Changes in myosin and C-protein isoforms proceed independently of the conversion to singly innervated neuromuscular...

Changes in contractile protein expression during myogenesis are usually categorized as developmentally programmed or neuronally dependent. Studies on aneurogenic chick embryos indicated that the neuronally dependent phase begins at about Embryonic Day 15, immediately prior to the fetal transition in myosin and C-protein expression. The prime candidate for the neuronal event that induces the fetal transition is the conversion to the adult form of singly innervated neuromuscular junctions (NMJs), which occurs contemporaneously with the fetal transition. Using curare to inhibit the conversion to focal innervation, we find that the fetal transition proceeds unimpaired, demonstrating that there is no causal link between the fetal transition and the conversion to focal innervation. Furthermore, because the doses of curare used inhibit motor activity by more than 80%, the fetal transition can occur in the absence of normal levels of motor activity. These observations show that the fetal transition in ovo is not induced by either a specific change in innervation or use. Rather, the dependence on innervation seems to be a consequence of the need for muscle activity to prevent atrophy, and the fetal transition appears to have characteristics more like the preprogrammed contractile protein transitions that precede it.