Tissue Response To Injury

Question 1

Describe the sequence of events involved in healing by first intention. What factors influence wound healing?

Answer 1

1. Healing by first intention
   A. Inflammation
   - Immediate: Formation of blood clot
   - 24 hours: Neutrophils infiltrate margins of clot
   - 24 - 48 hours: Edema caused by leaky blood vessels, wound closure by spurs of epithelial cells
   B. Proliferation
   - Day 5: Granulation tissue formed by fibroblast, neutrophils replaced by macrophages
   - Week 2: Scar formation - Fibroblast proliferation, collagen accumulation, leukocytic infiltration, 10% of strength
   - 1st month: Scar devoid of inflammatory infiltrates, covered by intact epithelium
   C. Maturation
   - Wound contraction
   - Connective tissue remodelling
   - Takes 3 months for wound to regain 70 - 80% of strength
2. Factors influencing wound healing
   - Local factors: Wound size, location, type (incision vs blunt trauma), motion of wound, infection, foreign body
   - Systemic factors: Malnutrition, circulatory/vascular supply, chronic disease (DM), metabolic status, age, drugs, hormonal state (steroids)

Question 2

How do skin wounds recover tensile strength? What is the approximate time frame for recovery of tensile strength in skin wounds?

Answer 2

1. Wound strength increases due to
   - Increase synthesis of type 1 collagen
   - Decrease collagen degradation
   - Connective tissue remodelling -> structural modification of collagen by increasing cross-linkage and fibres size
2. Time for recovery of tensile strength
   - 1 - 2 weeks: 10% tensile strength
   - Gradually increases in the next 3 weeks
   - 3 months: 70 - 80% tensile strength
   - But never 100% recovery

Question 3

Describe the phases of cutaneous wound healing. What is wound contraction?

Answer 3

1. 3 phases
   - Inflammation phase:
   + Immediate: Blood clot formation
   + 24 hours: Neutrophils infiltration into margin
   + 24 - 48hours: Edema, wound closure by spurs of epithelial cells
   - Proliferation phase:
   + Day 5: Granulation tissue by fibroblasts, neutrophils replaced by macrophages
   + Week 2: Scar formation - Fibroblast proliferation, collagen accumulation, leukocytic infiltration, 10% tensile strength
   - Maturation phase:
   + Wound contraction
   + Connective tissue remodelling
   + Recovery of 70 - 80% tensile strength by 3 months
2. Wound contraction usually occurs in large wounds
   - Wound contracts to reduce gap between dermal edges
   - Reduce wound surface area
   - Due to network of myofibroblasts at wound edge

Question 4

What important effects does platelet-derived growth factor (PDGF) have in wound healing?

Answer 4

• Monocytes chemotaxis
• Fibroblast migration and proliferation
• Collagen synthesis
• Collagenase secretion