Here we report the cloning of a Xenopus frizzled transmembrane receptor, Xfz7, and describe its expression pattern during early embryogenesis. Xfz7 mRNA is provided maternally and zygotic transcription peaks in gastrula stages. At that time, transcripts are preferentially localized to the marginal zone and become restricted to distinct regions of the tadpoles in tailbud stages. Overexpression of Xfz7 in embryos perturbs the morphogenesis of trunk and tail, blocks convergence-extension movements in animal caps induced with activin and dorsal lip explants and decreases cadherin-mediated cell adhesion. Xfz7 can interact specifically with Xwnt-8b and signal in the canonical, dorsalizing Wnt pathway. Overexpression of Xfz7 does not trigger the Wnt-1-type pathway but acts in a non-canonical Wnt or morphogenetic-effector pathway involving the activation of protein kinase C (PKC). Xfz7 seems to be involved in different aspects of Wnt signaling during the course of embryogenesis.

EpCAM confers “invasive” properties to embryonic cells through intracellular signaling.

In Xenopus laevis, patterning of the trunk mesoderm into the dorsal notochord and lateral somites depends on differential regulation of Wnt–β-catenin signaling. To study the cellular requirements for the physical separation of these tissues, we manipulated β-catenin activity in individual cells that were scattered within the trunk mesoderm. We found that high activity led to efficient cell sorting from the notochord to the somites, whereas reduced activity led to sorting in the opposite direction. Analysis of individual cells overexpressing β-catenin revealed that these cells were unable to establish stable contacts with notochord cells but could freely cross the boundary to integrate within the somitic tissue. Interference with cadherin-mediated adhesion disrupted tissue architecture, but it did not affect sorting and boundary formation. Based on these results, we propose that the boundary itself is the result of cell-autonomous changes in contact behavior that do not rely on differences in absolute levels of adhesion.

No abstract

When groups of cells from the inner marginal zone (mesendoderm) of the early Xenopus gastrula are placed on a fibronectin-coated substratum, the explants of the dorsal region spread into monolayers whereas those from the ventral region, though they adhere to the substratum, do not show this spreading reaction. This different behaviour is not reflected in the in vitro behaviour of the respective cells kept in isolation. No difference between dorsal and ventral cells was observed, when they were tested for lamellipodia-driven spreading, movement over the substratum or properties of integrin- and cadherin-mediated adhesion. However, cell contacts between individual dorsal cells are significantly less stable than those between ventral cells. The higher flexibility of the cell-cell contacts seems to determine the spreading behaviour of the dorsal explants, which includes lamellipodia-driven outward movement of the peripheral cells, rearrangements of the cells, building up a horizontal tension within the aggregate and intercalation of cells from above into the bottom layer. Ventral explants lack these properties. Staining for F-actin revealed a decisive difference of the supracellular organisation of the cytoskeleton that underlies the morphology of the different types of explants. Evidence for a higher flexibility of cell-cell contacts in the dorsal mesendoderm was also obtained in SEM studies on gastrulating embryos. Dorsal mesendodermal cells show stronger protrusive activity as compared to ventral mesendodermal cells. The meaning of these observations for the mechanisms of morphogenetic movements during gastrulation is central to the discussion.