Fibrosis and Repair

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Overview of Tissue Repair - Healing's ABCs

  • Goal: Restore tissue structure & function post-injury.
  • Two Paths:
    • Regeneration: Complete replacement by original cells; no scar.
    • Repair (Scarring): Fibrous tissue fills defect if regeneration fails.
  • Cell Proliferative Potential:
    • Labile: Always cycling (e.g., skin, gut, marrow).
    • Stable: Quiescent (G0), can divide (e.g., liver, kidney).
    • Permanent: No significant regeneration (e.g., neurons, cardiac muscle).
  • Outcome: Depends on injury severity & tissue's regenerative capacity.

Wound healing process: inflammation, granulation, scar

⭐ Cardiac muscle, being composed of permanent cells, typically heals by fibrosis (scar formation) after an infarction, not by regeneration of myocytes.

Repair Mechanisms & Angiogenesis - New Pipes & Scaffolds

  • Repair Phases:
    • Inflammation: Clears debris, initiates repair.
    • Proliferation: Granulation tissue formation, angiogenesis, re-epithelialization.
    • Remodeling: ECM deposition, scar maturation.
  • Granulation Tissue: Hallmark of healing.
    • Fibroblasts (collagen synthesis).
    • New thin-walled capillaries (angiogenesis).
    • Loose ECM.
    • Inflammatory cells (macrophages).
  • Angiogenesis (New Vessel Formation):
    • Key growth factors: VEGF, FGF-2.
    • Steps: Vasodilation & ↑permeability → Proteolytic degradation of BM → Endothelial cell migration & proliferation → Tube formation → Maturation.

Angiogenesis steps diagram

VEGF (Vascular Endothelial Growth Factor) is the most important growth factor in angiogenesis, stimulated by hypoxia via HIF-1α.

  • ECM Deposition & Remodeling:
    • TGF-β: Potent fibrogenic agent; stimulates collagen, fibronectin, proteoglycan synthesis.
    • MMPs (Matrix Metalloproteinases): Degrade ECM, crucial for remodeling. Balanced by TIMPs (Tissue Inhibitors of Metalloproteinases).

Wound Healing & Scars - Skin's Patchwork

  • First Intention (Primary Union): Clean, incised, apposed edges (e.g., surgical). Minimal granulation, fibrin clot, rapid re-epithelialization, less scarring.
  • Second Intention (Secondary Union): Large defects, infected wounds, ulcers. More intense inflammation, abundant granulation tissue, significant wound contraction (myofibroblasts), larger, more prominent scar.
  • Phases of Healing:
  • Scar Strength: Reaches ~70-80% of normal skin by 3 months; Type I collagen dominates.

⭐ Hypertrophic scars remain within wound boundaries; Keloids (📌 KEEPS growing) extend beyond, are often familial, more common in darker skin, and rich in Type III collagen. Healing by first and second intention

Pathologic Fibrosis - Scars Gone Wild

Aberrant wound healing: excessive collagen & ECM deposition leading to tissue scarring and organ dysfunction. Caused by chronic inflammation or persistent injury.

  • Key Cell: Myofibroblast (activated fibroblast) - primary collagen producer.
  • Key Mediator: TGF-β (Transforming Growth Factor-beta) - most potent fibrogenic cytokine.
    • Promotes fibroblast migration, proliferation, & collagen synthesis.
    • Inhibits ECM breakdown (↓MMPs, ↑TIMPs).
  • Consequences: Organ failure (e.g., cirrhosis, pulmonary fibrosis), constrictures, keloids.

⭐ TGF-β is the principal profibrogenic cytokine, pivotal in fibrosis development across nearly all tissues.

Keloid Histology and Appearance

Healing Factors & Flops - Fixer-Uppers & Fails

  • Systemic Factors:
    • Promote: Nutrition (Vit C, protein, zinc), adequate perfusion.
    • Impair: Diabetes, glucocorticoids, poor circulation, advanced age.
  • Local Factors:
    • Promote: Good blood supply, no infection, immobilization.
    • Impair: Infection (major delay!), foreign bodies, ischemia, movement.
  • Complications (Flops):
    • Deficient Scar: Dehiscence (rupture), ulceration.
    • Excessive Scar: Hypertrophic (raised, within borders), Keloid (grows beyond; 📌 Keloid Klimbs).
    • Contractures: Limits mobility (e.g., post-burn).

⭐ Keloids show haphazard collagen (Type I & III); common in African descent, earlobes, sternum.

High‑Yield Points - ⚡ Biggest Takeaways

  • Granulation tissue formation, with fibroblasts, angiogenesis, and macrophages, is absolutely crucial for repair.
  • TGF-β is the most potent fibrogenic cytokine, strongly stimulating collagen production.
  • Myofibroblasts are key cells for effective wound contraction.
  • Type III collagen (early) is replaced by Type I collagen (late, for strength).
  • Keloids and hypertrophic scars result from excessive collagen deposition.
  • Infection, diabetes, and steroids are major systemic inhibitors of wound healing.

Practice Questions: Fibrosis and Repair

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Wound contraction can be most effectively minimized by:

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Flashcards: Fibrosis and Repair

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The most common mechanism of vascular leakage in acute inflammation is the _____ of endothelial cells

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The most common mechanism of vascular leakage in acute inflammation is the _____ of endothelial cells

contraction

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