The Integumentary System: Structure, Function, and Pathology

The Integumentary System

Overview of the Integumentary System

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 and plays a vital role in homeostasis.

• Basic Structure of Skin: The skin consists of two main layers: the epidermis (outer layer) and the dermis (inner layer), with a subcutaneous layer (hypodermis) beneath.

• Functions of the Integumentary System: Protection, sensation, thermoregulation, excretion, vitamin D synthesis, and storage of lipids.

• Accessory Structures: Includes hair, nails, sweat glands, and sebaceous glands.

The Epidermis

Structure and Cell Types

The epidermis is a stratified squamous epithelium composed mainly of keratinocytes. It is organized into distinct layers, each with specialized functions.

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

  1. Stratum basale

  2. Stratum spinosum

  3. Stratum granulosum

  4. Stratum lucidum (only in thick skin)

  5. Stratum corneum

• Keratinocyte Life Cycle: Keratinocytes originate in the stratum basale and migrate upward, undergoing differentiation and eventually sloughing off at the surface.

• Other Cell Types: Melanocytes (produce melanin), Langerhans cells (immune defense), Merkel cells (sensory function).

• Thick vs. Thin Skin: Thick skin (palms, soles) contains all five layers; thin skin (most of the body) lacks the stratum lucidum and has a thinner stratum corneum.

The Dermis

Structure and Functions

The dermis is a connective tissue layer beneath the epidermis, providing structural support and housing blood vessels, nerves, and accessory structures.

• Layers of the Dermis:

  • Papillary Layer: Superficial, areolar connective tissue with dermal papillae that interlock with the epidermis.

  • Reticular Layer: Deeper, dense irregular connective tissue containing collagen and elastic fibers.

• Dermal Papillae: Projections that increase surface area for nutrient exchange and form fingerprints (epidermal ridges).

• Skin Markings: Created by the arrangement of dermal papillae and epidermal ridges; unique to each individual.

Skin Pigmentation

Melanin and Other Pigments

Skin color is determined by the presence and distribution of several pigments, with melanin being the most significant.

• Melanin: Produced by melanocytes in the stratum basale; protects against UV radiation.

• Other Pigments: Carotene (yellow-orange pigment from diet), hemoglobin (red pigment in blood).

• Pathological Coloration: Cyanosis (bluish skin due to low oxygen), jaundice (yellowing from bilirubin), erythema (redness from increased blood flow).

Accessory Structures of the Integument

Hair, Nails, and Glands

Accessory structures originate from the epidermis but are located within the dermis. They serve protective, sensory, and regulatory functions.

• Hair: Composed of dead keratinized cells; functions in protection, sensation, and thermoregulation.

• Nails: Modified epidermal cells; protect fingertips and enhance fine touch.

• Sweat Glands: Eccrine (thermoregulation, watery secretion) and apocrine (odor, found in axillary/genital areas).

• Sebaceous Glands: Secrete sebum (oily substance) for lubrication and waterproofing.

Pathology of the Skin

Burns and Skin Cancer

Skin disorders can range from acute injuries to chronic diseases, with burns and cancer being among the most clinically significant.

• Classification of Burns:

  • First-degree: Affects only the epidermis (redness, pain).

  • Second-degree: Involves epidermis and part of dermis (blisters).

  • Third-degree: Destroys both epidermis and dermis (loss of sensation, risk of infection).

• Estimating Severity: Rule of nines is used to estimate the percentage of body surface area affected.

• Types of Skin Cancer:

  • Basal cell carcinoma (most common, least dangerous)

  • Squamous cell carcinoma (can metastasize)

  • Malignant melanoma (most dangerous, high metastatic potential)

Summary Table: Layers of the Skin

Layer	Main Components	Functions
Epidermis	Keratinocytes, melanocytes, Langerhans cells, Merkel cells	Protection, barrier, UV defense, sensation
Dermis (Papillary)	Areolar connective tissue, dermal papillae	Nutrient supply, support, fingerprints
Dermis (Reticular)	Dense irregular connective tissue, collagen, elastic fibers	Strength, elasticity, houses glands/hair follicles