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 embryonic skeletal myocytes maintained in culture for 10 days. The cells were maintained under culture conditions that prevented cell division, fusion and cell contacts with neuroblasts. The cell surface was estimated morphometrically 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 voltage dependent ionic currents occurs during the first 3–5 days. Na⁺ maximum specific conductance at days 1–2 was low but by day 7 it showed a 20-fold increase. The magnitude of Na⁺ current densities increased 16-fold from day 1 (3.6 A/cm) to the day 7 (58.1 A/cm). The maximum specific K⁺ conductance increased almost 3-fold during the first 5 days. In contrast to the other types of currents, I_K undergoes qualitative changes. Sodium action potentials, whose amplitude and time course depend on g_Na/g_K ratio, appeared from day 4 in culture, when myofibrils and the T-system also developed. The amplitude of DHP-sensitive slow I_Ca increased in parallel with the development of the T-membrane. I_Ca,S density per unit of T-membrane area reached an equilibrium of ca., 17 A/cm² on the day 4 and then remained stable until the end of the period of observation. These studies demonstrate that muscle-specific characteristics including morphology and excitatory properties begin to develop on the third day and resemble those of adult muscle cells by the sixth day in culture.