Paraxis is required for somite morphogenesis and differentiation in Xenopus laevis

In most vertebrates, the segmentation of the paraxial mesoderm involves the formation of metameric units called somites through a mesenchymal-epithelial transition. However, this process is different in Xenopus laevis because it does not form an epithelial somite. Xenopus somitogenesis is characterized by a complex cells rearrangement that requires the coordinated regulation of cell shape, adhesion, and motility. The molecular mechanisms that control these cell behaviors underlying somite formation are little known. Although the Paraxis has been implicated in the epithelialization of somite in chick and mouse, its role in Xenopus somite morphogenesis has not been determined.

Using a morpholino and hormone-inducible construction approaches, we showed that both gain and loss of function of paraxis affect somite elongation, rotation and alignment, causing a severe disorganization of somitic tissue. We further found that depletion or overexpression of paraxis in the somite led to the downregulation or upregulation, respectively, of cell adhesion expression markers. Finally, we demonstrated that paraxis is necessary for the proper expression of myotomal and sclerotomal differentiation markers.

Our results demonstrate that paraxis regulates the cell rearrangements that take place during the somitogenesis of Xenopus by regulating cell adhesion. Furthermore, paraxis is also required for somite differentiation.