Morphological chnges of mi
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Morphological Changes of Myocardial Infarction (MI)
The morphological changes of MI follow a predictable, time-dependent sequence progressing from reversible ischemic injury through coagulative necrosis, inflammation, and finally fibrous scar formation. The primary mechanism of cell death is ischemic coagulative necrosis (though apoptosis also occurs). - Robbins, Cotran & Kumar Pathologic Basis of Disease
Progression of Necrosis Through the Wall
Necrosis begins in the subendocardial zone - the region most distal from epicardial vessels and exposed to the highest intramural pressures. Without intervention, a "wavefront" of necrosis expands outward, and within 3-6 hours the full infarct extent is reached (transmural infarct). A narrow rim immediately beneath the endocardium is spared because it can be oxygenated by diffusion from the ventricular cavity.
Timeline of Morphological Changes
The classic table (Table 12.5, Robbins & Cotran) is the standard reference:
Phase 1: Reversible Injury
| Time | Gross | Light Microscope | Electron Microscope |
|---|---|---|---|
| 0 - 0.5 hr | None | None | Relaxation of myofibrils; glycogen loss; mitochondrial swelling |
Phase 2: Irreversible Injury - Early (0.5 hr - 3 days)
| Time | Gross Features | Light Microscope (H&E) | EM |
|---|---|---|---|
| 0.5 - 4 hr | None | Usually none; variable waviness of fibers at border | Sarcolemmal disruption; mitochondrial amorphous densities |
| 4 - 12 hr | Occasionally dark mottling | Coagulative necrosis beginning; wavy fibers; early contraction band necrosis; early neutrophilic infiltrate | - |
| 12 - 24 hr | Dark mottling | Coagulative necrosis; pyknosis of nuclei; cytoplasmic hypereosinophilia; marginal contraction band necrosis; interstitial edema; neutrophilic infiltrate begins | - |
| 1 - 3 days | Mottling with yellow-tan infarct center | Coagulative necrosis with loss of nuclei and striations; brisk neutrophilic infiltrate in interstitium | - |
Key microscopic hallmarks of this phase:
- Wavy fibers - the stretching and buckling of dead, non-contractile fibers pulled by adjacent viable contracting myocardium
- Cytoplasmic hypereosinophilia - loss of cytoplasmic RNA → brighter pink on H&E
- Nuclear pyknosis, karyorrhexis, then karyolysis - progressive nuclear death
- Contraction bands - dense transverse bands of hypercontracted sarcomeres seen especially at borders and with reperfusion
Phase 3: Progressive Necrosis and Early Repair (3 days - 3 weeks)
| Time | Gross Features | Light Microscope |
|---|---|---|
| 3 - 7 days | Hyperemic border; central yellow-tan softening | Macrophage infiltration begins (replacing neutrophils); beginning disintegration of dead myofibers; granulation tissue starts at margins |
| 7 - 10 days | Maximally yellow-tan and soft; depressed borders | Well-developed granulation tissue with neovascularization at margins; progressive loss of necrotic debris |
| 10 - 14 days | Red-gray depressed infarct borders | Established granulation tissue (macrophages + fibroblasts + new vessels); collagen begins depositing |
| 2 - 8 weeks | Gray-white fibrous tissue | Fibrosis progressively replaces granulation tissue; collagen deposition advancing |
Phase 4: Healed Scar (>2 months)
| Time | Gross | Histology |
|---|---|---|
| >2 months | Firm, white fibrous scar | Dense collagenous scar - pale, acellular, avascular; no viable myocytes |
Early Detection Techniques
Because MIs less than 12 hours old are often not visible on gross examination alone, a histochemical trick is used: Triphenyl tetrazolium chloride (TTC) stain. Intact myocardium with preserved lactate dehydrogenase (LDH) stains brick-red; infarcted tissue (where LDH has leaked out) appears as an unstained pale zone. This test works if the infarct is at least 2-3 hours old. - Robbins, Cotran & Kumar
Gross Appearance Summary
| Time | Gross Appearance |
|---|---|
| 0 - 12 hr | No visible change (TTC needed) |
| 12 - 24 hr | Reddish-blue discoloration (congestion, extravasated blood) |
| 1 - 3 days | Dark mottling; yellow-tan center begins |
| 3 - 7 days | Yellow-tan, soft, surrounded by hyperemic border |
| 1 - 3 weeks | Yellow-tan, soft, depressed; granulation tissue at edges |
| >2 months | Gray-white firm fibrous scar |
Special Feature: Reperfusion Injury Morphology
When blood flow is restored (by thrombolysis or PCI), reperfused infarcts show additional/modified changes:
- Contraction band necrosis is greatly accentuated - a hallmark of reperfused MI
- Mitochondria show large amorphous densities
- Cell membranes are disrupted
- Large hemorrhagic areas due to vascular damage from returning blood
- Despite this, reperfusion is still beneficial as it limits overall infarct size
Complications Visible Morphologically
| Complication | Timing | Morphology |
|---|---|---|
| Fibrinous pericarditis | 2-3 days | Roughened, dark epicardial surface over infarct |
| Ventricular free wall rupture | 3-7 days | Tear through softened necrotic wall |
| Septal rupture | 3-7 days | Perforation of interventricular septum |
| Papillary muscle rupture | 3-5 days | Necrotic papillary muscle head detaches |
| Mural thrombus | Days to weeks | Thrombus overlying endocardium of infarct zone |
| Ventricular aneurysm | Weeks to months | Outward bulging of thinned fibrous scar |
Sources:
- Robbins, Cotran & Kumar Pathologic Basis of Disease (Table 12.5 - Evolution of Morphologic Changes in MI)
- Robbins & Kumar Basic Pathology (Chapter 9: Myocardial Response to Ischemia)
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