The location of central nuclei along muscle fibers suggests that degeneration/regeneration events are quite frequent in older animals and are apparently randomly distributed. A levator auris muscle from a 27-month-old mouse was labeled in whole mount for dystrophin to identify the outlines of the muscle fibers, with bungarotoxin to identify the NMJs, and with DAPI to identify nuclei. A montage of maximum projections of overlapping confocal stacks containing 17–27 fibers at one edge of the muscle was made. The total extent examined was 1500 μm along the length of the fibers from one tendon (bottom) to the band of NMJs (top); only 1000 μm is shown here. Central nuclei, some of which are in chains within individual fibers, are identified with arrows. The central location of the nuclei within the fibers was ascertained from examination of confocal z-sections. A total of 42 fiber segments with central nuclei was found of varying length (the longest, 67 μm), suggesting that fiber damage is a frequent event in aging muscle. Variation in the number of central nuclei is likely explained by the severity of the damage event and/or the time elapsed since the injury, as nuclei move to the periphery of the fibers. Scale bar, 30 μm.