Demonstration of neural induction using nuclear markers inXenopus

Abstract

Two nuclear markers were used to investigate the origin of cells in secondary embryos ofXenopus induced by dorsal lip transplants, and to determine the ability of the chordomesoderm to direct cells to change their fates.3H-thymidine was used to label cells transplanted between individualX. laevis embryos, and nuclear quinacrine fluorescence was used to distinguishX. borealis tissues transplanted toX. laevis hosts. In the first set of experiments, dorsal lip tissue (also known as the dorsal marginal zone; DMZ) was transplanted to the ventral marginal zone (VMZ) of host embryos. The marginal zone is the toroid of presumptive mesodermal cells which involutes during gastrulation. Examination of the secondary embryos resulting from these grafts revealed that their notochords were derived almost exclusively from transplanted cells whereas their nervous systems and somites were composed almost entirely of host cells. Next, the nuclear markers were used to show the normal fates of the tissue of the ventral equatorial region immediately above the VMZ by orthotopic grafting. This tissue was found to give rise to structures in the ventral posterior portions of the tailbud embryo. Finally, the same ventral tissue was labeled and transplanted to the dorsal equatorial region above the DMZ. As a result, it was induced to change its fate and become neural. These results lend unequivocal support to Spemann's theory of neural induction which has recently been questioned.