Type II atrophy

TYPE II MUSCLE FIBER ATROPHY

Clinical features
Pathology
Features
Childhood
Early
Elongated fibers
2B fiber atrophy
Moderately severe
Atrophy: General

ATPase pH 9.4 stain

Type II atrophy: Pathology

Type II (Dark at pH 9.4) Muscle fibers
- Small: Reduced cross-sectional area\
- Shape
  - Adult: Angular or Elongated
  - Childhood: Polygonal or Round
- Early changes
  - Atrophy: Type IIB (Intermediate staining at pH 4.6; Myosin 2A+2X) fibers
    - More IIB muscle fiber atrophy in males than females ²
  - Shape of muscle fiber cross-sections: Narrow; Elongated
- Disuse & weight loss atrophy: Small fibers are angular
- Congenital: Small fibers are polygonal or round
Type I (Lighter at pH 9.4) Muscle fibers
- Larger than type II
- Often atrophic compared to normal type 1 fibers
NADH stain: Small fibers usually pale
Nuclei
- Number: Unchanged
- Myonuclear domain size: Reduced
Recovery of fiber size after Disuse atrophy with exercise ¹
- Probably doesn't require satellite (Pax7) cells
Mitochondrial oxidative enzyme activity reduction ³
- Complexes II & II+III > I

Type II Muscle Fiber Atrophy: Clinical associations

Weakness: Proximal > Distal; Symmetric
Muscle size: Wasting > Weakness
Associated disorders
- Weight loss: > 15%
- Disuse
- Aging
- Systemic disease
  - Paraneoplastic
  - Corticosteroid myopathy
- Childhood
  - Congenital hypotonia
  - Myasthenia gravis

Type 2 Muscle Fiber Atrophy: Moderately Severe

H&E stain

Muscle fiber sizes
Bimodal distribution
Type 1 (Larger) muscle fibers are normal or somewhat small
Small muscle fibers
Shape: Often angular; Some are elongated
Distribution: May appear clustered

H&E stain

VvG stain

ATPase pH 9.4 stain

Small muscle fibers
Usually Type 2 (Dark stained at ATPase pH 9.4)
Shape: Often angular; Some are narrow and elongated
Distribution: May appear clustered

ATPase pH 9.4 stain

ATPase pH 4.3 stain

Small muscle fibers: Type 2
Pale stained at ATPase pH 4.6 & 4.3

ATPase pH 4.6 stain

NADH stain

Type 2 muscle fibers: Other stains
NADH: Pale
COX: Pale
Esterase: Not dark

Esterase stain

PAS stain

Type 2 muscle fibers: Other stains
PAS: Darker
Phosphorylase: Darker

Phosphorylase stain

Type 2 Atrophy: Early

H&E stain

Type 2 muscle fibers
Elongated
Narrow

H&E stain

Gomori trichrome stain

Type 2 muscle fibers
Elongated
Narrow

Congo red stain

Congo red stain

ATPase pH 9.4 stain

Type 2 muscle fibers
Shapes
Elongated & Narrow
Intermediate-sized & Polygonal

ATPase pH 9.4 stain

ATPase pH 4.3 stain

Atrophy: Type 2 muscle fibers
Small & Angular
Pale stained with ATPase pH 4.3
Smallest muscle fibers may be 2B (Intermediate stain with ATPase pH 4.6)