Abstract
We have studied the myosin types present in developing fast and slow muscles of the chicken embryo. Myosin light chains were characterized by their mobility on sodium dodecyl sulfate/polyacrylamide gels; myosin heavy chains were identified by their reaction with antibodies specific for adult fast or adult slow myosin heavy chains. During development, the pectoralis muscle, a fast muscle in the adult, contains heavy chains and two of the three light chains characteristic of adult fast muscle myosin. However, the anterior latissimus dorsi muscle, a slow muscle in the adult, also contains fast myosin light and heavy chains during early development. Only after the time of innervation does this muscle begin synthesizing predominantly the slow myosin heavy and light chains. We hypothesize that the synthesis of fast myosin in both early fast and slow muscles is the result of the endogenous program for muscle development; initiation of the synthesis of slow myosin, however, is dependent upon exogenous factors.