The Integumentary System: Structure, Function, and Clinical Relevance

The Integumentary System

Introduction

The integumentary system is the body's largest organ system, primarily composed of the skin and its accessory structures. It serves as the first line of defense against environmental hazards, regulates body temperature, and provides sensory information.

• Main components: Skin (cutaneous membrane), hair, nails, glands, sensory receptors, and arrector pili muscles.

• Functions: Protection, sensation, thermoregulation, excretion, vitamin D synthesis.

LO1: Integumentary System Organs

Main Organs and Structures

• Skin (Cutaneous membrane): Composed of the epidermis (outer layer) and dermis (inner layer).

• Hair: Provides protection, sensory input, and thermoregulation.

• Nails: Protect distal phalanges and aid in manipulation of objects.

• Glands:

  • Sudoriferous (Sweat) glands: Eccrine (thermoregulation), apocrine (odor production).

  • Sebaceous glands: Secrete sebum (oils) for lubrication and waterproofing.

• Sensory receptors: Detect touch, pressure, pain, and temperature.

• Arrector pili muscles: Cause hair to stand up (goosebumps).

LO2: The Epidermis

Structure and Cell Types

The epidermis is the superficial layer of the skin, composed of stratified squamous epithelium. It is avascular and relies on diffusion from the dermis for nutrients.

• Layers of the Epidermis (from deep to superficial):

  1. Stratum basale: Single layer of stem cells; site of mitosis; contains melanocytes and Merkel cells.

  2. Stratum spinosum: Several layers of keratinocytes; contains dendritic (Langerhans) cells.

  3. Stratum granulosum: Keratinocytes begin to die; contain keratohyalin granules.

  4. Stratum lucidum: Present only in thick skin (palms, soles); clear, dead keratinocytes.

  5. Stratum corneum: Multiple layers of dead, flattened keratinocytes; provides barrier function.

• Cell Types:

  • Keratinocytes: Most abundant; produce keratin for strength and waterproofing.

  • Melanocytes: Produce melanin pigment; protect against UV radiation.

  • Dendritic (Langerhans) cells: Immune defense; phagocytose pathogens.

  • Merkel cells: Sensory receptors for light touch.

• Keratinocyte Lifecycle:

  • Originates in stratum basale, migrates upward, accumulates keratin, dies, and is shed from stratum corneum.

  • Typical turnover time: 40-50 days.

LO3: The Dermis

Structure and Skin Markings

The dermis is a connective tissue layer beneath the epidermis, providing strength, elasticity, and housing accessory structures.

• Layers of the Dermis:

  1. Papillary layer: Superficial; loose connective tissue; contains dermal papillae (form fingerprints).

  2. Reticular layer: Deep; dense irregular connective tissue; contains collagen bundles, sweat glands, hair follicles.

• Skin Markings:

  • Dermal ridges: Formed by prominent dermal papillae; create fingerprints, enhance grip.

  • Tension lines (lines of cleavage): Gaps between collagen bundles; important for wound healing and surgical incisions.

  • Flexure lines: Where skin is tightly bound to underlying tissues (joints, palms).

LO4: Skin Pigments and Coloration

Melanin and Other Pigments

Skin color is determined by several pigments and can indicate underlying pathology.

• Melanin:

  • Produced by melanocytes in stratum basale.

  • Protects DNA from UV radiation.

  • Secondary function: reduces vitamin D synthesis in response to UV exposure.

  • Color depends on amount and type (eumelanin: brown-black; pheomelanin: yellow-red).

  • Distribution varies by body region, but total melanocyte number is similar among individuals.

• Carotene:

  • Orange-red pigment from diet (carrots, egg yolk).

  • Stored in stratum corneum and adipose tissue.

• Hemoglobin:

  • Red pigment in erythrocytes; gives skin a pinkish hue when oxygenated.

Clinical Significance of Skin Color

• Freckles: Localized higher melanin production.

• Moles: Local proliferation of melanocytes.

• Albinism: Melanocytes do not produce tyrosinase; lack of melanin.

• Erythema: Reddening due to increased blood flow (inflammation, fever).

• Pallor: Pale appearance due to decreased blood flow.

• Cyanosis: Bluish skin from low oxygen saturation.

Effects of UV Radiation

• Immediate: Increased melanin production, DNA damage.

• Delayed: Risk of skin cancer, sunburn, and changes in pigmentation.

• All skin types are susceptible to UV damage and skin cancer.

LO5: Accessory Structures of the Skin

Hair, Nails, and Glands

• Hair: Derived from epidermis; functions in protection, sensation, and thermoregulation.

• Nails: Protect fingertips, aid in manipulation.

• Glands:

  • Eccrine sweat glands: Most prevalent; secrete water, electrolytes, lactic acid for cooling.

  • Apocrine sweat glands: Release into hair follicles; rich in protein, produce odor (axillae, anal area, areolae).

  • Ceruminous glands: Modified apocrine glands in ear; produce cerumen (earwax).

  • Mammary glands: Produce milk.

  • Sebaceous glands: Secrete sebum (oily lipids); holocrine secretion; associated with hair follicles; factor in acne.

LO6: Wounds and Skin Pathology

Types of Wounds and Burns

• Wounds: Any disruption in skin integrity (lacerations, burns, skin cancers).

• Burns:

  • First-degree: Superficial; only epidermis; redness, mild pain, no permanent damage.

  • Second-degree: Partial thickness; epidermis and part of dermis; pain, blistering, scarring.

  • Third-degree: Full thickness; epidermis, dermis, and deeper tissues; may not be painful initially (nerve destruction); significant scarring, dehydration risk, infection risk.

Skin Cancer

• Basal cell carcinoma: Most common; arises from keratinocytes in stratum basale; rarely metastasizes; treatable by surgery.

• Squamous cell carcinoma: Second most common; arises from keratinocytes in stratum spinosum; may ulcerate and bleed; more likely to metastasize than basal cell carcinoma.

• Malignant melanoma: Cancer of melanocytes; highly metastatic; prognosis depends on size, depth, and spread; treated with surgery, radiation, chemotherapy.

LO7: Regional Hyper- and Hypopigmentation

Examples and Clinical Relevance

• Hyperpigmentation:

  • Overproduction of melanin; brown patches (melasma during pregnancy, Addison's disease).

• Hypopigmentation:

  • Decreased melanin production or destruction of melanocytes (vitiligo: autoimmune destruction; tinea versicolor: fungal infection).

  • Burns on darker skin may also cause hypopigmentation.

Summary Table: Skin Layers and Cell Types

Layer	Main Cell Types	Key Features
Stratum basale	Keratinocytes, Melanocytes, Merkel cells	Mitotic activity, pigment production, sensory function
Stratum spinosum	Keratinocytes, Dendritic cells	Strength, immune defense
Stratum granulosum	Keratinocytes	Keratinization, cell death
Stratum lucidum	Keratinocytes	Present only in thick skin
Stratum corneum	Dead keratinocytes	Barrier function

Key Equations and Concepts

• Vitamin D Synthesis: $\text{7-dehydrocholesterol} + UV \rightarrow \text{cholecalciferol (Vitamin D}_3)$

• Thermoregulation: $\text{Heat loss} = \text{Sweat evaporation} + \text{Vasodilation}$

Conclusion

The integumentary system is essential for protection, homeostasis, and sensory perception. Understanding its structure, function, and clinical relevance is fundamental for students of anatomy and physiology.