Cellular Pathology: Neuropathology I Notes

Learning Outcomes

  • Describe normal histology of neurons and glia in routine H&E stain
  • Discuss what routine H&E stain can and cannot reveal in histology of neurons and glia
  • Discuss the reactions of different types of cells in central nervous system to the injury and their corresponding morphological changes

Histology of Neurons in H&E Stain

  • Large neuron from spinal cord:
    • Large nucleus and nucleolus
    • Blue stippling - Nissl Substance
  • Neuron from cerebellar dentate nucleus:
    • Less Nissl substance than in large neurons
  • Large pyramidal neurons from hippocampus:
    • Large nuclei and nucleoli
    • Nissl substance
  • Two motor neurons from anterior horn of spinal cord
  • Hippocampal pyramidal neurons filled with a fluorescence dye
  • Neurons from cerebellar cortex
    • Cytoplasm of cells: eosinophilic (acidophilic), pink
    • Nuclei and nucleoli: hematoxylinophilic (basophilic), blue
    • Nissl substance: blue (hematoxylin), more distinctive with Nissl stain; basophilic material primarily composed of ribonucleic acid (RNA) and proteins in aggregate with rough endoplasmic reticulum

Neurons and Glia in Normal Cerebral Cortex

  • Neurones
  • Astrocytes - large pale nuclei
  • Oligodendrocytes - rounded nucleus, pale halo, “fried egg”
  • Microglia - small dark elongated nucleus
  • Capillaries
  • H&E: neuronal cell bodies are visible, the cytoplasm of glial cells is not well demonstrated, needs special stains.

Ependymal Cells

  • Ependymal-lined central canal of spinal cord
  • Ependymal-lined ventricle or central canal of spinal cord
  • Glial lineage, with epithelial characteristics: a basement membrane, cell-cell junctions and motile cilia.

Cerebellar Cortex with H&E Stain

  • White matter: Myelin stains dark pink
  • Nuclei and Nissl substance stain blue-purple
  • Everything else stains pink
  • Layers:
    • Molecular Layer
    • Purkinje Cell Layer
    • Granular Layer

Cerebellar Cortex Details

  • Folium: Long axis and transverse plane
  • Parallel fiber (PF)
  • Smooth branchlet
  • Spiny branchlet
  • Climbing fiber (CF)
  • Process of Purkinje cell (PC)
  • Sagittal plane
  • GoC: Golgi cell dendrite
  • GrC: Granule cell dendrites
  • PC axon
  • MF: Mossy fiber rosette
  • Golgi cell axon
  • SC: Stellate cell
  • BC: Basket cell
  • Mossy fiber
  • Glial capsule
  • Key:
    • Basket cell (BC), stellate cell (SC)
    • Climbing fiber (CF)
    • Purkinje cell (PC), Golgi cell (GoC)
    • Granule cell (GrC), parallel fiber (PF)
    • Mossy fiber (MF)
    • Multilayered fiber (MulF)

Cerebellar Cortex with Immunostain

  • Calbindin staining in cerebellar Purkinje cell somata and dendrites.
  • DAPI counterstaining of nuclei.
  • Calbindin staining showing spines on proximal and distal dendrites of Purkinje cells.
  • Calbindin staining showing abundant spines on distal dendrites of Purkinje cells.

Cerebellar Neurons with Golgi Stain

  • A-E: Golgi staining
  • BF: Bergmann fiber, a type of glial cell process
  • F: Electron microscopy

Histology of Peripheral Nerve

  • Cross section of peripheral nerve containing a single fascicle:
    • Schwann cells nuclei and pink myelin
  • Spinal nerve containing many fascicles

Histology of Neurons and Glia

  • Routine H&E staining reveals:
    • Main cell types can be identified, the main architectural arrangements are often distinctive.
    • Cytologic features of neurons in CNS can be readily characterised.
    • For glial cells, ependyma are easy to identify.
  • However:
    • Many specialised elements can only be seen with special stains or electron microscopy
    • Neuronal dendrites and axons are hard to identify in routine preparations. All cell processes merge into a background pink-stained matrix called the neuropil.
    • For other glial cells, only nuclei can be seen in routine preparations. Identification of glial cells in the brain requires special stains.

Response of CNS Tissues to Injury

  • Neurons are more vulnerable than neuroglia to environmental changes
  • Neuronal injury can be reversible – swelling of cell body, loss of Nissl body (chromatolysis)
  • Healing through granulation and fibrous scarring is uncommon in the CNS – lack of fibroblasts
  • Necrosis of brain tissue usually results in liquefaction, leaving a fluid-filled space

Inflammation

  • Common process:
    • Initial exudative response:
      • Serum, fibrin, and white blood cells move to injury site from circulatory system
    • Activation of local microglia, phagocytosis of dead tissue
    • Microgliosis:
      • Begins within hours of initial CNS injury
      • Oligodendrocyte precursor cells recruited to the site
      • After 3-5 days
      • May contribute to remyelination
    • Hypertrophy (↑\uparrow in size) and hyperplasia (↑\uparrow in number) of astrocytes – astrocytic scar
    • Astrogliosis, final component of gliosis
    • Insufficient to repair if necrosis is extensive – fluid filled space lined by gliosis

Reactions of Neurons to Injury - Morphology

  • Acute irreversible hypoxic-ischaemic injury in cerebral cortex:
    • Shrinkage of cell bodies
    • Condensation of nuclei
    • Nucleolus disappearance
    • Loss of Nissl substance
    • Cytoplasm eosinophilia
  • Axonal spheroids (swellings) are visible at points of disruption or altered axonal transport, in traumatic brain injury especially diffuse axonal injury.
  • Swelling of the cell body, and peripheral dispersal of the Nissl substance – central chromatolysis.

Reactions of Neurons to Injury

  • Central chromatolysis:
    • Usually occurs in response to axonal damage, also in ischaemia, metabolic or toxic insults, neurodegenerative diseases, trauma, infection, tumour, ECT
    • Soma (cell body) swells
    • Chromatin and nucleus are pushed to the cell periphery
    • Loss or dispersion of the Nissl substance starting near the nucleus
    • Peak at around 8 days
    • Increased RNA and protein synthesis – regenerative response
  • Anterograde degeneration:
    • Result of axonal transection
    • Anterograde degeneration of distal part of the axon
    • Usually accompanied by central chromatolysis
    • Fragmentation of myelin sheath
    • Phagocytosis of axonal and myelin debris by microglia and macrophages, remain around the injury site for several months
    • Axonal regeneration does NOT occur to a significant extent in the CNS
  • Transneuronal degeneration:
    • Anterograde (dying forward): caused by presynaptic injury or loss; e.g. AD
    • Retrograde (dying backward): caused by loss of trophic support from postsynaptic neuron/cell; e.g. amyotrophic lateral sclerosis (MND): rapid loss of lower motor neurons (dying forward) & slow loss of corticomotor neurons (dying backward)

Reactions of Astrocytes to Injury

  • Principle cells for repair and scar formation in the brain
  • Hypertrophy and hyperplasia
    • Following almost all forms of CNS damage
    • Reactive gliosis
    • Gliotic tissue translucent and firm, forming a barrier around injured area, e.g. at the edge of a cerebral infarct

Reactions of Astrocytes to Injury - Morphology

  • Neuronal death and astrocytic response:
    • Normal cells in the thalamus
    • Early hypoxic/ischaemic response: Dead neurons in the thalamus become shrunken and eosinophilic with pyknosis of the nuclei. Surrounding cells such as oligodendrocytes and microglia are unaffected.
    • Later response: No neurons are seen, a large number of astrocytes A with pink-stained cytoplasm are noted – gliosis.

Reactions of Astrocytes to Injury

  • The nucleus enlarges and becomes vesicular
  • The nucleolus becomes prominent
  • The previously scant cytoplasm expands and takes on a bright pink hue
  • The cells extends multiple stout, ramifying processes
  • In long standing gliosis, the cytoplasm shrinks, and the processes become more tightly interwoven
  • In chronic gliosis and some low-grade gliomas, thick, elongated, eosinophilic protein aggregates (rosenthal fibres, primarily glial fibrillary acidic protein (GFAP) and other heat-shock proteins) are found in astrocytic processes; special stains or IHC techniques provide much clearer and more specific visualisation.

Reactions of Microglia to Injury - Morphology

  • Microglial nodule in viral meningitis
  • Microglial nodule and multinucleate giant cell in HIV encephalitis

Reactions of Microglia to Injury

  • Bone-marrow-derived cells, resident phagocytes of CNS
  • Phagocytosis
    • Together with recruited blood monocytes
    • Ingest breakdown products of damaged myelin, distended with lipid droplets
    • Take on appearance of activated macrophages in areas of demyelination, infarct, or haemorrhage
  • In other settings (e.g. neurosyphilis or other infections), elongated nuclei (rod cells)
  • Microglial nodules – aggregates of elongated microglia at sites of injury

Reactions of Oligodendrocytes to Injury - Morphology

  • Section stained for myelin showing irregular, poorly defined areas of demyelination, which become confluent in places.
  • Enlarged oligodendrocyte nuclei stained for viral antigens surround an area of early myelin loss.
  • Limited capacity to react to injury

Reactions of Ependymal Cells to Injury

  • Limited capacity to react to injury
  • Certain pathogens, particularly cytomegalovirus (CMS), can causes ependymal injury
  • Viral inclusion seen inside the cells