Skeletal myogenesis

The fates of 10-day chick embryo myogenic cell populations cultured in conditions that maximize or suppress proliferation or fusion were examined. The repression of the cell cycle, prevalence of fusible cells, or expression of myofibril formation were monitored by autoradiography, counts of myotube nuclei, staining with fluorescein-labelled antibody against skeletal myosin heavy chain, or electron microscopy. Using a calcium chelator (EGTA) to block cell fusion did not prevent the accumulation of myoblasts blocked in G1 of the cell cycle that initiated skeletal myosin synthesis and myofibril formation. Trypsinization, dilution, and daily feeding with fresh medium and serum did not reverse this cell cycle block. Using cytochalasin B (CB) as an alternative fusion block confirmed these results. Using fluorodeoxyuridine (FUdR) to prevent the cycling of myogenic cells that normally would have multiplied in vitro did not prompt these cells to fuse. Muscle-conditioned medium could not prompt a switch in the commitment of replicating cells when in G1 to terminal differentiation. The indications are that myogenic cell populations contain definite mixtures of precursor phenotypes. The terminal phenotype is initiated coordinately with a relatively stable cue for the repression of the cell cycle. Differentiation-specified growth control is discussed within the context of a set of growth control mechanisms known to operate in culture.