Development of muscle-specific features in cultured frog embryonic skeletal myocytes

To study the development of muscle-specific features during myogenesis, we analysed the ultrastructure and voltage-dependent currents of frog embryoni c skeletal myocytes maintained in culture for 10 days. The cells were maint ained under culture conditions that prevented cell division, fusion and cel l contacts with neuroblasts. The cell surface was estimated morphometricall y and from cell capacity and the values obtained were used to calculate ion current densities. It was shown that the expression of all main types of v oltage dependent ionic currents occurs during the first 3-5 days. Na+ maxim um specific conductance at days 1-2 was low but by day 7 it showed a 20-fol d increase. The magnitude of Na+ current densities increased 16-fold from d ay 1 (3.6 mu A/cm) to the day 7 (58.1 mu A/cm). The maximum specific K+ con ductance increased almost 3-fold during the first 5 days. In contrast to th e other types of currents, I-K undergoes qualitative changes. Sodium action potentials, whose amplitude and time course depend on g(Na)/g(K) ratio, ap peared from day 4 in culture, when myofibrils and the T-system also develop ed. The amplitude of DHP-sensitive slow I-Ca increased in parallel with the development of the T-membrane. I-Ca,I-S density per unit of T-membrane are a reached an equilibrium of ca., 17 mu A/cm(2) on the day 4 and then remain ed stable until the end of the period of observation. These studies demonst rate that muscle-specific characteristics including morphology and excitato ry properties begin to develop on the third day and resemble those of adult muscle cells by the sixth day in culture.