Paper II
Question
Gross and microscopic features of myocardial infarction
Answer
Gross features (correlate with time since infarction):
- 0–12 hours: Usually no visible change (occasionally mottled with tetrazolium stain reaction absent).
- 12–24 hours: Pallor, sometimes with a reddish-blue tinge (dark mottling due to trapped blood).
- 1–3 days: Infarct becomes sharply defined, yellow-tan, and soft.
- 3–7 days: Hyperaemic (red-tinged) border develops around a yellow, softened centre; maximal softening (“yellow softening”) — the period of greatest risk of rupture.
- 7–10 days: Maximally yellow and depressed, with a red-tan granulation tissue margin.
- 2–8 weeks: Progressive grey-white scarring from the border inward.
- >2 months: Complete, firm, grey-white fibrous scar.
Microscopic features (correlate with time):
- 0–4 hours: Usually normal (may show waviness of fibres at the periphery).
- 4–12 hours: Early coagulative necrosis, oedema, haemorrhage.
- 12–24 hours: Coagulative necrosis (nuclear pyknosis, hypereosinophilic cytoplasm), early neutrophilic infiltrate begins.
- 1–3 days: Coagulative necrosis with loss of nuclei and striations; extensive neutrophilic infiltration (peaks around day 2–3).
- 3–7 days: Beginning disintegration of dead myofibres, dying neutrophils, early macrophage infiltration at the border, phagocytosis of dead cells begins.
- 7–10 days: Well-developed phagocytosis of dead cells by macrophages; early formation of granulation tissue (fibroblasts, new capillaries) at the margins.
- 10 days–2 weeks: Well-established granulation tissue with new blood vessels and collagen deposition.
- 2–8 weeks: Progressive collagen deposition and increasing fibrosis.
- >2 months: Dense, well-formed collagenous scar, cellular in the initial stages, becoming progressively acellular.