A&P I · Unit 5 · Guidebook
The Integumentary System
Skin layers, hair, nails, glands, functions — and burns & skin cancer
By the end of this unit you can…
- ✓Name the layers of the epidermis and dermis and the cells of the epidermis
- ✓Explain the pigments that determine skin colour and clinical colour changes
- ✓Describe the structure of hair, nails, sweat and sebaceous glands
- ✓List the protective, thermoregulatory, sensory and metabolic functions of skin
- ✓Compare the three skin cancers and classify burns using the rule of nines
Key terms
Practice →1Skin structure
The skin (cutaneous membrane) is the body's largest organ — about 1.5–2 m² and roughly 7% of body weight. It has two distinct regions:
- Epidermis — superficial; keratinized stratified squamous epithelium; avascular.
- Dermis — deep; mostly dense irregular connective tissue; vascular, with nerves, glands and hair follicles.
Below the skin lies the hypodermis (subcutaneous layer, superficial fascia) — mostly adipose tissue that anchors skin to muscle, stores fat and absorbs shocks. It is not part of the skin but shares its protective functions.
2The epidermis
Four cell types live in the epidermis:
- Keratinocytes
- Most cells. Produce keratin, the fibrous protein that toughens the skin. Born by mitosis in the deepest layer and pushed up; dead by the time they reach the surface.
- Melanocytes
- Spider-shaped cells in the deepest layer that make the pigment melanin, packaged in melanosomes and passed to keratinocytes to shield their nuclei from UV light.
- Dendritic (Langerhans) cells
- Star-shaped immune cells (from bone marrow) that ingest foreign substances and alert the immune system.
- Tactile (Merkel) cells
- At the epidermal–dermal junction; with a sensory nerve ending they form a tactile (Merkel) disc — a touch receptor.
| Layer (deep → superficial) | What happens there |
|---|---|
| Stratum basale (basal layer) | Single row of stem cells dividing rapidly; contains melanocytes and tactile cells |
| Stratum spinosum (prickly layer) | Several layers; cells joined by desmosomes; keratin filaments; dendritic cells abundant |
| Stratum granulosum (granular layer) | Cells flatten; keratohyaline and lamellar granules (waterproofing glycolipid); cells start to die |
| Stratum lucidum (clear layer) | Thin, clear band of dead cells — thick skin only (palms, soles) |
| Stratum corneum (horny layer) | 20–30 rows of dead, keratin-filled cells; ¾ of epidermal thickness; protects and sheds (dandruff) |
3The dermis
- Papillary layer
- Superficial ~20%; areolar connective tissue. Finger-like dermal papillae contain capillary loops and tactile (Meissner) corpuscles. On palms and soles they lie on dermal ridges that create friction ridges — fingerprints.
- Reticular layer
- Deep ~80%; dense irregular connective tissue with collagen (strength, binds water) and elastic fibers (stretch-recoil). Cleavage (tension) lines follow the collagen bundles — surgeons cut parallel to them so wounds heal with less scarring.
4Skin colour
Three pigments contribute to skin colour:
- Melanin
- The only pigment made in the skin (by melanocytes): yellow to reddish-brown to black. Everyone has about the same number of melanocytes — differences come from the kind and amount of melanin made. Sun exposure stimulates melanin production (tanning).
- Carotene
- Yellow-orange pigment from food (carrots); accumulates in the stratum corneum and hypodermis — most obvious on palms and soles.
- Hemoglobin
- In red blood cells of dermal capillaries; gives a pinkish hue to fair skin.
| Sign | Colour | Meaning |
|---|---|---|
| Cyanosis | Blue | Low blood oxygen (heart or lung problems) |
| Erythema | Redness | Fever, inflammation, allergy, embarrassment |
| Pallor (blanching) | Paleness | Fear, anemia, low blood pressure |
| Jaundice | Yellow | Liver disorder → excess bilirubin |
| Bronzing | Bronze | Addison's disease (adrenal cortex hypofunction) |
| Bruises (hematomas) | Black-and-blue | Blood escaped from vessels and clotted |
5Hair, nails & glands
A hair is a flexible strand of dead, keratinized cells. The shaft projects above the skin; the root is embedded in it. In cross-section a hair has a central medulla, a bulky cortex and an outer cuticle. Hair pigment comes from melanocytes at the base of the follicle.
- Hair follicle
- Extends from the epidermal surface into the dermis. Its deep end expands into the hair bulb; the hair matrix there is the actively dividing region that makes the hair. A dermal papilla supplies blood. A root hair plexus of sensory nerves wraps the bulb, so bending a hair is felt.
- Arrector pili muscle
- Smooth muscle attached to the follicle; contracts in cold or fear to pull the hair upright — goosebumps.
A nail is a scale-like modification of the epidermis — hard keratin. Parts: free edge, nail body (plate), nail root, proximal and lateral nail folds, the eponychium (cuticle), the hyponychium under the free edge, the nail bed, and the nail matrix (growth zone). The lunula is the white crescent over the thickened matrix.
| Gland | Secretion & duct | Location / function |
|---|---|---|
| Eccrine (merocrine) sweat | Sweat (99% water, NaCl, vitamin C, antimicrobial dermcidin, wastes) → pore on skin surface | Everywhere, esp. palms, soles, forehead; thermoregulation |
| Apocrine sweat | Sweat + fats and proteins → hair follicles; odourless until bacteria break it down | Axillary and anogenital areas; active from puberty |
| Ceruminous | Modified apocrine; cerumen (earwax) | External ear canal |
| Mammary | Modified sweat glands; milk | Breasts |
| Sebaceous (oil) | Sebum by holocrine secretion → usually into hair follicles | Everywhere except thick skin; softens hair/skin, slows water loss, kills bacteria |
6Functions of the skin
| Function | How |
|---|---|
| Protection — chemical | Low pH acid mantle; sebum and dermcidin (bactericidal); defensins; melanin blocks UV |
| Protection — physical/mechanical | Continuous keratinized surface; glycolipids block water loss and most water-soluble substances |
| Protection — biological | Dendritic cells and dermal macrophages; DNA absorbs UV and converts it to heat |
| Body temperature regulation | ~500 mL/day of insensible sweat; heavy sweating in heat; dermal blood vessels dilate (release heat) or constrict (conserve heat) |
| Cutaneous sensation | Receptors for touch, pressure, vibration, pain, temperature |
| Metabolic | Vitamin D synthesis: UV light converts a cholesterol derivative in keratinocytes into a vitamin D precursor |
| Blood reservoir | Dermal vessels hold ~5% of blood volume, shunted to muscles during exercise |
| Excretion | Small amounts of nitrogenous wastes (urea, ammonia), salt and water in sweat |
| Receptor | Senses | Where |
|---|---|---|
| Free nerve endings | Pain, temperature (and some touch) | Epidermis and dermis |
| Tactile (Merkel) disc | Light touch | Epidermal–dermal junction |
| Tactile (Meissner) corpuscle | Light (discriminative) touch | Dermal papillae |
| Lamellated (Pacinian) corpuscle | Deep pressure and vibration | Deep dermis / hypodermis |
| Bulbous (Ruffini) corpuscle | Deep, continuous pressure and stretch | Dermis |
| Root hair plexus | Hair movement (light touch) | Around hair follicles |
7Skin cancer & burns
| Cancer | Origin | Notes |
|---|---|---|
| Basal cell carcinoma | Stratum basale cells | Most common, least malignant; shiny dome-shaped nodules; slow growing; rarely metastasizes |
| Squamous cell carcinoma | Keratinocytes of stratum spinosum | Scaly reddened papule (scalp, ears, lip, hands); grows rapidly and can metastasize if untreated |
| Melanoma | Melanocytes | Most dangerous; metastasizes quickly; often from an existing mole |
| Burn | Depth | Appearance |
|---|---|---|
| First-degree (superficial) | Epidermis only | Red, swollen, painful (like sunburn); heals in 2–3 days |
| Second-degree (partial thickness) | Epidermis + upper dermis | Red, painful, blisters; heals in 3–4 weeks if no infection |
| Third-degree (full thickness) | Entire skin, maybe deeper | Gray-white, cherry red or black; not painful (nerve endings destroyed); needs skin grafts |
The immediate threat after a serious burn is loss of body fluids (with proteins and electrolytes) → dehydration, electrolyte imbalance, renal shutdown and circulatory shock. Later, infection is the leading cause of death. Fluid replacement is calculated using the rule of nines: head & neck 9%, each upper limb 9%, trunk 36% (front 18%, back 18%), each lower limb 18%, genitalia 1%.