Action of actinomycin D on grasshopper neuroblasts in culture

Portions of embryos of the grasshopper Chortophaga viridifasciata (DeGeer) were prepared in hanging drop cultures and maintained in a microscope incubator at 38±0.2 °C. Individual neuroblasts were mapped and observed for reaction to actinomycin D (AMD) in the presence and absence of grasshopper egg yolk. Midmitotic duration was not altered by the omission or addition of yolk or by 4 or 10 μg/ml AMD, therefore the number of cells in midmitosis at any one time indicated the number of cells normally progressing through mitosis (mitotic rate). Four and 10 μg/ml AMD depress the mitotic rate by more than 75% in cultures lacking yolk. In cultures containing yolk, 4 μg/ml had no effect but 10 μg/ml depressed the rate to the same extent as in cultures lacking yolk. These same concentrations had no effect on chromosome condensation or attenuation or upon nucleolar disappearance in very late prophase. Concentrations as low as 4 μg/ml, however, often inhibit nucleolar differentiation in late telophase. The presence of 10 μg/ml AMD in hanging-drop cultures containing yolk reduces RNA synthesis by 30% but does not significantly alter the durations of early, middle, or late prophase stages. Constant treatment with 10 μg/ml AMD did significantly increase cell cycle times, often preventing cells from completing one full cycle.