The Integumentary System: Structure, Function, and Clinical Relevance

The Integumentary System

Overview

The integumentary system comprises the skin and its accessory structures, including hair, nails, and glands. It plays a vital role in protection, sensation, thermoregulation, and personal identity. The system is studied at both the organ and organ system levels, with a focus on anatomy, physiology, and clinical relevance.

• Key components: Skin, hair, nails, sweat glands, sebaceous glands.

• Main functions: Protection, sensation, thermoregulation, excretion, and synthesis of vitamin D.

• Clinical relevance: Common conditions include acne, burns, and skin cancers.

Skin Structure

Layers of the Skin

The skin is the largest organ of the body, accounting for 10-15% of total body weight. It consists of two main layers: the epidermis and the dermis. Beneath these lies the hypodermis (subcutaneous tissue).

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

• Dermis: Contains connective tissue, blood vessels, nerves, and accessory structures. It is divided into the papillary layer (loose connective tissue) and reticular layer (dense irregular connective tissue).

• Hypodermis: Composed of loose connective tissue and adipose tissue, providing insulation and energy storage.

Accessory Structures

• Hair: Protects and insulates, found in most skin regions except palms and soles.

• Nails: Protect the tips of fingers and toes, aid in sensation and manipulation.

• Glands: Sweat glands (thermoregulation), sebaceous glands (sebum production for lubrication and protection).

Functions of the Integumentary System

Protection

• Mechanical: Keratinized epithelium forms a durable barrier against physical injury and pathogens.

• Chemical: Sebaceous gland secretions create an acidic environment, deterring pathogen growth.

• UV Protection: Melanin produced by melanocytes absorbs UV radiation.

Sensation

• Receptors: Sensory receptors in the skin detect touch, pressure, pain, and temperature.

• Merkel cells: Specialized for touch sensation.

Thermoregulation

• Sweat glands: Produce sweat to cool the body.

• Blood vessel constriction/dilation: Regulates heat loss.

Excretion

• Sweat: Removes waste products such as urea and salts.

Vitamin D Synthesis

• UV radiation converts 7-dehydrocholesterol in the skin to cholecalciferol (vitamin D3), which is then modified by the liver and kidneys to form calcitriol, essential for calcium absorption.

The Epidermis

Structure and Cell Types

The epidermis is composed of stratified squamous epithelium and contains several cell types:

• Keratinoctyes: Most abundant, produce keratin for strength and waterproofing.

• Melanocytes: Produce melanin pigment for skin color and UV protection.

• Langerhans cells: Dendritic cells involved in immune defense.

• Merkel cells: Sensory cells for touch.

Layers of the Epidermis

The epidermis consists of five layers (from deep to superficial):

• Stratum basale: Single layer of cuboidal/columnar cells; site of mitosis and new cell formation.

• Stratum spinosum: Several layers of keratinocytes; cells appear spiny due to desmosomes.

• Stratum granulosum: 3-5 layers of keratinocytes with granules; cells release lipid-based substances for water barrier.

• Stratum lucidum: Thin, clear layer found only in thick skin (palms, soles).

• Stratum corneum: Outermost layer; several layers of dead, flattened keratinocytes.

Thick vs. Thin Skin

• Thick skin: Found on palms and soles; contains all five epidermal layers and lacks hair follicles.

• Thin skin: Covers most of the body; lacks stratum lucidum and has fewer cell layers.

Keratinoctye Life Cycle

• Keratinoctyes originate in the stratum basale and migrate upwards, undergoing mitosis and differentiation.

• As they move up, they become flatter and are coated with lipid-based substances.

• By the time they reach the stratum corneum, they are dead cells filled with keratin.

• The process of cell shedding and replacement maintains epidermal thickness.

Other Cells of the Epidermis

• Langerhans cells: Immune cells found in the stratum spinosum and granulosum.

• Merkel cells: Sensory receptors for touch, located in the stratum basale.

• Melanocytes: Produce melanin pigment, contributing to skin color and UV protection.

Clinical Relevance

Burns

• First-degree burns: Affect only the epidermis.

• Second-degree burns: Affect both epidermis and part of the dermis.

• Third-degree burns: Extend through the dermis and potentially affect underlying tissues.

Skin Cancer

• Basal cell carcinoma: Originates from basal cells.

• Squamous cell carcinoma: Originates from keratinocytes in the stratum spinosum.

• Melanoma: Originates from melanocytes; most dangerous type.

Cellulite

• Cellulite is the dimpled appearance of skin caused by collagen bands forming around adipose tissue in the hypodermis.

• Commonly found on thighs, hips, and gluteal area.

• Factors include genetics, sex, adipose tissue amount, and distribution.

Topical Medications

• Lipid-based substances can penetrate the superficial layers of the epidermis, allowing for topical absorption.

• Polar compounds cannot pass through the epidermis, making topical use safe with minimal risk of systemic absorption.

• Nonpolar substances can easily cross the epidermis, useful for medications like hormones in patches.

Summary Table: Layers of the Epidermis

Layer	Cell Type	Main Function	Location
Stratum basale	Cuboidal/columnar keratinocytes, melanocytes, Merkel cells	Cell division, new cell formation	Deepest layer
Stratum spinosum	Keratinocytes, Langerhans cells	Strength, flexibility, immune defense	Above basale
Stratum granulosum	Keratinocytes with granules	Water barrier, keratin production	Middle layer
Stratum lucidum	Dead keratinocytes	Extra protection	Only in thick skin
Stratum corneum	Dead, flattened keratinocytes	Protection, waterproofing	Outermost layer

Key Terms and Definitions

• Keratin: A fibrous protein that strengthens the skin and makes it resistant to mechanical stress.

• Melanin: Pigment produced by melanocytes, responsible for skin color and UV protection.

• Desmosomes: Intercellular junctions that hold keratinocytes together.

• Apoptosis: Programmed cell death, important for cell turnover in the epidermis.

Important Equations

• Vitamin D Synthesis: $\text{7-dehydrocholesterol} \xrightarrow{UV} \text{cholecalciferol (Vitamin D}_3\text{)} \xrightarrow{liver, kidney} \text{calcitriol}$

Mnemonic for Epidermal Layers

• Mnemonic: "Brilliant Studying Gives Loads of Confidence" for: Basale, Spinosum, Granulosum, Lucidum, Corneum.

Additional info:

• Cellulite is not considered a disease but a normal skin condition.

• Thickness of the epidermis is maintained by mitosis in the stratum basale and spinosum.

• Accessory structures are derived from the epidermis and include hair and nails.