Chapter 5: The Integumentary System – Structure, Function, and Clinical Relevance

Chapter 5: The Integumentary System

Overview of the Integumentary System

The integumentary system, also known as the integument, is the body's first line of defense and the largest organ system. It serves as a protective barrier and is composed of the skin (cutaneous membrane) and accessory structures such as hair, nails, and exocrine glands.

• Cutaneous membrane: The skin itself, consisting of multiple tissue layers.

• Accessory structures: Includes hair, nails, and exocrine glands (sweat and oil glands).

Functional Anatomy of the Skin

Major Layers of the Skin

The skin is composed of three primary layers, each with distinct tissue types and functions:

• Epidermis: The outermost layer, made of stratified squamous epithelium. It is avascular and relies on diffusion from the underlying dermis for nutrients.

• Dermis: Located beneath the epidermis, it consists of two sublayers:

  • Papillary layer: Composed of areolar connective tissue, contains capillaries and sensory neurons.

  • Reticular layer: Made of dense irregular connective tissue, provides strength and elasticity.

• Hypodermis (Subcutaneous layer): Not technically part of the skin, but stabilizes the skin's position relative to underlying tissues. Dominated by adipose tissue, it serves as an energy reserve and insulator.

Accessory Structures

Accessory structures originate in the dermis and extend through the epidermis to the skin surface.

• Hair: Produced by hair follicles, provides protection and sensory input.

• Nails: Protect the tips of fingers and toes, and can reflect health status.

• Exocrine glands: Include sweat (sudoriferous) and oil (sebaceous) glands, which play roles in thermoregulation and lubrication.

Functions of the Integumentary System

• Protection: Shields underlying tissues from mechanical damage, pathogens, and chemical exposure. Melanin protects against UV radiation; keratin provides water resistance and abrasion protection.

• Excretion: Removes salts, water, and organic wastes through sweat.

• Temperature Regulation: Maintains body temperature via insulation and evaporative cooling.

• Vitamin D3 Synthesis: Epidermal cells produce vitamin D3 when exposed to sunlight, which is essential for calcium metabolism.

• Energy Storage: Adipocytes in the dermis and hypodermis store lipids.

• Sensory Input: Detects touch, pressure, pain, and temperature through specialized receptors.

• Immune Defense: Coordinates immune responses to pathogens and skin cancers.

Structure and Function of Skin Layers

Epidermis

• Stratified squamous epithelium with multiple strata (layers).

• Thin skin: Covers most of the body, has four layers, and is about 0.08 mm thick.

• Thick skin: Found on palms and soles, has five layers, and is about 0.5 mm thick.

• Avascular: Nutrients and oxygen diffuse from the dermis.

• Cell turnover: Takes 7–10 days for cells to migrate from the stratum basale to the stratum corneum.

Dermis

• Papillary layer: Areolar tissue, supports and nourishes the epidermis.

• Reticular layer: Dense irregular connective tissue, provides strength and elasticity via collagen and elastic fibers.

Hypodermis

• Adipose tissue: Energy storage, insulation, and shock absorption.

• Stabilizes skin: Anchors skin to underlying structures.

Pigmentation and Skin Color

• Carotene: Orange-yellow pigment, most visible in light-skinned individuals and found in orange vegetables.

• Melanin: Brown to black pigment produced by melanocytes in the stratum basale. Protects against UV radiation. Larger melanosomes result in darker skin.

• Hemoglobin: Red pigment in blood; increased blood flow causes redness, while decreased oxygenation leads to cyanosis (bluish skin).

Glands of the Skin

Sebaceous (Oil) Glands

• Secrete sebum via holocrine secretion (cell rupture).

• Composition: Triglycerides, cholesterol, proteins, electrolytes.

• Functions: Lubricates and waterproofs skin and hair; has antibacterial properties.

• Distribution: Most abundant on face, back, chest, nipples, and external genitalia.

Sweat (Sudoriferous) Glands

• Types: Eccrine (merocrine) and apocrine glands.

• Eccrine glands: Widely distributed, produce watery sweat for thermoregulation.

• Apocrine glands: Found in armpits, groin; secrete a thicker, odoriferous sweat.

Hair and Nails

• Hair: Produced by follicles; consists of a visible shaft and a root anchored in the skin. The arrector pili muscle causes "goosebumps."

• Nails: Protect digits; changes in appearance can indicate health conditions (e.g., clubbing, pitting, concavity).

Skin Cancers

• Basal cell carcinoma: Most common, originates in the stratum basale, rarely metastasizes.

• Squamous cell carcinoma: Second most common, more likely to metastasize, found in sun-exposed areas.

• Malignant melanoma: Most dangerous, arises from melanocytes, rapidly metastasizes. Early detection is critical for survival.

Vitamin D Synthesis and Calcium Homeostasis

• Sunlight: Epidermal cells convert a steroid precursor to vitamin D3 (cholecalciferol).

• Dietary sources: Fish, fish oils, fortified foods.

• Metabolism: Liver and kidneys convert vitamin D3 to calcitriol, which stimulates calcium and phosphorus absorption in the intestines.

Age-Related Changes in the Integument

• Decreased melanocyte activity: Increased risk of sunburn, gray/white hair.

• Reduced sebum production: Dry, scaly skin.

• Thinning epidermis and dermis: Increased risk of injury, slower healing, sagging, and wrinkling.

• Reduced sweat gland activity: Greater risk of overheating.

• Slower skin repair and reduced blood supply: Delayed wound healing and impaired temperature regulation.

Integumentary System Disorders and Abnormalities

• Lesion: Any abnormality or damage in tissue, often due to disease or trauma.

• Urticaria (Hives): Pale red, raised, itchy bumps, often due to allergic reactions.

• Parasitic infestations: Diseases caused or transmitted by parasites, affecting skin and other tissues.

• Tissue injuries: Includes cuts, abrasions, burns, and other forms of trauma.

Wound Healing and Repair

• Initial response: Bleeding and mast cell activation.

• Clot formation: Blood clot (scab) forms; cells of the stratum basale migrate to cover the wound; macrophages remove debris.

• Granulation tissue: Formed by fibroblasts, blood clot, and capillary network if the dermis is involved.

• Scab removal: Epidermal cells migrate, phagocytosis completes, and fibroblasts produce scar tissue.

• Scar formation: Inflexible, fibrous, noncellular material replaces normal tissue.

Criteria for Suturing Wounds

• Depth: Wounds exposing subcutaneous tissue may require stitches.

• Width: Wounds that cannot be easily closed by pinching need sutures.

• Location: Wounds on areas of frequent movement are more likely to need stitches.

Summary Table: Major Skin Cancers

Type	Origin	Metastasis Risk	Key Features
Basal Cell Carcinoma	Stratum basale	Rare	Most common, slow-growing, UV exposure
Squamous Cell Carcinoma	Sun-exposed epidermis	Moderate	Second most common, may metastasize
Malignant Melanoma	Melanocytes	High	Irregular shape, color, rapid growth, deadly if not detected early

Key Equations

• Vitamin D3 Synthesis:

$\text{7-dehydrocholesterol} + \text{UV light} \rightarrow \text{cholecalciferol (vitamin D}_3\text{)}$

• Calcium Homeostasis:

$\text{Vitamin D}_3 \xrightarrow{\text{liver, kidney}} \text{calcitriol} \rightarrow \uparrow \text{Ca}^{2+} \text{ absorption in intestines}$

Additional info: These notes synthesize and expand upon the provided slides and text, offering a comprehensive overview suitable for exam preparation in a college-level Anatomy & Physiology course.