Dual mode of paraxial mesoderm formation during chick gastrulation.

The skeletal muscles and axial skeleton of vertebrates derive from the embryonic paraxial mesoderm. In amniotes, paraxial mesoderm is formed bilaterally to the nerve cord as a result of primitive streak and tail-bud regression during body axis formation. In chick and mouse embryos, paraxial mesoderm was proposed to derive from a population of resident cells located in the regressing primitive streak and tail bud. In contrast, in lower vertebrates, paraxial mesoderm is formed as a result of the continuation of ingression movements of gastrulation. Here, we reinvestigate paraxial mesoderm formation in the chicken embryo and demonstrate that these two modes are concomitantly at work to set up the paraxial mesoderm. Although the medial part of somites derives from stem cells resident in the primitive streak/tail bud, the lateral part derives from continuous ingression of epiblastic material. Our fate mapping further shows that the paraxial mesoderm territory in the epiblast is regionalized along the anteroposterior axis as in lower vertebrates. These observations suggest that the mechanisms responsible for paraxial mesoderm formation are largely conserved across vertebrates.