Neuroectodermal specification and regionalization of the Spemann organizer in Xenopus.

During gastrulation in Xenopus convergence and extension movements, mediated by mediolateral intercalations, are the driving force for early neural plate morphogenesis. Here we show that the winged helix transcriptional regulator, Xfd-12' is dynamically expressed in medial neural plate precursors that undergo convergence and extension movements. These medial neuraxial progenitors are specified in and beyond the Spemann organizer prior to specification of the basal anlage of the neural plate. The initiation of Xfd-12' expression coincides with the induction of mesendoderm by Nodal-related growth factors at the late blastula stage. Comparative expression analysis suggests that cellular rearrangements at the pre-gastrulation stage account for regionalization of the Spemann organizer into head and trunk organizer compartments, the latter in which medial neural plate progenitors reside. While the maintenance of Xfd-12' expression in the dorsal non-involuting marginal zone requires FGF signalling, its subsequent positioning along the medial aspect of the neuraxis depends on signalling by Wnt and Nodal-related family members. Based on these findings we propose that XFD-12' is a trunk organizer component that might control convergence and extension movements of medial neural plate precursors during gastrulation.