Molecular and genetic regulation of convergence and extension in zebrafish

During vertebrate gastrulation the cell movements of convergence and extension mediolaterally narrow and anteroposteriorly lengthen the forming embryonic axis concurrent with the acquisition of cell fate. Patterning molecules direct both of these yet we know little of how they are coordinated. What genetic pathways underlie the movements is also unknown. To study convergence and extension I use the zebrafish, due to the optical transparency of the embryo and the presence of a number of mutants exhibiting gastrulation defects. Our work reveals three domains of convergence and extension within the wild-type zebrafish gastrula. Ventrally, cells migrate vegetally to the tailbud. Laterally, cells move with increasing speed towards dorsal. Dorsally, we correlate high levels of extension with mediolateral cell polarization in vivo . We characterized the convergence and extension defects in two mutants specifically affected in these processes, trilobite and knypek. We propose trilobite function is needed for the acceleration of convergence and extension movements in the lateral domain whereas Knypek, which potentiates Wnt11 signaling, is required for the dorsal cell polarization. The Bmp activity gradient specifies the C&E movement domains, in addition to directing cell fate specification along the dorso-ventral axis. It does so in part by cell autonomous regulation of cell behavior as well as downregulating secreted dorsal factors that promote C&E without affecting cell fates, wnt11 and wnt5. Therefore we propose that a single gradient of Bmp activity can regulate both cell movements and, possibly in parallel, cell fate specification.