## Irreversible Injury: Hallmark Ultrastructural Changes **Key Point:** Myelin figures (whorled phospholipid masses) and amorphous densities in mitochondria are pathognomonic ultrastructural features of irreversible cell injury and represent the point of no return. ### Mechanism of Irreversible Injury Once a cell sustains irreversible injury, the following occur: 1. **Mitochondrial dysfunction** — loss of oxidative phosphorylation capacity 2. **Membrane disruption** — phospholipid degradation and loss of selective permeability 3. **Myelin figure formation** — concentric whorls of phospholipid membranes accumulate in cytoplasm and lysosomes 4. **Amorphous densities** — calcium-phosphate precipitates in mitochondrial matrix ### Reversible vs Irreversible: Key Distinctions | Feature | Reversible Injury | Irreversible Injury | |---------|-------------------|--------------------| | **Cell membrane** | Intact, blebbing reversible | Disrupted, myelin figures present | | **Mitochondria** | Swollen, cristae intact | Amorphous densities, calcium deposits | | **ATP production** | Reduced but recoverable | Completely lost | | **Nuclear changes** | Pyknosis (reversible) | Karyorrhexis, karyolysis | | **Outcome** | Recovery possible with stimulus removal | Cell death inevitable | **High-Yield:** Myelin figures and amorphous mitochondrial densities are the **earliest ultrastructural markers** of irreversibility — once seen on electron microscopy, the cell cannot recover even if the injurious stimulus is removed. **Clinical Pearl:** In acute myocardial infarction, irreversible injury develops within 20–40 minutes of complete ischemia; myelin figures appear within this window and signal irreversible necrosis. **Mnemonic: MAID** — **M**yelin figures, **A**morphous densities, **I**rreversible injury, **D**eath inevitable.
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