[Effect of attachment to a solid substrate on the structural organization of skeletal muscle symplasts]

A study was made of the ultrastructural reorganization of myosymplasts passed from the primary suspension culture to the secondary monolayer culture. The microtubules in myoblasts and myosymplasts from suspension are very rare, the intermediate filaments form a perinuclear network, small bundles of myofilaments are arranged in disorder, often as whorls around nuclei. After attachment to the solid substrate the myosymplasts form pseudopodia to move as non-muscle fibroblast-like cells. On the leading end of moving symplasts some stress fiber-like structures are found. Numerous microtubules appear. The microtubules and intermediate filaments are arranged in parallel along the axis of a lengthening symplast. A stepwise reorganization of the non-muscle type cytoskeleton to sarcomeres of differentiated myotubes is observed later. The role of attachment and mechanical stress in myotube morphogenesis are discussed.