Formation process and fate of the nuclear chain after injury in regenerated myofiber.

Although it is well known that regenerated myofibers contain nuclear chains (arrayed nuclear clusters), details of its process of formation and fate are still remained unclear. In the present study, we isolated single myofibers from injured ICR mouse tibialis anterior muscles by the alkali maceration-based method, and carried out histological observation and bromodeoxyuridine (BrdU) pulse-chase analysis on the nuclear chains. The nuclear chains were formed after injury and remained stable for at least 6 months after injury. When BrdU was administered during the first 4 days after injury, up to 56% of nuclei in the nuclear chains were labeled with BrdU, whereas when BrdU was administered 5 days or later after injury, less than 3% of myonuclei were labeled with BrdU. Among BrdU-positive nuclei in the nuclear chains, the nuclei showing attenuated and strong BrdU signal were dominant when BrdU was administered at the time points of 0-2 and 3-4 days after injury, respectively. These results suggest that successive nuclear divisions occur during the first 4 days after injury and might be involved in the appearance of the stable nuclear chains in regenerated myofibers.