Integumentary System: Structure, Function, and Pathology

Module 5.1 Overview of the Integumentary System

Skin Structure

The skin, also known as the cutaneous membrane, is the largest organ of the body and serves as a protective barrier. It consists of two main layers and several accessory structures.

• Epidermis: The outermost layer, composed primarily of epithelial cells.

• Dermis: The deeper layer, made up of connective tissue.

• Accessory Structures:

  • Sweat glands

  • Sebaceous glands

  • Hair

  • Nails

• Avascular: The epidermis lacks blood vessels; nutrients diffuse from the dermis.

• About 50% of epidermal cells are too far from blood supply to survive; superficial layers are dead cells.

• Hypodermis: Also called superficial fascia or subcutaneous tissue, lies beneath the dermis and consists mainly of fat and connective tissue.

Functions of the Integumentary System

The skin performs several vital functions essential for homeostasis and protection.

• Protection: Acts as a barrier against pathogens, chemicals, and physical injury.

• Sensation: Contains sensory receptors for touch, pain, temperature, and pressure.

• Thermoregulation: Regulates body temperature through sweat production and blood flow.

• Excretion: Removes waste products via sweat.

• Vitamin D Synthesis: Initiates production of vitamin D when exposed to sunlight.

When body temperature rises above normal, the following sequence occurs:

• Receptor: Detects change in temperature.

• Control Center: Processes information and initiates response.

• Effector/Response: Sweat glands increase activity.

• Effector/Response: Blood vessels dilate.

• In Homeostatic Range: Temperature returns to normal.

The Epidermis

Cell Types and Structure

The epidermis is the most superficial layer of skin, composed of several cell types, with keratinocytes being the most numerous.

• Keratinocytes: Make up about 95% of the epidermis and produce keratin, a protein that strengthens the skin.

• Keratinocytes are linked by desmosomes, providing structural integrity.

Layers of the Epidermis

Keratinocytes are organized into five distinct layers (strata), from deep to superficial:

• Stratum basale

• Stratum spinosum

• Stratum granulosum

• Stratum lucidum (only in thick skin)

• Stratum corneum

The migration of keratinocytes from the stratum basale to the stratum corneum takes about 40–50 days.

Other Epidermal Cells

• Dendritic (Langerhans) cells: Immune cells that protect against pathogens.

• Merkel cells: Sensory cells associated with touch.

• Melanocytes: Produce melanin, the pigment responsible for skin color.

Thick and Thin Skin

Skin is classified based on the degree of mechanical stress it endures:

• Thick skin: Found on palms and soles; contains all five epidermal layers and is adapted to high stress.

• Thin skin: Covers most of the body; lacks the stratum lucidum.

• Callus: Localized thickening due to repeated friction.

Module 5.3 The Dermis

Dermal Structure

The dermis is composed of two layers of connective tissue: the papillary and reticular layers.

• Papillary layer: Contains dermal papillae and tactile (Meissner) corpuscles for touch sensation.

• Reticular layer: Contains lamellated (Pacinian) corpuscles for pressure sensation.

Skin Markings

• Epidermal ridges: Enhance grip and form fingerprints.

• Tension (cleavage) lines: Gaps between collagen bundles in the dermis; important in surgical incisions.

Module 5.4 Skin Pigmentation

Melanin

Skin color depends on the number and activity of melanocytes and the amount of melanin produced.

• Mole (nevus): Localized overgrowth of melanocytes.

Other Pigments Affecting Skin Color

• Carotene: Yellow-orange pigment from diet.

• Hemoglobin: Red pigment in blood; affects skin color based on blood flow.

Skin Color as a Diagnostic Tool

Changes in skin color can indicate disease:

• Erythema: Redness due to increased blood flow.

• Pallor: Paleness due to decreased blood flow.

• Cyanosis: Bluish color due to low oxygen levels.

Module 5.5 Accessory Structures of the Integument: Hair, Nails, and Glands

Hair (Pili)

Hair consists of the shaft and root, made of keratinized epithelial cells.

• Arrector pili: Smooth muscle that causes hair to stand up (goosebumps).

• Hair types:

  • Lanugo: Fine, unpigmented hair on fetus.

  • Terminal hair: Thick, coarse hair (scalp, eyebrows).

  • Vellus hair: Fine, pale hair covering most of the body.

• Terminal hair replaces vellus hair after puberty; more in males than females.

Nails

• Nail plate: Visible part of the nail.

• Nail root: Embedded under the skin.

• Nail matrix: Site of nail growth.

• Proximal nail fold: Eponychium (cuticle).

• Medial and lateral nail folds: Surround the nail plate.

• Distal, or free, edge: Hyponychium (under the nail tip).

• Lunula: Crescent-shaped area at the base of the nail.

• Function: Protects distal tips of fingers and toes.

Glands

• Sweat (sudoriferous) glands:

  • Eccrine sweat glands: Most numerous; regulate temperature.

  • Apocrine sweat glands: Found in axillary and genital areas; produce odor.

  • Ceruminous glands: Produce earwax.

  • Mammary glands: Produce milk.

• Sebaceous glands: Secrete sebum to lubricate skin and hair.

Module 5.6 Pathology of the Skin

Wounds and Burns

• Wound: Disruption of skin integrity.

• Burns:

  • First-degree burns: Affect only the epidermis; redness and pain.

  • Second-degree burns: Affect epidermis and part of dermis; blistering.

  • Third-degree burns: Destroy epidermis and dermis; may affect deeper tissues.

Skin Cancer

Skin cancer is one of the most common cancers worldwide, often caused by mutations in DNA due to UV exposure.

• Unchecked cell division leads to tumor formation.

• Cancerous cells can metastasize via blood or lymphatic vessels.

• Major types of skin cancer:

  • Basal cell carcinoma

  • Squamous cell carcinoma

  • Malignant melanoma:

    • (A) Asymmetrical shape

    • (B) Border irregularity

    • (C) Color variation

    • (D) Diameter > 6 mm

    • (E) Evolving shape and size

• Early detection is critical for successful treatment.

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Single row of stem cells; mitosis occurs here
Stratum spinosum	Above basale	Several layers; contains desmosomes
Stratum granulosum	Middle	Keratinization begins; cells flatten
Stratum lucidum	Only in thick skin	Clear, thin layer
Stratum corneum	Superficial	Many layers of dead, keratinized cells

Summary Table: Types of Skin Cancer

Type	Origin	Features
Basal cell carcinoma	Basal cells of epidermis	Most common; rarely metastasizes
Squamous cell carcinoma	Keratinocytes of stratum spinosum	May metastasize; scaly appearance
Malignant melanoma	Melanocytes	Most dangerous; rapid metastasis

Key Formula: Rule of Nines for Burn Assessment

The Rule of Nines is used to estimate the percentage of body surface area affected by burns:

$\text{Total Burn Area} = \sum \text{(Area of each region as % of body surface)}$

Example: Each arm = 9%, each leg = 18%, anterior trunk = 18%, posterior trunk = 18%, head = 9%, perineum = 1%.

Additional info: Academic context and definitions have been expanded for clarity and completeness.