A 32-year-old man sustains a laceration to his forearm during a motor vehicle accident. Histologic examination of the wound at day 7 post-injury shows granulation tissue with prominent fibroblasts, new blood vessels, and sparse inflammatory cells. Regarding the molecular regulation of this phase, all of the following are correct EXCEPT:

Explanation

## Molecular Regulation of the Proliferative Phase (Day 3–21) ### Clinical Context At day 7 post-injury, the wound is in the **proliferative phase**, characterized by active fibroblast recruitment, collagen deposition, and angiogenesis. Multiple growth factors orchestrate these events in a temporal and spatial sequence. ### Analysis of Growth Factor Roles | Growth Factor | Phase(s) | Primary Role | Accuracy | |---------------|----------|--------------|----------| | **FGF** | Proliferative, Remodeling | Fibroblast proliferation, migration, angiogenesis | ✓ Correct | | **PDGF** | Inflammatory, Early Proliferative | Chemotaxis, fibroblast recruitment; **NOT dominant throughout** | ✗ INCORRECT | | **VEGF** | Proliferative, Remodeling | Angiogenesis (via HIF-1α in hypoxia) | ✓ Correct | | **TGF-β** | All phases | Fibroblast recruitment, collagen synthesis, angiogenesis | ✓ Correct | | **MMP** | Proliferative, Remodeling | ECM degradation, cell migration | ✓ Correct | ### Key Point: **PDGF is NOT the dominant growth factor throughout wound healing.** PDGF is released from platelet α-granules **immediately after injury** (hemostasis/early inflammation) and is crucial for **early fibroblast and macrophage recruitment**. However, by the proliferative phase, **TGF-β and VEGF** become the dominant regulators. The dominance shifts temporally. ### High-Yield: **Temporal sequence of growth factor dominance:** 1. **Hemostasis/Early Inflammation (0–3 days):** PDGF, Thrombin, Fibrin 2. **Late Inflammation/Early Proliferation (2–7 days):** TGF-β, VEGF, FGF 3. **Proliferation/Remodeling (7+ days):** TGF-β, FGF, VEGF ### HIF-1α and Hypoxia Mechanism The wound microenvironment is hypoxic (PO₂ ~10–20 mmHg). HIF-1α is stabilized under hypoxia and transactivates genes encoding **VEGF, FGF, PDGF, and erythropoietin**, driving angiogenesis and fibroblast function. ### MMP Regulation MMPs (especially MMP-2 and MMP-9) degrade the provisional fibrin matrix and allow fibroblast infiltration. They are upregulated by TGF-β and hypoxia during proliferation and remodeling. ### Clinical Pearl: In diabetic wounds, impaired HIF-1α signaling and reduced VEGF expression contribute to poor angiogenesis and delayed healing. Topical growth factor therapy (e.g., PDGF-based products like becaplermin) is used to accelerate healing in chronic wounds.