CRITICAL REVISION OF THE MESODERM INDUCTION HYPOTHESIS

长期以来，人们认为两栖动物发育中最早的诱导过程发生在卵裂阶段，并被认为诱导了Spemann-Mangold组织者。我们提出了一种替代机制的组织者的发展，植物本地化的背决定因素赤道移动，形成组织者在边缘区的细胞自主的方式。本研究的主要发现是，背侧诱导发生在合子基因表达开始之后，我们对爪蟾胚胎背侧植物细胞产生背侧诱导的时间和空间细节进行了研究。将两个背侧植物细胞（D1细胞）移植到32细胞期受体的腹侧植物区。在移植后的不同时间，将腹侧动物赤道部分固定并培养。移植后5.5小时（时间5.5）分离的外植体伸长并形成体节。在RT-PCR分析中， ...更多信息 在dorsal基因上，在对照阶段10，在时间4.0（约4000细胞阶段，对应于囊胚中期过渡-MBT）后分离的外植体中，脊索蛋白被激活。在另一系列实验中，从4000细胞期胚胎中分离腹侧动物赤道和背侧植物部分，并将它们组合2.0-2.5小时。这些腹面动物赤道外植体伸长，形成体节。在外植体中也观察到脊索蛋白的表达。而32细胞期和256细胞期的背侧植物细胞在接合后2.0-2.5小时内不能发挥背侧化活性。这些结果表明，MBT后2小时的接触是必要的和足够的背部诱导从背部植物细胞，它发生作为合子基因表达的结果。与这一想法相一致，合子背基因，siamois和chordin上的移植的D1后代在第10阶段的上部区域表达。此外，当D1细胞以倒置方向移植时，该区域开始原肠胚形成。我们的数据表明，D1后代的上部区域本身作为Spemann组织者，而不是Nieuwkoop中心。少

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