The Integumentary System: Structure, Function, and Components

The Integumentary System

Overview of the Integumentary System

The integumentary system consists of the skin and its derivatives, including sweat glands, oil glands, hair, and nails. It serves as the body's primary barrier against the external environment and performs numerous vital functions.

• Skin is the largest organ of the body, providing protection, sensation, and regulation.

• Key properties: waterproof, durable, washable, air-conditioned, self-healing, and long-lasting.

• Functions include protection, temperature regulation, sensation, metabolic functions, blood reservoir, and excretion.

Structure of the Skin

Major Layers of the Skin

The skin is composed of two main layers: the epidermis and the dermis. Beneath these lies the hypodermis (subcutaneous tissue), which is not technically part of the skin but shares some of its functions.

• Epidermis: The outermost, avascular layer made of keratinized stratified squamous epithelium.

• Dermis: The deeper, vascularized connective tissue layer.

• Hypodermis (Subcutaneous tissue): Composed of areolar and adipose tissue, it anchors the skin to underlying structures, provides insulation, and stores fat.

Cells of the Epidermis

• Keratinocytes: The most abundant cell type; produce keratin, a protein that provides strength and waterproofing. Lifespan is 25-45 days. Epidermal growth factor stimulates their mitotic activity in the deepest layer.

• Melanocytes: Located in the deepest epidermal layer; produce melanin, which protects against UV damage and determines skin color.

• Dendritic (Langerhans) cells: Star-shaped immune cells that arise from bone marrow and activate the immune system.

• Tactile (Merkel) cells: Located at the epidermal-dermal junction; function as touch receptors.

Layers of the Epidermis

The epidermis is organized into distinct layers (from deep to superficial):

• Stratum basale: Deepest layer; site of mitosis and new cell production.

• Stratum spinosum: Several layers of keratinocytes; contains dendritic cells.

• Stratum granulosum: Cells begin to die; contain keratohyalin granules.

• Stratum lucidum: Present only in thick skin (palms, soles); clear layer containing eleidin, a lipid-rich, transparent protein providing water-proofing.

• Stratum corneum: Outermost layer; 20-30 layers of dead, keratinized cells.

Thick skin (palms, soles) contains all five layers, including the stratum lucidum. Thin skin lacks the stratum lucidum.

Layers of the Dermis

• Papillary layer: Superficial; made of areolar connective tissue, contains dermal papillae (projections) with capillaries and nerve endings. Forms friction ridges (fingerprints).

• Reticular layer: Deeper; dense irregular connective tissue with thick collagen bundles. Provides strength, elasticity, and houses lines of cleavage (important for surgical incisions).

Hypodermis (Subcutaneous Tissue)

• Composed of areolar and adipose tissue.

• Anchors skin, provides insulation, absorbs shock, and stores fat.

Skin Color and Pigments

Main Pigments

• Melanin: Only pigment produced in the skin; derived from tyrosine. Color ranges from reddish-yellow to brownish-black. Amount and type determine skin color and tanning ability.

• Carotene: Yellow to orange pigment from plant products; accumulates in the stratum corneum and hypodermis.

• Hemoglobin: Oxygenated pigment in red blood cells; gives a pinkish hue, especially in light-skinned individuals.

Cyanosis is a bluish discoloration of the skin due to low oxygenation of hemoglobin.

Accessory Structures of the Skin

Hair

• Functions: Senses insects, protects from trauma, heat loss, and sunlight, shields eyes, filters air.

• Structure: Composed of hard keratin. Three layers: medulla (core, air spaces), cortex (pigment), cuticle (outermost, overlapping cells).

• Hair shaft shape determines hair texture (straight, wavy, curly); follicle size determines thickness.

• Hair follicle: Contains root, bulb (with papilla and root hair plexus), surrounded by connective tissue and epithelial sheaths.

• Arrector pili muscle: Contracts to raise hair (goosebumps).

• Sebaceous glands: Secrete sebum (oily, waterproofing, bactericidal).

• Hair growth: Cycles through active and resting phases; affected by hormones, nutrition, and age.

Split ends occur when the cuticle wears away. Gray/white hair results from decreased melanin and air bubbles in the shaft.

Nails

• Protective, keratinized modifications of the epidermis.

• Parts: free edge, body, root, nail folds, lunula (crescent-shaped area), nail matrix (growth region), nail bed, eponychium (cuticle).

Glands

• Sudoriferous (Sweat) glands: Two main types:

  • Eccrine glands: Most numerous; secrete watery sweat for thermoregulation.

  • Apocrine glands: Found in axillary and anogenital areas; secrete sweat with fatty substances and proteins, leading to body odor when decomposed by bacteria.

• Other glands: Ceruminous (ear wax) and mammary (milk) glands.

Functions of the Skin

• Protection: Physical, chemical (acid mantle, melanin), and biological (immune cells) barriers.

• Temperature regulation: Sweating and blood flow adjustments.

• Cutaneous sensation: Touch, pressure, pain, and temperature receptors.

• Metabolic functions: Synthesis of vitamin D, activation of some hormones, and processing of carcinogens.

• Blood reservoir: Dermis holds about 5% of total blood volume.

• Excretion: Removal of wastes (ammonia, urea, uric acid) via sweat.

Wound Healing and Burns

Phases of Wound Healing

• Inflammation

• New vessel and tissue remodeling

• Healed wound

Burns

• First-degree: Only epidermis is damaged.

• Second-degree: Epidermis and superficial dermis are damaged.

• Third-degree: Entire thickness of skin is destroyed.

Rule of Nines (for estimating burn area)

Body Region	Percentage of Total Body Surface Area
Head and neck (anterior & posterior)	9%
Each upper limb (anterior & posterior)	9% each (18% total)
Trunk (anterior & posterior)	36%
Each lower limb (anterior & posterior)	18% each (36% total)
Perineum	1%
Total	100%

Additional info:

• Lines of cleavage in the dermis are important for surgical incisions to minimize scarring.

• Flexure lines are dermal folds at or near joints, allowing skin to accommodate movement.

• Fingerprints are formed by friction ridges in the papillary layer of the dermis.