FIBERS OF INTERMEDIATE TYPE-1C AND TYPE-2C ARE FOUND CONTINUOUSLY IN MDX SOLEUS MUSCLE UP TO 52 WEEKS

The mdx mutant is a murine homologous model of Duchenne muscular dystr ophy (DMD). Fibre types determined by the myosin-ATPase technique in s oleus muscles were compared in C57BL/10 control and mdx mice from 3 to 52 weeks of age. The control strain continuously presented 70% of typ e 2 fibres whereas the mdx strain showed an increase to 80% at 6 weeks and a subsequent decline. In mdx muscles, necrosis which begins at 3 weeks of age did not affect specifically one type of fibre. Type group ing was never observed when muscle regeneration occurred. Fibres of in termediate type (1C and 2C) were found continuously up to 52 weeks of age in the mdx mutant. The foci of small immature regenerating fibres were of type 2C but never 1C. A few mature fibres were either of type 2C or 1C. We suggest that the presence of intermediate type fibres cou ld result from the co-expression of type 1 and 2 myosin heavy chains, indicating a transition from type 2 to type 1 in regenerating fibres.