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Extraocular Muscles (EOM)

Normal Dysthyroid Orbital Myositis PEO

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Rectus EOM: Structure ¹

• Layers
  • Global
    • Location: Internal; Near eye globe
    • Insertion: Globe
  • Orbital
    • Location: Near orbital region
    • Insertion: Pulley
  • Marginal zone: External
• Other contents: Spindle fibers; Golgi tendon organs
• Axon innervation
  • Twitch: May drive eye movements
  • Non-twitch: Tonic muscle activity
  • Innervation types
    • All have innervation by single axon
    • 2 types of NMJ contact: Single & Multiple NMJ regions
• EOM Muscle fiber types
  • Orbital
    • Single NMJ (Single-innervated)
      • Frequency: 80% of fibers in orbital layer
      • ATPase stain: 4.3 pale; 9.4 dark
      • Myosin isoforms: Fast in entire length; Neonatal/Embryonic (developmental) in distal & proximal segments
      • Mitochondrial stains (COX & SDH): Dark, Clustered
      • Sarcoplasmic reticulum structure: Coarse
      • AChR types: Adult & Fetal
      • Physiology: Fast, Fatigue resistant
    • Multiple NMJ (Multi-innervated)
      • Frequency: 20% of fibers in orbital layer
      • ATPase stain: Ends 4.3 dark, 9.4 intermediate; Center 4.3 pale
      • Myosin isoforms: Fast in center; Neonatal/Embryonic (developmental) in distal & proximal segments
      • Mitochondrial stains (COX & SDH): Dark, Clustered
      • Sarcoplasmic reticulum structure: Fine in distal & proximal segments
      • AChR types: Adult & Fetal
      • Physiology: Fatigue resistant in center; Slow at ends
  • Global

    Red, Single NMJ Frequency: 30% of fibers in global layer ATPase stain: 4.3 pale; 9.4 dark Myosin isoforms: IIA-like Mitochondrial stains (COX & SDH): Dark, Clustered Sarcoplasmic reticulum structure: Coarse (C) AChR types: Adult Physiology: Fast, Fatigue resistant Intermediate/Pale Single NMJ Frequency: 60% of fibers in global layer ATPase stain: 4.3 pale; 9.4 dark Myosin stain: IIB-like Mitochondrial stains (COX & SDH): Dark, Clustered Sarcoplasmic reticulum structure: Granular (G) AChR types: Adult Physiology: Fast, Fatigue resistant Multiple NMJ Frequency: 10% of fibers in global layer ATPase stain: Ends 4.3 dark, 9.4 intermediate Myosin isoforms: Slow Mitochondrial stains (COX & SDH): Dark, Clustered Sarcoplasmic reticulum structure: Fine (F) AChR types: Adult & Fetal Physiology: Slow

    NADH stain

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Muscle patterns: Normal Adult EOM

H&E stain

Muscle Fiber size variation: Mild to moderate
Fascicles: Smaller than limb or trunk muscles
Endomysial connective tissue
  More between muscle fibers than limb or trunk muscles
    Especially in regions with intramuscular nerves
Perimysial Vessels: Normal structure
Intramuscular nerves: May be abundant

VvG stain

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Muscle fibers: Sizes & Regional patterns

H&E stain

Internal architecture
  Varied: Among, & within, fascicles
  May be smaller at surface of muscle (Above)

H&E stain

NADH stain

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Muscle fibers: Morphology

Muscle fibers
  Size: Varied; 10 to 50 μM in diameter

H&E stain

H&E stain

Muscle fibers
  Size: Varied; 10 to 50 μM in diameter
  Myonuclei: Large or Small
  Neuromuscular junctions: Pale regions around muscle fibers (Arrow)
  Motor point: Endomysial connective tissue increased

H&E stain

H&E stain

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Congo red stain

Muscle fibers
  Internal architecture: Irregular
  Myonuclei: Large

Congo red stain

Neuromuscular junctions: Have appearance of sarcoplasmic pads

Congo red stain

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VvG stain

Internal architecture: Variable in different regions; Coarse

VvG stain

Neuromuscular junctions: Dark-stained cytoplasm around surface of muscle fibers

VvG stain

Neuromuscular junctions: Dark-stained cytoplasm on surface of muscle fibers

VvG stain

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Gomori trichrome stain

Mitochondria
  Abundant in muscle fibers
  Some muscle fibers appear "ragged red"

Gomori trichrome stain

Some muscle fibers appear "ragged red"

Gomori trichrome stain

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Internal architecture

NADH stain

Internal architecture: Coarse or Punctate patterns

NADH stain

Internal architecture
  Some muscle fibers are very darkly stained
  Sarcoplasmic reticulum staining: Coarse; Punctate

NADH stain

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EOM: Mitochondria

SDH stain

Mitochondria are more abundant than in limb muscles
  Some muscle fibers are very dark stained for SDH

SDH stain

SDH stain

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COX stain

Mitochondria are more abundant than in limb muscles
  Some muscle fibers are very dark stained for COX
  Mitochondria appear large

COX stain

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EOM: Fiber types

ATPase pH 9.4 stain

Fiber types
    ATPase pH 9.4: Dark stained

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ATPase pH 4.3 stain

Fiber types
  Most similar to type 2C
    ATPase pH 4.6 & 4.3: Intermediate stained
  Few fibers similar to type I
    ATPase pH 4.6 & 4.3: Dark stained
    Often are: Muscle fibers with multiple innervation regions by individual axons

ATPase pH 4.3 stain

ATPase pH 4.6 stain

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EOM: Neuromuscular Junctions

Esterase stain

Neuromuscular Junctions
  Dark stained by esterase
  Large
  May extend around surface of muscle fibers

Esterase stain

Neuromuscular Junctions
  Varied sizes
  Usually have 1 or 2 patches on single muscle fiber

Esterase stain

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Acid phosphatase stain

Acid phosphatase positive granules in muscle fibers

Acid phosphatase stain

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EOM: Endomysial capillaries

Alkaline phosphatase stain

Alkaline phosphatase stain: Increased compared to limb muscles

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EOM: Lipid in muscle fibers

Muscle fibers have scattered, small, lipid droplets

Sudan black stain

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References
1. Invest Ophthalmol Vis Sci 2000 Apr;41(5):980-90

11/27/2024