Left–right patterning in the mouse requires Epb4.1l5-dependent morphogenesis of the node and midline

The mouse node is a transient early embryonic structure that is required for left–right asymmetry and for generation of the axial midline, which patterns neural and mesodermal tissues. The node is a shallow teardrop-shaped pit that sits at the distal tip of the early headfold (e7.75) embryo. The shape of the node is believed to be important for generation of the coherent leftward fluid flow required for initiation of left–right asymmetry, but little is known about the morphogenesis of the node. Here we show that the FERM domain protein Lulu/Epb4.1l5 is required for left–right asymmetry in the early mouse embryo. Unlike other genes previously shown to be required for left–right asymmetry in the mouse, lulu is not required for specification of node cell identity, for Nodal signaling in the node or for ciliogenesis. Instead, lulu is required for proper morphogenesis of the node and midline. The precursors of the wild-type node undergo a series of rapid morphological transitions. First, node precursors arise from an epithelial-to-mesenchymal transition at the anterior primitive streak. While in the mesenchymal layer, the node precursors form several ciliated rosette-like clusters; they then rapidly undergo a mesenchymal-to-epithelial transition to insert into the outer, endodermal layer of the embryo. In lulu mutants, node precursor cells are specified and form clusters, but those clusters fail to coalesce to make a single continuous node epithelium. The data suggest that the assembly of the contiguous node epithelium from mesenchymal clusters requires a rapid reorganization of apical–basal polarity that depends on Lulu/Epb4.1l5.