During Xenopus gastrulation, the dorsal mesoderm undergoes two types of cell behaviour in two regions: active cell migration of prechordal mesoderm and convergent extension of chordamesoderm. Now Kwan and Kirschner report, on p. 1961, thatXbra might act as a switch to keep these two cell behaviours mutually exclusive during gastrulation. To study the role of Xbra and of other genes in these two processes, the authors used dissociated cells and explants in which cell migration and convergent extension were induced with activin. Their results revealed that Xbra — a T-box transcription factor that is necessary for convergent extension and that is not expressed in prechordal mesoderm — actively inhibits cell migration. From their findings, the authors propose that the ground state for dorsal mesodermal cells might be one of cell migration, which is inhibited in the chordamesoderm by Xbraexpression.

© 2003.
2003