Properties of the dorsal activity found in the vegetal cortical cytoplasm of Xenopus eggs.

The Xenopus egg contains a maternal dorsal determinant that is specifically localized to the vegetal cortex. We have previously shown that vegetal cortical cytoplasm can generate a full dorsal axis when it is injected into ventral vegetal blastomeres of a cleavage-stage embryo. In this study, we have defined further the properties of the dorsal activity. The cortical dorsal activity arises during oocyte maturation after germinal vesicle breakdown. When injected into the four extreme animal pole blastomeres of ultraviolet-ventralized 32-cell embryos, vegetal cortical cytoplasm partially rescued dorsal axial structures. As revealed by lineage tracing, these axial structures formed ectopically from the progeny of the cells that were injected. Injection of animal cortical cytoplasm had no effect. When mid-blastula (stage 8) animal caps from these injected embryos were isolated and cultured, both vegetal cortex-enriched and animal cortex-enriched animal caps produced only epidermis. Therefore vegetal cortex, on its own, is not a mesoderm inducer. Between stage 8 (blastula) and stage 10 (gastrula), a ventral mesoderm-inducing signal spreads from vegetal cells towards the animal pole. We tested whether this natural mesoderm-inducing factor interacts with the activity found in the vegetal cortex. Injection of vegetal cortex enhanced the formation of neural tissue and cement gland when animal caps were isolated at stage 10. When cultured from stage 8 in the presence of the ventral mesoderm-inducing fibroblast growth factor, animal caps enriched in vegetal cortex developed significantly more neural tissue and cement gland than ones enriched in animal cortex. These results indicate that the dorsal activity localized to the egg vegetal cortex alters the response of cells to mesoderm inducers.