The Integumentary System: Structure, Function, and Components

The Integumentary System

Introduction

The integumentary system is composed of the skin and its derivatives, including sweat glands, oil glands, hair, and nails. It serves as the primary barrier between the body and the external environment, providing protection, sensation, and various physiological functions.

Structure of the Skin

Major Regions of the Skin

• Epidermis: The outermost layer, composed of keratinized stratified squamous epithelium. It is avascular and provides a tough, protective barrier.

• Dermis: The deeper layer, made of connective tissue (CT), which is vascularized and contains nerves, glands, and hair follicles.

• Hypodermis (Subcutaneous Tissue): Not technically part of the skin, but closely associated. Composed of areolar and adipose tissue, it anchors the skin to underlying structures and provides insulation and shock absorption.

Cells of the Epidermis

• Keratinocytes: The most abundant cell type; produce keratin, a protein that gives skin its strength and waterproofing. Lifespan is approximately 25-45 days. Their proliferation is stimulated by epidermal growth factor.

• Melanocytes: Located in the deepest layer of the epidermis; produce melanin, the pigment responsible for skin color and protection against UV radiation. Melanosomes are transferred to adjacent keratinocytes.

• Dendritic (Langerhans) Cells: Star-shaped immune cells that originate from bone marrow; act as macrophages to activate the immune system.

• Tactile (Merkel) Cells: Located at the epidermal-dermal junction; associated with sensory nerve endings and function as touch receptors.

Layers of the Epidermis

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

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

• Stratum Granulosum: Cells contain granules of keratohyalin and lamellar granules; involved in keratin formation and waterproofing.

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

• Stratum Basale: Deepest layer; a single row of stem cells that undergo mitosis. Contains melanocytes and Merkel cells.

Thick Skin vs. Thin Skin: Thick skin (palms, soles) has all five layers, including the stratum lucidum; thin skin lacks the stratum lucidum.

Layers of the Dermis

• Papillary Layer: Superficial; composed of areolar connective tissue with collagen and elastic fibers. Contains dermal papillae, which form friction ridges (fingerprints) and house capillaries and sensory receptors.

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

Hypodermis (Subcutaneous Tissue)

• Composed of areolar and adipose tissue.

• Anchors skin to underlying structures, allows skin to slide, acts as a shock absorber, and insulates the body.

• Stores fat and contains blood vessels.

Skin Pigments

• Melanin: Only pigment produced in the skin; derived from the amino acid tyrosine. Color varies from reddish-yellow to brownish-black, depending on type and amount. Protects against UV damage.

• Carotene: Yellow to orange pigment from plant products (e.g., carrots); accumulates in the stratum corneum and hypodermis.

• Hemoglobin: Oxygenated pigment in red blood cells; gives a pinkish hue to fair skin.

Cyanosis: A bluish discoloration of the skin due to low oxygenation of hemoglobin.

Accessory Structures of the Skin

Hair

• Functions: Senses insects, protects against trauma, heat loss, and sunlight, shields eyes, and filters inhaled particles.

• Structure: Composed of hard keratin. The shaft (visible part) has three layers: medulla (core), cortex (middle, contains pigment), and cuticle (outer, overlapping cells).

• Hair Texture: Determined by follicle shape (round = straight, oval = wavy, flat = curly) and size (thicker follicles = thicker hair).

• Hair Follicle: Extends from the epidermis into the dermis; contains the hair bulb, papilla, and root hair plexus (sensory nerve endings).

• Arrector Pili Muscle: Smooth muscle that contracts to raise hair (goosebumps).

• Sebaceous Gland: Associated with hair follicles; secretes sebum for lubrication and waterproofing.

• Hair Growth: Occurs in cycles (growth, regression, rest); affected by hormones and nutrition. Age and genetics influence hair thinning and baldness.

Nails

• Protective, scale-like modifications of the epidermis.

• Structure: Free edge, body, root, nail folds (lateral and proximal), lunula (crescent-shaped area), nail bed, and nail matrix (growth zone).

• Eponychium: Also known as the cuticle; protects the area between the skin and the nail plate.

Glands

• Sudoriferous (Sweat) Glands: Two main types:

  • Eccrine Glands: Most numerous; found on palms, soles, and forehead. Simple coiled tubular glands that secrete sweat (99% water, salts, vitamin C, antibodies, metabolic wastes) for thermoregulation.

  • Apocrine Glands: Found in axillary and anogenital areas; larger, ducts empty into hair follicles. Secrete sweat plus fatty substances and proteins, which are odorless until decomposed by bacteria (body odor). May function as sexual scent glands.

• Other Glands: Ceruminous glands (produce earwax), mammary glands (produce milk).

• Sebaceous (Oil) Glands: Holocrine glands that secrete sebum; lubricates and waterproofs skin and hair, has bactericidal properties.

Functions of the Skin

• Protection: Acts as a physical, chemical, and biological barrier.

  • Chemical: Acid mantle (low pH), dermcidin, and melanin protect against bacteria and UV radiation.

  • Physical: Keratinized cells provide waterproofing and resistance to trauma.

  • Biological: Dendritic cells and macrophages provide immune defense.

• Temperature Regulation: Sweat glands and blood vessel dilation/constriction help maintain body temperature. Sweat output can range from 0.5 to 12 liters per day.

• Cutaneous Sensation: Sensory receptors detect touch, pressure, pain, and temperature.

• Metabolic Functions: Synthesis of vitamin D, activation of some hormones, and conversion of carcinogens.

• Blood Reservoir: The dermis can hold about 5% of the body's blood volume.

• Excretion: Sweat eliminates ammonia, urea, and uric acid.

Wound Healing

• Occurs in natural phases: inflammation, new vessel and tissue formation, and tissue remodeling.

• Skin is capable of self-repair after injury, involving both the epidermis and dermis.

Burns and the Rule of Nines

• Burns: Damage caused by heat, electricity, radiation, or chemicals; can denature proteins and kill cells.

• Degrees of Burns:

  • First-degree: Only epidermis is damaged.

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

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

• Rule of Nines: Used to estimate the percentage of body surface area affected by burns.

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

Example: Application of the Rule of Nines

If a patient has burns on the entire left arm and the anterior trunk, the affected area would be 9% (one arm) + 18% (anterior trunk) = 27% of the total body surface area.

Additional info: Some details, such as the precise structure of the nail and the full process of wound healing, were expanded for academic completeness.