Neuropathology Fundamentals
Neuropathology Fundamentals
What You'll Learn
- Cell reactions to injury — red neurons (acute ischemia), chromatolysis (axonal injury), Wallerian degeneration (anterograde), gliosis (astrocytic scar)
- Neuropathology stains — H&E, Nissl, Luxol fast blue (myelin), Congo red (amyloid → apple-green birefringence), silver stains (tangles/plaques), GFAP (astrocytes)
- Immunohistochemical markers — synaptophysin/NeuN (neurons), CD68 (microglia), S-100 (Schwann cells), EMA (meningiomas), Ki-67 (proliferation), IDH1 R132H, ATRX, 1p/19q
- Protein aggregates — tau (AD, PSP, CBD, Pick), alpha-synuclein (PD, DLB, MSA), amyloid-beta (AD, CAA), TDP-43 (ALS, FTD), prion protein (CJD)
- Hallmark pathology findings — Lewy bodies (PD), Pick bodies (FTD), Negri bodies (rabies), Cowdry type A (HSV), Rosenthal fibers (Alexander disease, pilocytic astrocytoma, chronic reactive gliosis — not specific), Verocay bodies (schwannoma)
- Muscle & nerve pathology buzzwords — ragged red fibers (mitochondrial), rimmed vacuoles (IBM), perifascicular atrophy (dermatomyositis), onion bulbs (CMT/CIDP)
- Selective vulnerability — hippocampal CA1, Purkinje cells, watershed zones, cortical laminar necrosis (layers 3, 5, 6)
- Patterns of CNS injury — global vs focal ischemia, hemorrhagic transformation, transtentorial herniation (PCA compression, Duret hemorrhages)
HighYield Pearls
- Red neurons: eosinophilic shrunken neurons appearing 12–24 h after hypoxia/ischemia — first histologic sign of irreversible ischemic injury; most vulnerable = CA1 hippocampus, Purkinje cells, neocortical layers III/V
- Chromatolysis = axonal injury response (NOT ischemia): swollen cell body, eccentric nucleus, dispersed Nissl substance — potentially reversible
- Lewy body (α-synuclein): cytoplasmic eosinophilic inclusion with pale halo → PD/DLB; GCIs (oligodendroglial α-synuclein) → MSA — same protein, different cell
- 4R tauopathies have signature glial lesions: tufted astrocytes → PSP, astrocytic plaques → CBD — Gallyas silver highlights both
- Negri bodies (eosinophilic cytoplasmic, Purkinje + hippocampal pyramidals) = rabies; Lafora bodies (PAS+ polyglucosan) = Lafora disease; Bunina bodies = ALS motor neurons
- Rosenthal fibers are NOT specific: Alexander disease + pilocytic astrocytoma + chronic reactive gliosis — eosinophilic, carrot-shaped, GFAP+/αB-crystallin+
- Congo red → apple-green birefringence under polarized light = amyloid (CAA, amyloid neuropathy); routine boards stem
- Ragged red fibers on Gomori trichrome + COX-negative fibers = mitochondrial myopathy (subsarcolemmal mito accumulation)
- Microglial nodules + neuronophagia = viral encephalitis (HSV, CMV, HIV, polio); perivascular T-cell cuffing also classic
- IDH1 R132H IHC & ATRX loss — IDH-mutant astrocytoma; BRAF V600E IHC — pilocytic, ganglioglioma, PXA; routine neuro-oncology buzzwords
🔍 Quick ReferenceInclusion bodies / cells · Cellular reactions · Stains
Inclusion bodies / cells
- Lewy body (cytoplasmic, eosinophilic with halo, α-synuclein+) → Parkinson disease, DLB
- Pick body (rounded, 3R tau+, silver/Bielschowsky+) → Pick disease (FTLD-tau)
- Negri body (cytoplasmic eosinophilic, Purkinje + hippocampal pyramidals) → rabies
- Lafora body (PAS+ basophilic polyglucosan, intracytoplasmic) → Lafora disease (PME)
- Bunina body (small eosinophilic, motor neuron) → ALS
- Rosenthal fibers (eosinophilic carrot-shaped, GFAP+) → Alexander disease, pilocytic astrocytoma, chronic gliosis
- Tufted astrocytes (Gallyas+, 4R tau) → PSP
- Astrocytic plaques (4R tau) → corticobasal degeneration (CBD)
- Glial cytoplasmic inclusions (GCIs, oligodendroglial α-synuclein) → MSA
- Ballooned neurons → CBD, Pick, neuronal storage disease
- Ghost tangles (extracellular NFTs) → end-stage AD, PSP
- Marinesco bodies (intranuclear, substantia nigra) → normal aging
- Corpora amylacea (basophilic polyglucosan) → normal aging (subpial/perivascular)
- Cowdry type A intranuclear inclusion → HSV encephalitis
- Verocay bodies (palisading nuclei) → schwannoma
- Alzheimer type II astrocytes (pale swollen nuclei) → hepatic encephalopathy
Cellular reactions to injury
- Red neurons (eosinophilic, shrunken, pyknotic) → acute ischemia 12–24 h (CA1, Purkinje, cortex layers III/V)
- Chromatolysis (central Nissl loss, eccentric nucleus, swollen soma) → axonal injury / anterior horn cell reaction
- Gemistocytes (plump eosinophilic reactive astrocytes, eccentric nucleus) → chronic injury / peritumoral reaction
- Axonal spheroids / retraction bulbs (Bielschowsky+) → diffuse axonal injury (TBI)
- Wallerian degeneration (distal axon + myelin fragmentation) → distal to axonal transection
- Ferruginous bodies (iron/Ca encrusted neurons) → old infarct / abscess wall
- Gitter cells (foamy lipid-laden macrophages) → infarct (subacute), demyelination
- Rod cells (elongated microglia) → neurosyphilis, viral encephalitis
- Microglial nodules + neuronophagia → viral encephalitis (HSV, CMV, HIV, polio)
- Perivascular T-cell cuffing → MS, viral/autoimmune encephalitis, CNS vasculitis
- Granulomatous inflammation → TB, sarcoid, fungal (Crypto, Coccidio)
- Vasogenic edema (BBB disruption) → tumor, abscess, late stroke; cytotoxic edema → acute ischemia
Stains / IHC
- Luxol fast blue (LFB) → myelin (pale = demyelination); often paired with PAS or H&E
- Bielschowsky silver → neurofibrillary tangles, neuritic plaques, axonal spheroids
- Gallyas silver → 4R tau lesions (PSP tufts, CBD astrocytic plaques)
- Congo red → apple-green birefringence (polarized) → amyloid (CAA, amyloid neuropathy)
- PAS → glycogen, Lafora bodies, fungal walls, corpora amylacea
- Prussian blue → iron / hemosiderin (old hemorrhage, ferruginous bodies)
- Gomori trichrome → ragged red fibers → mitochondrial myopathy; COX-negative fibers = mito
- Oil red O / Sudan black → lipid (storage disease, myelin breakdown)
- GMS / PAS → fungal organisms
- GFAP → astrocytes (Rosenthal fibers, glial scar, gliomas)
- Synaptophysin / NeuN / chromogranin → neuronal & synaptic markers
- α-synuclein IHC → Lewy bodies, GCIs (PD, DLB, MSA)
- p-tau (AT8) IHC → AD NFTs, PSP, CBD, Pick; TDP-43 IHC → ALS, FTLD-TDP
- Aβ IHC → AD plaques, CAA; ubiquitin / p62 → FTLD-TDP, intranuclear inclusions
- IDH1 R132H + ATRX loss → IDH-mutant astrocytoma; BRAF V600E → pilocytic, ganglioglioma, PXA
- EMA → meningioma, ependymoma (dot-like); S-100 → Schwann cells, melanocytes; Ki-67 → proliferation index
Cell Reactions to Injury
Neuronal Injury Responses
- Red neurons (eosinophilic neuronal necrosis): hallmark of acute ischemic injury (12–24 hours); shrunken cell body, pyknotic nucleus, bright eosinophilic cytoplasm on H&E
- Chromatolysis: cell body response to axonal injury; cell swelling, eccentric nucleus displaced to periphery, dissolution of Nissl substance (rough ER); represents metabolic shift toward repair proteins
- Apoptosis: programmed cell death; cell shrinkage, nuclear condensation, no inflammation; mediated by caspases; occurs in development and neurodegenerative disease
- Necrosis: cell swelling, membrane rupture, inflammatory infiltrate; occurs in ischemia, trauma, infection
- Ferrugination (mineralization): calcium and iron deposition in dead neurons; seen as basophilic encrustation on old infarcts
Board Pearl
Red neurons = acute ischemia (12–24 hours). Chromatolysis = axonal injury response (swollen cell body, peripheral nucleus, Nissl dissolution). These are the two most-tested neuronal injury patterns. Red neurons indicate irreversible injury; chromatolysis is potentially reversible if the axon regenerates.
Pathological Evolution of Infarction
| Time After Ischemia | Pathological Findings | Key Cell Types |
|---|---|---|
| 0–12 hours | No visible changes on H&E (too early) | — |
| 12–24 hours | Red neurons (eosinophilic necrosis); early neutrophil infiltration | Neutrophils (PMNs) |
| 1–3 days | Tissue edema; neutrophilic infiltration peaks; coagulation necrosis | Neutrophils |
| 3–7 days | Macrophage invasion begins; liquefactive necrosis starts; gitter cells appear | Macrophages / gitter cells |
| 1–4 weeks | Reactive gliosis at periphery; macrophages clear necrotic debris; neovascularization | Macrophages, reactive astrocytes |
| Months–years | Cystic cavity lined by glial scar (GFAP+ astrocytes); hemosiderin-laden macrophages | Astrocytes |
Wallerian vs Retrograde Degeneration
| Feature | Wallerian (Anterograde) | Retrograde |
|---|---|---|
| Definition | Degeneration distal to injury site | Degeneration proximal to injury site (toward soma) |
| Mechanism | Axon and myelin fragment distally; macrophages clear debris | Cell body undergoes chromatolysis; may progress to death |
| Timeline | 3–5 days: Schwann cell proliferation; 7–10 days: complete | Hours to days post-injury |
| NCS/EMG | Reduced/absent distal CMAP/SNAP by 7–10 days; fibrillations at 2–5 weeks | Loss of proximal function |
| PNS recovery | Schwann cells form bands of Büngner → axon regrows at ~1 mm/day | Depends on severity of cell body injury |
Glial Responses to Injury
- Reactive gliosis (astrocytic response): GFAP+ astrocytes hypertrophy and proliferate → form a glial scar; barrier to axonal regeneration in the CNS
- Alzheimer type II astrocytes: swollen, pale nuclei; seen in hepatic encephalopathy (ammonia → glutamine synthetase → osmotic swelling)
- Rosenthal fibers: eosinophilic, corkscrew-shaped inclusions in astrocytes; seen in Alexander disease and pilocytic astrocytoma
- Corpora amylacea: round, basophilic polyglucosan bodies in astrocytic processes; increase with normal aging — not pathological
Microglial Responses
- Rod cells: elongated, activated microglia aligned along damaged neurons; seen in viral encephalitis, neurosyphilis
- Gitter cells (foamy macrophages): lipid-laden macrophages that phagocytose myelin and necrotic debris; seen in infarcts, demyelination
- Microglial nodules: clusters of activated microglia; hallmark of viral encephalitis, especially HIV
- Neuronophagia: microglia surrounding and engulfing a dying neuron; classic finding in viral encephalitis (especially poliovirus)
Demyelination vs Dysmyelination
- Demyelination: destruction of previously normal myelin (acquired) — MS, GBS, CIDP, ADEM, PML
- Dysmyelination: defective myelin formation from the start (hereditary) — leukodystrophies (MLD, Krabbe, ALD, Pelizaeus-Merzbacher, Alexander, Canavan)
- Key MRI distinction: demyelination = focal, asymmetric lesions; dysmyelination = diffuse, symmetric white matter changes
Board Pearl
Gitter cells (foamy macrophages) in an infarct = subacute stage (days to weeks). Neuronophagia (microglia engulfing neurons) is the hallmark of viral encephalitis. Alzheimer type II astrocytes = hepatic encephalopathy. Rod cells = viral encephalitis or neurosyphilis.
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