Neurohistology is the study of the microscopic anatomy of the nervous system — the brain, spinal cord, peripheral nerves, and sensory organs. Nervous tissue presents unique challenges for histopathology due to its high lipid content, complex cytoarchitecture, and selective vulnerability of different cell types to disease.
Organization of the Nervous System
The central nervous system (CNS) — brain and spinal cord — consists of gray matter (neuronal cell bodies, dendrites, synapses, glial cells) and white matter (myelinated axons and oligodendrocytes). The peripheral nervous system (PNS) comprises cranial nerves, spinal nerves, and peripheral ganglia, with Schwann cells replacing oligodendrocytes as the myelinating cells.
Neurons are the functional units — large cells with specialized dendritic arbors and long axons. Their size, shape, and neurotransmitter content vary by region: pyramidal neurons (cerebral cortex), Purkinje cells (cerebellum), motor neurons (spinal cord anterior horn), and sensory neurons (dorsal root ganglia). Glial cells outnumber neurons 10:1 and include: astrocytes (support, glutamate metabolism, blood-brain barrier — GFAP-positive), oligodendrocytes (CNS myelin production — Olig2-positive), microglia (resident immune cells — Iba1, CD68-positive), and ependymal cells (lining ventricles).
Tissue Handling and Fixation
Brain tissue is extremely soft and must be adequately fixed before sectioning. Whole brain fixation requires 7-14 days in 10% NBF. Coronal sectioning of the cerebrum, transverse sectioning of the brainstem and spinal cord, and sagittal sectioning of the cerebellum are standard. Biopsy specimens (stereotactic brain biopsies) are small (1-2 mm cores) and require careful handling — often wrapped in lens paper before processing to prevent fragment loss.
Special fixatives for neurohistology include Bouin’s solution (pituitary and endocrine-related brain regions), glutaraldehyde (for electron microscopy of nerve and muscle), and buffered formalin (routine). Paraffin processing is standard for diagnostic neuropathology; frozen sections are used for intra-operative consultation (smear preparations or cryostat sections).
Standard Neurohistological Stains
H&E is the primary stain for cellular detail — neuronal Nissl substance (rough ER — basophilic granules), nuclear morphology, and glial cell identification. Luxol Fast Blue (LFB) stains myelin blue-green and is the standard stain for evaluating myelination and demyelination. LFB with H&E counterstain (LFB-HE) or LFB-PAS demonstrates both myelin and cellular morphology in a single section. LFB with Cresyl Violet (LFB-CV) — the “luxol fast blue-cresyl violet” combination stains myelin blue and neurons violet, used for cytoarchitectural studies.
Bielschowsky silver stain impregnates neurofibrils and neuritic plaques, demonstrating neurofibrillary tangles (Alzheimer disease), neuritic plaques, and Pick bodies (Pick disease). Bodian silver stain is similar, visualizing axons and neurofibrils.
Glial Cell Identification by IHC
IHC has largely replaced traditional histochemical stains for cell-type identification in neuropathology. GFAP (glial fibrillary acidic protein) — the defining marker of astrocytes and astrocytic tumors (astrocytoma, glioblastoma). Olig2 — nuclear marker of oligodendrocytes and oligodendroglial tumors. Iba1 and CD68 — microglia/macrophage markers, upregulated in inflammation and neurodegeneration. NeuN — neuronal nuclear protein, identifies mature neurons and is lost in many neurodegenerative diseases. Neurofilament (NF) — axonal marker, demonstrates axonal density, spheroids, and torpedoes. Synaptophysin — synaptic vesicle protein, identifies synaptic density in neurodegenerative conditions.
Special Regions
Hippocampus — the CA1 region is selectively vulnerable to anoxia and ischemia. Hippocampal sclerosis is associated with temporal lobe epilepsy. Substantia nigra — pigmented neurons that degenerate in Parkinson disease; neuromelanin pigment is visible on H&E without special stains. Cerebellum — Purkinje cells are selectively vulnerable to hypoxia, toxins, and paraneoplastic syndromes. Spinal cord — motor neuron loss in amyotrophic lateral sclerosis (ALS).
Artifacts
Dark neurons — shrunken, hypereosinophilic neurons resulting from delayed fixation or traumatic handling of unfixed brain. They can be mistaken for ischemic or hypoxic injury. Swirling artifact — from traumatic tissue disruption during stereotactic biopsy. Cautery artifact — from electrocautery used during surgical resection, causing coagulative necrosis at tissue edges. Freeze artifact — from inadequate cryoprotection in frozen sections.
