THE EFFECTS OF ACTINOMYCIN D ON MUSCLE CELLS OF ASCIDIAN EMBRYO.

The effect of actinomycin D on muscle cells development of the ascidian, Herdmania momus was studied ultrastructurally. No myofilament was formed when the drug was given at any stage before early tail-bud stage (stage 3). Some aggregates of myofilaments in various size were formed when the treatment was started at stage 4 (4.5 hr after fertilization at about 28°C). Above 60% of myofibrils of fully differentiated muscle cells were formed when the treatment was initiated at stage 5 (5 hr after fertilization). Muscle cells of the tadpoles treated from stage 7 (6 hr after fertilization), at which myofilaments were first detectable in normal development, differentiated almost normally. It is therefore suggested that most mRNAs for muscle proteins are synthesized preceding the onset of myofilament formation and are relatively stable. It is also suggested that mRNAs for myosin, actin and Z band materials are almost simultaneously synthesized. One of the characteristic features of the muscle cell development of the ascidian embryo is almost synchronous differentiation of relatively small numbers of cells (about 54-60 cells). The significance of these results is discussed in relation to mosaic natures of the muscle development.