Development of the sarcotubular system in fusion-arrested myoblasts.

We have compared the effect of two different procedures, equally effective in preventing muscle cell fusion in culture, on the development of the sarcotubular system in rat muscle cells. Whereas in myoblasts grown in low Ca++ medium the T system was poorly developed and diadic or triadic couplings between T tubules and sarcoplasmic reticulum were rare, in cytochalasin B-treated myoblasts the development of the sarcotubular system was comparable to that seen in myotubes of the same age. We conclude that (a) muscle cell fusion is not essential for the development of the sarcotubular system, and (b) procedures used to prevent cell fusion in vitro may affect directly muscle cell differentiation by a process independent of the fusion block.