Neuromuscular

AXON LOSS

Axon loss
  Myelinated
  Large
  Large & Small
  Large vs Small
  Differential fascicular
  Skin
  Schwann cell Δ
    Bungner bands
    Collagen pockets
Injury patterns
Wallerian degeneration

Gomori trichrome stain
Myelinated Axons (Red)
Loss: Moderatly servere

Oppenheim 1894


Nerve Injury

Degrees

Electrophysiology: Changes after nerve transection

Wallerian degeneration


Myelinated axons: Loss


Myelinated Axon loss: Moderate



VvG stain

Gomori trichrome stain

Myelinated Axon loss: Severe

Axons, Large & Small: Comparative changes

Myelinated Axon loss: Large > Small

Toluidine blue stain
Myelinated Axon loss: Small > Large

VvG stain

Large axons: Moderately severe loss
Small Axons: Relatively preserved:
  Many Small axons, but Few large, myelinated axons
  Small axons are diffusely distributed, not clustered, in endoneurium

Neurofilament stain
Remaining Large Myelinated Axon (Arrow)

Neurofilament stain

Axon loss, severe: Large & Small axons are both markedly reduced

Neurofilament stain
Myelinated axons: Severe loss

VvG stain

Wallerian Degeneration 5

Pathology
  Stages
    Axon degeneration & loss
      Early
        Neurofilament loss
        Myelin
    Myelin
      Irregular structure
      Degradation
        Fragmentation
        Intermediate
        Ovoids
        To lipids
      Remnants
      Also: Eosinophilic vasculitis
    Macrophages
    Schwann cells
    Ultrastructure
      Early
      Lipid debris
      Ovoids
      Schwann cells, Autophagic
  Post-WD
    Axon regeneration
    Bungner bands
    Collagen pockets
Molecules
Principles
Alternate Degeneration Process
  Trophic-Withdrawal
Axon Loss & Wallerian Degeneration
  Event Timing in Nerve

Myelin Ovoids

Waller

Remak


Wallerian Degeneration: Principles & Features

  • Wallerian degeneration: Definition
  • Wallerian Degeneration: Morphological & other changes in nerve constituents
    • Stimulus for Wallerian degeneration
      • Distal axon loses connection with proximal axon
    • Wallerian degeneration: Axon changes 7
      • Early
        • Minutes: Changes in axon segments near transsection
          • Short-distance (200 μM): Acute axon degeneration (AAD)
          • Mediated by
            • Extracellular Ca++ influx
              • Anterogradely conduced wave in distal axon
            • Activation of calpain
              • Ca++-dependent serine-threonine protease
              • Intracellular enzyme
          • Inhibited by: Ca++ channel blockers
        • Hours to 2 days
          • Regions near injury: Accumulation of organelles & mitochondria
            • Dystrophic bulbs
            • Occur at both transected ends
            • Mechanism: Anterograde & retrograde axon transport
          • Proximal & Distal axon: Retraction from injury site
          • Distal axon
            • Remains morphologically intact & electrically excitable
            • Axon transport (anterograde & retrograde) continues
          • Proximal stump: Produces sprouts within hours after axotomy
      • Later (≥ 1 to 3 days): Changes in Distal axon
        • Endoplasmic reticulum: Loss of structure
        • Cytoplasm: Neurofilament & Cytoskeleton Degradation
          • Associated with influx of Ca++
          • Activation of calpain
          • Autophagy-related
        • Mitochondria: Swelling
        • Distal axon morphology: Granular degeneration
          • Becomes fragmented
          • Phagocytosis
          • Direction after focal injury: Proximal to Distal; Rate of up to 24 mm/hr
      • Timing
        • Before degeneration
          • Distal axon segment may remain electrically excitable
          • Sensory responses persist 2 to 3 days longer than motor
          • Conduction failure may precede axon degeneration
          • Time to loss of excitability
            • Facial nerve: 4 to 7 days
            • Arm nerves, Motor: 5 to 7 days
            • Arm nerves, Sensory: 7 to 10 days
        • Degeneration
          • Begins several days (4 to 10) after axonal transection
          • Progression
            • Possible length dependence
            • May be from proximal to distal axon or diffuse
          • More rapid with shorter distal stump
        • WD Delayed by: Molecules & Factors
          • Temperature: Reduced
          • Extracellular Ca++: Lowered
          • Ca++ channel (L-type) blockers
          • Mutations or Loss
            • NMNAT1: WldS
            • NMNAT2 8
              • General
                • Chaperone
                • Aids in refolding of misfolded proteins
              • NAD+ biosynthesis
                • Catalyzes
                  • Nicotinamide mononucleotide (NMN) →
                      Nicotinamide adenine dinucleotide (NAD+)
              • Controls SARM1 activation
                • NAD+: Inhibits SARM1 (Inactive TIR domains)
                • NMN: Associated with SARM1 activation (Active TIR domains)
              • Present in axon cytoplasm
                • Carried down axon by anterograde axon transport
                • Short half-life: Rapidly lost after axon transection
                • 2 pools: Vesicular & Non-vesicular
              • Axons
              • Disease: Polyneuropathy & Erythromelalgia
            • NOS knockout
            • DR6 (TNFRSF21) 9
            • SARM1 ↓ Activity (Inhibition) 8
              • Toll-like receptor adapter protein
              • SARM1 structures
                • Forms octameric ring: via SAM domain
              • SARM1 Loss
                • Axon degeneration: Slowed
                • Types of axon degeneration affected
                  • Transsection
                  • Dying back (Chemotherapy)
              • SARM1 activity
                • Required for early injury induced axon degeneration
                  • Causes damage by: Reducing NAD+ in axon cytoplasm
                • SARM1 activated by
                  • High NMN/NAD+ ratio
                  • TNF-α (Neuro-Inflammatory signal)
                  • MLKL (Necroptosis-like signal): Via inducing loss of
                  • Axotomy
                    • Blocks delivery of labile axon survival factors (NMNAT2)
                    • Factors normally inhibit SARM1
                • Mechanism of SARM1 action: Stimulates NAD+ cleavage & loss
                  • Active SARM1 protein domain
                    • Toll/Interleukin-1 receptor (TIR)
                    • Possesses intrinsic NAD+ cleavage (NADase) activity
                  • NADase activity increased
                    • NAD+ converted to ADPR (ADP Ribose) products
                    • Leads to Ca++ influx & Axon degeneration
              • SARM1 required for: Vincristine & Bortezomib induced axon degeneration 12
                • Vincristine
                  • Stimulates axon autonomous degeneration via MAPK pathway
                  • Mediated by MAP3K12 & MAP3K13
                • Bortezomib
                  • Induces axon degeneration via neuron cell body
                  • Mediated by
                    • Activated caspases (Caspase-3 cleavage) in axon
                    • Transcriptional regulation
                  • Apoptosis-like
                  • Similar mechanism to NGF withdrawal
              • Disease association: ALS & SPG 13
                • Gain of function polymorphisms
                • Frequency: 0.12% vs Not seen in controls
            • DLK (MAP3K12) : Loss of function
            • jnk (MAPK8) : Signaling requires DR6
          • Inhibition: JNK kinase ; GSK3; IKKB (IKBKB)
        • WD More rapid
          • Galectin-3 loss
            • Increased pro-inflammatory cytokines
              • IL-1β, TNF-α, Toll-like receptor (TLR)-2 & -4
            • Increased phagocytic capacity of Schwann cells & macrophages
        • No effect: NGF
      • WD: Molecular events 1
        • Early in axons
          • Loss of m-Calpain
          • Ca++ entry
        • Associated cytokines
          • Early: TNFα & IL-1α
          • After delay: IL-1β
        • Inhibitory molecule: MOX2 (CD200) inhibits macrophage lineage cells
        • Not related to bcl-2 or caspase activation
      • Molecules upregulated in neurons after axotomy
        • STAT-3 protein : Associated with CNTF stimulation
        • Activating transcription factor 3 : Acts as heterodimer with jun proteins
        • Nna1 (ATP/GTP-Binding protein 1; AGTPBP1) : Motor neurons
          • Putative zinc carboxypeptidase
          • Presumed nuclear localization
          • Adenosine triphosphate/guanosine triphosphate binding motif
          • Genetics
            • Disorder: CONDCA
            • Mutations (Animal): Purkinje cell degeneration (pcd) mouse
        • Nerve injury associated kinase: Sensory neurons
      • Molecules upregulated in nerve distal to transection
        • Early activation of erbB2 4
          • Related to: Schwann cell demyelination after axotomy
          • Time course
            • Early activation: Occurs 10 to 180 minutes after nerve damage
            • erbB2 also increased late (days) after nerve transection
          • Anatomy
            • Originates in microvilli of Schwann cells, in direct contact with axon
            • Localized to nodal region of myelinating Schwann cells
            • Activation occurs near & distal to nerve transection site
          • Related features
            • MAPK is also activated early after nerve transection
            • ATP mimetic PKI166 (Blocks erbB2 activation)
              • Reduces ovoid accumulation in Schwann cell cytoplasm
            • Neuregulin coreceptor erbB3 participates in the rapid activation
            • Neuregulin in vitro
              • Induces demyelination
              • Mimics early response of Schwann cells to nerve damage
        • Ninjurin1
          • Adhesion molecule
          • Induced in injured DRG neurons & Schwann cells
        • Ninjurin2
          • Adhesion protein
          • Expressed constitutively by mature sensory neurons
          • Induced in Schwann cells in distal segment of lesioned nerve
        • Glial cell line-derived neurotrophic factor (GDNF)
        • GDNF family receptor α1 (GFRα1)
        • Disintegrin CRII-7/rMDC15
          • ADAM (a disintegrin and metalloprotease) gene family
        • FGF-2
        • IL-6: Pain-inducing cytokine
        • TNF-α : Macrophage recruitment from the periphery
        • SDF-1γ (Stromal cell-derived factor (SDF)-1 isoform)
      • Molecules reduced in nerve distal to transection
        • SCG10 (Stathmin-like 2; STMN2) 14
          • Reduced in distal stump before morphologic changes
          • Promotes regeneration in proximal stump
          • Heterozygous knockout: Motor axon damage
          • Reduced in spinal cord in ALS
      • Mice with slow Axon degeneration
        • C57BL/Wlds
          • Genetics
            • Mutation: 85 kb tandem triplication on distal mouse chromosome 4
            • Mutated region contains 2 associated genes
              • Nicotinamide mononucleotide adenylyltransferase (NMNAT1; D4Cole1e)
              • 5' end of ubiquitination factor E4B (Ube4b)
          • Proteins
            • NMNAT1
              • Subcellular location: Nuclear; May act in cytoplasm
              • Expressed in: Skeletal muscle, Heart, Liver, Kidney & Brain
              • Function: NAD biosynthesis
              • Probably the component responsible for axonal protection
                • NMNAT1 enzyme activity required for axon protection
                • Sirt1 (NAD-dependent deacetylase)
                  • Downstream of NNMNAT
                  • Contributes to axonal protection
            • E4B
              • Subcellular location: Cytoplasmic
              • Expressed in: Skeletal muscle, Ovary, Testis & Heart
              • Functions
                • Binds to ubiquitin moieties of conjugates
                • Catalyzes ubiquitin chain assembly
            • WldS mutation
              • Chimeric gene product: 1st 70 AA Of Ube4b + NMNAT1 full sequence
              • Causes ectopic localization of NMNAT1 (NAD+ biosynthesis enzyme) to axons
              • May augment NMNAT2
                • Maintains NMNAT enzyme activity in distal axons after injury
              • Loss of NMNAT2 causes axon loss in vitro
              • Site of action may be axonal ER/Golgi or mitochondria
          • Mutation effects on proteins: Increased expression
          • Mouse effects
            • Wallerian degeneration delayed by 3 to 4 weeks
            • Axons less susceptible to vincristine toxicity
            • pmn mouse: Slower progression of disease
            • SOD1/ALS (SOD1-G93A) mouse 3
              • Slightly longer survival
              • Delayed denervation at NMJ
        • Neuronal nitric oxide synthase knockout 2
          • Slow Wallerian degeneration
          • Delayed regeneration
          • Incomplete pruning of axon sprouts: Enhanced number of axons
    • Wallerian degeneration: Myelin changes 10
      • Early
        • Anatomic
          • Schmidt-Lantermann incisura: Widened
          • Myelin layers: Irregular
          • Ovoid formations begin at these loci
        • Molecular
          • Expression of Phospholipase A2 (Lipolytic enzyme)
          • Activation of neuregulin-ErbB2 signaling (Demyelinating mechanisms)
          • Actin polymerization
          • E-cadherin recycling
      • Paranodal myelin retraction
      • Myelin "collapse" & fragmentation
        • Myelin Degeneration
        • Degenerative changes of myelin: Patterns
          • Initial
            • Within "Demyelinating" (Post-myelinating) Schwann cells
            • Subcelllular: Begins along Schmidt-Lanterman clefts
            • Associated with: c-Jun activation; Increased autophagic activity
          • Anatomic along nerve: Distal to proximal direction
          • Axon types: Small axons before Large axons
          • Molecular markers of Schwann cell & Macrophage activity
            • Mixed lineage kinase domain-like protein (MLKL) 11
              • Location: Increased in Schwann cells & Macrophages
              • Binds to sulfatide
              • Final executioner of canonical necroptosis
              • Induces loss of axon survival factors NMNAT2 & STMN2 to activate SARM1
              • Myelin degradation associated with phosphorylation of MLKL serine 441
              • MLKL knockout: Reduces or delays myelin breakdown & axon regeneration
            • Myelin basic protein (MBP): Present in myelin fragments
          • Consequence: Often signal, or attract, phagocytic macrophages
          • Ovoid formation
        • Primary ovoids: Early change (1 to 2 days)
          • Intracellular (Schwann cell) myelin: Fragmentation
          • Abaxonal Schwann cell cytoplasm
            • Dilated
            • Contains rough endoplasmic reticulum (RER) & vesicles
        • Secondary ovoids (Myelinosomes): Later change (3 to 7 days)
          • Pinched off from primary ovoid
          • Compact myelin structures in Schwann cell cytoplasm
          • Exocytosed from Schwann cell cytoplasm
            • Into abaxonal extracellular space
            • Myelin debris: May be further degraded by macrophages
        • Macrophages
          • ? Attracted by cytokines
          • Phagocytosis & degradation of myelin debris
    • Schwann cells
      • Changes
        • Proliferation: Especially non-myelinating Schwann cells
        • De-differentiation: Myelinating Schwann cells
          • Develop autophagic properties
          • Degrade myelin sheath
      • Auto-autophagy: Myelinating Schwann cells 6
        • Contain & Degrade: Myelin debris (Myelinophagy); MPZ & MBP proteins
        • Autophagy markers increased: LC3-II; Wipi2
        • Most prominent in 1st week after nerve injury
        • Less autophagy by CNS oligodendroglia after axon damage
      • Form Bands of Büngner
        • Definition: Arrays of Schwann cells & processes within basement membrane
        • Molecular: Büngner band Schwann cells express both NCAM & P0 protein
        • Provide substrate for axonal regeneration
        • Long-term: Schwannn cells atrophy and disappear if axonal regeneration does not occur
    • Phagocytes (Macrophages): Degradation of myelin breakdown products to lipid debris
      • Origin
        • Mostly hematogenous incoming
        • Recruitment regulated by: Raf–MEK–ERK mitogen-activated protein kinase
        • Invasion of nerve 3 to 4 days after axon transection
      • Phagocytosis of sudanophilic (lipid) debris: Appear as foamy cells
      • Complement is necessary for phagocytosis
      • Clear axonal & myelin debris
      • Course: Cells may persist for 3 to 7 months
    • Fibroblasts
      • Proliferate during 1st week
      • Migrate adjacent to degenerating fibers
      • Produce some collagen
    • Blood-nerve barrier
      • Loses integrity during early degeneration & regeneration
      • Re-established over months

Alternate axon degeneration pathway: Trophic withdrawal induced
  • Stimulus: Loss of trophic factors (NGF)
  • Site of degeneration: Distal axon
  • Pathway components
    • Membrane related: p75; DR6
    • Bax
    • Caspases: 9; 6; 3

Remak

Oppenheim 1894

Wallerian Degeneration: Pathology

Axons
  Early
    Damage & Loss
    Relationship to myelin
  Regeneration
Myelin
  Early
    Structure changes
  Degeneration
    Early
    Fragmented in Schwann cells
  Degradation
    Lipid & Myelin debris
  Ovoids
End Stage
  Collagen pockets
  Bungner bands
Schwann cells
  Autophagic
Ultrastructure

Waller illustration



5 days after nerve section (Hypoglossal nerve)

Axon Degeneration: Patterns & Morphology


Axons: Degeneration, Ongoing, Early

Axon Loss
  Histology
  Morphology/Ultrastructure
    Very Early
    Axoplasm loss
    Organelle aggregation
Myelin Pathology
  Histology
  Periphery
    C5b-9 deposition
    MxA+ cells
  Myelin Ovoids/Remnants
Schwann cells
  Autophagic
  Ultrastructure
    Inner layers
    Compact
    Outer layers
    Later stages
      Ovoids
      Autophagic Schwann cells
      Cells with Lipid debris

Degeneration of Myelinated Axons: Ultrastructure


Axon Loss: Early

Neurofilament stain of Axons: Reduced or Absent
Neurofilament-stained axons: Lost within regions of myelin (MBP stain)
Axons: Neurofilament stain
  Reduced or Absent staining for Large axons (Yellow)
  Staining for Small axons (Green) remains
  Mechanism of axon loss: SARM1
Myelin-Basic Protein (MBP) (Red): Present in remaining myelin
  Regions with MBP stain are abnormal: Costain with NCAM
Also see: Control Nerve

Neurofilament stain (Green or Yellow); Myelin Basic Protein stain (Red)

Control Nerve
  Myelin basic protein (Red) surrounds large axons (Yellow)
  Normal clusters of small unmyelinated axons (Green)

Neurofilament stain = Green; Myelin basic protein stain = Red
Also see
  Control nerve
  Moderately later pathology
  Large Axon loss: Chronic


Neurofilament stain
Reduced, or absent, staining of large axons within pale areas that normally contain axons & myelin
  Clusters of small, unmyelinated, axons are normally stained
Compare to: Control nerve

Neurofilament stain

Myelinated Axon Degeneration: Very Early

Axon Pathology


From: R Schmidt

WD Very Early: Axoplasm Aggregates

WD Very Early: Axoplasm: Dark & Homogeneous

WD Very Early: Axoplasm
  Pale
  Few organelles

WD Very Early: Axoplasm, Ultrastructure
  Aggregated organelles
    Clustered regionally in axon: Near mildly abnormal myelin structure


From: R Schmidt
WD Very Early: Axoplasm, Ultrastructure
  Volume: Reduced
  Structure
    Irregular or Pale
    May contain vesicles or organelles
Myelin sheath: May appear very thick compared to axon size

From: R Schmidt


From: R Schmidt
WD Very Early: Axoplasm, Ultrastructure
  Volume: Reduced
  Structure
    Irregular or Pale
    May contain vesicles or organelles

WD Moderately Early: Axoplasm loss & Pathology

From: R Schmidt
Axoplasm
  Volume: Reduced
  Structure: May be irregular
Myelin sheath: Appears very thick compared to axon size

From: R Schmidt

From: R Schmidt
Axoplasm
  Volume: Reduced
  Structure: May be irregular
Myelin sheath: Appears thick compared to axon size


From: R Schmidt

Axoplasm
  Blebs: May be present between damaged axon & myelin (Below)

Myelin ± Axon Pathology: Early


From: R Schmidt
Axoplasm
  Clustered organelles
Myelin: Early WD Changes
  Internal Pathology: Bleb, from internal layers of myelin sheath, indents axon
  Sheath: Thick compared to axon size

WD Very Early
  Myelin
    Abnormal structure of internal layers
  Axon
    Aggregated organelles
      Clustered regionally in axon: Near abnormal myelin structure
WD Very Early
  Axon
    Aggregated Organelles
    Remnants of axon within remaining compact myelin
  Myelin: Irregular internal areas
  No associated phagocytic cells

From: Robert Schmidt MD

From: Robert Schmidt MD

WD Early
  Myelin: Blebs from inner myelin layers indent axons

Compact Myelin: Abnormal Structure
Axoplasm Pallor
  Axoplasm is pale
  Organelle aggregates are present
Myelin Early WD Changes
  Structure of compact myelin is disrupted

Outer Myelin Layers: Abnormal Structure
Axoplasm
  May be: Pale or Aggregated
Myelin Early WD Changes
  Myelin outer layers: Abnormal structure
  Remaining myelin: Thick compared to axon size


WD, Early: Ab-Axonal Schwann cell Cytoplasm
  Pale background
  Irregular aggregates

Wallerian Degeneration

Wallerian Degeneration, Days to Few Weeks
  Regions of Myelin basic protein (Red) often have no associated axons
  Loss of small unmyelinated axons (Green)

Neurofilament stain = Green; Myelin basic protein stain = Red

Loss of Large Axons: Moderately later than above
  Axons (Yellow & Green; Neurofilament stain)
    Absent, or Reduced large axons: Axons inside Myelin basic protein stained myelin (Schwann cells)
    Areas where axons are lost are black
    Small axons are relatively preserved
  Myelin basic protein stain (Red)
    Abundant
    Shapes: Variable and irregular
    No associated axons
    Central dark areas where axons are lost

Regions of Myelin Basic Protein (Red) have no associated Neurofilament-stained (Yellow) axons

Neurofilament (Green) & Myelin basic protein (Red) stain
Also See
  Control nerve for comparison
  Axon loss: Earlier

Axon Loss: Myelin Pathology


H&E stain
Myelin
  Stain: Reduced or lost
  Structure: Lost or clumped

Gomori trichrome stain

VvG stain

H&E stain
Myelin
  Stain: Reduced or lost
  Structure: Lost or clumped

Gomori trichrome stain

Gomori trichrome stain

Myelin is still present

VvG stain

Many Myelin sheaths are paler stained

VvG stain

Wallerian Degeneration: Schwann cells, Myelinating: Molecular pathology
  NCAM expressed in myelin sheath

NCAM stain
  Axon Degeneration, Early/Myelin Pathology (Above): Increased NCAM expression in Schwann cell cytoplasm/Myelin sheath
  Normal (Below): NCAM expressed mainly in adaxonal cytoplasm

NCAM stain
Normal nerve: Schwann cells, Non-myelinating
  Normal (NCAM+) Non-myelinating Schwann cells
    Present in clusters between myelinated axons
    Not present in most regions of myelin sheath
  Also see: Large axon loss, Chronic

Abnormal Co-localization of NCAM & MBP in myelin remnants (Yellow)

NCAM (Green) + Myelin basic Protein (MBP) (Red) stain
Also see: Control nerve

Autophagic Schwann cells


Acid phosphatase stain
Acid phosphatase stains
  Fragmented myelin
  Scattered endoneurial histiocytes

Acid phosphatase stain

Myelin & Axon degeneration: Ongoing, Early

  Fixed nerve

Toluidine blue stain
Axon & Myelin Degeneration: Many circular compact myelin profiles with
  Irregular, pale or dark, central regions
  No phagocytes or fragmentation
  Irregular morphology

Toluidine blue stain
Axon degeneration: Early
  Irregular myelin figures
  Some remaining axons have dark axoplasm
  No histiocytes (with lipid droplets in cytoplasm)

Toluidine blue stain

Wallerian degeneration: Schwann cell pathology

C5b-9 stain
C5b-9: Stains Schwann cell processes around myelin sheaths

C5b-9 stain


MxA stain
MxA: Stains Schwann cell processes around myelin sheaths

MxA stain

MxA stain

Wallerian Degeneration: Intermediate stages

Myelin Breakdown: Fragmentation & Degeneration in Schwann cells

Gomori trichrome stain
Wallerian degeneration: Early; Myelin fragmentation
  Myelin in phagocytic, post-myelinating cells: Clustered red (GT) or Black (VvG) endoneurial stain
Also see: Normal control nerve

VvG stain


Acid phosphatase stain
Wallerian degeneration: Myelin fragmentation stage
  Large endoneurial cells (red) contain prominent lysosomal activity

Acid phosphatase stain

Large, Post-myelinating (Autophagic) Schwann cells
  Autophagic
  Contain
    Fragmented myelin (Light arrow): In various shapes & stages of degeneration
    Lipid droplets (Dark arrow): Small, Round & Clear

Toluidine blue stain

Myelin fragmentation in Post-myelinating (Autophagic) Schwann cells


Electron micrograph: From Robert E Schmidt MD
Early phase: Autophagic Schwann cell
  Myelin, compact: Fragmentation inside Schwann Cell
  Schwann cell characteristic: Surrounded by Basal lamina (Below; Arrow)
  Axon is lost

Autophagic Schwann cell
  Contains: Myelin fragments & Lipid droplets

Robert E Schmidt MD


Myelin Degradation: Later phase
  Autophagic Schwann Cells contain
    Lipid debris
    Myelin fragments, small
  Axons: Lost

Electron micrograph: From Robert E Schmidt MD

Electron micrograph: From Robert E Schmidt MD

Myelin & Axon Degeneration: Ongoing, Weeks; Myelin Degradation to Lipids

Ultrastructure

Macrophages & Phagoocytic cells 15


Acid phosphatase stain
Phagocytic cells, Endoneurial



Toluidine blue stain


CD68 stain
Macrophages


Toluidine blue stain
Wallerian degeneration: Ongoing
  Irregular, large myelin figures
  Many histiocytes: Contain small round lipid droplets in cytoplasm
  Axons are degraded & lost

Toluidine blue stain

Toluidine blue stain
Wallerian degeneration: Ongoing myelin degradation
  Irregular, large myelin figures
  Many histiocytes with lipid droplets in cytoplasm
  Axons are degraded & lost

Toluidine blue stain

Toluidine blue stain

Toluidine blue stain

Myelin Ovoids & Remnants

Myelin ovoids: Longitudinal section of nerve

Toluidine blue stain

Myelin Ovoids: Teased axons
  Top: Myelinated axon, control (Node of Ranvier at Arrow)
  2nd row: Schwann cell sheath with no remaining myelin fragments or axon
  Below: Myelin ovoids & remnants along paths of previously degenerated myelinated axon

Myelin Remnants: Features
  Irregular laminated structure
  Within Schwann cell
  No associated axon

Histiocytes: Debris- & Lipid-containing

Phagocytes (vacuolated) in endoneurium
Large cells: Contain myelin debris or round, clear lipid droplets.
Normal axons with thin & thick myelin sheaths may also be present.



Toluidine blue stained plastic sections
Myelin profiles & Lipid droplets in phagocytic (Probably Schwann) cells

Toluidine blue stained plastic sections

Early WD: Histiocytes
  Myelin
    Myelin outer layers: Irregular structure
  Schwann cell, Ab-Axonal cytoplasm
    Contains myelin fragments
  Histiocytes
    Present in areas around axon


Histiocytes with Myelin fragments around a cell with a large, partially degraded Myelin sheath


Myelin Degradation: Late stage with Lipid droplets & some Myelin fragments

Ultrastructure: Myelin debris & Lipid droplets in Schwann cells & Histiocytes
  Some cells have mostly myelin debris (Upper right)
  Some cells have many clear, round lipid droplets

Histiocyte: Endoneurial
  Contains many clear, round Lipid droplets
  Surface: Shows several processes extending into endoneurium

Lipid droplets in Schwann Cells & Macrophages
Schwann cells (Dark arrows) &
  Macrophages (Light arrows)
    Closely-apposed
    Phagocytic
    Contain lipid droplets &
      Myelin debris




Phagocytic cells: Contain Myelin & Lipid debris

Wallerian Degeneration, Later stage: Histiocytes with Lipid debris around Endoneurial Microvessels


From R E Schmidt MD
Macrophages, Perivascular

From R E Schmidt MD

Axons, Regenerating


  Surrounded by Schwann cell cytoplasm
  Unmyelinated (Below)
  Thinly myelinated axon (Far right)




From R E Schmidt MD

From R E Schmidt MD

Denervated Schwann cell bands
  (Bands of Büngner)

  • Due to: Myelinated axon loss
  • Basement membrane: Irregular
  • Size: > 3 μM

From R E Schmidt MD

Collagen Pockets

  Collagen fibrils surrounded by Schwann cell processes
  More common with
    Loss of small axons
    Increased age
  No axon regeneration

RE Schmidt MD

Electron micrograph: From Robert E Schmidt MD
Collagen Pockets
  Collagen fibrils surrounded by Schwann cell processes

Electron micrograph: From Robert E Schmidt MD

Schwann cell processes within & around collagen fibrils

Loss of Myelinated, & Some Unmyelinated, Axons

Unmyelinated axons: Mild loss
  Normal: Yellow (Green axon + Red NCAM stained Schwann cell)
  Lost: Non-myelinating Schwann cells (NCAM stained, Red) without associated axons (Arrow)
Myelinated axons: Severe loss
  Markedly reduced numbers of larger green (neurofilament) axons
    Few remaining large axons are small
  See
    More severe small axon loss
    Control (Below)

Neurofilament stain (Green) + NCAM stain (Red)

Large Axons: Nearly complete loss (Chronic)
  Large axons: None stained
  Small axons: Many preserved
  Myelin basic protein: Co-stains (Yellow) on many smaller axons

Neurofilaments (Green); MBP (Red)

Control nerve

Neurofilaments (Green); NCAM (Red)
Myelinated Axons (Large, Green)
  2 sizes: Large & Intermediate
  No associated NCAM positive (red or yellow) cells
Unmyelinated axons (Small, Yellow)
  Occur in clusters
  Co-stain (Yellow) for neurofilaments & surrounding non-myelinating Schwann cells

Also see
  Large axon loss
  Large & Small axon loss

Skin: Normal & Axon loss

Biopsy
  General
  Images
  Technical features

Skin Innervation & Biopsy: General

Skin: Normal innervation


PGP 9.5 stain: Glenn Lopate

 

Skin: Pathologic innervation

Beaded axons


PGP 9.5 stain: Glenn Lopate
Axon loss

 

Myelin Artefact

Vesiculated myelin


Go to Normal nerve
Return to Pathology & Illustrations
Return to Neuromuscular Home Page

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3/25/2025