Comparative evolution of muscular dystrophy in diaphragm, gastrocnemius and masseter muscles from old male mdx mice

X chromosome-linked muscular dystrophic mdx mouse lacks the sarcolemmal pro tein dystrophin and represents a genetic homologue of human Duchenne muscul ar dystrophy (DMD). The present study analysed some aspects of pathological processes such as fibrosis, frequency of centralized nuclei, presence of d egenerative or regenerative fibres, expression of utrophin and associated p rotein complexes, and myosin heavy chain isoforms in three muscles [diaphra gm (DIA), gastrocnemius (GTC) and masseter (MAS)] from old male mdx mice. A ll parameters investigated comparatively in these pathological muscles prov ided evidence that the MAS mdx muscle presents a slight deterioration patte rn in comparison to that of DIA and GTC muscles. Utrophin and associated pr oteins are present in many cell clusters with continuous membrane labelling in MAS muscle. Respective proportions of myosin heavy chain isoforms, meas ured by electrophoresis/densitometry, showed only slight change in GTC musc le, significant evolution in DIA muscle but drastic isoform conversions in MAS muscle. These results highlighted the difference in deterioration susce ptibility of various muscles to muscular dystrophy. The reason why this occ urs in MAS muscles is still obscure and discussed in terms of the comparati ve developmental origins of these muscles.