[Structural differentiation of dissociated skeletal embryonal myocytes of frog under conditions of cell culture].

The development of membrane structures, providing E-C coupling, and the contractile apparatus organization were investigated in frog skeletal myocytes cultured for 1 to 10 days in conditions preventing both myocyte division and fusion. Ruthenium red was used to determine the membranous structures being in contact with the extracellular environment. The marked membrane structures (vesicles and short tubules) appeared to be near the cell membrane on the first days of culturing. The increase in the ratio of the surface area of all internal membranous structures, marked by Ruthenium red, to the external membrane area with aging was proven by morphometric calculations, that means a progressive development. Contractile filaments were found near the cell membrane on the first days of development. Bundles of filaments with initial signs of sarcomere organization were observed on the 3rd-4th days, and myofibrils with highly organized sarcomeres occupied the main part of the sarcoplasm on the 6th day of culturing. The triads appeared also on the sixth day, being regularly inserted into the sarcomere structure. Degenerative signs in the myocytes (sarcomere disorganization and T-tubule swelling) were observed on the 8-10th days, but the area occupied by contractile elements was increased. These results show that the myocyte fusion into myotubules is not a necessary condition for either sarcomere formation, or the formation of all membranous structures providing the E-C coupling.