Integumentary System

Introduction to the Integumentary System

• The integument is the largest system of the body, accounting for:

  • 16% of body weight

  • Approximately 20 ft² in area

• Composed of two main parts:

  • Cutaneous membrane (skin)

  • Epidermis: Outer layer (epithelium)

  • Dermis: Underlying connective tissues

  • Accessory structures:

  • Hair

  • Nails

  • Exocrine glands (sweat glands, sebaceous glands, etc.)

Functions of the Integumentary System

• Protection:

  • Provides a barrier against mechanical stress, abrasion, and chemicals

  • Keratin production safeguards against abrasion and fluid loss

  • Serves as the first line of defense against microorganisms and skin cancers through:

  • Tight junctions resistant to microbial invasion

  • Dendritic (Langerhans) cells and dermal macrophages

  • Low pH and bactericidal skin secretions

  • Melanin production protects against UV damage

  • Excretion of water, salts, and organic wastes

• Regulation of body temperature (thermoregulation)

  • Dermal blood vessels hold 8-10% of the body’s total blood flow

• Sensory detection

  • Senses touch, pressure, pain, vibration, and temperature stimuli

  • Sensory information conveyed to the nervous system

• Synthesis of Vitamin D3 (cholecalciferol)

  • Needed for calcium and phosphate absorption in the small intestine

• Storage of lipids in dermis and underlying connective tissues

Skin Anatomy

1. Skin (cutaneous membrane) consists of:

   • Epidermis:

     • Superficial stratified squamous epithelium

     • Avascular; nutrients diffuse from dermal capillaries

     • Contains epidermal ridges over dermal papillae

   • Dermis:

     • Underlying connective tissue

     • Contains dermal papillae that increase surface area for layer attachment

Layers of the Epidermis

• Stratum Corneum:

  • Composed of 15-30 layers of keratinized cells

  • Keratinocytes remain here for about 2 weeks before shedding

  • Forms a water-resistant barrier

• Stratum Lucidum:

  • Present only in thick skin, composed of dead cells filled with keratin

• Stratum Granulosum:

  • 3-5 layers of flattened keratinocytes

  • Produces keratin and keratohyaline; cells undergo apoptosis

• Stratum Spinosum:

  • 8-10 cell layers; keratinocytes proliferate

  • Contains dendritic cells for immune defense

• Stratum Basale:

  • Single layer of basal cells, secured to the basement membrane

  • Site of mitotic activity and melanin production

  • Interlocks with dermal papillae for strength and unique fingerprints

Skin Color and Melanin

• Melanin is produced by melanocytes in the stratum basale:

  • Transferred to keratinocytes; affects skin pigmentation

  • Dark skin: More melanosome transfer in superficial layers

  • Light skin: Transfer occurs deeper

  • Albinism: Absence or deficiency of melanin production

  • Freckles: Areas of increased melanin production due to UV exposure

Blood Supply to the Skin

• Supplied by two horizontal plexuses:

  • Superficial vascular plexus just below dermis-epidermis junction

  • Deep vascular plexus at dermis-hypodermis interface

  • Interconnected vessels running vertically promote nutrient exchange

Dermis Composition

1. Papillary Layer:

   • Areolar connective tissue with loosely woven fibers

   • Contains dermal papillae for sensory receptors and blood vessel loops

2. Reticular Layer:

   • Dense irregular connective tissue; predominant collagen bundles

   • Provides strength, resiliency, and extensibility (stretch)

   • Major contribution (80%) to skin’s thickness

   • Orientation of fibers defines cleavage lines for surgical incisions

Skin Appendages

• Sebaceous glands:

  • Function holocrine; secrete sebum to lubricate skin/hair

• Sweat glands:

  • Apocrine: Develops at puberty, located in specific body areas (e.g., axilla)

  • Eccrine: Predominantly found on palms, soles, forehead; produces visible sweat

• Hair follicles: Wrapped in connective tissue, associated with sebaceous glands and arrector pili muscles

• Nails: Protect tips of fingers/toes; composed of dead cells filled with keratin

Wound Healing

• Epidermal wound healing phases:

  1. Inflammatory Phase: Immediate response; bleeding and inflammation

  2. Migratory Phase: Scab formation; cells of stratum basale migrate

  3. Proliferation Phase: Epidermal cells proliferate, covering underlying tissue

  4. Scarring Phase: Synthesis of fibroblast collagen, completion of epithelium

Burns Overview

• First Degree: Affect surface of epidermis (e.g., sunburn)

• Second Degree: Involve entire epidermis and part of the dermis

• Third Degree: Full thickness burn affecting epidermis and dermis; may require grafting

• Fourth Degree: Extends through skin to underlying muscle/bone, life-threatening due to loss of protective functions.