.
Pacinian corpuscles: Deep pressure/vibration, in reticular layer and hypodermis.
Free nerve endings: Pain, temperature, itch, throughout dermis and epidermis.
Arrector pili muscle: Smooth muscle attached to hair follicles; causes goosebumps.

Accessory Structures

Glands

Gland Type	Location	Secretion	Function
Eccrine (Merocrine) Sweat	Everywhere (palms, soles, forehead)	Watery sweat (water, salts, waste)	Thermoregulation, excretion
Apocrine Sweat	Armpits, groin, nipples	Thick sweat (water, fats, proteins)	Body odor, active at puberty
Sebaceous (Oil)	Connected to hair follicles	Sebum (oily, waxy)	Lubricates, waterproofs, antibacterial
Ceruminous	Ear canal	Cerumen (ear wax)	Traps dust, protects eardrum

Hair

Hair shaft: Visible part above skin.
Hair root: Below skin, within follicle.
Hair follicle: Tube-like structure in dermis; produces hair.
Hair bulb/papilla: Base of follicle; site of hair growth (cell division).
Arrector pili: Smooth muscle causing hair to stand (goosebumps).

Nails

Nail body/plate: Visible, dead keratinized cells.
Nail root: Hidden, site of growth.
Nail matrix: Growth zone under root.
Nail bed: Skin under nail plate (pink from blood vessels).
Lunula: White half-moon at base.
Cuticle (eponychium): Skin fold at base.
Hyponychium: Skin under free edge.

Functions of the Integumentary System

Protection: Physical, chemical, and biological barrier (keratin, sebum, Langerhans cells, melanin).
Prevention of Water Loss: Keratin and lipids in stratum corneum prevent dehydration.
Thermoregulation: Sweat glands and blood vessel dilation/constriction regulate temperature.
Excretion: Removal of waste via sweat (urea, ammonia, salts).
Vitamin D Synthesis: UV light converts cholesterol to vitamin D precursor; completed in liver and kidneys.
Metabolic Functions: Activation of hormones, breakdown of carcinogens.
Sensation: Detection of touch, pressure, pain, temperature via specialized receptors.

Skin Color and Pigmentation

Pigment	Color	Source	Key Facts
Melanin	Brown/black or yellow/red	Melanocytes (stratum basale)	Primary determinant; protects from UV; same number of melanocytes in all humans
Hemoglobin	Red/pink	Blood in dermal vessels	Gives pink/rosy color; cyanosis = blue (low O2); erythema = red (vasodilation)
Carotene	Yellow/orange	Diet (carrots, squash)	Accumulates in stratum corneum and hypodermis; not jaundice

Clinical terms: Albinism = no melanin; cyanosis = blue skin (low oxygen); jaundice = yellow skin (liver dysfunction).

Burns: Classification and Rule of Nines

Degrees of Burns

Degree	Layers Affected	Signs/Symptoms	Healing
1st (Superficial)	Epidermis only	Red, painful, no blisters	Heals in 3–5 days, no scarring
2nd (Partial thickness)	Epidermis + upper dermis	Blisters, severe pain, redness, swelling	Heals in 2–3 weeks, may scar
3rd (Full thickness)	Entire epidermis + dermis (may reach muscle/bone)	White/red/black, painless, dry/leathery, no blisters	Requires surgery/skin grafts, major scarring

Rule of Nines (Adult)

Body Area	% BSA
Head & Neck	9%
Each Arm	9%
Front Trunk	18%
Back Trunk	18%
Each Leg	18%
Perineum	1%
Total	100%

Clinical application: Used to estimate fluid needs in burn patients.

Skin Cancer

Type	Origin Cell/Layer	Appearance	Danger
Basal Cell Carcinoma	Keratinocytes, stratum basale	Pearly/waxy bump, central ulcer	Low; rarely metastasizes
Squamous Cell Carcinoma	Keratinocytes, stratum spinosum	Firm red nodule, scaly patch	Medium; can metastasize
Melanoma	Melanocytes, stratum basale	Dark, uneven, raised, color variation	High; highly metastatic, fatal if not caught early

ABCDE Rule for Melanoma Detection

Letter	Stands For	Normal	Warning Sign
A	Asymmetry	Both halves match	Halves do not match
B	Border	Smooth, even	Irregular, ragged
C	Color	Uniform shade	Multiple colors
D	Diameter	<6mm	>6mm
E	Evolving	No change	Changing size, shape, color

Aging and Wound Healing

Effects of Aging

Decreased collagen and elastin: Wrinkles, sagging, easy bruising.
Fewer melanocytes: Less melanin, gray hair, age spots.
Fewer Langerhans cells: Reduced immune defense, slower healing.
Decreased sweat and sebaceous glands: Dry, itchy skin, poor thermoregulation.
Reduced blood supply: Slower wound healing, increased risk of pressure ulcers.

Wound Healing Process

Inflammation (0–3 days): Blood clot forms, vasodilation, macrophages clean debris.
Proliferation (3–21 days): Fibroblasts produce collagen, new blood vessels form, epithelial cells migrate.
Remodeling (21 days–2 years): Collagen reorganizes, scar tissue forms (about 80% strength of original skin).

Keloid: Overproduction of collagen, raised thick scar (more common in darker skin). Scar tissue: Collagen-rich, fewer blood vessels, no hair/glands.

Practice Questions (Sample)

Q: What percentage of body weight is skin? A: 16%.
Q: List the five layers of the epidermis (deep to superficial). A: Basale, Spinosum, Granulosum, Lucidum, Corneum.
Q: Which epidermal layer is avascular? A: Epidermis.
Q: Which sweat gland is responsible for thermoregulation? A: Eccrine gland.
Q: What is the Rule of Nines for both legs and front trunk? A: 36% (legs) + 18% (front trunk) = 54%.
Q: What is the ABCDE rule for melanoma? A: Asymmetry, Border, Color, Diameter (>6mm), Evolving.

Key Equations and Clinical Formulas

Parkland Formula for Burn Fluid Replacement:

Additional info: This formula is used to calculate IV fluid needs in severe burn patients.

Summary Table: Integumentary System Quick Facts

Topic	Key Fact	Memory Trick
Largest organ	Skin = 16% body weight, 1.5–2 m2	Bigger than liver or brain
3 layers	Epidermis → Dermis → Hypodermis	EDH
5 epidermal layers	Basale, Spinosum, Granulosum, Lucidum, Corneum	"Bad Boys Love Cool Girls"
Thick vs thin skin	Thick = 5 layers (palms/soles), Thin = 4 layers	Thick = palms/soles only
4 cell types	Keratinocytes, Melanocytes, Langerhans, Merkel	"Kings Must Look Magnificent"
3 pigments	Melanin, Hemoglobin, Carotene	MHC
Burn degrees	1st = epidermis, 2nd = +dermis, 3rd = all layers	Blisters = 2nd degree
Rule of Nines	Head=9, Arm=9, Trunk=18, Leg=18, Perineum=1	HAAT BLPP
Skin cancers	Basal (least dangerous), Squamous, Melanoma (most dangerous)	Common to deadly
ABCDE rule	Asymmetry, Border, Color, Diameter, Evolving	"Always Be Checking Dangerous Evolving moles"