Wnt genes play important roles in regulating patterning and morphogenesis during vertebrate gastrulation. In zebrafish, slb/wnt11 is required for convergence and extension movements, but not cell fate specification during gastrulation. To determine if other Wnt genes functionally interact with slb/wnt11, we analysed the role of ppt/wnt5 during zebrafish gastrulation. ppt/wnt5 is maternally provided and zygotically expressed at all stages during gastrulation. The analysis of ppt mutant embryos reveals that Ppt/Wnt5 regulates cell elongation and convergent extension movements in posterior regions of the gastrula, while its function in more anterior regions is largely redundant to that of Slb/Wnt11. Frizzled-2 functions downstream of ppt/wnt5, indicating that it might act as a receptor for Ppt/Wnt5 in this process. The characterisation of the role of Ppt/Wnt5 provides insight into the functional diversity of Wnt genes in regulating vertebrate gastrulation movements.

During vertebrate gastrulation, the three germ layers, ectoderm, mesoderm and endoderm are formed, and the resulting progenitor cells are brought into the positions from which they will later contribute more complex tissues and organs. A core element in this process is the internalization of mesodermal and endodermal progenitors at the onset of gastrulation. Although many of the molecules that induce mesendoderm have been identified, much less is known about the cellular mechanisms underlying mesendodermal cell internalization and germ layer formation. Here we show that at the onset of zebrafish gastrulation, mesendodermal progenitors in dorsal/axial regions of the germ ring internalize by single cell delamination. Once internalized, mesendodermal progenitors upregulate E-Cadherin (Cadherin 1) expression, become increasingly motile and eventually migrate along the overlying epiblast (ectodermal) cell layer towards the animal pole of the gastrula. When E-Cadherin function is compromised,mesendodermal progenitors still internalize, but, with gastrulation proceeding, fail to elongate and efficiently migrate along the epiblast,whereas epiblast cells themselves exhibit reduced radial cell intercalation movements. This indicates that cadherin-mediated cell-cell adhesion is needed within the forming shield for both epiblast cell intercalation, and mesendodermal progenitor cell elongation and migration during zebrafish gastrulation. Our data provide insight into the cellular mechanisms underlying mesendodermal progenitor cell internalization and subsequent migration during zebrafish gastrulation, and the role of cadherin-mediated cell-cell adhesion in these processes.

We have identified a novel role of a signaling pathway comprised of PDGF, PI3K, and PKB in the control of morphogenetic cell movements during gastrulation. Furthermore, our findings provide insight into the relationship between cell polarization and directed cell migration at the onset of zebrafish gastrulation.