PSEUDOMYOPATHIC CHANGES

• Increased endomysial connective tissue
• Internal nuclei: One or several in many muscle fibers
• Pyknotic nuclear clumps
• Grouped atrophy of muscle fibers
  • Groups are less well demarcated than in ongoing denervation
  • Increased endomysial connective tissue is especially prominent in these regions
• Small muscle fibers may be rounded or angular
• Hypertrophic muscle fibers are common

SPLIT MUSCLE FIBERS

INTERNAL NUCLEI

• "Split" muscle fibers probably result from partial fusion of regenerating fibers
• These split muscle fibers are very large (hypertrophic)
• Split muscle fibers often have internal nuclei

FIBER TYPE GROUPING

NECROTIC & CLUSTERED MUSCLE FIBERS

• A hypertrophied muscle fiber may become necrotic (left)
• Several different muscle fibers regenerate in its place and cluster within the old basal lamina

PYKNOTIC NUCLEAR CLUMPS

• Definition
  • Clumps of myonuclei are the end product of denervation atrophy without reinnervation.
  • The remainder of the myofiber, such as the contractile apparatus and most of the cytoplasm, has largely disappeared.
• Histochemical staining
  • Nuclei: Basophilic on H & E
  • Muscle fiber: Dark on NADH & esterase

REGENERATING AXONAL CLUSTERS

• Clusters of regenerating axons are
  often seen in the nerves when muscles
  show chronic partial denervation