Myogenesis in the Mouse

The first striated muscle to form during mouse embryogenesis is theheart followed by skeletal muscle which is derived from the somites. The expressionof genes encoding muscle structural proteins and myogenic regulatory sequencesof the MyoD1 family has been examined using 35S-labelled riboprobes. In thecardiac tube, actin and myosin genes are expressed together from an early stage,whereas in the myotome, the earliest skeletal muscle, they are activatedasynchronously over days. They are not expressed in the somite prior to myotomeformation. One potential muscle marker, carbonic anhydrase III, is expressed inearly mesoderm and subsequently in the notochord, similarly to the Brachyurygene. The myogenic sequences are not detectable in the heart. In the myotomethey show distinct patterns of expression; this is discussed in the context of theirrole as muscle transcription factors. myf-5 is the only myogenic factor sequencepresent in the somite prior to muscle formation and thus is potentially involvedin an earlier step of muscle determination. It is also present in the early limb bud,but the status of myogenic precursor cells in the limb in this context is less clear.