The integumentary system is your body's largest organ — skin layers, accessory structures, thermoregulation, and protection. These memory tricks combine the anatomy and physiology of the skin so every concept sticks for lab practicals and exams.
Adipose + loose CT. NOT part of skin. Subcutaneous injection site.
Epidermal Layers
Come, Let's Get Sun Burned — Corneum · Lucidum · Granulosum · Spinosum · Basale
Five epidermal strata from superficial to deep
The five epidermal layers — from dead surface to living base
Stratum Corneum (outermost): 20-30 layers of dead, flattened, fully keratinized cells — the waterproof barrier. Shed continuously. Stratum Lucidum: only in thick skin (palms, soles) — clear layer of dead cells. Stratum Granulosum: cells begin dying, keratohyalin granules form — transition zone. Stratum Spinosum: several layers of living cells with desmosomes — gives "spiny" appearance. Contains Langerhans cells (immune). Stratum Basale (deepest): single layer of stem cells that continuously divide — new keratinocytes produced here. Contains melanocytes (melanin pigment) and Merkel cells (touch receptors).
Four epidermal cell types — each with a distinct role
Four cell types in the epidermis — what each does and where it lives
Keratinocytes (90% of epidermis): produce keratin — waterproof structural protein. Born in stratum basale → pushed upward → die → shed. Langerhans cells (in stratum spinosum): dendritic immune cells — capture antigens and present to T cells. First immune line in skin. Melanocytes (in stratum basale): produce melanin via melanosomes → transferred to keratinocytes via long dendrites → absorbs UV radiation, protects DNA. Everyone has same number of melanocytes — skin color difference is melanin AMOUNT. Merkel cells (stratum basale): touch receptors — detect light touch and pressure. Associate with sensory nerve endings.
Keratinocytes
90% of epidermis. Make keratin. Waterproofing and protection.
Langerhans cells
Dendritic APCs in spinosum. Immune surveillance — activate T cells.
Melanocytes
In basale. Make melanin. Same number in all skin types — amount varies.
Merkel cells
In basale. Light touch receptors. Form Merkel discs with sensory neurons.
Skin Functions
WIPES D — Waterproofing · Immunity · Protection · Excretion · Sensation · D vitamin synthesis
Six functions of the integumentary system
What the skin actually does — six functions beyond just covering the body
Protection: physical barrier against microbes, UV, chemicals, mechanical injury. Waterproofing: keratin prevents water loss and entry — prevents dehydration. Thermoregulation: sweat glands cool the body, blood vessel dilation/constriction, arrector pili muscles (goosebumps — vestigial hair raising for insulation). Sensation: Meissner's corpuscles (light touch), Pacinian corpuscles (pressure and vibration), free nerve endings (pain, temperature). Vitamin D synthesis: UV light converts cholesterol derivative → Vitamin D3 → liver and kidney activate → essential for calcium absorption. Excretion: small amounts of waste (urea, NaCl) in sweat.
Three skin glands — where each is found and what it secretes
Eccrine sweat glands: most numerous — all over body, most on palms and soles. Secrete watery sweat (99% water + NaCl + urea) directly to skin surface via duct. Thermoregulation — activated by heat and emotional stress. Innervated by sympathetic cholinergic fibers. Apocrine sweat glands: in armpits, groin, areola, pubic region. Larger than eccrine — open into hair follicles. Produce thicker, protein-rich secretion → bacteria metabolize → body odor. Activated at puberty and by stress/sexual arousal. Sebaceous (oil) glands: everywhere except palms and soles. Produce sebum (oil) — keeps skin and hair soft, antibacterial (lowers pH), prevents drying. Holocrine secretion (whole cell ruptures). Plugged sebaceous gland = acne.
Eccrine
All over body. Thermoregulation. Merocrine secretion. No body odor.
Apocrine
Axilla, groin. Into hair follicles. Active at puberty. Odor from bacterial breakdown.
Sebaceous
Sebum = oil. Holocrine. Everywhere except palms/soles. Acne when blocked.
Hair Structure
From inside out — Medulla · Cortex · Cuticle · Follicle · Root · Shaft
Hair shaft anatomy from core to surface plus follicle structure
Hair anatomy — structure, growth cycle, and function
Hair shaft: visible part above skin. Hair root: below skin surface, within follicle. Hair follicle: tube of epithelium extending into dermis — wall cells produce hair. Inner structure (deep to superficial): medulla (central core, pigment), cortex (bulk of hair, keratin), cuticle (outer scale-like layer — overlapping). Hair matrix: rapidly dividing cells at base of follicle bulb. Hair papilla: supplies nutrients via blood vessels. Hair growth cycle: anagen (active growth, 2-6 years), catagen (transition, 2-3 weeks), telogen (resting, 3 months → hair shed). Arrector pili: smooth muscle attached to follicle — contracts → goosebumps + hair stands up (piloerection).
Hair matrix
Rapidly dividing cells at follicle base — makes new hair. Chemo targets these cells (hair loss).
Anagen phase
Active growth — 2-6 years. 85-90% of scalp hairs in this phase at any time.
Melanin from melanocytes in hair matrix. Air bubbles replace melanin → gray/white hair.
Nail Structure
Nail plate on nail bed · Lunula is the white crescent · Matrix grows the nail
Nail plate · Nail bed · Nail matrix · Lunula · Eponychium (cuticle)
Nail anatomy — each structure and its clinical significance
Nails are scale-like modifications of the epidermis — heavily keratinized. Nail plate: visible hard portion — dead, keratinized cells. Nail bed: epidermis beneath nail plate — highly vascular (pink color from blood). Nail matrix: proximal region beneath nail (under the cuticle) — actively divides to produce nail plate. Lunula: whitish crescent at base — visible part of matrix (less vascular). Eponychium (cuticle): narrow band of epidermis covering nail root. Nail growth: ~3 mm/month fingernails, 1 mm/month toenails. Clinical: clubbing (O2 deficiency), koilonychia (spoon nails — iron deficiency), splinter hemorrhages (endocarditis), yellow nails (fungal infection).
Nail matrix
Produces nail plate. Under cuticle. Damage → permanent nail deformity.
Lunula
White crescent — visible part of matrix. Most prominent on thumb.
Nail bed
Vascular — pink color. Pale nail beds = anemia. Cyanotic = low O2.
Clubbing
Nail curves over bulbous fingertip — chronic hypoxia (COPD, heart disease, lung cancer).
Five warning signs of melanoma — the most dangerous skin cancer
Three types of skin cancer — and the ABCDE rule for melanoma detection
Basal cell carcinoma (BCC): most common skin cancer (80%) — from stratum basale. Pearly, raised nodule with rolled border. Grows slowly, rarely metastasizes. Best prognosis. Squamous cell carcinoma (SCC): from stratum spinosum — scaly, ulcerated lesion. Can metastasize — second most common. Melanoma: from melanocytes — least common but most dangerous. Highly metastatic. ABCDE warning signs: Asymmetry (one half unlike other), Border (irregular), Color (multiple colors), Diameter (>6 mm = pencil eraser), Evolving (changing over time). Risk factors: UV exposure, fair skin, family history, many moles. SPF sunscreen blocks UV.
BCC
80% of skin cancer. From basale. Pearly nodule. Rarely metastasizes.
SCC
From spinosum. Can metastasize. Scaly, ulcerated. Second most common.
Melanoma
From melanocytes. Most deadly — highly metastatic. ABCDE rule for detection.
Three degrees of burns — depth of damage and clinical features
First-degree (superficial): only epidermis damaged. Red, dry, painful. Sunburn. Heals in 3-5 days with no scarring. Second-degree (partial thickness): epidermis + part of dermis. Blisters, very painful (nerve endings intact and exposed). Heals in 2-3 weeks — may scar. Third-degree (full thickness): epidermis, dermis, and possibly hypodermis destroyed. Leathery, waxy, or charred — NOT painful (nerves destroyed). Requires skin grafting. Rule of Nines: used to estimate burned body surface area — head 9%, each arm 9%, each leg 18%, anterior trunk 18%, posterior trunk 18%, perineum 1%. Critical burns: >25% BSA second-degree, any third-degree. Major risk = infection and fluid loss.
First degree
Epidermis only. Red, dry, painful. No blisters. Heals in days. Sunburn.
Second degree
Into dermis. Blisters, very painful. 2-3 weeks healing. May scar.
Third degree
Full thickness. Painless (nerves gone). Requires grafting. Infection risk.
Rule of Nines
Head 9%, each arm 9%, each leg 18%, each trunk half 18%, perineum 1%.
🎓 Common Exam Questions
Q: What are the layers of the epidermis from deep to superficial?
A: Mnemonic 'Come Let's Get Sun Burned': Stratum Corneum (outermost, dead keratinized cells, waterproofing), Stratum Lucidum (only in thick skin — palms, soles — clear layer of dead cells), Stratum Granulosum (3-5 layers, cells filling with keratin granules, dying), Stratum Spinosum (several layers, desmosomes create spiny appearance, Langerhans cells), Stratum Basale (deepest, single layer of stem cells, mitotically active, melanocytes present, attached to basement membrane). Journey: new cell born in basale → pushed upward by new cells → keratinizes → reaches corneum → shed (desquamation). Full turnover: ~4-6 weeks. Thick skin (palms, soles): all 5 layers, no hair follicles. Thin skin (everywhere else): 4 layers (no lucidum), has hair follicles.
Q: What are the functions of the skin?
A: Protection: physical barrier against pathogens, chemicals, abrasion. Keratin and lipids prevent water loss (prevents dehydration). Acid mantle (pH 4-6) inhibits bacteria. Melanin absorbs UV radiation. Temperature regulation: sweat glands produce sweat → evaporative cooling. Dermal blood vessels dilate (release heat) or constrict (conserve heat). Arrector pili muscles raise hair → trap warm air layer (goosebumps in humans — vestigial). Sensation: Meissner corpuscles (light touch), Pacinian corpuscles (deep pressure/vibration), Merkel discs (sustained touch), Ruffini endings (skin stretch), free nerve endings (pain, temperature). Vitamin D synthesis: UV → converts 7-dehydrocholesterol in skin → vitamin D3 → kidney/liver activate. Immune function: Langerhans cells (APCs) in epidermis. Excretion: small amounts of salt, water, urea in sweat.
Q: What are the ABCDEs of skin cancer and the three types?
A: ABCDEs of melanoma: Asymmetry (one half unlike the other), Border (irregular, ragged, notched), Color (varied from one area to another — shades of tan, brown, black, red, white, blue), Diameter (>6mm — size of pencil eraser), Evolving (changing in size, shape, color, or new symptom). Three types: Basal cell carcinoma (BCC): most common (80%), arises from stratum basale, 'pearly' or flesh-colored nodule with telangiectasias, rarely metastasizes, sun-exposed areas. Squamous cell carcinoma (SCC): second most common, arises from stratum spinosum, scaly red plaque or ulcerated nodule, can metastasize (especially lip, ear), sun-exposed skin, associated with actinic keratosis (precursor). Melanoma: least common but most dangerous, arises from melanocytes, high metastatic potential, prognosis based on Breslow depth (depth of invasion).
Q: How are burns classified and what are the rules of nines?
A: Classification by depth: Superficial (first degree) — epidermis only, red, dry, painful, no blisters. Heals in 3-5 days. Example: sunburn. Partial thickness (second degree) — epidermis + part of dermis. Superficial partial: blisters, moist, very painful. Heals in 2-3 weeks. Deep partial: may need grafting. Full thickness (third degree) — entire epidermis and dermis destroyed. Leathery, dry, white/brown/black, painless (nerves destroyed). Requires skin grafting. Fourth degree: extends to muscle and bone. Rules of Nines (adult): Head = 9%, each arm = 9%, chest = 9%, abdomen = 9%, upper back = 9%, lower back = 9%, each thigh = 9%, each lower leg = 9%, genitalia = 1%. Total = 100%. Burns >20% BSA: risk of systemic complications. Burns >50% BSA: life-threatening. Lund-Browder chart is more accurate, especially for children (larger head relative to body).
Q: What are the skin appendages and what does each do?
A: Hair: composed of medulla, cortex (keratin), cuticle. Follicle produces hair — matrix cells divide. Arrector pili muscle (smooth) — attached to follicle, causes goosebumps (piloerection). Hair color from melanocytes in follicle matrix. Functions: thermal insulation (scalp), protection (eyelashes, nose hairs filter particles), sensory (detect air movement). Nails: modified stratum corneum. Nail plate overlies nail bed. Nail matrix (lunula) is the growth zone. Nails grow ~1mm/week (fingernails faster than toenails). Clinical: nail changes indicate systemic disease (koilonychia = iron deficiency, clubbing = chronic hypoxia, Beau lines = physiologic stress, Terry nails = liver disease). Sebaceous glands: secrete sebum (oil) into hair follicle. Lubricates skin and hair, inhibits bacterial growth. Holocrine secretion. Acne = blocked sebaceous follicle + Propionibacterium acnes. Sweat glands: Eccrine (most of body, thermoregulation, merocrine), Apocrine (axilla, groin, nipple, modified in ear as ceruminous glands — produce waxy secretion).