Diversity of myosin heavy chain expression in satellite cells from mouse soleus and EDL muscles.

Postnatal myoblasts, the satellite cells, originating from slow and fast skeletal muscle fibres differentiate and fuse into myotubes expressing different phenotype of myosin heavy chain (MyHC) isoforms. Little is known, however, of factors which establish and maintain this phenotypic diversity. We used immunofluorescent labelling and Western blotting to examine the expression of slow and fast MyHC isoforms in myotubes formed in vitro from satellite cells isolated from mouse fast twitch extensor digitorum longus (EDL) and slow twitch soleus muscles. Satellite cells were cultured in serum-rich growth medium promoting myoblast proliferation until cross-striated and self-contracting myotubes were formed. We report that in both cultures myotubes expressed slow as well as fast MyHC isoforms, but the level of slow MyHC was higher in soleus culture than in EDL culture. Hence, the pattern of expression of slow and fast MyHC was characteristic of the muscle fibre type from which these cells derive. These results support the concept of phenotypic diversity among satellite cells in mature skeletal muscles and suggest that this diversity is generated in vitro irrespectively of serum mitogens.