Neuromuscular

DENERVATION & REINNERVATION OF MUSCLE

Fiber properties

Chronic partial denervation
Reinnervation: Fiber type grouping
  Early
  Late
Grouped atrophy
  Large
  Small
  ALS
Pyknotic nuclear clumps
Target fibers

Denervating disorders
  Amyotrophic lateral sclerosis (ALS)
  Bulbospinal Muscular atrophy
  Spinal muscular atrophy

Denervated muscle fibers: General properties

PYKNOTIC NUCLEAR CLUMPS

TARGET FIBERS 1

Description
Histochemistry
Ultrastructure
Botulinum toxin
  • Clinical correlations
    • Often seen with
    • Rare with: Diffuse denervation
    • Etiology: ? Related to stretching of muscle fibers
  • Histochemistry
    • Targets more common in
      • Gastrocnemius muscle
      • ? Type 1 muscle fibers
    • Shape
      • Transverse view: Rounded, polygonal or irregular
      • Longitudinal
        • Extend along length of muscle fiber: Up to 500 μM
        • Not generally from end-to-end of fiber
    • NADH stain
      • Best visualization of target architecture
      • Architecture
        • Typical: 3 concentric zones
        • Central zone: May be clear, dark, or loculated
      • Fiber size
        • Often in normal-sized or hypertrophied muscle fibers
        • ALS: Targets common in small muscle fibers
      • Differences from cores
        • Central zone
          • Non-homogeneous: Several different regions
          • Central NADH stain: May be dark or light
        • Periphery of target: Often dark stained
        • Frequency
          • Cores: Occur in all or most muscle fibers
          • Targets: Regional or Scattered
        • LC3 aggregates: Smaller or absent in cores
        • Fiber type association
          • Cores
            • Occur with marked type I fiber predominance
          • Targets
            • Both fiber types usually present
            • May occur selectively in one fiber type
    • SDH stain
      • Most sensitive for targets
        • Internal rounded clear regions in muscle fibers
      • Doesn't stain internal architecture
    • Associated structures: Cytoplasmic bodies (or ? Rods)
      • May occur within, or around, targets
  • Targets: Molecular
  • "Targetoid" fibers: NADH stain
    • Irregular internal clear regions in muscle fibers
    • Less distinct regional organization than targets
    • Contain similar molecular components to targets


NADH stain
Target Fibers
  Clear central regions
  Surrounded by layers with varied staining

NADH stain

Targets: Histochemistry


NADH stain

NADH stain
Targets
NADH stain
Targetoid fibers


Gomori trichrome stain
Targets
Targets: Varied forms

NADH stain

NADH stain
Target fibers

NADH stain

NADH stain
Targets: Varied stages of development

NADH stain

Targets: May be visible on H&E stain as aggregates (Above) or internal clear regions (Below)

H&E stain
 
Targets: May be visible on ATPase

ATPase pH 9.4 stain
 Target & Targetoid fibers

NADH stain
Targets
  May be visible on Gomori trichrome and VvG stains as rounded aggregates with pale surround

VvG stain

VvG stain
Targets: AMPDA
  Targets are visible on AMPDA stain
  AMPDA is present inside targets

AMPDA stain

Gomori trichrome stain

Gomori trichrome stain
Targets
  May be associated with Cytoplasmic bodies (or Rods)

Gomori trichrome stain
 
 

 
Aggregated LC3 in Target formations

Target Fibers: Ultrastructure


From: Namita Sinha
Target Fiber: Zones
  Outer area: Normal structure
  Intermediate zone: Mild Z-line irregularities
  Central zone: Contains Z-line-like material; No mitochondria or glycogen

From: Namita Sinha

From: Namita Sinha
Dark stained material in central zone of targets: May show continuity with Z-line

From: Namita Sinha

From: Namita Sinha
Targets contain
  Dark, electron-dense Z-band-like material
  Myofilaments between Z-band material
  Autophagic material: Stains strongly for LC3

From: Namita Sinha

From: Namita Sinha
Disordered sarcomeres in targets (Top half of images)
  Disorganization of Z-disks
Target: Central region may contain tubules & vacuoles

From: Oliver Ni

Grouped Atrophy of Muscle Fibers

H&E stain
NADH stain
ATPase pH 9.4 stain
Non-specific esterase stain
  • Small angular muscle fibers in clusters
    • Basophilic: H&E stain (Top left)
    • Dark Stain: NADH; Esterase; Desmin, Spectrin & Leu-19 (Myoblast & satellite cell related antigen)
    • Fiber Shapes
  • Denervation in ALS: Grouped atrophy with little type grouping (ATPase)
    • Atrophic groups: Multiple; Small (Often < 20 muscle fibers)
    • Small fibers: Often
      • Angular
      • Esterase positive
      • Contain targets
    • Within individual atrophic groups: Both types I & II fibers often present
    • Remaining large muscle fibers: Both fiber types
  • Denervation in SMA
    • Atrophic groups: Larger
    • Small fibers: Often rounded
    • Remaining large muscle fibers: Often type I
  • Partial denervation, Chronic
  • Differential Diagnosis

Denervation-Reinnervation: Sub-Acute

Partial Denervation with Reinnervation: Recent
  Many intermediate-sized angular muscle fibers: Present in clusters
  Larger fibers are normal-size to slightly large
Partial Denervation with Reinnervation: Recent
  Smaller angular fibers: Stain dark; Contain targets

ATPase pH 4.3 stain
Partial Denervation with Reinnervation: Recent
  Fiber type changes
  Early fiber type grouping
    Incomplete type switching
      Many fibers in groups are type 2C
      Intermediate staining with ATPase pH 4.3 & 9.4

ATPase pH 4.3 stain
Early reinnervation: Intermediate staining fibers on ATPase pH 9.4

ATPase pH 9.4 stain
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References
1. Neuropathol Appl Neurobiol 2023;49:e12853

2/11/2025