Guillot, C.; Djeffal, Y.; Serra, M.; Pourquie, O.

In amniotes, embryonic tissues originate from multipotent epiblast cells, arranged in a mosaic of presumptive territories. How these domains fated to specific lineages become segregated during body formation remains poorly understood. Using single cell RNA sequencing analysis and lineage tracing in the chicken embryo, we identify epiblast cells contributing descendants to the neural tube, somites and lateral plate after completion of gastrulation. We show that intercalation after cell division generates important movements of epiblast cells which lead to their relocation to different presumptive territories, explaining this broad spectrum of fates. This tissue remodeling phase is transient, being soon followed by the establishment of boundaries restricting cell movements therefore defining the presumptive territories of the epiblast. Finally, we find that the epiblast faces distinct mechanical constraints along the antero-posterior axis, leading to cell fate alterations when challenged. Together, we demonstrate the critical role of mechanical cues in epiblast fate determination.