Dorsoventral polarization and formation of dorsal axial structures in Xenopus laevis: analyses using UV irradiation of the full-grown oocyte and after fertilization.

Xenopus laevis embryos which had been UV-irradiated as full-grown oocytes (UV-O) or after fertilization (UV-F) showed typical UV syndrome, namely dorsal axial deficiency. Morphological comparison revealed that UV-O embryos showed a clear dorsoventral polarity from early cleavage to gastrula stage, but UV-F embryos showed radially symmetrical development throughout embryogenesis. Although UV-O embryos developed morphologically normal-looking dorsal lips of the blastopore, they failed to develop dorsal axial structures at later stages. Implantation of dorsal lips demonstrated that the dorsal lip of UV-O embryos had less activity as Spemann's organizer than the dorsal lip of normal embryos. It is thus suggested that a morphological differentiation of the dorsal lip of the blastopore does not necessarily imply a functional differentiation of Spemann's organizer. Dorsal or ventral cytoplasm from normal embryos at the 8-16 cell stage was injected into a blastomere of UV-F and UV-O embryos at the same stage as the donor. The injection of the dorsal cytoplasm could rescue partially the UV syndrome of UV-F but not of UV-O embryos.