Mitochondrial Localization Around Central Nuclei in Regenerating Skeletal Muscle Following Eccentric Contraction in Rat Model

This study aimed to characterize the spatial distribution and ultrastructural changes of mitochondria in regenerating muscle following ECC-induced injury, utilizing photothermal microscopy (PTM) and transmission electron microscopy (TEM). ECC was applied to the gastrocnemius muscles of male rats (13 weeks old, 284.8 ± 8.9 g), and regenerating muscles were harvested seven days post-injury. PTM, featuring a high-sensitivity optical system, was employed to visualize the wide-range and three-dimensional distribution of mitochondria within the white gastrocnemius muscle region. Concurrently, TEM was used for quantitative analysis of mitochondrial ultrastructural morphology, including cristae density. In regenerating muscle, the regular lattice-like network observed in normal tissue was disrupted and replaced by fragmented, randomly distributed mitochondria. Notably, both PTM and TEM analyses revealed a high concentration of mitochondria specifically around “central nuclei”, a hallmark of regenerating muscle (i.e. within 0.1- 1.0 µm: Normal 1.8 ± 2.0% vs. Regeneration 5.5 ± 3.6%, p < 0.0001, by TEM data). Detailed morphological analysis further demonstrated that mitochondria in the immediate vicinity of the central nucleus (< 0.1 µm) had significantly lower cristae density (inner and outer membranes ratio, 1.10 ± 0.43) compared to those in distal regions (> 2.0 µm) (1.80 ± 0.65, p < 0.0001), indicating that they may be structurally immature. In conclusion, during the muscle regeneration process, mitochondria specifically localize around the central nucleus. Given their low cristae density, these potentially represent newly synthesized (biogenesis-derived) mitochondria. This perinuclear accumulation is thought to function as a critical energy source for the nuclear transcriptional and translational activities required for muscle differentiation, while also serving as a hub for organelle coordination during the regeneration process.