The three-dimensional cytoarchitecture and pattern of motor innervation of branched striated myotubes.

Three-dimensional reconstructions of "regenerating" myotubes in the "degeneration-regeneration" regions and in the "regenerative" foci of the extensor digitorum longus muscle of the C57BL6J/dy2J myopathic mutant mouse were made from spaced serial ultrathin section. Complex branching and recombination occurred, involving myotubes which for extensive regions along their length appeared to be independent. No accumulation of specialized organelles occurred at the branching site. Continuous branches displayed multiple discrete motor endplates.