The Integumentary System: Structure, Function, and Clinical Relevance

The Integumentary System

Overview and Functions

The integumentary system consists of the skin and its derivatives, including sweat and oil glands, hair, and nails. It serves as the primary barrier between the body and the external environment, providing protection, sensation, and other vital functions. m'/

• Protection: Acts as a physical, chemical, and biological barrier against pathogens, trauma, and UV radiation.

• Regulation: Maintains body temperature through sweating and blood flow regulation.

• Sensation: Contains sensory receptors for touch, pain, and temperature.

• Metabolic Functions: Synthesizes vitamin D and participates in other metabolic processes.

• Excretion: Removes metabolic wastes such as urea and ammonia via sweat.

• Blood Reservoir: Stores up to 5% of the body's blood volume in the dermis.

Structure of the Skin

Major Regions

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 protective functions.

• Epidermis: Outermost layer; made of keratinized stratified squamous epithelium; avascular.

• Dermis: Deeper layer; composed of connective tissue; vascularized.

• Hypodermis: Subcutaneous tissue; consists of areolar and adipose tissue; anchors skin to underlying structures and provides insulation.

Cells of the Epidermis

• Keratinocytes: Main cell type; produce keratin for protection and waterproofing. Lifespan: 25-45 days.

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

• Dendritic (Langerhans) Cells: Star-shaped immune cells; act as macrophages and activate the immune system.

• Tactile (Merkel) Cells: Located at the epidermis/dermis boundary; function as touch receptors.

Layers of the Epidermis

The epidermis is organized into distinct layers, each with specific functions and cell types.

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

• Stratum Lucidum: Clear layer found only in thick skin (palms, soles); contains eleidin, a lipid-rich protein.

• Stratum Granulosum: Contains keratohyalin granules; cells begin to die and lose organelles.

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

• Stratum Basale: Deepest layer; single row of stem cells; site of mitosis and melanocyte activity.

Additional info: Eleidin in the stratum lucidum provides some waterproofing and is converted to keratin as cells move outward.

Thick Skin vs. Thin Skin

• Thick Skin: Found on palms and soles; contains all five epidermal layers, including stratum lucidum.

• Thin Skin: Covers most of the body; lacks stratum lucidum and has fewer layers in the stratum corneum.

Layers of the Dermis

• Papillary Layer: Superficial; made of areolar connective tissue; contains dermal papillae, capillaries, and sensory receptors. Forms friction ridges (fingerprints).

• Reticular Layer: Deep; dense irregular connective tissue; contains thick bundles of collagen and elastic fibers, providing strength and elasticity. Source of lines of cleavage (important for surgical incisions).

Hypodermis (Subcutaneous Tissue)

• Composition: Areolar and adipose tissue, blood vessels.

• Functions: Anchors skin, absorbs shock, insulates, stores fat.

Skin Pigments

Types of Pigments

• Melanin: Only pigment produced in the skin; derived from tyrosine; color ranges from reddish-yellow to brownish-black. Protects against UV damage.

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

• Hemoglobin: From blood; gives skin a pinkish hue, especially in light-skinned individuals.

Cyanosis: Bluish discoloration due to low oxygen levels in blood.

Accessory Structures of the Skin

Hair

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

• Structure: Composed of hard keratin; includes shaft (visible part), root (embedded in skin), and bulb (expanded base with papilla and root hair plexus).

• Layers: Medulla (central core), cortex (pigment and strength), cuticle (outer layer).

• Texture: Determined by follicle shape (round = straight, oval = wavy, flat = curly).

• Thickness: Determined by follicle size.

• Color: Melanin in cortex; gray/white hair results from decreased melanin and air bubbles in shaft.

• Growth: Rate ~2 mm/week; cycles of active growth and rest; affected by hormones and nutrition.

• Arrector pili muscle: Contracts to raise hair and produce "goosebumps."

• Sebaceous glands: Secrete sebum for lubrication and waterproofing; bactericidal.

Nails

• Structure: Hard keratin modification of epidermis; includes free edge, body, root, nail folds, lunula (crescent-shaped area), and eponychium (cuticle).

• Function: Protects distal phalanges; aids in grasping and manipulation.

Glands

• Sudoriferous (Sweat) Glands: Over most skin surface; 2 types:

  • Eccrine: Most common; found on palms, soles, forehead; secrete watery sweat for thermoregulation.

  • Apocrine: Found in axillary and anogenital areas; secrete sweat with fatty substances and proteins; responsible for body odor when decomposed by bacteria.

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

• Sebaceous (Oil) Glands: Holocrine glands; secrete sebum into hair follicles; lubricate and waterproof skin and hair.

Functions of the Skin

Major Functions

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

• Temperature Regulation: Sweating (0.5-12 L/day) and blood flow adjustments.

• Sensation: Detects touch, pain, temperature, and pressure.

• Metabolic: Vitamin D synthesis, conversion of chemicals (e.g., hydrocortisone).

• Blood Reservoir: Dermis holds ~5% of blood volume.

• Excretion: Removes wastes via sweat.

Clinical Relevance

Burns

• First-degree: Only epidermis affected.

• Second-degree: Epidermis and superficial dermis affected.

• Third-degree: Entire thickness of skin (epidermis and all of dermis) affected.

Rule of Nines

The Rule of Nines is used to estimate the percentage of body surface area affected by burns.

Body Region	Percentage
Head and Neck (anterior & posterior)	9%
Upper Limbs (anterior & posterior)	18%
Trunk (anterior & posterior)	36%
Perineum	1%
Lower Limbs (anterior & posterior)	36%
Total	100%

Summary Table: Layers of the Skin

Layer	Location	Main Features
Stratum Corneum	Surface	Dead, keratinized cells; protection
Stratum Lucidum	Thick skin only	Clear, lipid-rich cells; waterproofing
Stratum Granulosum	Middle	Keratohyalin granules; cell death begins
Stratum Spinosum	Below granulosum	Keratinocytes, dendritic cells
Stratum Basale	Deepest	Stem cells, melanocytes; mitosis
Papillary Dermis	Superficial dermis	Areolar CT, dermal papillae
Reticular Dermis	Deep dermis	Dense irregular CT, collagen bundles
Hypodermis	Below dermis	Adipose, areolar CT; insulation

Key Equations

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