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Pseudo-Onion Bulbs

Circumferential Schwann cell processes often surrounding Axons or Axon sprouts

Associations

• Axon Regeneration
  • Pseudo-Onion Bulb Structure
    • Central: Axon(s)
      • Regenerated axons & Axon Sprouts: Individual or Clusters, or
      • Larger myelinated axon
    • Peripheral region of Pseudo-Onion Bulbs
      • Layers of Schwann cell processes
      • Small, Unmyelinated axons: May be surrounded by Schwann cell processes
  • Other features
    • Axon Loss in nerve: Often moderate to severe
    • No demyelination
• Chronic Demyelination without Remyelination
  • Central: Demyelinated axon
  • Peripheral: Multiple concentric Schwann cell processes

Differential diagnosis

• Perineurioma
  
• Thinly myelinated axons surrounded by regions of collagen: No other associated cells or structures
• Onion bulbs
  
• Basal lamina onion bulbs (BLOBs): Very chronic demyelination (Demyelinating CMT)
  
• Büngner bands: Clustered Schwann cells, & their interdigitated processes, with no axons
  

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H&E stain

Whorled structures: Contain nuclei & connective tissue; Myelinated axon may be in center

VvG stain

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Beaded axon sprouts extend circumferentially around central large axon Neurofilament stain

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Toluidine blue stain

Whorled structures: Central myelinated axon is present within some pseudo-onion bulbs

Toluidine blue stain

Toluidine blue stain

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Pseudo-Onion Bulbs
  Around regenerated axons with thin myelin sheaths
  Also present in regions with no axons

Toluidine blue stain

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Pseudo-Onion Bulbs
  Around regenerating axon clusters

Toluidine blue stain

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Pseudo-Onion bulbs: Ultrastructure

Schwann cell processes surround central axon

From: Chunyu Cai

Pseudo-Onion bulbs
Multiple Schwann cell processes surround regenerated, thinly myelinated, axons

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Schwann cell processes may surround collagen

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Büngner Bands
Interdigitated Schwann cell processes

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Collagen Bulbs

Anatomy
  Collagen bulbs: Unstructured; Size 10 to 80 μM
  Axons: Within some collagen clusters
    Myelin: Thin; Similar thickness on all axons
    Number: 0 to 5
  Schwann cells: Few or Scattered small processes
  Schwann cell basal lamina: Few or None
  Fibroblast processes: May surround collagen clusters

Correlations
  Clinical: Polyneuropathy
    Anatomic pattern: Multifocal; Asymmetric
    Motor + Sensory
    Course: Slow progression
    Nerve Size: Enlarged, Multifocfal
  Electrodiagnostic
    Demyelinating Features
    Axon Loss: Patchy

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Toluidine blue stain

Collagen Bulbs
  Multiple clusters of endoneurial collagen: Some have myelinated axons inside

Toluidine blue stain

Toluidine blue stain

Collagen Bulbs
  Multiple clusters of endoneurial collagen: Some have myelinated axons inside

From: Sara Stone

From: Sara Stone

Collagen Bulbs
  Clusters of endoneurial collagen: Some have thinly myelinated axons inside

From: Sara Stone

From: Sara Stone

Collagen Bulbs
  Thinly myelinated axons surrounded by collagen

From: Sara Stone

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Collagen bulb
  No associated axon
  Mostly composed of collagen
  Surrounded by fibroblast processes
  Contains normal & large Schwann cell proceses

from: R Schmidt

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Collagen Bulb
  Central: Thinly myelinated axon
  Mostly composed of collagen
  Contains scattered Schwann cell proceses & nuclei
  Surrounded by fibroblast processes

from: R Schmidt

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from: R Schmidt

Collagen bulb
  Collagen & Schwann cell processes around thinly myelinated axon

from: R Schmidt

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Collagen Bulb
  Central: Thinly myelinated axon
  Mostly composed of collagen
  Contains scattered Schwann cell proceses & nuclei
  Surrounded by fibroblast processes

from: R Schmidt

from: R Schmidt

Collagen Bulb
  Collagen & Schwann cell processes around thinly myelinated axon

from: R Schmidt

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Return to: Onion bulbs

7/15/2024