The Integumentary System

Overview

The integumentary system is a complex organ system that serves as the body's primary barrier against the external environment. It consists of the skin, sweat glands, oil glands, hair, nails, and subcutaneous tissue.

• Skin: The largest organ, providing protection and sensory input.

• Sudoriferous (Sweat) Glands: Regulate temperature and excrete wastes.

• Sebaceous (Oil) Glands: Lubricate skin and hair.

• Hairs: Protect and sense environmental changes.

• Nails: Protect fingertips and enhance fine touch.

• Subcutaneous Tissue: Anchors skin, provides insulation and shock absorption.

Skin Structure

Layers of the Skin

The skin is composed of two main layers:

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

• Dermis: The deeper layer, composed mainly of dense connective tissue. It is vascular and houses nerves, glands, and hair follicles.

• Subcutaneous Tissue (Hypodermis): Not technically part of the skin, but closely associated. Composed mostly of adipose tissue, it anchors the skin, absorbs shock, and insulates.

The Epidermis

Cell Types

The epidermis contains four main cell types:

• Keratinocytes: Produce keratin, the protein that gives skin its strength and waterproofing.

• Melanocytes: Produce melanin, the pigment that protects against UV radiation.

• Dendritic (Langerhans) Cells: Immune cells that patrol for pathogens.

• Tactile Epithelial (Merkel) Cells: Sensory receptors for touch.

Layers of the Epidermis

The epidermis is organized into distinct strata:

• Stratum Basale: Deepest layer; contains stem cells and melanocytes. Site of active cell division.

• Stratum Spinosum: Several layers thick; contains keratinocytes and dendritic cells. Cells resist tension.

• Stratum Granulosum: Cells flatten and keratinization begins. Contains keratohyaline and lamellar granules.

• Stratum Lucidum: Present only in thick skin (palms, soles); consists of clear, dead keratinocytes.

• Stratum Corneum: Outermost layer; 20–30 rows of dead, keratinized cells. Provides barrier against environmental damage.

The Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer containing fibroblasts, macrophages, mast cells, and white blood cells. It provides structural support and houses blood vessels, nerves, glands, and hair follicles.

• Papillary Dermis: Superficial layer of areolar connective tissue. Contains dermal papillae, capillary loops, and touch receptors.

• Reticular Dermis: Deep layer of dense irregular connective tissue. Contains collagen and elastic fibers for strength and elasticity.

Dermal Modifications

• Friction Ridges: Formed by dermal papillae; enhance grip and create fingerprints.

• Cleavage (Tension) Lines: Parallel collagen fibers; important for surgical incisions.

• Flexure Lines: Dermal folds at joints; visible on hands and feet.

• Striae (Stretch Marks): Dermal tears from extreme stretching.

Skin Color

Pigments

Skin color is determined by three main pigments:

• Melanin: Produced by melanocytes; protects against UV damage. Varies from reddish yellow to brownish black.

• Carotene: Yellow to orange pigment; accumulates in stratum corneum and subcutaneous tissue.

• Hemoglobin: Gives pinkish hue to fair skin; visible when melanin levels are low.

Clinical Significance

Changes in skin color can indicate disease:

• Cyanosis: Blue tint; low oxygenation.

• Pallor: Pale; stress, anemia, or low blood pressure.

• Erythema: Redness; fever, inflammation, allergy.

• Jaundice: Yellow; liver disorder.

• Bruises: Blood leakage under skin.

Hair

Structure and Function

Hair is composed of dead, keratinized cells and serves protective and sensory functions.

• Regions:

  • Root: Within scalp; keratinization ongoing.

  • Shaft: Above scalp; keratinization complete.

• Parts of Hair Shaft:

  • Medulla: Central core.

  • Cortex: Surrounds medulla.

  • Cuticle: Outermost layer.

• Pigmentation: Melanocytes produce melanin; variations create different hair colors.

Hair Follicle

• Hair Bulb: Expanded area at deep end.

• Hair Papilla: Supplies nutrients.

• Hair Matrix: Actively dividing cells.

• Arrector Pili: Smooth muscle causing "goose bumps".

Types and Growth

• Vellus Hair: Fine, pale hair.

• Terminal Hair: Coarse, long hair (scalp, eyebrows, pubic regions).

• Growth Cycles: Follicles alternate between active and resting phases.

Nails

Structure and Function

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal phalanges and aid in fine touch.

• Root: Embedded in skin.

• Nail Plate/Body: Visible attached portion.

• Free Edge: Extends beyond finger/toe.

• Nail Matrix: Responsible for growth.

• Nail Folds: Skin overlapping nail borders.

• Cuticle (Eponychium): Protects nail body.

• Hyponychium: Under free edge; collects dirt.

Skin Appendages: Glands

Sweat Glands

• Eccrine (Merocrine) Glands: Most numerous; regulate temperature; secrete watery sweat.

• Apocrine Glands: Located in axillary and anogenital areas; secrete viscous sweat; responsible for body odor.

• Modified Apocrine Glands:

  • Ceruminous: Produce earwax.

  • Mammary: Produce milk.

Sebaceous (Oil) Glands

• Holocrine glands: Secrete sebum into hair follicles.

• Functions: Lubricate skin/hair, antibacterial properties.

• Inactive until puberty; stimulated by androgens.

Functions of Skin

Protection

The skin acts as a barrier through chemical, physical, and biological mechanisms.

• Chemical: Acid mantle, antimicrobial proteins, melanin.

• Physical: Keratinized cells, glycolipids.

• Biological: Dendritic cells and macrophages activate immune responses.

Body Temperature Regulation

• Insensible Perspiration: Baseline sweat production.

• Sensible Perspiration: Increased sweat for cooling.

• Vasoconstriction: Reduces heat loss in cold environments.

Cutaneous Sensations

• Touch: Meissner’s corpuscles and Merkel cells.

• Pressure: Pacinian corpuscles.

• Pain: Free nerve endings.

• Hair Movement: Hair follicle receptors.

Metabolic Functions

• Vitamin D Synthesis: Essential for calcium absorption.

• Activation of Hormones: Keratinocytes convert cortisone to hydrocortisone.

• Collagenase Production: Prevents wrinkles.

Blood Reservoir

• Holds up to 5% of blood volume.

• Can shunt blood to other organs as needed.

Excretion

• Removes nitrogenous wastes (ammonia, urea, uric acid).

• Loss of salt and water through sweat.

Skin Cancer and Burns

Skin Cancer

• Basal Cell Carcinoma: Most common, least malignant; arises from stratum basale.

• Squamous Cell Carcinoma: Arises from stratum spinosum; can metastasize.

• Melanoma: Cancer of melanocytes; highly metastatic and dangerous.

ABC(D) Rule for Early Detection

• A: Asymmetry

• B: Border irregularity

• C: Color variation

• D: Diameter > 6 mm

• E: Evolution (changes over time)

Burns

• First-degree: Epidermal damage only.

• Second-degree: Epidermal and upper dermal damage; blisters.

• Third-degree: Entire skin thickness; requires grafting.

Rule of Nines

Used to estimate fluid loss and burn severity.

• Body divided into 11 sections, each representing 9% of surface area.

• Critical burns: >25% second-degree, >10% third-degree, or third-degree on face/hands/feet.

Developmental Aspects of the Integumentary System

Embryonic Development

• Epidermis: Develops from ectoderm.

• Dermis and Subcutaneous Tissue: Develop from mesoderm.

• Lanugo: Fine hair covering fetus, replaced by vellus hair.

Infancy to Adulthood

• Vernix Caseosa: Protective substance from sebaceous glands.

• Mila: Small white spots on newborn skin.

• Skin thickens, gland activity increases, acne may occur.

Aging Skin

• Epidermal replacement slows; skin becomes thin, dry, and itchy.

• Subcutaneous fat and elasticity decrease; wrinkles and cold intolerance increase.

• Risk of cancer increases due to fewer melanocytes and dendritic cells.

• Hair thinning occurs.

Check Your Understanding

• Which layer of the skin – dermis or epidermis – is better nourished? Dermis, due to its vascularity.

• Layers pierced by a nail: Stratum corneum, stratum granulosum, stratum spinosum, stratum basale, papillary dermis.

• Major function of stratum basale: Cell division and regeneration.

• Importance of desmosomes and tight junctions: Maintain integrity and prevent water loss.

• Tissue types in dermis: Papillary (areolar), reticular (dense irregular connective tissue).

• Third pigment for skin color: Hemoglobin, found in blood.

• Alteration indicating liver disorder: Jaundice (yellow skin).

• Chemicals providing bacterial barriers: Sweat, sebum, defensins, acid mantle.

• Immunity-related epidermal cells: Dendritic (Langerhans) cells.

• Sunlight and bone health: Stimulates vitamin D synthesis for calcium absorption.

Additional info: Academic context and explanations have been expanded for clarity and completeness.