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PA1.1-3,PA2.1-2 | Introduction to Pathology & Mechanisms of Cell Injury — SDL Guide (Part 3)

Reversible Cell Injury — Morphology and Function

Comparison diagram of reversible cell injury showing hydropic cellular swelling, fatty change in hepatocytes, reversibility, and functional consequences. — **Panel A:** Cellular hydropic swelling; enlarged cells; pale vacuolated cytoplasm; dilated ER cisternae; swollen mitochondria; pale turgid enlarged organ; 400x H&E.. **Panel B:** Fatty change / steatosis; hepatocytes; clear lipid vacuoles; macrovesicular fat droplet; peripheral nucleus; microvesicular droplets; yellow greasy enlarged liver; 400x H&E.. **Top banner:** Normal cell; injurious stimulus; reversible injury; stimulus removed; recovery possible.. **Bottom strip:** Decreased ATP; decreased protein synthesis; decreased membrane ion transport; cellular swelling..

Reversible cell injury is injury in which the cell can return to normal if the stimulus is removed. The key morphological hallmarks are:

1. Cellular (hydropic) swelling
- The most common and earliest change.
- Gross: organ appears pale, turgid, increased weight.
- Microscopy: cells enlarged, cytoplasm pale and vacuolated; organelle changes visible on EM — ER cisternae dilated, mitochondria swollen with small amorphous densities.
- Reversible if the injurious stimulus is removed promptly.

2. Fatty change (steatosis)
- Occurs predominantly in cells with active lipid metabolism: hepatocytes, cardiac myocytes, renal tubular cells.
- Mechanism: impaired lipoprotein export (↓apolipoprotein synthesis), increased fatty-acid mobilisation from adipose, impaired β-oxidation.
- Gross: liver appears yellow, greasy, enlarged (fatty liver).
- Microscopy: clear cytoplasmic vacuoles (lipid dissolved out during processing); large droplets displace the nucleus to periphery (macrovesicular) or small droplets (microvesicular).

Side-by-side H&E histology comparison showing hydropic cellular swelling on the left and fatty change in hepatocytes with lipid vacuoles displacing nuclei on the right. — **Panel A:** Hydropic change with swollen cells, pale vacuolated cytoplasm, intact central nucleus, softened cell borders, narrowed intercellular spaces, and 400x magnification marker.. **Panel B:** Fatty change in hepatocytes with clear lipid vacuoles, peripherally displaced nuclei, hepatocyte plates, sinusoids, microvesicular droplets, and 400x magnification marker.. **Shared inset:** Functional correlates of reversible injury: decreased protein synthesis, impaired membrane transport, altered ion gradients, and recovery possible if injury stops..

Functional correlates: during reversible injury, cells show ↓protein synthesis, ↓membrane transport, altered ion gradients — but these recover. The cell has not yet activated irreversible death pathways.

Irreversible Cell Injury — The Point of No Return

A sequential pathology diagram compares normal, reversible, and irreversible cell injury, highlighting mitochondrial flocculent densities, membrane rupture, nuclear changes, calcium influx, lysosomal rupture, and progression to necrosis. — **Panel A:** Normal cell with intact plasma membrane, normal nucleus, healthy mitochondria, rough ER, lysosome, and normal ATP/ion balance.. **Panel B:** Reversible injury with cellular swelling, continuous membrane blebs, dilated rough ER, ribosome detachment, swollen mitochondria without flocculent densities, and reversible ATP depletion.. **Panel C:** Irreversible injury with severe plasma membrane disruption, organelle membrane damage, mitochondrial permeability transition, large flocculent mitochondrial densities, massive Ca2+ influx, lysosomal rupture, pyknosis, karyorrhexis, karyolysis, and enzyme leakage.. **Panel D:** Biochemical cascade showing severe/prolonged injury leading to irreversible mitochondrial dysfunction, irreversible ATP depletion, sustained phospholipase/protease activation, autolytic enzyme release, membrane disruption, necrosis, and inflammatory response, with necrosis contrasted against apoptosis..

Irreversible cell injury occurs when damage is severe or prolonged enough that the cell cannot recover even if the injurious stimulus is removed. Distinguishing features from reversible injury:

Morphological hallmarks of irreversibility:
- Severe membrane damage — large discontinuities in plasma and organelle membranes.
- Mitochondrial permeability transition (MPT) with large, flocculent (amorphous) densities in the mitochondrial matrix — the most reliable early EM marker of irreversibility.
- Nuclear changes: pyknosis (condensation), karyorrhexis (fragmentation), karyolysis (dissolution).

Biochemical triggers of the point of no return:
1. Inability to reverse mitochondrial dysfunction → irreversible ↓ATP.
2. Massive Ca²⁺ influx → sustained phospholipase/protease activation.
3. Lysosomal membrane rupture → autolytic enzyme release.

The end result is necrosis — a pathological cell death characterised by enzymatic digestion of cellular contents, membrane disruption, and inflammatory response (contrast with apoptosis, which is regulated and non-inflammatory).

Three-panel comparison showing a normal cell, reversible cell injury with swelling and dilated ER, and irreversible cell injury with mitochondrial flocculent densities, membrane disruption, and nuclear pyknosis. — **Panel A:** Normal plasma membrane, normal mitochondria, rough endoplasmic reticulum with attached ribosomes, normal nucleus, balanced ion gradients. **Panel B:** Cellular swelling, cytoplasmic vacuolation, dilated endoplasmic reticulum, detached ribosomes, mild mitochondrial swelling, intact plasma membrane, ATP depletion, Na+/K+ pump failure, Na+ and water influx, reduced protein synthesis, lactic acidosis. **Panel C:** Plasma membrane disruption, mitochondrial flocculent densities, mitochondrial permeability transition, nuclear pyknosis, lysosomal rupture, Ca2+ influx, irreversible ATP loss, membrane phospholipid damage, enzyme leakage.

Clinical translation: the window between reversible and irreversible injury is time-sensitive. In myocardial ischaemia, irreversible cardiomyocyte injury begins at ~20–30 minutes of sustained ischaemia; in the brain, neurons begin to die within 4–6 minutes. This is why time is muscle and time is brain are clinical mantras.

SELF-CHECK

Which ultrastructural finding is considered the most reliable early marker of irreversible cell injury?

A. Dilatation of the endoplasmic reticulum

B. Detachment of ribosomes from rough ER

C. Large amorphous (flocculent) densities in mitochondrial matrix

D. Cytoplasmic vacuolation with increased cell volume

Reveal Answer

Answer: C. Large amorphous (flocculent) densities in mitochondrial matrix

Large amorphous (flocculent) densities within the mitochondrial matrix — corresponding to the mitochondrial permeability transition and precipitated proteins — are the hallmark EM marker of irreversible injury. Options A, B, and D (ER dilatation, ribosome detachment, cytoplasmic vacuolation/swelling) are all features of reversible injury and do not by themselves indicate that the cell has crossed the point of no return.

Ischaemia-Reperfusion Injury

Flow diagram showing how ischaemia primes tissue through hypoxanthine accumulation and enzyme conversion, while reperfusion triggers ROS, neutrophil activation, calcium overload, mitochondrial permeability transition, microvascular plugging, and clinical injury. — **Panel A:** Ischaemic phase showing low O2, ATP depletion, ATP catabolism to hypoxanthine, xanthine dehydrogenase conversion to xanthine oxidase, early Ca2+ influx, and vulnerable parenchymal cell.. **Panel B:** Reperfusion phase showing sudden O2 delivery, xanthine oxidase-mediated hypoxanthine conversion to xanthine, ROS burst with O2•− and H2O2, neutrophil adhesion and respiratory burst, protease release, reverse-mode Na+/Ca2+ exchanger, Ca2+ overload, mitochondrial permeability transition pore opening, mitochondrial swelling, and cell death.. **Panel C:** Consequences and clinical relevance showing complement activation, platelet aggregation, endothelial swelling, microvascular plugging/no-reflow, myocardial IR after PCI with arrhythmias, cerebral IR after thrombolysis, organ transplantation IR injury with early graft dysfunction, and therapeutic targets including allopurinol, NAC/antioxidants, Ca2+ overload reduction, and MPT inhibition..

Ischaemia-reperfusion (IR) injury is the paradox at the heart of the opening case: restoring blood flow to an ischaemic tissue can itself cause additional cell death beyond what ischaemia alone produced.

Why?

During ischaemia:
- Xanthine dehydrogenase is converted to xanthine oxidase.
- ATP catabolism produces hypoxanthine.

At reperfusion:
- Sudden O₂ delivery → xanthine oxidase converts hypoxanthine → xanthine + massive ROS burst (O₂•⁻ and H₂O₂).
- Simultaneously: (a) neutrophil infiltration (activated by ROS and complement) → respiratory burst adds more ROS + proteases; (b) Ca²⁺ overload (reverse-mode Na⁺/Ca²⁺ exchanger); (c) mitochondrial permeability transition (MPT) triggered by Ca²⁺ + ROS together; (d) complement and platelet activation → microvascular plugging.

Flow diagram showing ischaemic xanthine oxidase priming and calcium influx followed by reperfusion ROS burst, MPT pore opening, neutrophil activation, and microvascular plugging with heart, brain, and transplant examples. — **Panel A:** Main ischaemia-to-reperfusion pathway: ATP depletion, xanthine dehydrogenase to xanthine oxidase conversion, hypoxanthine accumulation, Ca2+ influx, oxygen re-entry, ROS burst, MPT pore opening, neutrophil activation, endothelial swelling, microvascular plugging, no-reflow, necrosis/apoptosis.. **Panel B:** Clinical examples: heart showing myocardial ischaemia-reperfusion after PCI with no-reflow and arrhythmias; brain showing cerebral ischaemia-reperfusion after thrombolysis in stroke; transplanted kidney/graft showing early graft dysfunction.. **Panel C:** Therapeutic targets: allopurinol inhibiting xanthine oxidase, NAC/antioxidants reducing ROS, ischaemic preconditioning, hypothermia slowing MPT kinetics, cyclosporin A inhibiting cyclophilin D and blocking MPT pore.. **Panel D:** Clinical pearl: allopurinol before cardiac surgery to blunt xanthine oxidase-mediated ROS burst when cardiopulmonary bypass is discontinued..

Clinical examples:
- Myocardial IR: post-PCI no-reflow phenomenon; reperfusion arrhythmias.
- Cerebral IR: after thrombolysis in stroke.
- Organ transplantation: donor organ IR injury is a major cause of early graft dysfunction.

Therapeutic strategies targeting IR injury: antioxidants (NAC, allopurinol — inhibits xanthine oxidase), ischaemic preconditioning, hypothermia (slows MPT kinetics), cyclosporin A (inhibits cyclophilin D → blocks MPT pore).

CLINICAL PEARL

Allopurinol before cardiac surgery: some centres administer allopurinol pre-operatively to inhibit xanthine oxidase and blunt the reperfusion ROS burst when cardiopulmonary bypass is discontinued. This is a direct clinical application of the xanthine oxidase mechanism of IR injury — a pathway you can now trace from biochemistry to the operating theatre.

SELF-CHECK

A kidney is successfully transplanted after 18 hours of cold ischaemia. On reperfusion, the transplant team observes sluggish urine output and rising creatinine over the next 24 hours. The primary mechanism responsible is:

A. Acute T-cell mediated rejection against donor HLA antigens

B. Sudden ROS burst from xanthine oxidase activation causing tubular cell injury

C. Calcineurin inhibitor nephrotoxicity from cyclosporin induction

D. Hyperacute rejection from preformed antibodies

Reveal Answer

Answer: B. Sudden ROS burst from xanthine oxidase activation causing tubular cell injury

In ischaemia-reperfusion injury, the xanthine oxidase pathway generates a massive ROS burst on reperfusion, damaging tubular cells and endothelium — producing 'delayed graft function.' This is a biochemical, not immunological, mechanism. Acute T-cell rejection (A) typically manifests days–weeks later. Calcineurin inhibitor toxicity (C) is dose-dependent and later onset. Hyperacute rejection (D) would occur within minutes and is pre-screened by cross-matching.