Cardiac progenitor migration and specification: The dual function of Wnts.

The heart is the first organ to function during vertebrate development and cardiac progenitors, are among the first cell lineages to be established from mesoderm cells emerging from the primitive streak during gastrulation. Cardiac progenitors have been mapped in the epiblast of pre-streak embryos. In the early chick gastrula they are located in the mid-primitive streak, from which they enter the mesoderm bilaterally. However, migration routes of cardiac progenitors have never been directly observed within the embryo and the factor(s) controlling their movement are not known. Furthermore, it is not understood how signals controlling cell movement are integrated with those that determine cell fate. Long-term video microscopy combined with GFP labelling and image processing enabled us to observe the movement patterns of prospective cardiac cells in whole embryos in real time. Embryo manipulations and the analysis of explants suggest that Wnt3a plays a crucial role in guiding these cells through a RhoA dependent mechanism involving negative chemotaxis. Wnt3a is expressed at high levels in the amniote primitive streak and ectopic signalling activity caused wider movement trajectories resulting in cardia bifida, which was rescued by dominant-negative Wnt3a. Our studies revealed Wnt3a-RhoA mediated chemo-repulsion as a novel mechanism guiding cardiac progenitors. This activity can act at long-range and does not interfere with cardiac cell fate specification.