Fast myosin heavy chain expression during the early and late embryonic stages of chicken skeletal muscle development

The development of embryonic skeletal muscles in the chick can be divided into two periods of fiber specialization-an early one during which the different muscles of the limb are formed and an initial round of fiber specialization occurs and a late or fetal period during which there is extensive growth of this previously established fiber pattern. This latter period of growth is dependent on the establishment and maintenance of functional neuromuscular contacts. As has been described for other developmental stages, we show here that there are different embryonic fast skeletal muscle myosin heavy chain (MHC) isoforms expressed during the different embryonic periods of muscle growth. The identification of these isoforms was based on differences in their reactivity with various fast MHC monoclonal antibodies and on their different peptide banding patterns. The in ovo accumulation of the late embryonic MHC isoform pattern was similar to the time course of the previously described changes in α-actin and troponin T isotype switching during embryogenesis. The appearances of the late embryonic isoforms were blocked by chronic treatment with the neuromuscular blocking agent, d-tubocurarine, and cell cultures of embryonic chicken skeletal muscle which differentiated in the absence of motorneurons expressed little of the late embryonic isoform, indicating that the expression of the late embryonic isoform was dependent on functional nerve-muscle interactions. These different embryonic fast MHC isoforms provide important markers for monitoring the progression of muscle through its embryonic stages and its interaction with motorneurons.