Muscle fibers: Variable involvement in different regions
  Regional necrosis zone: In border regions between vessels
  Regeneration zone: Clustered regeneration after necrosis
  Purlieu zone: Region between vessels & necrosis zone
  Capillaries: Necrotic
  Veins with damaged walls

MRI images

Regional pathology: Necrotic areas

All tissues are involved
  Muscle fibers: Necrosis
  Capillaries: Necrosis
  Perimysial & endomysial connective tissue: Abnormal structure & CA9 staining

H&E stain
Necrotic muscle fibers
  Pale stained or Hypercontracted
  No nuclear staining
  None have macrophage infiltration
Perimysium (Far right): Pale stained
Vessels (arrow): Not contiguous with necrosis zone

H&E stain

Regional Necrosis

H&E stain

RIIM: Borderzone Necrosis

C5b-9 (Green) & Collagen IV (Red) stains
Regional necrosis
  Zones of necrosis
    Surrounding but not neighboring vessels (Arrow)
    Do not respect fascicular borders
  Necrotic muscle fibers
    Clustered in borderzones between large arteries & veins (Arrows)
    Muscle fiber cytoplasm: Stains for C5b-9 complement (Green; Above)
  Connective tissue (Perimysium & endomysium) is also damaged
    CA9: Abnormal staining of perimysial & endomysial connective tissue & muscle fibers(Below)
    C5b-9 complement: Punctate deposition in perimysium (Above)
  Capillaries, endomysial: Necrotic & effaced
Purlieu zone: Muscle fibers between vessels & necrotic zone have no C5b-9 deposition
Vessels (Arrows): Inside purlieu zone & Distant from necrotic zone

CA9 stain
CA9 staining in necrotic zone: Abnormal on perimysium & endomysium

C5b-9 (Green) & Collagen IV (Red) stains
Borderzone regions of necrosis: No relation to fascicular borders

C5b-9 stain

H&E stain
Muscle fibers in Necrotic zones
  General: Most in similar, early stage, of necrosis
    Pale staining or Hypercontracted
    No nuclear staining
  Edges: Some are invaded by phagocytic cells

H&E stain

H&E stain

Esterase stain

Necrotic muscle fibers: Absent SDH staining

SDH stain

Necrotic muscle fibers: Reduced or Absent NADH staining

NADH stain

Muscle fibers in Necrotic zones
Terminal components of complement ((C5b-9; Membrane attack complex)
  C5b-9: Deposited in cytoplasm of clusters of necrotic muscle fibers

Toluidine blue stain
Necrotic muscle fibers
  Myofiber cytoplasm shows
    Loss of myofibrillar apparatus
    Granular debris
    Lipid droplets

Electron microscopy

RIIM: Capillaries

Capillary, Endomysial: RIIM
  Small & Necrotic with Effaced architecture (Open arrowhead)

Electron microscopy
Capillary, Endomysial: Control
  Normal lumen contains RBCs

Electron microscopy

RIIM: Vessel pathology

VvG stain
RIIM vessels
  Veins (v) have swollen endothelial cells, thickened walls, intramural inflammatory cells & damaged elastic lamina
  Arteries (a) are normal

VvG stain
Artery (a) & Vein (v)

VvG stain

Vein (v)
  Smooth muscle actin (Green): Multifocal loss (Arrows)
  PECAM1: Remains present around vein
Artery (a): Normal wall structure

Smooth muscle actin (Green) & PECAM1 (Red) stains

Regional Ischemic Immune Myopathy

Purlieu zone: Between Vessels & Zone of necrosis

H&E stain
Muscle fibers
  Varied size
  Internal architectureL Irregular or Vacuoles
  Aggregates: Caveolin & LC3 aggregates

H&E stain

  Cytoplasmic Aggregates: Caveolin & LC3

Caveolin-3 stain
Capillaries: C5b-9 deposition on larger endomysial vessels

C5b-9 stain

Regional Ischemic Immune Myopathy

Muscle fiber regeneration zone

H&E stain
Regenerating muscle fibers: Small; Coarse internal architecture; Large nuclei; Clustered; In similar stages

VvG stain

ATPase pH 4.3 stain
Immature muscle fibers
  Many type 2C (Intermediate-stained) muscle fibers (Above)
  Dark stain & Coarse internal architecture on NADH (Below)

NADH stain

Immature muscle fibers: Cytoplasm of all fibers stained by alkaline phosphatase

Alkaline phosphatase stain

Immature muscle fibers & related structures
  Myofibers: Small (Black arrowheads) with excessive sarcolemmal folds (Inset)
  Endomysium: Widened
  Capillaries: Large with disrupted architecture (Open arrowhead)

Toluidine blue stain

Capillary: Regenerating zone
  Markedly enlarged
  Disrupted architecture
  Endothelial cytoplasm (E): Swollen; Extended
  Red blood cells (Black arrows): Extravasated

Electron microscopy

Regional Ischemic Immune Myopathy: MRI

T2: No contrast

Increased T2 signal in patchy regions of muscle & connective tissue

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