ABSTRACT
Neurogenesis begins in amphibian embryos around the time of gastrulation when a portion of the ectoderm receives an inducing signal from dorsal mesoderm. Two different proposals have been made for how ectoderm must come into contact with dorsal mesoderm in order for the inducing signal to pass between the two tissues. Induction in one proposal would require normal gastru-lation movements to bring dorsal mesoderm under-neath, and into apposition with, the overlying ectoderm. The inducing signal in this case would pass between dorsal mesoderm and ectoderm as apposed tissue layers. The other proposal is that induction requires only a small contact between ectoderm and dorsal mesoderm at the boundary they share before gastrulation. The in-ducing signal by this proposal would pass laterally across this small area of contact between mesoderm and ectoderm, perhaps before gastrulation, and spread within the ectodermal cell layer. Since it is not known to what extent neurogenesis depends on each of these proposed contacts between ectoderm and dorsal meso-derm, we have generated explants of embryonic tissue in which one or the other type of contact between meso-derm and ectoderm is favored. The amount of neural tissue formed under these various conditions was then assessed using a quantitative RNase protection assay to measure the levels of two neural-specific RNA tran-scripts. The results show that neural tissue forms efficiently when ectoderm and dorsal mesoderm only interact laterally within a plane of tissue. In contrast, neural tissue, forms extremely poorly when ectoderm is placed experimentally in apposition with involuting, anterior-dorsal mesoderm. Finally, a synergistic effect is obtained when both types of contacts can occur between ectoderm and mesoderm indicating that two signals may be required for neural induction in Xenopus embryos.
