Multicellular rosettes link mesenchymal-epithelial transition to radial intercalation in the mouse axial mesoderm

Mesenchymal-epithelial transitions are fundamental drivers of development and disease, but how these be-haviors generate epithelial structure is not well understood. Here, we show that mesenchymal-epithelial tran-sitions promote epithelial organization in the mouse node and notochordal plate through the assembly and radial intercalation of three-dimensional rosettes. Axial mesoderm rosettes acquire junctional and apical po-larity, develop a central lumen, and dynamically expand, coalesce, and radially intercalate into the surface epithelium, converting mesenchymal-epithelial transitions into higher-order tissue structure. In mouse Par3 mutants, axial mesoderm rosettes establish central tight junction polarity but fail to form an expanded apical domain and lumen. These defects are associated with altered rosette dynamics, delayed radial inter-calation, and formation of a small, fragmented surface epithelial structure. These results demonstrate that three-dimensional rosette behaviors translate mesenchymal-epithelial transitions into collective radial inter-calation and epithelial formation, providing a strategy for building epithelial sheets from individual self -orga-nizing units in the mammalian embryo.