A 52-year-old man with acute myocardial infarction is brought to the emergency department 3 hours after symptom onset. Cardiac biopsy obtained during angiography shows coagulation necrosis with preserved architecture and early neutrophilic infiltration. Which histological feature best distinguishes this reversible injury phase from irreversible injury in the same tissue?

Explanation

## Distinguishing Reversible from Irreversible Myocardial Injury ### Key Morphological Differences **Key Point:** The sarcolemmal (cell membrane) integrity is the critical dividing line between reversible and irreversible injury. Once the membrane is breached, the cell cannot be salvaged. | Feature | Reversible Injury | Irreversible Injury | |---------|-------------------|---------------------| | **Sarcolemmal integrity** | Intact | Disrupted / breached | | **Mitochondrial changes** | Swelling (reversible) | Calcification, dense matrix densities | | **Nuclear changes** | Intact, normal staining | Pyknosis, karyorrhexis, karyolysis | | **Contraction bands** | Absent | Present (hallmark of reperfusion) | | **Timeline in MI** | 0–4 hours | >4 hours | | **Electron microscopy** | Mitochondrial swelling, glycogen loss | Myelin figures, flocculent densities | ### Why Sarcolemmal Integrity Matters **High-Yield:** Once the cell membrane ruptures, intracellular contents leak out, extracellular calcium floods in, and the cell cannot maintain ionic gradients. This is the **point of no return**. **Clinical Pearl:** In acute MI, the first 4–6 hours represent a window where reperfusion can salvage myocardium. After this, irreversible changes (contraction bands, coagulation necrosis with inflammation) dominate. ### Timeline in Acute Myocardial Infarction ```mermaid flowchart TD A["0–1 hour: Reversible injury<br/>Mitochondrial swelling<br/>Glycogen depletion"]:::action B["1–4 hours: Still reversible<br/>Sarcolemma intact<br/>Early coagulation necrosis"]:::action C["4–12 hours: Irreversible injury<br/>Sarcolemma breached<br/>Contraction bands appear"]:::urgent D["12–72 hours: Coagulation necrosis<br/>Neutrophilic infiltration<br/>Myelin figures"]:::outcome E["Days–weeks: Granulation tissue<br/>Fibrosis"]:::outcome A --> B B --> C C --> D D --> E ``` **Mnemonic:** **RIMI** = **R**eversible **I**njury = **M**itochondrial swelling + **I**ntact membrane. [cite:Robbins 10e Ch 1]