Appearance of complex branched fibers following repetitive muscle trauma in normal rat skeletal muscle.

BACKGROUND

To examine whether the complex branched fibers observed in mdx mutant mice were formed in normal skeletal muscles, long-term repetitive muscle trauma was applied to muscle of normal Wistar male rats.

METHODS

Three kinds of artificial muscle trauma--crush injury, bupivacaine hydrochloride treatment, and forced stretching of contracting muscle (eccentric contraction)--were performed once a week for 10 weeks to achieve a state of repetitive degeneration and regeneration in the muscles. Two weeks after the final treatment, numerical, histochemical, and three-dimensional analyses by scanning electron microscopy were performed.

RESULTS

Mean numbers of total branched fibers of the three groups were increased compared with normal control values, especially in the bupivacaine treatment group (three- to fivefold greater than in the other two groups). Aggregations of fibers of the same type which usually appear in mdx mice were observed in various parts of histological sections of the bupivacaine treatment group and only in a part of the crush injury group. No aggregations were observed in sections of the forced stretching group. In the three-dimensional analysis, complex branched fibers appearing as an "anastomosing syncytial reticulum" were observed only in the bupivacaine treatment group.

CONCLUSIONS

These findings suggest that the formation of an anastomosing syncytial reticulum is one of the adaptation mechanisms of normal skeletal muscle rather than a specific event in mdx mutant mice, and long-term repeated trauma of the same fiber is necessary for this formation. Adaptive changes in the muscles with the three different types of muscle trauma are discussed.