FIGURE 2.

The myogenic waves and the myotome formation in amniotes and Xenopus. (A) The myotome is essentially derived from the dermomyotome in amniotes. The primary myotome is made of mononucleated cells arising from the four borders of the dermomyotome. Next cells coming from the central dermomyotome invade the primary myotome and contribute to the myotome growth. Hence, two myogenic waves are at the origin of myotome formation. Modified from Lagha et al. (2008a). (B) Myotome is the main somite compartment in Xenopus and is formed by at least three myogenic waves. The first myogenic wave is made up of two subpopulations, a medial and a lateral one, constitutes the primitive myotome and arises directly from paraxial mesoderm. The second myogenic wave arises from epaxial and hypaxial border of the dermomyotome at stage 28–30. The third myogenic wave has been visualized by myf5 mRNA staining that marked isolated round cells inside the myotome at stage 37–38. The myotome is initially made up of mononucleated fibers until stage 45 when the first multinucleated muscle fibers were observed. Hence, it can be considered that both the first wave of primitive myotome and the second wave of hypaxial and epaxial dermomyotome contribute to the formation of primary myotome. The third could participate to plurinucleated fibers formation and myotome growth. St., stage.