Cellular Reaction Patterns

The CNS has a remarkably limited repertoire of cellular reactions to injury. Neurons, astrocytes, microglia, oligodendrocytes, and the vasculature each respond to insult in a small number of stereotyped ways, and almost every neuropathologic diagnosis is built from recognizing these reactions, identifying their combinations, and reading their anatomic distribution. This page covers the major cellular reaction patterns of the nervous system, the appearance that allows them to be recognized on H&E, and the clinical contexts in which they appear.

Neuronal Reactions

Acute Neuronal Injury — the “Red Neuron”

The most rapidly recognizable change in injured CNS tissue. After acute ischemia or hypoxia (4-12 hours minimum), neurons:

  • Shrink and become angulated.
  • Develop deeply eosinophilic (pink-red) cytoplasm.
  • Lose their pale Nissl substance.
  • Develop a small, dark, pyknotic nucleus.
  • Eventually disappear, leaving a microglial nodule or simple gliosis.

This is the classical red neuron or ischemic neuronal change. It is a stereotyped indicator of hypoxic-ischemic injury at the cellular level — seen after stroke, cardiac arrest, status epilepticus, severe hypoglycemia, severe metabolic insult. It appears earliest in the most vulnerable neurons: CA1 hippocampal pyramidal cells, Purkinje cells of the cerebellum, layer III and V pyramidal neurons of cortex, large neurons of caudate and putamen.

Central Chromatolysis

After axonal injury, the cell body responds by becoming swollen, with the nucleus displaced eccentrically and the Nissl substance dispersed from the center toward the periphery (or lost). This is the “axon reaction” — central chromatolysis — and is the cell body’s attempt to ramp up protein synthesis to regenerate the cut axon. Most visible in motor neurons after peripheral nerve injury. The reaction is reversible if regeneration succeeds; the neuron dies if it does not.

Neuronal Atrophy / Loss

The end stage of many chronic neurodegenerative processes. The neuron simply disappears, leaving the surrounding glia and parenchymal architecture behind. Often associated with reactive astrogliosis. Diagnostic specificity comes from:

  • Distribution: which neurons are lost? (e.g., substantia nigra pars compacta in PD; anterior horn motor neurons in ALS; striatal neurons in HD).
  • Inclusions: are there specific deposits in the surviving or remaining cells?
  • Astrogliosis: the density and pattern of reactive astrocytes.

Trans-synaptic Degeneration

Loss of input causes the receiving neuron to atrophy or die. Examples: lateral geniculate nucleus atrophy after long-standing eye loss; thalamic atrophy after large cortical strokes; cerebellar atrophy after pontine infarction (with secondary olivary hypertrophy as the disinhibited downstream response).

Neuronophagia

A dying or recently dead neuron is surrounded by microglia that consume it. The visible result: a small cluster of microglia where a neuron used to be, sometimes with a few neuronal remnants in the center. Classical of polio, viral encephalitides, and many causes of acute neuronal injury. The microglial cluster persists as a “microglial nodule” after the neuron is fully phagocytosed.

Inclusions

Many neurodegenerative diseases are defined by specific cytoplasmic or nuclear inclusions. These are covered in detail in the [[neurodegenerative-inclusions]] page, but the major categories:

  • Tau: neurofibrillary tangles (AD), Pick bodies (Pick disease, FTLD-tau), globose tangles (PSP), astrocytic plaques (CBD), tufted astrocytes (PSP, CBD), thorn-shaped astrocytes (aging-related tauopathy).
  • α-synuclein: Lewy bodies and Lewy neurites (PD, DLB); glial cytoplasmic inclusions of Papp-Lantos (MSA).
  • TDP-43: cytoplasmic inclusions in motor neurons and cortical neurons (ALS, FTLD-TDP).
  • FUS: cytoplasmic inclusions in some FTLD and ALS subsets.
  • Polyglutamine: intranuclear inclusions in Huntington disease and some SCAs.
  • Bunina bodies: small eosinophilic cytoplasmic inclusions in ALS motor neurons.
  • Hirano bodies: rod-shaped eosinophilic inclusions in CA1 hippocampus (aging, AD).
  • Marinesco bodies: small intranuclear eosinophilic inclusions in pigmented brainstem neurons (aging — generally non-specific).
  • Lafora bodies: PAS-positive polyglucosan inclusions (Lafora disease, progressive myoclonic epilepsy).
  • Negri bodies: eosinophilic cytoplasmic inclusions in Purkinje cells and hippocampal neurons in rabies.
  • Cowdry type A: large intranuclear viral inclusions in HSV encephalitis.
  • Owl’s eye: large intranuclear inclusions with surrounding clear halo in CMV-infected cells.

Astrocyte Reactions

Reactive Astrogliosis

The single most common reaction in the CNS, accompanying almost every chronic insult. Reactive astrocytes:

  • Become enlarged, with prominent eosinophilic cytoplasm.
  • Develop visible cytoplasmic processes radiating outward.
  • Express increased GFAP (the workhorse IHC marker).
  • Proliferate in some contexts.

Reactive astrogliosis surrounds infarcts, plaques in MS, neoplasms, areas of neuronal loss, and chronic inflammation. Its density and distribution are clues, but reactive astrocytosis itself is non-specific — it reports that something is wrong, not what.

Gemistocytic Astrocytes

A subtype of reactive astrocyte with prominent globular eosinophilic cytoplasm — “gemistocyte” (Greek “stuffed cell”). Seen in reactive gliosis around chronic lesions, in low-grade astrocytomas (gemistocytic astrocytoma is a distinct pattern), and in some demyelinating plaques. Always raise the question of an underlying neoplasm.

Alzheimer Type II Astrocytes

A specific reactive change of hepatic encephalopathy (and other hyperammonemic states). The astrocytes have:

  • Enlarged, pale nuclei.
  • Visible nucleoli.
  • Often paired or clustered (2-3 nuclei together).
  • Minimal cytoplasmic change.

They are seen mostly in the deep gray (basal ganglia, thalamus, dentate). The name is misleading — these are NOT seen in Alzheimer disease; they were named by Alzheimer in the early 20th century in a different context.

Rosenthal Fibers

Brightly eosinophilic, irregular, elongated or globular structures formed by aggregation of GFAP and αB-crystallin in chronically reactive astrocytes. Found in:

  • Pilocytic astrocytoma: characteristic finding (along with eosinophilic granular bodies).
  • Chronic gliosis of any cause — around syringomyelia cavities, old infarcts, in margins of slow-growing lesions.
  • Alexander disease: a leukodystrophy of GFAP gene mutation, with Rosenthal fibers throughout brain especially the subependymal, perivascular, and subpial regions.

Corpora Amylacea

Round, basophilic, PAS-positive polyglucosan bodies seen in increasing numbers with aging. Concentrated in subpial cortex, around ventricles, near vessels. Generally non-specific markers of “wear and tear,” though massively increased in Lafora disease (where the polyglucosan bodies are more widespread and clinically meaningful).

Gliosis Subtypes by Disease Pattern

  • Fibrillary gliosis: classic dense gliosis around chronic lesions.
  • Isomorphic gliosis: organized arrangement, often around tracts.
  • Anisomorphic gliosis: disorganized, surrounding mass lesions.
  • Subpial / Chaslin gliosis: subpial astrocyte band, often in chronic temporal lobe epilepsy.

Microglial Reactions

Resting (Ramified) Microglia

Normal microglia have small cell bodies with thin, branching processes. They are scattered throughout the parenchyma. IHC: CD68 (lysosomal, increases with activation), IBA1 (constitutive), HLA-DR.

Activated Microglia

In response to injury, microglia:

  • Retract their processes.
  • Enlarge their cell bodies.
  • Become phagocytic, accumulating debris.
  • Proliferate locally.

Rod Cells

Elongated microglial nuclei oriented in parallel rows, classical of:

  • HIV encephalitis.
  • Subacute and chronic viral encephalitis.
  • Neurosyphilis (general paresis).

Microglial Nodules

Small clusters of microglia, sometimes with a degenerating neuron at the center. Classic of:

  • Viral encephalitis (especially HSV, HIV, arboviruses).
  • Toxoplasmosis.
  • Listeria rhombencephalitis.

Foamy Macrophages

Lipid-laden macrophages, indicating active phagocytosis of myelin or other lipid debris. Classical of:

  • Active MS plaques.
  • Subacute infarcts (foamy macrophages clear necrotic tissue 1-4 weeks post-event).
  • PML.
  • Some chronic infections.

Multinucleated Giant Cells

Specific to:

  • HIV encephalitis: multinucleated giant cells with characteristic syncytial appearance, IHC positive for p24.
  • Granulomatous disease: tuberculosis (with caseation), sarcoidosis (without caseation), some fungal infections.
  • Foreign body reaction.

Oligodendrocyte and Myelin Reactions

Demyelination

Loss of myelin with relative preservation of axons. Recognized on:

  • Luxol fast blue: pale areas of myelin loss.
  • H&E: pale, somewhat translucent regions.
  • Bielschowsky: axons preserved.

Active demyelination shows foamy macrophages with luxol-positive myelin debris inside. Chronic plaques show sharply demarcated demyelinated zones with loss of oligodendrocytes and reactive astrogliosis.

Axonal Damage with Secondary Myelin Loss

Distinguished from primary demyelination by loss of both axons (Bielschowsky) and myelin (LFB) in the same area. Common in ischemia, trauma, neurodegenerative diseases.

Acute Disseminated Encephalomyelitis (ADEM) Pattern

Perivenous demyelination — small zones of demyelination centered on small veins, with surrounding inflammation. Distinguishes ADEM histologically from MS (which has confluent plaques).

Viral Inclusions in Oligodendrocytes

Classical of progressive multifocal leukoencephalopathy (PML): enlarged “smudgy” basophilic nuclei in oligodendrocytes infected with JC virus. IHC for JC virus large T antigen confirms.

Vascular Reactions

  • Fibrinoid necrosis: vessel wall deposition of fibrin-rich material; pathognomonic of vasculitis.
  • Thrombosis: occluding fibrin or platelet-fibrin thrombus.
  • Amyloid deposition: Congo red-positive material in vessel walls (CAA).
  • Lipohyalinosis: small vessel arteriolosclerosis from chronic hypertension; substrate of lacunar infarction.
  • Atherosclerosis: classical lipid plaques in large vessels.
  • Angiocentric inflammation: lymphocytic cuffing around vessels — viral encephalitis, autoimmune encephalitis.

Necrosis Patterns

  • Coagulative necrosis: preservation of tissue outlines with loss of cell nuclei; acute infarction.
  • Liquefactive necrosis: tissue dissolves into fluid; later infarction, abscess. The end-stage of CNS necrosis (unlike most other organs).
  • Caseating necrosis: cheesy granular debris; tuberculosis.
  • Fibrinoid necrosis: vessel-centered; vasculitis.
  • Geographic necrosis: irregular sharp-bordered areas; classical of glioblastoma (with pseudopalisading) and PCNSL.

Inflammatory Patterns

  • Acute neutrophilic: bacterial meningitis, abscess.
  • Lymphocytic (T-cell predominant): viral encephalitis, autoimmune encephalitis, MS.
  • Plasma cells: chronic syphilis, some autoimmune conditions.
  • Granulomatous: TB, fungal, sarcoid.
  • Eosinophilic: parasites, hypersensitivity, eosinophilic granulomatosis.
  • Mixed: many chronic and resolving processes.

Integrating the Reactions

A neuropathologic diagnosis is rarely a single feature. The pathologist asks:

  1. What cell types are reacting?
  2. What is the dominant pattern (ischemic, inflammatory, demyelinating, neoplastic, neurodegenerative)?
  3. What is the temporal stage (acute, subacute, chronic)?
  4. What is the anatomic distribution?
  5. Are there specific inclusions or organisms?
  6. What IHC and molecular studies are informative?

The combination — pattern + tempo + topography + specifics — narrows the differential and often yields a specific diagnosis.

🔍 Did You Know?

The red neuron (ischemic neuronal change) takes a precise minimum time to develop — typically 4 to 12 hours after the ischemic insult, and not before. This has practical forensic and clinical implications. A patient who dies within minutes of a cardiac arrest will show no red neurons; the brain looks histologically normal despite catastrophic clinical reality. Red neurons begin to appear by 6-12 hours, become prominent by 24-48 hours, and persist for days. By 4-7 days they are accompanied by neutrophil infiltration; by 1-2 weeks by foamy macrophages clearing debris; by months to years by cystic cavitation and gliosis. This temporal evolution allows the neuropathologist to estimate the age of an ischemic injury with reasonable precision — and to interpret findings in patients who present after sudden death or unwitnessed events. The forensic implication is that histology cannot date acute injury at the bedside: a patient who is pulled from a cold lake apparently dead may have suffered an injury hours earlier that has not yet expressed itself at the cellular level. The clinical implication is that early imaging changes (DWI restricted diffusion within minutes) are far more sensitive than histology for hyperacute injury — but that days later, the histologic age can corroborate or contradict the clinical history.

Pitfalls and Pearls

  • The red neuron is the marker of ischemic-hypoxic injury. 4-12 hours minimum to appear; weeks to evolve.
  • Reactive astrogliosis is non-specific. It reports injury, not cause.
  • Alzheimer type II astrocytes = hepatic encephalopathy / hyperammonemia. NOT Alzheimer disease.
  • Rosenthal fibers: pilocytic astrocytoma, chronic gliosis, Alexander disease.
  • Rod microglia: HIV, viral encephalitis, neurosyphilis.
  • Microglial nodules with neuronophagia: viral encephalitis.
  • Multinucleated giant cells: HIV (p24+), granulomatous infection, foreign body.
  • Foamy macrophages with myelin debris: active MS plaque, evolving infarct, PML.
  • Fibrinoid necrosis: vasculitis (until proven otherwise).
  • Smudgy enlarged oligodendrocyte nuclei: PML — confirm with JC virus IHC.
  • Negri bodies in Purkinje cells: rabies.
  • Owl-eye intranuclear inclusion: CMV.
  • Cowdry A inclusion: HSV.
  • Lafora bodies (PAS+): progressive myoclonic epilepsy of Lafora disease.
  • Neuronal loss + gliosis + inclusions = neurodegenerative; the inclusion identifies the disease.

References

  1. Love S, Budka H, Ironside JW, Perry A, eds. Greenfield’s Neuropathology. 9th ed. CRC Press; 2015.
  2. Ellison D, Love S, Chimelli L, et al. Neuropathology: A Reference Text of CNS Pathology. 3rd ed. Mosby; 2013.
  3. Sofroniew MV, Vinters HV. Astrocytes: biology and pathology. Acta Neuropathol. 2010;119(1):7-35.
  4. Kreutzberg GW. Microglia: a sensor for pathological events in the CNS. Trends Neurosci. 1996;19(8):312-318.
  5. Liddelow SA, Barres BA. Reactive astrocytes: production, function, and therapeutic potential. Immunity. 2017;46(6):957-967.