Transient Epithelial Mimicry by Neural Crest Mesenchyme Anchors Cell Condensations Across Avian Beaks

Multicellular morphogenesis must balance organismal cohesion with local tissue differentiation. In avian beaks, conserved epithelial-mesenchymal crosstalk underlies the formation of condensations of migratory neural crest mesenchymal (NCM) cells, yet how these cells acquire precise positional information without compromising stemness is unclear. Using high-throughput quantification of protein expression and morphology of 2.1 million cells and 16 stereotypical condensations across upper and lower beaks, we resolve the temporal sequence of condensation anchoring. We find that a subset of mesenchymal cells at each condensation site transiently matches protein expression in the overlying epithelium, which diverges as development proceeds. Propagation of these location-specific expression profiles into mesenchyme establishes signaling boundaries that anchor forming condensations. As NCM cells accumulate within these boundaries, they progressively erase location-specific protein profiles and restore their region- and tissue-specific protein expression. These transient location-matching and cell-homogenization phases show how migrating NCM cells achieve precise positional anchoring while retaining stemness needed for regional specifications. Ultimately, spatiotemporal modulations of a conserved regulatory network by predictable patterns of cell proliferation and migration can underpin the remarkable evolutionary diversification of avian beaks.