PERIPHERAL NERVE: Active Demyelination 1,2

Myelin stripping, Cell mediated

Cell Processes inside Schwann cell basal lamina
  Phagocytic cell process (White Arrow): Extends beneath a layer of myelin
  Other, larger cell processes contain myelin breakdown products (Dark Arrow)
  Multiple, very small, round Schwann cell processes are located immediately under the basal lamina
Axon within myelin sheath: Intact
Macrophage & Processes (Top, Left): Outside the basal lamina
  Some macropahge processes extend near, but do not penetrate, the Schwann cell basal lamina

Electron micrograph: From Robert Schmidt MD
Also See: AIDP

Demyelinated axons
  Axoplasm is dark: Contains closely spaced neurofilaments.
  Without surrounding myelin
  Surrounded by cell processes: Contain inclusions
  Entire structure is surrounded by a Schwann cell basal lamina (Arrow)
    A pale Schwann cell process extends outward, but inside the Basal lamina (Bottom, Left)
    2 small, round cell processes are apparent inside the basal lamina (Bottom, Left)
Demyelinated Axon

Electron micrographs: From Robert Schmidt MD

Active Demyelination: Histiocytic Schwann cells & Myelin

Acid phosphatase stain
Most Myelin sheaths: Acid phosphatase+
  Compare to: Wallerian degeneration

MBP = Myelin Basic Protein; P0 = P0 protein
Myelin Structure: Irregular P0 & P0+MBP Sheaths


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1. Neurology 2018;91:1051-1060
2. J Neurol Neurosurg Psychiatry 2020;9:650-659