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Methyl-bupivacaine is a local anaesthetic with a selective myotoxic action. A single subcutaneous injection of the drug into the hind leg of adult rats produces a uniform, complete and irreversible destruction of superficial layers of fibres in the underlying extensor digitorum longus muscle. The degeneration of muscle fibres is followed by phagocytosis and a rapid and complete regeneration. The first stage in the regeneration process is the appearance of presumptive myoblasts within the original basement membrane of the sarcolemmal tube. On the second day after injury aggregates of myoblasts are present and fusion is observed between the cells. The myotubes thus formed increase in size by fusing with additional myoblasts. Myotubes are also observed to fuse with one another. On the fifth day after injury the regeneration process has proceeded to the stage of early muscle fibres with fully differentiated myofibrils with typical sarcomere structures. By ten days only mature muscle fibres of about normal size are present and regeneration appears complete. In previously denervated and methyl-bupivacaine treated muscles the stages of regeneration are similar to those observed in innervated muscles, the only apparent difference being a slowing of cell differentiation and incomplete maturation. An electrophysiological study shows that the motor nerve at the third day after injury forms synaptic contacts with regenerating muscle cells. At that stage of myogenesis the myotubes are highly sensitive to applied acetylcholine. 1 (1-n-butyl-DL-piperidine-2-carboxylic acid-2,6-dimethyl-anilide-hydrochloride); Marcaine®, manufactured by AB Bofors, Nobel-Pharma, Mölndal, Sweden. [ABSTRACT FROM AUTHOR]

Mice embryos from day 9 to 15 p.c. have been studied by Nile blue sulfate staining revealed segmental necroses in the back of mice embryos centrally between the intersegmental arteries. Necroses appeared in the cranial myotomes and extended to the caudal ones during the days 9 to 13 p.c. By light microscopy the necroses extended in the centre of the myotome to the ventromedial margin. Electron microscopy revealed in this area myoblast-like cells rich in filaments, arranged in parallel without striation. A part of these myoblasts were electron-opaque and the nucleus was no longer evident. Forming vortices of filaments, these cells on the cranial and caudal end of the myotomes ceased fusion with the more centrally situated myoblasts. Vortices of filaments were also found in phagolysosomes of macrophages on day 12 p.c. On the same day of gestation we found an unsegmented myoblastema with irregular long myoblasts and striated polynuclear muscle fibres, arranged parallel to the neural tube. Between and parallel to the muscle fibres elongated individual cells could be seen, growing necrotic. Their nuclei were electron-dense with the chromatin clumping cap-like on the inner nuclear membrane. Fragments of myofibrils, corresponding approximately to sarcomeres, were dispersed irregularly as filament logs in the cytoplasm. Logs of filaments were bent at an obtuse angle, partly showing remnants of Z line material. In this stage of necrosis cells were phagocytized by macrophages. After disintegration of the segmental arrangement in scleroblastemata and myoblastemata a segmental rearrangement was not achieved by segmental shifting or by necrotic liquefaction zones. Whether the described cell necroses are due to differentiation, or even to phylogenetic causes is discussed. [ABSTRACT FROM AUTHOR]