Gastrulation occurs when a single-layered blastula transitions to a gastrula containing all three germ layers: ectoderm, endoderm and mesoderm. Many aspects of vertebrate gastrulation have been well-characterised using the model vertebrate Xenopus, but surprisingly little is known about how the dorsal mesoderm moves from the outer surface to the interior of the gastrula. Now, on p. , Rudolf Winklbauer and colleagues analyse the cellular and molecular dynamics that are central to this involution process and provide several insights into the mechanism of Xenopus gastrulation. First, the authors show that the blastopore lip is capable of autonomous shape changes and that different domains, for example those marked by Goosecoid and Brachyury (Xbra), show differences in cellular migration patterns. Second, the authors investigate the molecular requirements during these events and show that the tyrosine kinase ephrin receptor EphA4 is transiently required to maintain appropriate levels of Xbra expression, but that this occurs indirectly via p21-activated kinase 1 (Pak1). These data highlight a role for Xbra during mesoderm involution in Xenopus and shed light on the general features of dynamic cell migration during gastrulation.
