How is myogenesis initiated in chordates?

Expression of transcriptor factors MyoD, Myf5, myogenin and MRF4 forms the basis of myogenesis. In Acrania, Pisces and Amphibia, as in Aves, myogenesis is initiated by MyoD. In Mammalia expression of Myf5 initiates myogenesis. Signal proteins Wnt and Shh induce the expression of genes encoding for MyoD or Myf5. In fishes and amphibians expression of MyoD starts in non-segmented mesoderm and then in myotomal cells. In birds and mammals expression of MyoD or Myf5 is initiated in the cells of the dermatomyotome. Embryonic myotomes are post-mitotic. Proliferating cells Pax3 and Pax7-positive and mesenchymal cells take part in the growth of myotomal muscles. Cells migrating to the limb bud contain regulatory proteins Six4/Six1, Pax3, Lbx1 and c-met. Rectus abdominis develops from cells that contain Pax3 and Lbx1.