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Wound healing occurs in overlapping phases: hemostasis (platelet plug formation), acute inflammation (neutrophil and macrophage infiltration), proliferation (angiogenesis, collagen deposition by fibroblasts), and remodeling (collagen crosslinking and reorganization). The process involves coordinated signaling by growth factors (TGF-β, VEGF, FGF), cytokines, and extracellular matrix components. Chronic wounds and pathologic scars result from dysregulation of these phases.
Use skin wound healing as the paradigm, then explore differences in muscle, bone, and other tissues. Consider how age, nutrition, infection, and chronic diseases impair healing. Study the role of myofibroblasts.
Wound healing is not a linear sequence—the phases overlap significantly. Complete 'remodeling' takes months to years; initial tensile strength returns much faster than final tissue strength.
Your prerequisites give you the two anchors for understanding wound healing: cell injury and adaptation (what happens when tissue is damaged) and acute inflammation (the first-response system). Wound healing is what happens after those two processes have run their initial course — the body shifts from emergency control to reconstruction. Think of it as a building project with four overlapping contracts that share a timeline, not a strict sequence.
Phase 1 — Hemostasis begins within seconds of injury. Platelet activation triggers the clotting cascade, forming a fibrin clot that serves two functions simultaneously: it stops bleeding and creates a provisional scaffold that cells will use to migrate into the wound. The clot is not just a plug — it is a bioactive matrix releasing growth factors (PDGF, TGF-β) from degranulating platelets that recruit the next wave of cells.
Phase 2 — Acute inflammation (hours to days) brings neutrophils first, then macrophages. Neutrophils debride the wound — they kill bacteria and clear debris using proteases and reactive oxygen species. Macrophages arrive and shift the wound environment: they phagocytose neutrophils and switch from pro-inflammatory signaling to releasing growth factors (TGF-β, VEGF, FGF) that initiate repair. This macrophage phenotypic switch — from M1 (inflammatory) to M2 (reparative) — is a critical checkpoint. In chronic wounds (diabetic ulcers, pressure injuries), macrophages stay locked in the M1 state, releasing proteases that degrade the matrix faster than it is rebuilt.
Phase 3 — Proliferation (days to weeks) involves three parallel processes: angiogenesis (VEGF drives new capillary sprouting to supply the metabolically active wound), fibroplasia (fibroblasts migrate into the provisional matrix and deposit type III collagen, creating granulation tissue), and epithelialization (keratinocytes at the wound margins migrate under the scab to resurface the defect). Myofibroblasts — fibroblasts that have differentiated under TGF-β signaling and acquired contractile actin — begin pulling wound edges together, mechanically closing the defect.
Phase 4 — Remodeling (weeks to years) is the most underappreciated phase. The type III collagen deposited rapidly in phase 3 is mechanically weak. Matrix metalloproteinases (MMPs) break it down and replace it with stronger type I collagen in a cross-linked, aligned pattern. Tensile strength recovers slowly: a wound has roughly 50% of normal skin strength at one month and never fully recovers — mature scar reaches only about 80% of original strength even after years of remodeling. Keloids and hypertrophic scars represent failures of remodeling regulation, where myofibroblasts persist and continue depositing collagen beyond what is needed.