Behavior of sarcotubular system formation in experimentally induced regeneration of muscle fibers.

To examine the behavior of transverse (T)-tubule formation in experimentally-induced regenerating fibers, a local anesthetic, bupivacaine hydrochloride, was injected directly into the rat soleus muscle to cause myonecrosis. The regenerating fibers following necrosis were then examined by electron microscopy using lanthanum nitrate which clearly demonstrated the sarcotubular system. In the early stage of regeneration within 7 days after muscle necrosis, the T-tubules seemed to be composed of markedly proliferated subsarcolemmal caveolae with occasional honeycomb structure formation. Around 10 days, the T-tubules in regenerating fibers were tortuously and irregularly arranged with focal dilatation in diameter, and extended longitudinally along the axis of well organized myofibrils. As the regenerating fibers matured, the sarcotubular system, irregular in course and in shape, gradually became organized into a regular transverse position against the myofibrils, along with a marked decrease in longitudinally arranged tubular components. These morphological findings of the early T-tubule formation seen in the present study were similar to those found in early myogenesis, and in biopsied muscles from cases of polymyositis and progressive muscular dystrophy.