Ventricular myosin expression in developing and regenerating muscle, cultured myotubes, and nascent myofibers of overloaded muscle in the chicken.

In embryonic chicken skeletal muscle, the presence of a ventricular myosin heavy chain (MHC) is demonstrated by reactivity with an anti-ventricular MHC antibody. Developmental repression of the ventricular MHC expression occurs earlier in embryonic fast twitch posterior latissimus dorsi (PLD) than in embryonic slow-tonic anterior latissimus dorsi (ALD) muscles. Ventricular MHC expression also occurs in developing myotube cultures and in regenerating ALD muscles. Following the application of a weight overload, a population of nascent myofibers emerges in ALD but not PLD muscles which express a ventricular MHC. Localization of cells reactive with anti-ventricular and anti-fast MHC antibodies suggests that satellite cells participate in nascent myofiber formation. In addition, some mature ALD muscle fibers demonstrate reactivity with these antibodies, suggesting that satellite cell fusion with mature myofibers also occurs in overloaded ALD muscles and results in the reinitiation of the embryonic phenotype in hypertrophying fibers. The presence of cells reacting exclusively with the anti-fast antibody after removal of the overload for 9 wk indicates that nascent myofiber formation results in the establishment of a new myofiber population with an abnormal phenotype in overloaded ALD muscles. These cells form a lattice-like network around normal fascicles.