DURING FETAL MUSCLE DEVELOPMENT, CLONES OF CELLS CONTRIBUTE TO BOTH PRIMARY AND SECONDARY FIBERS

All mammalian muscle fibers are formed by cell fusion, but fiber formation occurs in two successive waves. Initially a small number of large fibers (primaries) are produced and are used as a scaffold for the formation of a larger number of smaller later forming fibers (secondaries). It has been reported that different populations of myoblasts are present at different ages, but it is unclear whether separate populations of cells contribute to primary and secondary fibers during the period of fetal development. We have tested this by injecting two replication-deficient retroviruses, carrying a marker gene (lac Z), into the hind limbs of Embryonic Day (E) 15 and E17 mouse fetuses. All fetuses were killed at E19. Clusters of marked fibers (assumed to correspond to single clones) found after injection at both ages contained both primary and secondary fibers. This indicates that at these ages, a single population of cells contributes to both primary and secondary fibers. (C) 1994 Academic Press, Inc.