AN INHIBITORY EFFECT OF XENOPUS-GASTRULA ECTODERM ON MUSCLE-CELL DIFFERENTIATION AND ITS ROLE FOR DORSOVENTRAL PATTERNING OF MESODERM

In Amphibia, mesoderm cells such as notochord, muscle, and blood are f ormed as a result of mesoderm induction, the first known inductive int eraction during the embryonic development of Vertebrates. Recent evide nce shows, however, that, in addition to mesoderm induction, further c ell-cell interactions during gastrulation also play an important role in the differentiation of mesoderm cells in Xenopus embryos. Here we r eport the existence of an inhibitory effect of gastrula ectoderm on mu scle cell differentiation. When placed in contact with a muscle progen itor cell population, ectoderm from the ventral side of a gastrula emb ryo exerts an inhibitory effect on MyoD expression and muscle differen tiation. This inhibitory effect is not observed on notochord different iation. Dorsal ectoderm shows the same extent of inhibitory effect if it is isolated at the early-gastrula stage but gradually loses its eff ect during gastrulation as it is induced to become neural tissue. Cell mixing experiments have shown that this inhibitory effect, clearly se en in ventral ectoderm, is not observed in ventral mesoderm cells. We propose that the ectoderm emits a signal counteracting dorsalization a nd the community effect and that this signal plays an important role i n the correct dorsoventral patterning of mesoderm. (C) 1994 Academic P ress, Inc.