Competence prepattern in the animal hemisphere of the 8-cell-stage Xenopus embryo

Activins are known to be potentially important regulators in Xenopus developmental processes. It has been shown that activins exist maternally in the egg and can induce mesodermal tissues in blastula animal cap explants. However, the blastula ectoderm is known to possess a predisposed local response pattern to activin, and the process of the prepatterning is not understood. We isolated animal hemispheres from late 8-cell-stage embryos and treated them briefly with activin A. Expression of the muscle-specific actin gene was induced after a 30-min activin treatment, even when it was followed by treatment with follistatin, an activin-specific binding protein. This suggests that the animal-half blastomeres become competent to activin A before the 16-cell stage. In the normal embryo, the 8-cell-stage animal dorsal blastomeres populate neural ectoderm and most of the dorsal lip of the gastrula blastopore, the region of Spemann's organizer, and are the major progenitor for dorsal mesodermal tissues. When the dorsal and ventral animal-half blastomeres of the 8-cell stage were isolated and treated with activin independently, significant differences in tissue differentiation were observed. Dorsal blastomeres gave rise to trunk and tail structures containing dorsal mesoderm, whereas the ventral blastomere explants formed spheres containing solely ventral mesoderm. Further, both muscle actin transcription and goosecoid transcription were induced primarily in dorsal blastomeres. Our results suggest that a competence prepattern of response to activin exists as early as the 8-cell stage.