Planar signalling is not sufficient to generate a specific anterior/posterior neural pattern in pseudoexogastrula explants from Xenopus and Triturus

Early observations on the morphology of total exogastrulae from urodeles (A xolotl) had provided evidence for essential vertical signalling mechanisms in the process of neural induction. Conversely, more recent studies with an urans (Xenopus laevis) making use of molecular markers for neural-specific gene expression appear to support the idea of planar signalling as providin g sufficient information for neural differentiation along the anterior-post erior axis. In an attempt to resolve this apparent contradiction, we report on the comparative analysis of morphology and gene expression characterist ics with explants prepared from both urodeles (Triturus alpestris) and anur ans (Xenopus laevis). For this purpose, we have made use of a refined exper imental protocol for the preparation of exogastrulae that is intended to co mbine the advantages of the Holtfreter type exogastrula and the Keller sand wich techniques, and which we refer to as pseudoexogastrula explants. Analy sis of histology and expression of several neural and ectodermal marker gen es in such explants suggests that neural differentiation is induced in both species, but only within the intermediate zone between ectoderm and endome soderm. Therefore, experiments with Xenopus and Triturus explants described in this communication argue against planar signalling events as being suff icient to generate a specific anterior/posterior neural pattern. (C) 2000 E lsevier Science Ireland Ltd. All rights reserved.