Immunofluorescence analysis of the primordial myosin detectable in embryonic striated muscle.

Immunofluorescence analysis showed that the earliest myosin detectable in both the embryonic chicken heart and somitic myotome, the precursor to skeletal muscle, was strongly reactive with two different monoclonal antibodies specific for the heavy chain of cardiac ventricular myosin, but it showed no reactivity with affinity-purified polyclonal antibodies specific for the heavy chains of either fast-twitch or slow-tonic skeletal myosins. The heart remained reactive exclusively with the antibodies to cardiac myosin throughout development, while late embryonic (day 20) skeletal muscles were strongly reactive only with their homologous skeletal myosin antibodies. Our findings suggest that the primordial myosin heavy chain detectable in both forms of embryonic chicken striated muscle, the myotome and the heart, is immunologically distinct from myosins expressed in later embryonic as well as adult skeletal muscles, but it contains antigenic determinants similar to those present in cardiac ventricular myosin.