An electron microscope study of myofibril formation in embryonic rabbit skeletal muscle.

Trunk and limb muscles from fetal and newborn rabbits were investigated by means of light and electron microscopes. At 14 days gestation, the presumptive myoblasts migrate away from the myotome to form the anlage of the muscle of the trunk and limb. Among the population of undifferentiated cells, the myoblasts were recognized due to the presence of actin and myosin filaments. The aggregates of thin and thick filaments appear at the periphery of the cells. There is a great variety of filament assembly. The presence of Z band material appears to be essential for sarcomere formation. At 14 days of gestation the myotubes are more numerous in the limb than in the trunk. The presence of unmaturated fibrils with absence of the M line in the sarcomeres was observed. By day 18 of gestation the myotubes are wider and aggregate to form small bundles. The myofibrils were more numerous and the vesicles of the SR precursor, partly incrustated with ribosomes were dispersed among them. At day 22 of gestation the myotubes are thicker because of the myofibrils which are far more numberous. The sarcomeres were more fully developed, with the M line present. At day 28 of gestation and 3 days after delivery the already developed myofibers were present with a well organized SR system and fully developed sarcomeres.