Characteristics of muscle fibers reconstituted in the regeneration process of masseter muscle in an mdx mouse model of muscular dystrophy.

Mdx mice, which lack dystrophin, were examined for changes in the properties of muscle fibers in the growth process of the masseter muscle at the morphological, protein and transcriptional levels. The slow-type isoform, MyHC-1, and the fast-type isoforms, MyHC-2a, MyHC-2d and MyHC-2b, were examined at the protein and the transcriptional level. Morphological examination showed that in the mdx mouse masseter muscle, degeneration, necrosis, and regeneration occurred, particularly at the age of 4 weeks, and many regenerated muscle fibers with centrally located nuclei were observed at the age of 9 weeks. The results of examination at the protein and the transcriptional level showed that in the process of muscle fiber degeneration, necrosis, and regeneration, the mdx mouse masseter muscle acquires muscle fiber characteristics entirely different from those in the normal mouse masseter muscle. In particular, MyHC-1, which is rarely found in normal mice, was very strongly expressed.