EXPRESSION OF DIFFERENTIATIVE TRAITS IN ABSENCE OF CELL-FUSION DURING MYOGENESIS IN CULTURE

Fusion of chick myoblasts is inhibited in cultures at low Ca++ concentration (0.44 mM); creatine phosphokinase and myokinase activities as well as myosin synthesis and the appearance of post-myoblasts do not significantly differ from those of control cultures (grown at 1.04 mM Ca++) which undergo cell fusion. When Ca++ concentration is increased to the control value after the 2nd day of culture, fusion occurs very rapidly and it is not inhibited by actinomycin D or cycloheximide. Treatment with 0.06 mM bromodeoxyuridine strongly inhibits creatine phosphokinase activity and myotubes formation. The study of the kinetics of reversal of cell fusion and of creatine phosphokinase activity after removal of the analog, shows that this process is slower than the decrease of the relative content of bromodeoxyuridine incorporated into DNA. The expression of the differentiative characters examined apparently does not require cell fusion. The process of myotube formation seems to imply 2 subsequent stages consisting first of a slow maturative process, which is followed by the actual fusion of cell membranes; the former is Ca++ independent, the latter is Ca++ dependent and does not require RNA or protein synthesis.