The final determination of Xenopus ectoderm depends on intrinsic and external positional information

Traditionally the whole animal cap (ventral plus dorsal ectoderm) of amphibian blastula and gastrula stages was considered as a homogeneous cell mass, because both the isolated dorsal and ventral ectoderm without induction differentiated into ciliated (atypical) epidermis. Recent results suggest a predisposition of the dorsal and ventral ectoderm. We used a special experimental approach, i.e. injection of activin as inducer into the blastocoel of intact Xenopus blastulae before the isolation of animal caps and fluorescein-dextran-amine (FDA) as a lineage tracer. In recombinants of FDA-labeled and unlabeled ectoderm we showed that the cells of the dorsal ectoderm mainly differentiate into neural tissue and notochord when they remain at their original dorsal position. In contrast, when small pieces of dorsal ectoderm are transplanted to the ventral part of animal caps, most of the descendants form epidermis. However, when small pieces of the ventral ectoderm are transplanted to the dorsal side, they significantly contribute to neural tissue and notochord. These results suggest that the prepattern in Xenopus animal caps of the late blastula and early gastrula stages is labile and reversible. Still more important is the fact that the fate of individual cells depends on the site of their localization within the animal cap. This means that cells in the dorsal most or ventral most part of the animal cap, respectively, will not randomly differentiate into all cell types, but predominantly into dorsal or ventral derivatives, respectively.