Visualizing the cellular components of the nervous system requires specialized stains that highlight myelin sheaths, neuronal processes, and glial cells. These techniques are essential for diagnosing demyelinating diseases, neurodegenerative disorders, and brain tumors.
Myelin Stains
Luxol Fast Blue (LFB) is the standard histochemical stain for myelin. The copper-phthalocyanine dye binds to the basic protein component of myelin sheaths, producing a blue-green color. LFB is used to evaluate myelination patterns (normal vs. hypomyelination in leukodystrophies), demyelination (multiple sclerosis plaques — well-demarcated areas of myelin loss with relative axonal preservation), and white matter pathology (edema, infarction, leukoencephalopathy). LFB is typically combined with H&E or PAS for cellular detail.
Solochrome cyanine is a rapid myelin stain that produces blue-black myelin staining in 10-15 minutes. It is used for frozen sections and rapid assessment of myelination.
Myelin basic protein (MBP) IHC detects the MBP component of myelin sheaths. MBP IHC is more sensitive than LFB for detecting early demyelination and residual myelin debris. It is used to confirm demyelination in multiple sclerosis biopsies and to identify central myelin in nerve sheath tumors.
Neuronal Stains
Cresyl Violet (CV) — a Nissl stain that binds to ribosomal RNA in neuronal cytoplasm, producing violet Nissl substance. CV demonstrates neuronal density, Nissl pattern (loss of Nissl substance — chromatolysis — indicates axonal injury), and nuclear morphology. The classic LFB-CV combination stains myelin (blue) and neurons (violet) in the same section.
Bielschowsky silver impregnation — a reduction silver method that stains neurofibrils (intracellular) and neuritic processes (axons and dendrites) black against a pale brown background. It demonstrates neurofibrillary tangles (intracellular, flame-shaped tangles in Alzheimer disease), neuritic plaques (extracellular deposits of amyloid with dystrophic neurites), Pick bodies (round, intracytoplasmic inclusions in Pick disease), and neuropil threads (dystrophic neurites in the neuropil).
NeuN IHC (Neuronal Nuclei) — a specific marker of mature neurons. NeuN is a nuclear protein expressed in most neuronal types (exceptions include cerebellar Purkinje cells, retinal photoreceptors, and inferior olivary neurons). Loss of NeuN staining indicates neuronal injury or death.
Neurofilament (NF) IHC — detects the axonal cytoskeleton. Neurofilament stains demonstrate axonal spheroids (swollen, NF-positive axons in axonal injury) and torpedoes (swollen Purkinje cell axons in cerebellar degeneration). NF is also used for assessing axonal density in peripheral nerve biopsies.
Glial Stains
GFAP (glial fibrillary acidic protein) IHC is the defining marker of astrocytes. In normal brain, GFAP stains protoplasmic and fibrous astrocytes with fine, branching processes. In reactive astrogliosis (response to any CNS injury), GFAP upregulation produces hypertrophy of cell bodies and processes — “gemistocytic” astrocytes. GFAP is also the lineage marker for astrocytic tumors (astrocytoma, glioblastoma, pleomorphic xanthoastrocytoma).
Olig2 IHC — nuclear marker of oligodendrocytes and their precursors. Olig2 is expressed in oligodendrogliomas and is a sensitive marker for glial neoplasms. It is also expressed in a subset of pineal parenchymal tumors.
Iba1 IHC (ionized calcium-binding adaptor molecule 1) — a microglia-specific marker that stains both resting (ramified) and activated (amoeboid) microglia. Activated microglia are upregulated in neurodegenerative diseases (Alzheimer, Parkinson, ALS), multiple sclerosis, and infection. CD68 — a lysosomal marker in macrophages and activated microglia, used to assess the degree of inflammatory infiltrate.
Applications in Demyelinating Disease
Multiple sclerosis — LFB demonstrates well-demarcated, perivenular demyelinating plaques in white matter. Neurofilament IHC shows relative axonal preservation within the plaque. MBP IHC confirms myelin loss. CD68 IHC highlights macrophage/microglial infiltration. GFAP IHC shows reactive astrogliosis at the plaque edge.
Progressive multifocal leukoencephalopathy (PML) — JC virus infection of oligodendrocytes causes patchy demyelination with enlarged, virally infected oligodendroglial nuclei. LFB shows myelin loss; SV40 IHC (cross-reacts with JC virus) confirms the diagnosis.
Leukodystrophies — inborn errors of myelin metabolism (metachromatic leukodystrophy, adrenoleukodystrophy, Krabbe disease) show diffuse myelin loss on LFB with specific storage material identified by special stains. Quality assurance for neuropathology stains includes known positive controls (brain with Alzheimer pathology for Bielschowsky, multiple sclerosis plaque for LFB).
