The Integumentary System: Structure, Function, and Clinical Aspects

The Integumentary System

Overview

The integumentary system is a complex organ system primarily composed of the skin (cutaneous membrane) and its accessory structures. It serves as the body's first line of defense and performs multiple vital functions essential for homeostasis and protection.

Functions of the Integumentary System

• Protection

  • Physical Protection: Acts as a barrier against mechanical injury and pathogens.

  • Chemical Protection: Prevents excessive water loss (desiccation) and blocks many chemicals.

  • Biological Protection: Defends against bacteria, viruses, and other invasive organisms.

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

• Excretory: Removes waste products such as urea and salts through sweat.

• Secretory: Secretes useful substances like sweat and mucin.

• Regulation: Involved in thermoregulation and water balance.

• Synthesis: Produces vitamin D, keratin, and melanin.

• Storage: Stores lipids in associated adipose tissue.

Structure of the Skin

Epidermis

The epidermis is the thin, outermost layer of the skin, composed of keratinized stratified squamous epithelium. There are two types: thin skin (covers most of the body) and thick skin (palms and soles, lacks hair).

Layers of the Epidermis (from deep to superficial)

1. Stratum basale (germinativum): Deepest layer; single row of columnar cells; site of active cell division; contains Merkel cells.

2. Stratum spinosum: 8–10 layers; cells begin to flatten; contains abundant Langerhans cells.

3. Stratum granulosum: 3–5 layers; keratinization begins; cells produce keratin (a water-insoluble, fibrous protein).

4. Stratum lucidum: 3–5 layers; present only in thick skin.

5. Stratum corneum: 15–30 layers; dead, flattened, keratin-filled cells; forms a waterproof, protective barrier; cells are continuously shed.

Note: It takes about 35 days for a cell to move from the stratum basale to the surface and be shed. Increased abrasion accelerates this process, forming calluses.

Major Cell Types in the Epidermis

• Keratinocytes: ~90% of epidermal cells; produce keratin.

• Melanocytes: ~8%; produce melanin pigment; absorb/filter UV light; all humans have similar numbers, but melanin production varies.

• Langerhans Cells: Immune cells (macrophages) from bone marrow; phagocytize foreign substances; most abundant in stratum spinosum.

• Merkel’s Cells: Sensory receptors for touch; found in the stratum corneum.

Epidermal–Dermal Junction

The basement membrane (basal lamina) connects the epidermis to the dermis, providing mechanical support and acting as a partial barrier to large molecules.

Dermis

The dermis is the thicker, inner layer of the skin, providing mechanical strength and elasticity. It is composed of two layers:

• Papillary Layer: ~20% of dermis; made of areolar tissue; contains a network of blood vessels (nourishes epidermis, aids in waste removal); forms dermal papillae (ridges) that create fingerprints.

• Reticular Layer: Thicker, deeper layer; composed of dense irregular connective tissue (collagen, elastic, and reticular fibers); contains smooth and skeletal muscle; provides strength and elasticity.

Hypodermis (Subcutaneous Layer)

The hypodermis (subcutaneous layer or superficial fascia) is primarily adipose tissue with some areolar tissue. It is not always considered part of the integumentary system but serves as a fat storage depot, insulator, and shock absorber. Collagen fibers from the dermis anchor to the hypodermis and deeper tissues.

Accessory Structures of the Integumentary System

Hair

• Found only in thin skin; composed mainly of keratin.

• Produced by hair follicles (epidermal cells growing downward into the dermis/hypodermis).

• Hair root: Anchored in the skin; hair shaft: Visible part.

• Cross-section: Cuticle (outer), cortex (middle), medulla (inner); all keratinized.

Nails

• Composed of keratinized cells.

• Visible parts: Free edge (white tip), body (pink due to blood vessels), lunula (white, moon-shaped), cuticle/eponychium (skin fold).

• Nail root is beneath the cuticle.

• Fingernails grow faster than toenails; both grow faster in summer.

Glands

• Sweat (Sudoriferous) Glands:

  • Over 3 million in humans; sweat is >95% water plus salts, urea, and other ions.

  • Functions: Thermoregulation and excretion.

  • Types:

    • Apocrine: Larger; found in armpit and genital areas.

    • Eccrine: Smaller, more numerous; up to 3,000 per square inch on palms.

• Sebaceous (Oil) Glands: Secrete sebum (oil); lubricates and waterproofs skin.

• Ceruminous Glands: Modified sweat glands; produce cerumen (ear wax).

Damage and Repair of the Integumentary System

Wound Healing

Epidermal Healing

• Basal cells detach from the basement membrane, enlarge, and migrate across the wound.

• Migration stops when cells contact each other (contact inhibition).

Deep Wound Healing

Involves the dermis and is slower and more complex, resulting in scar tissue formation and some loss of function. Four phases:

1. Inflammatory phase: Blood clot forms; inflammation removes debris and prepares for repair.

2. Migratory phase: Clot becomes a scab; epithelial cells migrate beneath; fibroblasts produce scar tissue; new blood vessels form (granulation tissue).

3. Proliferative phase: Extensive epithelial growth; more collagen deposition; continued blood vessel growth.

4. Maturation (Scarring) phase: Scab falls off; collagen organizes; fibroblasts decrease; circulation normalizes.

Scar Tissue

• Fibrosis: Formation of scar tissue.

• Hypertrophic Scar: Remains within original wound boundaries.

• Keloid Scar: Extends beyond original wound into surrounding tissue.

• Scar tissue has denser collagen, fewer blood vessels, and less hair/glands/sensory structures than normal skin.

Burns

Burns are classified by depth and severity:

• First Degree (Superficial): Affects only the epidermis; no blisters.

• Second Degree (Partial Thickness): Involves all epidermal layers and possibly dermis; blisters present.

• Third Degree (Full Thickness): Destroys epidermis and dermis, may reach hypodermis; area is painless due to nerve destruction; does not regenerate naturally.

Severe burns can lead to infection, loss of thermoregulation, and dangerous fluid-electrolyte imbalances, potentially causing organ failure.

Rule of Nines

The Rule of Nines is used to estimate the percentage of body surface area affected by burns in adults (age 13+):

Body Region	Percentage
Head	9%
Each Arm	9%
Each Leg	18%
Back	18%
Chest	18%

This method helps guide treatment and fluid replacement.

Skin Cancer

Types of Skin Cancer

• Basal Cell Carcinoma: Most common; arises from basal cells of the epidermis; rarely fatal or metastatic; caused mainly by UV exposure.

• Squamous Cell Carcinoma: Second most common; arises from squamous epithelial cells; can metastasize; also linked to UV exposure.

• Melanoma: Rarest but deadliest; arises from melanocytes; highly metastatic; early detection is critical.

Recognizing Skin Cancer: The ABCDEs

• A: Asymmetry

• B: Border Irregularity

• C: Color variation

• D: Diameter (usually >6mm)

• E: Evolving (changes over time)

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Cell division, Merkel cells
Stratum spinosum	Above basale	Langerhans cells, some division
Stratum granulosum	Middle	Keratinization begins
Stratum lucidum	Thick skin only	Clear, 3–5 layers
Stratum corneum	Superficial	Dead, keratinized, protective

Key Terms

• Keratin: Water-insoluble, fibrous protein providing strength and waterproofing.

• Melanin: Pigment produced by melanocytes; protects against UV radiation.

• Granulation tissue: New tissue formed during wound healing.

• Fibrosis: Formation of scar tissue.

Additional info: The notes above include expanded explanations and context for wound healing, burn classification, and the Rule of Nines, as well as a summary table for the epidermal layers for clarity and exam preparation.