| Abstract: | This review outlines recent advances in understanding the program of muscle specialization during development and discusses some of the controls over this process. The scheme of fiber heterogeneity is laid down very early in myogenesis and, at least in the chicken, it appears to be intrinsically determined by the limb, independent of neuromuscular contact. In mature muscle, specialized fibers are usually intermingled in a mosaic. This results from the pattern of muscle assembly from primary and secondary generation cells as these distinct generations commonly express different phenotypes. The muscle develops in this way as secondary cells use the walls of primary myotubes as a scaffold to support their differentiation. This process may be regulated by transient expression of N CAMs. Myoblasts left after N CAMs are no longer expressed may become the satellite cells of mature muscle. In the rat, primary cells all initially express slow myosin, whereas most secondary cells are fast in phenotype. Post-partum, the initial plan of fiber specialization is modulated. In developing slow muscles many secondary generation fibers convert from a fast to a slow phenotype, whereas in developing fast muscles, primary slow fibers transform to a fast phenotype. Hormones, particularly thyroid hormone, play a significant role in this modification. The role of the nerve is less well understood. |
