In metabolically active skeletal muscle, mitochondria occupy,5% of the fibervolume and are rigidly located between bundles of myofilaments. This specific arrangement makes the mitochondria show little motility which is required for mitochondrial fusion by colliding end-to-end or end-to-side in many cells. Therefore, the question whether mitochondrial dynamics exists in skeletal muscle in vivo is raised. Until now, there are no direct experimental data to address this question and only several limited morphological results that indirectly showed that mitochondrial morphology was changed under pathophysiological conditions. Bortezomib Romanello V et al. showed that mitochondrial network is changed in atrophying muscle in vivo and inhibition of mitochondrial fission protects from muscle loss during fasting. Further, mitochondria in fusion machinery deficient skeletal muscle were fragmented into round spheres and accumulated into aggregates, indicating that the balance of mitochondrial fusion, fission and distribution are disrupted. Recently, smaller and shorter mitochondria were found in gastrocnemius skeletal muscle from both genetic-induced ob/ob and high-fat diet -induced obese mice, indicating increased mitochondrial fission under these pathological conditions. Inhibition of mitochondrial fission by Drp 1 pharmacological inhibitor midivi-1 improved insulin signaling in C2C12 cells and obese mice. Recent results showed that OPA1 appears essential for the normal adaptive response of skeletal muscle to training, manifested by blunted mitochondrial biogenesis in OPA1+/2 mice when they were adapted to exercise training. Therefore, these data suggest that mitochondrial dynamics may be existed and plays an important role in maintaining the function of skeletal muscle. To directly address whether mitochondrial dynamics exists in skeletal muscle in vivo, we expressed mitochondrial matrix-targeted photoactivatable green fluorescent protein in skeletal muscle using electroporation and examined mitochondrial dynamics in living anesthetic mice in real-time under confocal microscope. We found that mitochondria are quite dynamic and communicate with one another in skeletal muscle in vivo via mitochondrial fusion mediated by nanotunneling. Mitochondrial contents are transferred rapidly through mitochondrial network. Furthermore, the dynamic behavior is impeded in skeletal muscle in HFD-induced mice, accompanying with impairment of mitochondrial respiratory function and decrease of ATP content. Mitochondria are unique double membrane-bound organelles with their own genetic materials, mtDNA. Recent advances revealed that they play a central role in many cellular functions, including regulation of reactive oxygen species, calcium homeostasis and signal transduction, in addition to their most prominent roles in energy production. However, these functions are determined by their structures, morphologies and distributions, which are referred to mitochondrial dynamics.