Skin Anatomy and Wound Healing

Snapshot

• A 25-year-old African American woman presents to her dermatologist for a bothersome scar. She reports that a few months ago, she injured her arm after falling off a bike. Since then, her cut has healed and a raise flesh-colored scar has grown over the wound. It is often itchy or even painful. On physical exam, she has a 7 cm raised scar with irregular borders that extends beyond the original wound. (Keloid)

Overview

• Skin is the largest organ of the body
• Functions
  • photoprotection
  • barrier formation
  • homeostasis
  • thermoregulation
  • immunologic protection
• Skin is made of the epidermis and dermis

 Anatomy

• Epidermis
  • layers from top to bottom
    • stratum Corneum
      • serves as a physiologic barrier from
        • chemical and microbiologic invasion
        • fluid and solute loss
    • stratum Lucidum
    • stratum Granulosum
    • stratum Spinosum
    • stratum Basale
      • separates the epidermis and dermis
    • “Californians Like Girls in String Bikinis”
  • major cells
    • keratinocytes
      • mature from the stratum basale to the stratum corneum over the course of 4 weeks
    • melanocytes
      • produces melanin, which protects the skin from ultraviolet radiation
    • Langerhan cells
      • immune cells that present antigens
    • Merkel cells
      • contain neuroendocrine peptides
• Dermis
  • layers from top to bottom
    • papillary dermis
    • reticular dermis
  • functions as structural support and provides nutrition via the vasculature
  • contains subdermal epithelial structures 
    • sebaceous glands
    • apocrine glands
    • hair follicles
• nerves

Normal Wound Healing

• Wound disrupts structure and function of epidermis
• Activates keratinocytes, fibroblasts, endothelial cells, platelets, and macrophages
• Hemostasis and inflammatory phase (0-3 days)
  • clot forms with platelet aggregation, trigger of clotting cascade, and small vessel constriction
  • mast cells release histamine and other vasodilators to increase permeability in vasculature
  • neutrophils and other inflammatory cells migrate into tissue
    • edema or swelling is seen clinically
  • macrophages clear debris
• Proliferative phase (3 days to weeks)
  • fibroblasts
    • deposits glycoprotein and mucopolysaccharides
    • produce myofibroblasts
      • myofibroblasts help to contract the wound to pull the edges together
    • synthesize collagen (type III)
      • the resulting collagen matrix stimulates angiogenesis
  • keratinocytes re-epithelialize the wound 
  • the resulting collagen deposition and angiogenesis form granulation tissue
  • abnormal wound healing in this phase 
    • deficiency in vitamin C or copper can result in delayed wound healing
    • keloids and hypertrophic scars may result from persistent myofibroblasts or excessive matrix synthesis
• Remodeling (weeks to months)
  • type III collagen is replaced by type I collagen, resulting in increased tensile strength of the tissue
    • hypertrophic type 1 collagen deposition can result in formation of a contracture 
  • vessels mature
  • remodeling is mediated by fibroblasts
  • abnormal wound healing in this phase
• deficiency in zinc can result in delayed wound healing

Wound Closure Intentions

• Primary intention
  • wound edges are re-approximated
  • faster healing and less scarring
• Secondary intention
  • wounds are left open and allowed to granulate and fill in over time
• Tertiary intention (delayed closure)
  • wounds are left open and observed
  • wounds are then approximated and closed later