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Age: Muscle & Nerve

Muscle   Child   Adult   Aging     Morphology     Sarcopenia Nerve NMJs

From Bramwell: Atlas of Clinical Medicine

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Children: Muscle Features Muscle fibers Size: Smaller than adult Types 1st year: More 2C fibers (Up to 3%) General: Type 2 fibers 5% smaller than type 1 Nuclei Larger Capillaries Lower #/Muscle fiber 1st 3 years: Many fibers with no adjacent capillary Adults: Muscle Features Nuclei Small Round Capillaries All muscle fibers have adjacent capillary

NORMAL MUSCLE Age (Years) Mean fiber Diameter (μM) Birth 12 to 15 1 18 3 20 6 27 8 35 10 40 12 45 15 50 Adult 50 to 60

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Aging (> 65 years): Muscle Changes 1 Type 2 muscle fibers Reduced size Fewer associated satellite cells Mitochondrial SDH+ muscle fibers: More frequent COX- muscle fibers: More frequent Complex I: Reduced activity 2 mtDNA 3 Levels: Reduced More damage: mtDNA deletions, multiple Nuclei Shape: More irregular Size: Often increased Strength correlations Muscle mass: Correlates with Bioavailable testosterone levels Muscle fiber size Type 2 muscle fiber size NOT: Age per se Iron content: Increased Sarcopenia

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Increased age (> 85 years): Common features in muscle

Nuclear shapes: Irregular H&E stain

H&E stain Muscle fiber size: Varied

Gomori trichrome stain

ATPase ph 4.3 stain Type 2 fibers small

ATPase ph 4.6 stain

Fiber types: Non-random distribution ATPase ph 9.4 stain

Acid phosphatase stain Lipofuscin: Punctate sub-sarcolemmal regions

Esterase stain

Internal architecture   Mildly irregular   Pale (Type 2) fibers small NADH stain

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Age: Nerve changes

• Development
  • Myelination
    • Onset: 18 weeks of gestation
    • Birth: All fibers initially myelinated
    • Thickness: Increases through childhood
      • Absolute thickness
      • Relative to axon size
    • Internodal length
      • Birth: 200–300 μm
      • Adult: 200 to 1,800 μm
      • Increases in parallel with somatic growth
  • Axons
    • Number: Full at birth
    • Density: Reduced with increasing age
    • Diameter: Increases up to 5 years
    • Size distribution: Myelinated axons
      • Birth: Unimodal
      • Adult: Bimodal
    • Conduction velocity
  • Endoneurial area: Increases with age
• Full maturity: 10 years
• Aging
  • Axons
    • Myelinated fiber densities: Decrease continuously through adult life
      • By ~50% from 2nd to 8th decade
    • Regeneration
      • Regenerated axons: May become up to 25%
      • Regenerating clusters: More frequent with increased age
    • Conduction velocity
    • Axonal transport: Slowed ⁴
  • Myelin & Schwann cells
    • Denervated Schwann cells: Increase with age
    • Myelin thickness & internode length: Increased scatter with age
    • Pi granules: May be increased numbers in older patients
  • Endoneurium
    • Endoneurial area & Collagen: Increase with age
    • Endoneurial nuclei: Stable number with age
    • Endoneurial capillaries: Age changes
      • Hyalinization: Increased
      • Endothelial & Pericytic basement membrane: More duplication
  • Perineurial basement membrane
    • Increased thickness with age: Most at outer layers

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Age: Neuromuscular Junction Changes

NMJs: Rat

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Age: 2 Months

Silver-Esterase stain

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Age: 18 Months

Silver-Esterase stain

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Age: 28 Months

Silver-Esterase stain

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Return to Neuromuscular

References
1. J Am Geriatr Soc 2010;58:2069–2075, Eur J Transl Myol 2022;32:10416
2. Muscle Nerve 2017;56:122-128
3. Cells 2019;8(12)
4. Exp Neurol 1980;70:65-82

7/9/2024