DEVELOPMENTAL APPEARANCE OF MYOSIN HEAVY AND LIGHT CHAIN ISOFORMS INVIVO AND INVITRO IN CHICKEN SKELETAL-MUSCLE

Three myosin H chain isoforms with unique peptide maps appear sequentially in the development of the chicken pectoralis major muscle. An embryonic isoform is expressed early and throughout development in the embryo. A 2nd isoform appears just after hatching and predominates by 10 days ex ovo. A 3rd isoform, indistinguishable from adult myosin H chain, predominates by 8 wk after hatching. This sequence of myosin isoform change does not appear during myogenesis in vitro. In cultures prepared from embryonic myoblasts, only embryonic myosin H chain is expressed. This is true even in cultures maintained for 30 days. Myosin L chain expression also changes in vivo with a progressive increase in fast L chain 3 accumulation. In vitro, however, this shift to increasing fast L chain 3 accumulation does not occur. The myosin H chain and L chain pattern observed in vitro is identical to that of the embryonic muscle and that the conditions necessary for the shift in expression to a more mature myosin phenotype are not present in myogenic cultures. These cultures are therefore potentially of great value in probing further the neural and humoral determinants of muscle fiber maturation and growth.