( C) Schematic representation of all the muscles present in one hemisegment the LT muscles are highlighted in red. ( A) Stage 16 embryo showing the musculature in green (Tropomyosin) and the nuclei of the lateral transverse (LT) muscles in red (DsRed). The Drosophila embryo is divided in hemisegments, each containing 30 muscles. Specific motoneurons innervate each muscle, leading to coordinated contraction at the final stage of embryogenesis. As myoblast fusion concludes, the polarized syncytial myotube extends processes toward specific tendon cells and forms stable myotendinous junctions. Myoblast fusion is an iterative process depending on the particular muscle, body wall muscles in Drosophila embryos contain between 2 and 25 myonuclei. Upon fusion to an FC, FCMs become reprogrammed to the specific developmental program of the FC, as evidenced by the observation that each newly incorporated FCM nucleus begins to express the same combination of identity genes as the FC. The combination of identity genes and chromatin regulators expressed by a particular FC regulates the final morphology of the specific muscle. FCs can be identified by the expression of identity genes, such as the transcriptional regulators even-skipped and apterous. Each FC contains the information necessary to direct the formation of a specific muscle. Formation of these individual body wall myofibers depends on the specification and fusion of two myoblast cell types: founder cells (FCs) and fusion-competent myoblasts (FCMs). There are 30 individual muscles per abdominal hemisegment of the Drosophila embryo ( Fig. Lastly Drosophila embryos are amenable to time-lapse microscopy without perturbing development such that cell biological processes, including myonuclear dynamics, can be assessed. In addition to their powerful genetics, Drosophila embryos are also amenable to drug treatments commonly used to manipulate cell biological processes in culture, providing additional opportunities to probe the cellular machinery responsible for myonuclear movement. The Drosophila system also offers unparalleled genetic approaches to manipulate genes of interest within the whole organism or specifically within muscle tissue. Drosophila muscles share the conserved myofiber structure found in vertebrates: Drosophila myofibers are multinucleated, maintain particular sizes and shapes, attach to particular tendon cells, become innervated by specific motoneurons, build sarcomeres, and are essential for locomotion (reviewed in ). To identify the cellular mechanisms of myonuclear movement in vivo, we have utilized the musculature of Drosophila embryos and larvae. Moreover, mispositioned myonuclei correlate with muscle disease, and recent data suggest that mispositioned myonuclei may cause muscle weakness, illustrating the functional importance of proper positioning. Myofibers, the cellular units of the skeletal muscle, are multinucleated and position their myonuclei to reside above the sarcomeres at the cell periphery and along the length of the fiber to maximize internuclear distance. The skeletal muscle provides an important system in which to study mechanisms of myonuclear positioning.
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