Comparative roles of Wnt/PCP pathway in midline morphogenesis and cellular flows during non-amniote and amniote gastrulation

The Wnt/Planar cell polarity (PCP) pathway is evolutionarily conserved and plays crucial roles in coordinating collective cell behaviors during embryonic development. Across the animal kingdom, the bilateral body plan is built upon the midline, whose formation begins during gastrulation, a stage characterized by large-scale cellular flows (extensive collective cell movements). In non-amniotes, midline morphogenesis is tightly coupled to these cellular flows, and this coupling is mediated by the Wnt/PCP pathway. By contrast, during amniote embryogenesis, the Wnt/PCP pathway is essential for morphogenesis of the initial midline structure, the primitive streak, particularly in avian embryos. However, its role in regulating cellular flows during primitive streak development has yet to be fully elucidated. This review integrates historical and recent findings on the Wnt/PCP pathway in midline morphogenesis and cellular flows across non-amniotes and amniotes, with a particular focus on amniote (especially avian) primitive streak development. Conserved mechanisms and species-specific developmental processes are highlighted, and the interface between the Wnt/PCP pathway and collective cell behaviors is discussed in the context of vertebrate body-plan patterning.