Ischaemic and hypoxic cell injury: reversible vs irreversible, coagulative necrosis

Definition / Overview

Ischaemic and hypoxic cell injury represents one of the most clinically significant and frequently examined forms of cell damage in anatomical pathology. Ischaemia refers to inadequate tissue perfusion resulting in both oxygen deprivation and failure to deliver metabolic substrates (glucose) while simultaneously preventing removal of metabolic waste products. Hypoxia is the broader term encompassing reduced oxygen availability from any cause, including anaemia, respiratory failure, or histotoxic mechanisms (e.g. cyanide poisoning), without necessarily implying reduced perfusion.

The distinction matters diagnostically: pure hypoxia (without ischaemia) allows continued glucose delivery and partial glycolytic ATP generation, making it somewhat less injurious than complete ischaemia at equivalent oxygen levels.

The cellular response to ischaemia/hypoxia follows a predictable temporal sequence: functional impairment precedes biochemical derangement, which precedes ultrastructural change, which precedes light-microscopic change, which precedes gross morphological change. This hierarchy is fundamental to understanding why early infarcts may appear histologically normal.

Pathophysiology and Mechanisms

ATP Depletion: The Central Event

Oxygen deprivation halts mitochondrial oxidative phosphorylation. ATP levels fall rapidly. The consequences cascade:

• Failure of the Na$^+$/K$^+$-ATPase pump: sodium accumulates intracellularly, water follows osmotically, and the cell swells (hydropic change / oncosis).
• Failure of the Ca$^{2+}$-ATPase pump: cytosolic calcium rises, activating destructive phospholipases, proteases (including calpains), endonucleases, and ATPases.
• Anaerobic glycolysis: generates lactic acid, reducing intracellular pH; this inhibits enzymatic function and causes chromatin clumping.
• Ribosomal detachment from the endoplasmic reticulum (ER): reduces protein synthesis.
• Lipid accumulation: in metabolically active organs (liver, myocardium), disruption of lipoprotein synthesis leads to triglyceride accumulation (fatty change).

The Point of No Return: Irreversibility

The transition from reversible to irreversible injury is defined by two critical events:

1. Severe mitochondrial dysfunction: the inner mitochondrial membrane develops high-conductance permeability transition pores (mPTP). This uncouples oxidative phosphorylation irreversibly and triggers release of pro-apoptotic factors including cytochrome c.
2. Plasma membrane disruption: loss of structural integrity allows uncontrolled ion flux and leakage of intracellular contents (enzymes, proteins) into the extracellular space, triggering inflammation.

Lysosomal membrane rupture releases hydrolytic enzymes (cathepsins) into the cytoplasm, contributing to autodigestion.

Reperfusion Injury

Restoration of blood flow to ischaemic tissue paradoxically worsens injury through:

• Generation of reactive oxygen species (ROS) from activated neutrophils and dysfunctional mitochondria
• Calcium overload of reversibly injured cells (blood delivers calcium to cells with already-compromised Ca$^{2+}$ handling)
• Neutrophil-mediated microvascular plugging and endothelial injury
• Complement activation

Reperfusion injury is clinically relevant in myocardial infarction, stroke, and transplantation. Morphologically, contraction band necrosis (hypercontracted sarcomeres with dense eosinophilic transverse bands) is the hallmark of reperfusion in myocardium and distinguishes reperfused from non-reperfused infarcts.

Reversible Injury: Morphological Features

Gross Appearance

Early reversible injury produces subtle or no gross changes. Affected organs may show pallor, slight increase in weight, and increased turgor due to cellular oedema.

Light Microscopy

The term hydropic change (also called oncosis or vacuolar degeneration) describes the pale, swollen appearance. In renal proximal tubular epithelium, this is a classic early ischaemic finding.

Ultrastructural Features (Electron Microscopy)

• Plasma membrane blebbing, loss of microvilli
• Mitochondrial swelling with small, amorphous (flocculent) densities in the matrix
• ER dilation with polyribosome detachment
• Nuclear chromatin margination
• Accumulation of myelin figures (whorled phospholipid membranes derived from damaged cellular membranes)

These changes are reversible if the injurious stimulus is removed promptly.

Irreversible Injury and Necrosis

Defining Features of Irreversibility

The morphological hallmarks of irreversibility at ultrastructural level are:

• Large, flocculent (amorphous) densities in mitochondria: represent precipitated calcium and denatured proteins; indicate mPTP opening
• Plasma membrane disruption: loss of selective permeability
• Lysosomal swelling and rupture

At light microscopy, irreversible injury manifests as necrosis.

Coagulative Necrosis: Definition and Mechanism

Coagulative necrosis is the predominant pattern following ischaemia in most solid organs (heart, kidney, spleen, adrenal). It is characterised by:

• Preservation of the architectural ghost of the tissue (cell outlines, tubular or acinar structures remain recognisable)
• Denaturation of structural proteins and enzymes, which paradoxically inhibits proteolysis and preserves tissue architecture
• Loss of nuclear staining (karyolysis, pyknosis, karyorrhexis)
• Intense cytoplasmic eosinophilia

The term "coagulative" reflects the protein coagulation (denaturation) that maintains structural scaffolding, analogous to heat-fixing tissue.

Exception: the brain undergoes liquefactive necrosis following ischaemia because of its high lipid content and abundant hydrolytic enzymes from microglia/macrophages, which liquefy the tissue rather than preserve architecture.

Nuclear Changes in Necrosis

These changes occur sequentially and are not specific to any single type of necrosis.

Other Patterns of Necrosis (Differential Context)

Temporal Evolution of Myocardial Infarction: A Diagnostic Prototype

Myocardial infarction is the canonical model for ischaemic coagulative necrosis and is a high-yield topic for both Part 1 and Part 2 examinations.

Exam pitfall: at 0-4 hours post-infarction, the myocardium may appear entirely normal on routine H&E. Triphenyltetrazolium chloride (TTC) staining of fresh tissue (used in autopsy/research) demonstrates viable myocardium as magenta and infarcted tissue as pale/white, exploiting intact dehydrogenase enzyme activity.

Contraction band necrosis: hypercontracted sarcomeres with dense eosinophilic transverse bands; seen in reperfused infarcts, catecholamine excess (phaeochromocytoma), and resuscitation injury. Distinguishes reperfused from non-reperfused infarcts on histology.

Necrosis vs Apoptosis: Diagnostic Distinction

Apoptosis can coexist with necrosis in ischaemic injury, particularly at the border zone of infarcts and following reperfusion (cytochrome c release activates caspase-9 via the intrinsic pathway).

Organ-Specific Diagnostic Considerations

Kidney

• Proximal tubular epithelium is most vulnerable to ischaemia (high metabolic demand, limited glycolytic capacity)
• Early: hydropic change with loss of brush border (microvilli) on PAS stain
• Established: coagulative necrosis of tubular epithelium with preserved basement membranes (facilitates regeneration)
• Severe/prolonged ischaemia: cortical necrosis (glomeruli and tubules both necrotic); irreversible renal failure

Liver

• Zone 3 (centrilobular) hepatocytes are most susceptible to ischaemia (furthest from portal blood supply)
• Fatty change is a prominent reversible feature in toxic/metabolic injury
• Coagulative necrosis in ischaemia; liquefactive tendency with superimposed infection

Brain

• Neurons are exquisitely sensitive; irreversible injury within 3-5 minutes of complete ischaemia
• Selective neuronal necrosis (red neurons: shrunken, hypereosinophilic, pyknotic nuclei) precedes pan-necrosis
• Established infarct: liquefactive necrosis with cystic cavity formation; reactive astrogliosis at margins

Biomarkers of Irreversible Cell Injury: Clinico-Pathological Correlation

When plasma membranes rupture, intracellular enzymes and proteins leak into the circulation. These form the basis of serum biomarkers:

The pathologist should correlate histological necrosis with clinical enzyme profiles, particularly at autopsy where timing of infarction is relevant to cause-of-death determination.

Complications and Special Considerations

Reperfusion and Its Morphological Consequences

• Contraction band necrosis as described above
• Haemorrhagic transformation of infarcts (particularly cerebral and intestinal)
• Accelerated neutrophil infiltration

Hypoxia-Inducible Factor-1 (HIF-1) Response

Cells mount an adaptive transcriptional response to hypoxia via HIF-1$\alpha$ stabilisation (normally degraded by prolyl hydroxylases under normoxia via VHL-mediated ubiquitination). HIF-1 target genes include VEGF (angiogenesis), erythropoietin, and glycolytic enzymes. This pathway is relevant to tumour biology (VHL mutation in clear cell renal cell carcinoma constitutively activates HIF-1 signalling) and to understanding why some tissues tolerate hypoxia better than others.

Frozen Section and Intraoperative Considerations

• Ischaemic tissue at frozen section may show artefactual changes mimicking necrosis; correlation with clinical context is essential
• Autolysis (post-mortem or warm ischaemia in resection specimens) produces changes superficially resembling coagulative necrosis: nuclear loss, cytoplasmic eosinophilia; distinguished by clinical context, absence of inflammatory response, and uniform distribution

Autopsy Pathology

• TTC staining of fresh cardiac slices: standard technique for macroscopic delineation of acute infarcts at autopsy
• Histological confirmation required; early infarcts ($<$4-6 hours) may require ancillary techniques or clinical correlation
• Documentation of infarct age, extent, and complications (rupture, mural thrombus, papillary muscle necrosis) is required for cause-of-death certification

Key Exam Points

• Reversible injury = hydropic swelling, fatty change, mitochondrial swelling with small amorphous densities, ER dilation, membrane blebbing; all potentially reversible if stimulus removed
• Irreversible injury = large flocculent mitochondrial densities, plasma membrane disruption, lysosomal rupture; leads to necrosis
• Coagulative necrosis = ghost architecture preserved; most ischaemic organs; exception is brain (liquefactive)
• Contraction band necrosis = reperfusion marker in myocardium
• Myocardial infarction timeline: histologically normal at $<$4-6 hours; neutrophils by 12-24 hours; macrophages by day 3-7; granulation tissue by day 7-10; scar complete by $>$2 months
• Necrosis vs apoptosis: necrosis is passive, inflammatory, affects groups; apoptosis is active, non-inflammatory, affects individual cells
• Biomarker leak reflects plasma membrane disruption (irreversible injury threshold)
• Autolysis mimics necrosis: absence of inflammatory response and clinical context distinguish them