In vivo satellite cell activation via Myf5 and MyoD in regenerating mouse skeletal muscle

Regeneration of adult skeletal muscle is an asynchronous process requiring the activation, proliferation and fusion of satellite cells, to form new mu scle fibres, This study was designed to determine the pattern of expression in vivo of the two myogenic regulatory factors, Myf5 and MyoD during this process. Cardiotoxin was used to induce regeneration in the gastrocnemius a nd soleus muscles of heterozygous Myf5-nlacZ mice, and the muscles were ass ayed for the presence of P-galactosidase (Myf5) and MyoD. Adult satellite c ells identified by M-cadherin labelling, when activated, initially express either MyoD or Myf5 or both myogenic factors. Subsequently all proliferatin g myoblasts express MyoD and part of the population is P-galactosidase (Myf 5) positive. Furthermore, we demonstrate that activated satellite cells, wh ich express either Myf5 or MyoD, do not accumulate selectively on fast or s low muscle fibres.