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PA2.4-5 | Cell Death — Necrosis, Apoptosis, Gangrene & Calcification — SDL Guide (Part 3)

Necroptosis and Pyroptosis — Emerging Cell Death Modes

Diagram comparing apoptosis with necroptosis and pyroptosis, highlighting membrane rupture, inflammatory mediator release, and key molecular mediators RIPK3, MLKL, NLRP3 inflammasome, gasdermin D, IL-1 beta, and IL-18. — **Panel A:** Apoptotic cell shrinkage, apoptotic bodies, intact plasma membrane, no inflammation, swollen inflammatory dying cell, membrane rupture, DAMP release, inflammatory mediators. **Panel B:** TNF receptor, Fas, toll-like receptor, caspase blockade, RIPK3, MLKL, plasma membrane disruption, necrosis-like swelling, membrane rupture. **Panel C:** Microbial products, danger signals, NLRP3 inflammasome, gasdermin D, gasdermin D pores, IL-1 beta, IL-18, DAMP release, cell swelling. **Panel D:** Necroptosis clinical links: ischaemia-reperfusion injury, Crohn disease, viral infections; pyroptosis clinical links: host defence, sepsis, autoinflammatory syndromes.

Two additional death programmes have clinical relevance in Year-2 pathology:

Necroptosis (programmed necrosis):
• Triggered by TNF receptor, Fas, or toll-like receptors when caspases are blocked (e.g., viral inhibition of caspases).
• Mediated by RIPK3 and MLKL (mixed-lineage kinase domain–like protein), which disrupt the plasma membrane.
• Morphology: necrosis-like (cell swelling, membrane rupture) but genetically regulated.
• Significance: contributes to inflammation in ischaemia-reperfusion injury, Crohn's disease, and some viral infections.

Pyroptosis (inflammatory programmed death):
• Triggered by intracellular pattern recognition (NLRP3 inflammasome) detecting microbial products or danger signals.
• Mediated by gasdermin D — forms pores in the plasma membrane.
• Releases large amounts of IL-1β and IL-18 — highly pro-inflammatory.
• Morphology: cell swelling, pore formation, DAMP and cytokine release.
• Significance: critical in host defence against intracellular pathogens; dysregulated in sepsis and autoinflammatory syndromes.

For Year-2 exams: you are expected to name and briefly define necroptosis and pyroptosis and state they cause inflammation (contrast with apoptosis). The mechanistic detail above is for understanding, not memorisation.

SELF-CHECK

Which of the following is the MOST accurate feature that distinguishes apoptosis from necrosis in a tissue section?

A. Presence of nuclear fragmentation

B. Increased eosinophilia of the cytoplasm

C. Loss of nuclear basophilia (karyolysis)

D. Absence of inflammatory infiltrate with single-cell death

Reveal Answer

Answer: D. Absence of inflammatory infiltrate with single-cell death

The combination of single-cell death WITHOUT an inflammatory response is the hallmark of apoptosis. Nuclear fragmentation (A) occurs in both processes — karyorrhexis in necrosis and internucleosomal fragmentation in apoptosis. Increased eosinophilia (B) and karyolysis (C) are features of necrosis. The key distinguishing point is that apoptotic bodies are cleared by phagocytosis before inflammation is triggered — hence the 'clean' microscopic appearance of apoptosis.

Pathological Calcification — Dystrophic and Metastatic

A four-panel diagram compares dystrophic and metastatic pathological calcification by tissue status, serum calcium level, mechanisms, sites, and clinical examples. — **Panel A:** Pathological calcification, calcium salts in non-osseous tissue, dystrophic calcification branch, metastatic calcification branch. **Panel B:** Necrotic tissue, normal serum Ca2+, dead cell membranes, mitochondria releasing Ca2+, phospholipid nucleation sites, hydroxyapatite crystals. **Panel C:** Normal viable tissue, elevated serum Ca2+, hypercalcaemic blood, renal tubule, lung alveolar septum, gastric mucosa, systemic artery wall. **Panel D:** TB caseous lymph node, atherosclerotic plaque, psammoma body, calcific aortic valve, breast microcalcification, hyperparathyroidism, bone destruction, vitamin D excess, chronic renal failure.

Pathological calcification is the deposition of calcium salts in non-osseous tissues. It is classified as dystrophic or metastatic based on the status of systemic calcium metabolism.

Dystrophic calcification:
• Definition: calcium deposition in dead or damaged tissue with normal serum calcium and phosphate.
• Mechanism: cell death → mitochondria release calcium (they normally sequester it) + phospholipid breakdown exposes nucleation sites → hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) crystals form in situ.
• Serum calcium: NORMAL.
• Tissue: dead/necrotic/damaged.
• Examples:
— Caseous necrosis of TB lymph node (the chalky-white deposit in the hook scenario)
— Atherosclerotic plaques (Mönckeberg's medial sclerosis is a variant)
— Old infarcts, dead parasites (Schistosoma, Echinococcus — "encysted" calcification)
— Psammoma bodies in papillary thyroid carcinoma, meningioma, ovarian serous carcinoma
— Calcific aortic stenosis (degenerative calcification of valve)
— Breast microcalcification on mammography (benign sclerosing adenosis or DCIS)

Metastatic calcification:
• Definition: calcium deposition in normal viable tissue due to hypercalcaemia.
• Mechanism: elevated serum calcium precipitates in tissues that normally maintain an alkaline pH (sites of acid secretion) or have high metabolic activity.
• Serum calcium: ELEVATED (hypercalcaemia).
• Tissue: normal (viable).
• Preferred sites (alkaline microenvironment): renal tubules (nephrocalcinosis), gastric mucosa, pulmonary alveolar walls, systemic blood vessel media, cornea (band keratopathy).
• Causes of hypercalcaemia: primary hyperparathyroidism, malignancy (PTHrP, lytic bone mets), sarcoidosis (↑vitamin D activation), vitamin D toxicity, milk-alkali syndrome, immobilisation.

Side-by-side comparison of dystrophic calcification in damaged tissue with normal serum calcium versus metastatic calcification in normal tissue with elevated serum calcium. — **Panel A:** Dystrophic calcification in damaged or dead tissue, normal serum Ca2+, granular calcium deposits, psammoma body inset, caseating TB lymph node inset.. **Panel B:** Metastatic calcification in normal viable tissue, elevated serum Ca2+, renal tubule calcification, gastric mucosa calcification, hypercalcaemia badge..

CLINICAL PEARL

Psammoma bodies — the concentric, laminated calcific spherules seen in papillary thyroid carcinoma, papillary serous ovarian carcinoma, and meningioma — are a classic exam spotter for dystrophic calcification. Their presence does NOT indicate metastatic spread (despite the name); they represent focal dystrophic calcification within tumour papillae. However, their presence in a lymph node cytology sample can indicate metastatic papillary thyroid carcinoma even when no tumour cells are identified — the calcium is a diagnostic footprint of the primary tumour.

SELF-CHECK

A 45-year-old woman has chronic renal failure with a serum calcium of 3.1 mmol/L (elevated). Imaging shows calcium deposits in the lungs and gastric mucosa. Her renal biopsy shows calcium in tubular cells. The most likely diagnosis is:

A. Dystrophic calcification due to renal tubular necrosis

B. Psammoma body formation due to malignancy

C. Fat necrosis secondary to elevated lipase

D. Metastatic calcification due to hypercalcaemia

Reveal Answer

Answer: D. Metastatic calcification due to hypercalcaemia

This is metastatic calcification. The defining features are: (1) elevated serum calcium (3.1 mmol/L), (2) calcium depositing in normal viable tissues — lungs, gastric mucosa, renal tubules — all sites of alkaline pH that favour precipitation. Dystrophic calcification (A) requires dead/damaged tissue with normal serum calcium. Psammoma bodies (B) are a histological pattern of dystrophic calcification associated with specific tumours, not a cause. Fat necrosis (C) is unrelated to hypercalcaemia and affects adipose tissue via lipase action.

Integration: Linking Cell Death to Disease

A pathology infographic links major cell death patterns to clinical diseases, morphology, and exam-relevant biomarkers. — **Panel A:** Cell injury, irreversible injury, necrosis, apoptosis, DAMP release, inflammation, enzyme leakage, troponin, serum lipase, minimal inflammation. **Panel B:** Coagulative necrosis, myocardial infarction, ghost cardiac myocytes, anucleate cells, preserved architecture, troponin release. **Panel C:** Liquefactive necrosis, brain abscess, pus cavity, neutrophils, macrophages, loss of architecture. **Panel D:** Caseous necrosis, tuberculosis lymphadenitis, amorphous caseous debris, epithelioid granuloma, Langhans giant cell, Ziehl-Neelsen stain. **Panel E:** Fat necrosis, acute pancreatitis, damaged adipocytes, chalky saponification, calcium soaps, serum lipase, Grey Turner sign. **Panel F:** Fibrinoid necrosis, immune vasculitis, malignant hypertension, fibrin-like vessel wall deposits, immune complexes, narrowed lumen, RBC leakage. **Panel G:** Apoptosis, cell shrinkage, chromatin condensation, membrane blebbing, apoptotic bodies, phagocytosis, Councilman body, chemotherapy response.

The table below consolidates the clinico-pathological links — this is the Year-2 synthesis level.

Cell death type	Canonical clinical example	Key morphology on slide	Examination link
Coagulative necrosis	Myocardial infarction	Ghost cells, anucleate, intact architecture	MI staging, troponin release (DAMPs)
Liquefactive necrosis	Brain abscess	Pus cavity, neutrophils, no architecture	Bacterial meningitis → abscess
Caseous necrosis	TB lymphadenitis	Cheese-like debris, granuloma wall	ZN stain, caseation in primary complex
Fat necrosis	Acute pancreatitis	Chalky deposits, saponification	Serum lipase, Grey Turner sign
Fibrinoid necrosis	SLE vasculitis	Vessel wall smudgy pink	ANCA vasculitis, immune complex disease
Dry gangrene	Diabetic foot (PVD)	Mummification, line of demarcation	Buerger's, arterial Doppler
Wet gangrene	Strangulated bowel	Rapid spread, no demarcation	Emergency laparotomy
Gas gangrene	Trauma + Clostridium	Gas in soft tissue on X-ray	Crepitus, alpha-toxin (lecithinase)
Apoptosis (physiologic)	Thymic negative selection	Single cells, no inflammation	Immune tolerance
Apoptosis (pathologic)	Viral hepatitis	Councilman bodies in liver	Hepatitis B/C, ALT elevation
Dystrophic calcification	TB, atherosclerosis	Basophilic deposits, normal Ca	Psammoma bodies, valve calcification
Metastatic calcification	Hyperparathyroidism	Basophilic deposits, viable tissue, ↑Ca	Nephrocalcinosis, band keratopathy