Evidence for the presence of a distinct embryonic isoform of myosin heavy chain in chicken skeletal muscle.

Immunochemical studies have identified a distinct myosin heavy chain (MHC) in the chicken embryonic skeletal muscle that was undetectable in this muscle in the posthatch period by both immunocytochemical and the immunoblotting procedures. This embryonic isoform, identified by antibody 96J, which also recognises the cardiac and SM1 myosin heavy chains, differs from the embryonic myosin heavy chain belonging to the fast class described previously. Although the fast embryonic isoform is a major species present in the leg and pectoral embryonic muscles, slow embryonic isoform was present in significant amounts during early embryonic development. Immunocytochemical studies using another monoclonal antibody designated 9812, which is specific for SM1 MHC, showed this isoform to be restricted to only presumptive slow muscle cells. From these studies and those reported on the changes in SM2 MHC, it is proposed that as is the case for the fast class, there also exists a slow class of myosin heavy chains composed of slow embryonic, SM1 and SM2 isoforms. The differentiation of a muscle cell involves transitions in a series of myosin isozymes in both presumptive fast and slow skeletal muscle cells.