Introduction to Neuropathology
What You'll Learn
- A systematic way to read any neuropathology slide — orient, low power, high power, name the pattern, then order stains
- When to use low vs high magnification
- The core stains and a decision guide for which stain answers which question
- How tissue is prepared (permanent / frozen / smear) and the artifacts that fool you
- Low power first. Architecture, distribution, and what is abnormal are decided at low power; save cytology and inclusions for high power.
- H&E answers most questions; a special stain answers a specific one (myelin→LFB, amyloid→Congo red, iron→Prussian blue, fungus→GMS).
- Red neurons (shrunken, bright-pink, pyknotic) are the earliest light-microscopic sign of ischemia (12–24 h).
- Dark (“hippocampal”) neurons are a handling artifact — do not overcall them as red neurons.
- IHC names the cell/protein; molecular markers (IDH, 1p/19q, ATRX) increasingly define the diagnosis in neuro-oncology.
How to Approach a Slide
Read every slide the same way — a fixed routine keeps you from anchoring on the first odd cell:
- Orient. What tissue and region? Gray vs white matter? Is there an obvious lesion or a diffuse process?
- Scan at low power (2–10×). Architecture, cellularity, and distribution: focal vs diffuse, perivascular, laminar (cortical layers), demarcated vs infiltrative. Compare with adjacent normal tissue.
- Zoom to high power (40×). Cytology, nuclear features, mitoses, vessels, and inclusions/organisms.
- Name the pattern — injury, inflammation, demyelination, vascular, or neoplasia — before naming a disease.
- Order the stain that answers the specific question the pattern raises (see the decision guide below).
Magnification: Low vs High Power
| Power | What it's for | What you read |
|---|
| Low (2–10×) | Screening & architecture | Orientation, lesion distribution (focal/diffuse/perivascular/laminar), cellularity, demarcation, normal-vs-abnormal comparison |
| High (20–40×) | Cytology & detail | Nuclear atypia, mitoses, microvascular proliferation, inclusions, organisms, single-cell reactions (red neurons, chromatolysis) |
Rule of thumb: a question about where / how much / what pattern is answered at low power; a question about what is inside the cell is answered at high power.
Normal Tissue Architecture — Orientation
You cannot call a slide abnormal until you know normal. A quick orientation (full detail in the Normal Histology chapter):
- Cortex — six layers; large pyramidal neurons in layers III/V (metabolically demanding → selectively vulnerable).
- White matter — myelinated axons + oligodendrocytes + astrocytes; pale-blue on LFB.
- Cerebellum — molecular / Purkinje / granular three-layer pattern; Purkinje cells in a single row.
- Hippocampus — Cornu Ammonis pyramidal ribbon; CA1 (Sommer sector) most hypoxia-vulnerable.
- Peripheral nerve / muscle — read on transverse sections with dedicated stains (Normal Histology + Muscle chapters).
Core Stains
Histochemical Stains
| Stain | Target | Key use |
|---|
| H&E | General morphology | Nuclei blue, cytoplasm pink; red neurons bright eosinophilic |
| Nissl (cresyl violet) | Neuronal rough ER | Identifies neurons; Nissl dissolves in chromatolysis |
| Luxol fast blue | Myelin | Myelin blue; pale = demyelination/dysmyelination |
| Silver (Bielschowsky/Bodian) | Axons, tangles, plaques | Argyrophilic; Alzheimer pathology |
| Congo red / thioflavin | Amyloid | Apple-green birefringence (polarized light) |
| PAS | Glycogen, fungi, BM | Lafora bodies, corpora amylacea, fungi |
| GMS / mucicarmine | Fungi | GMS black; mucicarmine = Cryptococcus capsule |
| Ziehl-Neelsen | Acid-fast bacilli | Tuberculosis (red bacilli) |
| Prussian blue (Perls) | Iron / hemosiderin | Old hemorrhage, remote infarct, siderosis |
| Gomori trichrome | Muscle / mitochondria | Ragged red fibers; nemaline rods |
| Masson-Fontana | Melanin | Metastatic melanoma; neuromelanin |
| von Kossa / Alizarin | Calcium | Calcification (psammoma bodies, Fahr) |
| Oil Red O | Neutral lipid | Frozen tissue only; foamy macrophages, myelin debris |
Immunohistochemical Markers (starter set)
| Marker | Marks | Use |
|---|
| GFAP | Astrocytes | Astrocytoma/GBM, reactive gliosis, ependymoma |
| Synaptophysin | Neuronal/neuroendocrine | Medulloblastoma, neurocytoma, ganglioglioma |
| S100 | Schwann/melanocyte/glia | Schwannoma (diffuse), melanoma |
| EMA | Epithelial/meningothelial | Meningioma; dot-EMA in ependymoma |
| IDH1 R132H, ATRX, 1p/19q, Ki-67 | Molecular / proliferation | Glioma classification & grading |
Which Stain Answers Which Question?
| The question on the slide | Reach for… |
|---|
| Is myelin lost? | Luxol fast blue |
| Is this amyloid? | Congo red (apple-green birefringence) |
| Old blood / iron? | Prussian blue (Perls) |
| A fungus? | GMS / PAS; mucicarmine for Cryptococcus |
| Acid-fast (TB)? | Ziehl-Neelsen |
| Calcium? | von Kossa / Alizarin red |
| Melanin? | Masson-Fontana |
| Axons / tangles / plaques? | Silver (Bielschowsky, Bodian) |
| Glycogen / polyglucosan? | PAS |
| Mitochondrial myopathy? | Gomori trichrome (ragged red) + SDH/COX |
Tissue Preparation & Artifacts
| Preparation | When used | Notes |
|---|
| Permanent (FFPE) | Standard diagnosis | Best morphology; enables IHC and molecular testing |
| Frozen section | Intra-operative | Fast but distorts; freezing artifact (holes), dark neurons — interpret cautiously |
| Smear / squash | Intra-operative cytology | Excellent for glioma vs lymphoma vs normal; shows process detail (pilocytic bipolar cells) |
Don't be fooled: dark neurons (basophilic, shrunken, from rough handling) mimic red neurons but lack true eosinophilia and appear immediately (not after 12–24 h); freezing artifact leaves clear holes; autolysis softens all nuclei uniformly.
| Topic | Key fact |
|---|
| Reading order | Orient → low power (pattern) → high power (cytology) → stain |
| Red neurons | Earliest ischemic change, 12–24 h; not artifactual dark neurons |
| Congo red | Amyloid → apple-green birefringence under polarized light |
| Luxol fast blue | Myelin blue; pale = demyelination |
| Frozen section | Fast, intra-op, artifact-prone — confirm on permanent |
References
- Love S, Budka H, Ironside JW, Perry A. Greenfield's Neuropathology. 9th ed. CRC Press; 2015.
- Prayson RA. Neuropathology. 2nd ed. Elsevier; 2012.
- Ellison D, Love S, et al. Neuropathology: A Reference Text of CNS Pathology. 3rd ed. Mosby; 2013.
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