FAST MYOSIN HEAVY-CHAINS EXPRESSED IN SECONDARY MAMMALIAN MUSCLE-FIBERS AT THE TIME OF THEIR INCEPTION

Mammalian skeletal muscle is generated by two waves of fiber formation , resulting in primary and secondary fibers. These fibers mature to gi ve rise to several classes of adult muscle fibers with distinct contra ctile properties. Here we describe fast myosin heavy chain (MyHC) isof orms that are expressed in nascent secondary, but not primary, fibers in the early development of rat and human muscle. These fast MyHCs are distinct from previously described embryonic and neonatal fast MyHCs. To identify these MyHCs, monoclonal antibodies were used whose specif icity was determined in western blots of MyHCs on denaturing gels and reactivity with muscle tissue at various stages of development. To fac ilitate a comparison of our results, with those of others obtained usi ng different antibodies or species, we have identified cDNAs that enco de the epitopes recognized by our antibodies wherever possible. The re sults suggest that epitopes characteristic of adult fast MyHCs are exp ressed very early in muscle fiber development and distinguish newly fo rmed secondary fibers from primary fibers. This marker of secondary fi bers, which is detectable at the time of their inception, should prove useful in future studies of the derivation of primary and secondary f ibers in mammalian muscle development.