CRITICAL REVISION OF THE MESODERM INDUCTION HYPOTHESIS

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The earliest inductive process in amphibian development has long been thought to occur during the cleavage stage and thought to induce Spemann-Mangold organizer. We propose an alternative mechanism for the organizer development in which vegetally localized dorsal determinants move equatorially to form the organizer in the marginal zone in a cell-autonomous manner. The principal finding in the present project is that the dorsal induction occurs after zygotic gene expression commences.We performed some experiments to investigate the temporal and spatial details of the dorsal induction exerted by dorsal vegetal cells in Xenopus embryo. Two dorsal vegetal cells (D1 cells) were transplanted into the ventral vegetal region of a recipient at the 32-cell stage. At various times after transplantation, the ventral animal-equatorial part was explanted and cultured. The explants isolated later 5.5 hours after transplantation (time 5.5) elongated and formed somites. In RT-PCR analysis, the expressi … More on of dorsal gene, chordin was activated in the explants isolated after time 4.0 (about the 4000-cell stage which corresponds to the mid blastula transition-MBT) at control stage 10. In another series of experiments, ventral animal-equatorial and dorsal vegetal parts were isolated from the 4000-cell stage embryos and they were combined for 2.0-2.5 hours. These ventral animal-equatorial explants elongated and formed somites. The chordin expression was as well observed in the explants. But the 32- and 256-cell stage dorsal vegetal cells failed to exert the dorsalizing activity within the 2.0-2.5 hours of the conjugation. These results suggest that 2 hours contact after MBT is necessary and sufficient for the dorsal induction from the dorsal vegetal cells and it occurs as a result of the zygotic gene expression. Consistent with this idea, the zygotic dorsal genes, siamois and chordin were expressed on the upper regions of the transplanted D1 descendants at stage 10. Furthermore this region began to gastrulate when the D1 cell was transplanted with upside-down orientation. Our data indicate that the upper region of the D1 descendants by itself act as the Spemann organizer rather than the Nieuwkoop center. Less