Skeletal myogenesis results in the formation of diverse cell types (muscle fibers) characterized by specific differences in protein composition and function. Diversity of skeletal muscle fibers has generally been attributed to the influence of the central nervous system, the assumption being that all muscle fibers form from a single precursor cell type (myoblast) which multiplies and then fuses to form a muscle fiber or myotube that is diversified by an innervating motoneuron. This hypothesis has been recently challenged by observations of muscle fiber formation in avian embryos. These observations show fiber diversification in the embryo occurs in the absence of innervation and that myoblasts are of different types. Myoblasts appear to be committed to distinct fast, mixed fast/slow, and slow myogenic cell lineages, because the progeny of an individual myoblast form only a single type of myotube based on myosin heavy chain isoform content. Myoblasts committed to the fast lineage form only fast myotubes; myoblasts committed to the fast/slow lineage form only fast/slow myotubes; and myoblasts committed to the slow lineage form only slow myotubes. Commitment to a particular myogenic cell lineage is stably inherited during the period of myoblast proliferation that precedes myotube formation. We propose it is the myoblasts that must carry the genetic information required to form the diverse types of embryonic myoblasts from the quail, chicken and mouse. We will investigate the nature of the commitment process in embryonic quail myoblasts of different types, determine the migratory pathways that myogenic cells traverse from somites to developing muscles, determine the lineage interrelationships between myoblasts of different types and the myoblast origin of the two waves of muscle fiber formation that occur in the development of all vertebrates. The approaches include: transfection of myoblast DNA; tracing the migration and developmental fate of pure, cloned populations of different myoblast types marked with fluorescent microbeads, fluorescent vital dyes, or transfected marker genes, and identification of myogenic cells and their progeny using immunohistochemistry and immunoblotting with myogenic-cell and myosin heavy chain specific monoclonal antibodies.
