Revisions to the Xenopus gastrula fate map: Implications for mesoderm induction and patterning

A revised fate map of the gastrula Xenopus embryo predicts the existence of patterning mechanisms that operate within the animal/vegetal axis of the mesoderm-forming marginal zone. We review here molecular and embryologic data that demonstrate that such mechanisms are present and that they operate independently of the Spemann organizer. Evidence suggests that polarized fibroblast growth factor activity in the animal/vegetal axis patterns this axis. We present a model of mesoderm induction and patterning that integrates the new data on Spemann organizer-independent animal/vegetal patterning with data on other inductive pathways known to act on the gastrula marginal zone.