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PA4.1 | Healing & Repair — Regeneration, Granulation Tissue, Wound Healing — SDL Guide (Part 2)

Growth Factors and ECM — The Molecular Orchestra

Diagram showing how VEGF, PDGF, FGF-2, and TGF-beta from inflammatory and repair cells coordinate angiogenesis, fibroblast recruitment, collagen deposition, and fibrosis during wound healing. — **Panel A:** Wound bed overview showing fibrin clot, macrophages, platelets, hypoxic cells, endothelial cells, fibroblasts, smooth muscle cells, capillary sprouts, collagen-rich ECM, and color-coded VEGF, PDGF, FGF-2, and TGF-beta signalling arrows.. **Panel B:** VEGF released by macrophages and hypoxic cells causing angiogenesis through endothelial proliferation and migration.. **Panel C:** PDGF released by platelets, macrophages, and endothelium recruiting fibroblasts and smooth muscle cells and initiating repair signalling.. **Panel D:** FGF-2 released by macrophages, mast cells, and endothelium promoting angiogenesis and fibroblast proliferation.. **Panel E:** TGF-beta released by platelets, macrophages, and fibroblasts activating fibroblasts, increasing collagen synthesis, reducing inflammation, and driving fibrosis..

Four growth factors deserve explicit memory at Year-2 level:

Growth factor	Source	Primary role in healing
VEGF (Vascular Endothelial Growth Factor)	Macrophages, hypoxic cells	Angiogenesis — endothelial proliferation and migration
PDGF (Platelet-Derived Growth Factor)	Platelets, macrophages, endothelium	Fibroblast + smooth-muscle recruitment; initiates repair signalling
FGF (Fibroblast Growth Factor, esp. FGF-2)	Macrophages, mast cells, endothelium	Angiogenesis + fibroblast proliferation
TGF-β (Transforming Growth Factor-beta)	Platelets, macrophages, fibroblasts	Fibroblast activation, collagen synthesis, anti-inflammatory; chief driver of fibrosis

ECM roles: fibronectin provides provisional scaffold and promotes cell migration; hyaluronan retains water, resists compression early on; proteoglycans (e.g., decorin) regulate collagen fibrillogenesis; type III collagen provides early tensile strength, replaced by type I in remodelling.

Overarching concept: TGF-β is the master fibrogenic cytokine. Its unchecked activity drives pathological fibrosis in liver (cirrhosis), lung, and kidney.

SELF-CHECK

Which growth factor is the primary driver of angiogenesis during granulation tissue formation?

A. TGF-β

B. VEGF

C. PDGF

D. FGF-2

Reveal Answer

Answer: B. VEGF

VEGF (Vascular Endothelial Growth Factor) is the dominant angiogenic signal, produced by macrophages and hypoxic cells. It drives endothelial proliferation and capillary sprouting. PDGF recruits fibroblasts; TGF-β drives collagen synthesis; FGF-2 supports angiogenesis but is secondary to VEGF.

Wound Healing by Primary Intention

Primary intention (healing by first intention) applies to clean, surgically apposed wounds (e.g., a sutured surgical incision):

Wound edges are close together — minimal tissue defect
Inflammatory response is brief and localised
Granulation tissue volume is small
Epithelial coverage occurs within 24-48 hours (epithelial bridge across incision)
Collagen deposition is organised and aligned
Minimal wound contraction
End result: thin, hairline scar with rapid functional restoration

Timeline (skin incision, primary closure):

Time	Event
0-24 h	Fibrin clot; neutrophil influx
24-48 h	Macrophage arrival; epithelial cells migrate across incision
Day 3-5	Granulation tissue; fibroblast proliferation begins
Week 1	Collagen bridges; sutures removed (~50-60% tensile strength)
Week 4+	Remodelling; scar pales
Month 3	~70-80% original tensile strength (maximum achievable)

Diagram showing primary intention wound healing in a sutured clean incision, progressing from fibrin clot and inflammation to epithelial bridging, collagen deposition, remodelling, and a thin hairline scar. — **Panel A:** Clean sutured incision with closely apposed wound edges, epidermis, dermis, subcutaneous tissue, sutures, fibrin clot, epithelial bridge, small granulation tissue, organised collagen fibres, minimal tissue defect, minimal wound contraction.. **Panel B:** Early phase timeline: 0-24 h fibrin clot and neutrophil influx; 24-48 h macrophage arrival and epithelial cell migration across the incision.. **Panel C:** Proliferative phase: Day 3-5 granulation tissue, fibroblast proliferation, capillary buds, collagen deposition; Week 1 collagen bridge, suture removal, 50-60% tensile strength.. **Panel D:** Remodelling phase: Week 4 scar paling and collagen reorganisation; Month 3 thin hairline scar with approximately 70-80% original tensile strength..

Wound Healing by Secondary Intention

A four-panel medical diagram explains secondary intention wound healing, showing an open wound filled by granulation tissue, comparison with primary intention, myofibroblast contraction, and clinical contracture risk. — **Panel A:** Large open wound, separated epidermal edges, necrotic debris, inflammatory cells, macrophages, abundant granulation tissue, new capillaries, fibroblasts, collagen deposition, wound contraction arrows.. **Panel B:** Primary intention with approximated wound edges, minimal granulation tissue, small scar; secondary intention with large tissue defect, more inflammation, abundant granulation tissue, contraction, larger scar.. **Panel C:** TGF-beta stimulation, fibroblast, myofibroblast, alpha-smooth muscle actin stress fibers, contractile force arrows, inward movement of wound edges.. **Panel D:** Healthy pink granular wound bed, maturation to pale firm scar, normal wound contraction, cicatricial contracture across flexor joint, restricted movement..

Secondary intention (healing by second intention) occurs when wounds have:
• Large tissue defects that cannot be approximated (e.g., burn, abscess cavity, pressure ulcer)
• Infected wounds
• Wounds left open deliberately

Key differences from primary intention:

More inflammation — extensive necrotic debris must be phagocytosed
Abundant granulation tissue — fills the large defect from the base upward
Wound contraction — a critical mechanism unique to large secondary wounds

Wound contraction is mediated by myofibroblasts — fibroblasts that acquire smooth-muscle-like actin filaments (α-smooth muscle actin, α-SMA) under TGF-β stimulation. They generate contractile force, drawing wound edges inward. This reduces the surface area requiring scar formation but can cause contracture if it overshoots.

Larger, wider scar — more collagen deposited; scar may be hypertrophic

Clinical teaching point: The pink, granular, raised tissue filling an open wound bed IS healthy granulation tissue doing its job. The transition to pale, firm scar is maturation — not deterioration.

CLINICAL PEARL

Wound contraction vs cicatricial contracture — know the difference:

Wound contraction = normal, beneficial process during healing of large open wounds; myofibroblasts pull edges inward, reducing the wound area.

Contracture = a complication that occurs when wound contraction deforms a joint, digit, eyelid, or other functional structure. Burns across flexor surfaces of joints are the classic example. Prevention: early skin grafting, splinting in position of function, physiotherapy.

SELF-CHECK

A 45-year-old woman undergoes mastectomy. The wound is large and left to heal by secondary intention. Which cell type is primarily responsible for wound contraction?

A. Macrophages secreting TGF-β

B. Mast cells releasing histamine

C. Myofibroblasts expressing α-smooth muscle actin

D. Endothelial cells forming new capillaries

Reveal Answer

Answer: C. Myofibroblasts expressing α-smooth muscle actin

Myofibroblasts — fibroblasts induced by TGF-β to express α-smooth muscle actin — are the contractile effectors of wound contraction. They physically shorten, pulling wound edges toward the centre. Macrophages produce TGF-β (the inducer) but do not contract directly. Mast cells and endothelial cells play no direct role in contraction.