The Integumentary System

Overview

The integumentary system is composed of the skin and its accessory structures, including hair, nails, and glands. As the largest organ system, it plays a critical role in protection, sensation, thermoregulation, excretion, and vitamin D synthesis.

Skin Structure

Main Components

• Skin (Cutaneous Membrane): Composed of two main layers:

  • Epidermis: Superficial, avascular layer of keratinized stratified squamous epithelium resting on a basement membrane.

  • Dermis: Deep, vascularized layer of loose and dense irregular connective tissue.

• Accessory Structures: Hair, nails, sweat glands, sebaceous glands, sensory neurons, and arrector pili muscles.

• Hypodermis (Subcutaneous Tissue): Not part of the skin; consists of loose connective and adipose tissue, highly vascularized.

Functions of the Integumentary System

Protection

• Mechanical Trauma: Keratinized epithelium provides a durable, flexible barrier against stretching, pressure, and abrasion.

• Pathogen Defense: Continuous skin structure, immune cells, and antimicrobial secretions (acid mantle) deter pathogen growth.

• Environmental Protection: Absorbs UV radiation, secretes lipids to repel water and salts, preventing dehydration or overhydration.

Sensation

• Contains sensory receptors that detect touch, temperature, pain, and other stimuli, crucial for homeostasis.

Thermoregulation

• Maintains stable internal temperature via negative feedback loops.

• When body temperature rises: Thermoreceptors signal the hypothalamus, which triggers sweating and vasodilation to release heat.

• When body temperature falls: Thermoreceptors signal the hypothalamus, which triggers vasoconstriction to conserve heat.

Excretion

• Eliminates small amounts of metabolic waste (e.g., lactic acid, urea) in sweat.

Vitamin D Synthesis

• UV exposure converts a precursor in the epidermis to cholecalciferol, which is modified by the liver and kidneys to form calcitriol (active vitamin D), essential for calcium absorption.

The Epidermis

Cell Types

• Keratinocytes: 95% of epidermal cells; produce keratin for strength and water resistance; linked by desmosomes.

• Dendritic (Langerhans) Cells: Immune phagocytes in the stratum spinosum.

• Tactile (Merkel) Cells: Sensory receptors for light touch in the stratum basale.

• Melanocytes: Produce melanin pigment in the stratum basale.

Layers (Strata) of the Epidermis

• Stratum Basale (Germinativum): Deepest, single layer of stem cells; mitotically active; produces keratin and vitamin D precursors.

• Stratum Spinosum: Thickest layer; spiky cells; active in keratin and vitamin D production.

• Stratum Granulosum: 3–5 rows; contains keratin and membrane-coating granules; forms water barrier.

• Stratum Lucidum: Clear, dead keratinocytes; only in thick skin (palms, soles).

• Stratum Corneum: Multiple layers of dead, flattened keratinocytes; cells are shed (sloughed) daily.

Mnemonic for Epidermal Layers

• Brilliant – Stratum Basale

• Studying – Stratum Spinosum

• Gives – Stratum Granulosum

• Loads of – Stratum Lucidum

• Confidence – Stratum Corneum

Keratinocyte Life Cycle

• Cells divide in the stratum basale/spinosum, are pushed upward, and are eventually shed from the stratum corneum (cycle lasts 40–50 days).

Thick vs. Thin Skin

• Thick Skin: All five strata; found on palms, soles; lacks hair follicles; many sweat glands.

• Thin Skin: Lacks stratum lucidum; thinner stratum corneum; contains hair follicles, sweat, and sebaceous glands.

• Callus: Localized thickening of stratum corneum due to repeated pressure.

The Dermis

Layers of the Dermis

• Papillary Layer: Superficial 20%; loose connective tissue; contains dermal papillae (blood supply, tactile corpuscles).

• Reticular Layer: Deep 80%; dense irregular connective tissue; contains blood vessels, glands, hair follicles, lamellated corpuscles (deep pressure/vibration).

Skin Markings

• Dermal Ridges: Prominent in thick skin; form epidermal ridges (fingerprints).

• Tension (Cleavage) Lines: Gaps between collagen bundles; important for surgical incisions.

• Flexure Lines: Deep creases in areas like palms.

Pigmentation and Skin Color

Melanin

• Produced by melanocytes in the stratum basale; stored in melanosomes.

• Protects DNA from UV radiation; must be continually replaced.

• UV exposure increases melanin synthesis (immediate darkening and delayed production).

• Regulates vitamin D synthesis to maintain homeostasis.

Other Pigments

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue in fair-skinned individuals.

Skin Color as a Diagnostic Tool

• Erythema: Increased blood flow; red appearance.

• Pallor: Decreased blood flow; pale appearance.

• Cyanosis: Low oxygen; bluish color.

• Jaundice: High bilirubin; yellow-orange color.

Accessory Structures

Hair

• Functions in protection (e.g., UV, foreign objects) and sensation.

• Structure: Shaft (dead cells), root (living cells), follicle, arrector pili muscle, bulb, and papilla (blood supply).

• Growth: Alternates between growth and resting stages; alopecia is hair loss.

• Pigment: Determined by melanin; changes with age.

Nails

• Protect fingertips; tools for manipulation.

• Structure: Nail plate (body and root), nail bed, matrix, folds, cuticle, lunula.

• Growth: Continuous; lunula is the opaque region with more keratin.

Glands

• Eccrine Sweat Glands: Most common; thermoregulation; watery secretion via pores.

• Apocrine Sweat Glands: Axillae, anal, areolae; thick, protein-rich secretion onto hair follicles; odor develops after bacterial metabolism.

• Ceruminous Glands: Modified apocrine glands; produce ear wax (cerumen).

• Mammary Glands: Modified sweat glands; produce milk.

• Sebaceous Glands: Secrete sebum (oily, hydrophobic barrier) via holocrine secretion; increases at puberty.

Clinical Aspects

Burns

• First-Degree: Only epidermis; erythema, minor pain, no blisters.

• Second-Degree: Epidermis and part/all of dermis; pain, blisters, possible scarring.

• Third-Degree: Full thickness; all skin layers and possibly deeper; not initially painful due to nerve damage; risk of dehydration and infection; may require grafting.

Rule of Nines

• Used to estimate burn surface area for fluid replacement; body divided into regions representing 9% each (genital area is 1%).

Skin Cancer

• Basal Cell Carcinoma: Most common; arises from stratum basale; rarely metastasizes.

• Squamous Cell Carcinoma: From stratum spinosum; may metastasize; forms scaly plaques.

• Malignant Melanoma: Most dangerous; from melanocytes; high metastatic potential; identified by ABCDE rule (Asymmetry, Border, Color, Diameter, Evolving).

Summary Table: Epidermal Layers

Additional info:

• Cellulite is a normal anatomical variation, not a disease.

• Topical medications can be absorbed through the skin if they are nonpolar; this is also a route for some toxins.

• Wrinkles result from decreased collagen, elastic fibers, and adipose tissue; accelerated by UV exposure and smoking.

• Acne is caused by sebum and dead cell accumulation in sebaceous glands, often influenced by hormones.