Tissue flow acts as a guidance cue for immune cell polarization and directional migration.

Embryonic myeloid (EM) cells are the first immune-cell population to emerge during development and must disperse throughout the embryo to act as the first line of defence against infection. Although EM cells migrate directionally toward wounds, how they navigate through unwounded tissues during early colonization remains unclear. Here we show that EM cell dispersion in Xenopus embryos is driven, at least in part, by cell-on-cell migration, an important yet underappreciated phenomenon, guided by dynamic tissue flows. We have established a new ex vivo EM cell migration system that allows for live imaging, computational analyses and optogenetic manipulation. We find that local ectodermal tissue flows repolarize EM cell protrusions and bias their directional migration. Disrupting these flows, both ex vivo and in vivo, either genetically or mechanically, impairs EM cell dispersion. Our findings reveal that mechanical cues generated by surrounding tissue flows coordinate immune-cell migration during development, highlighting an overlooked mechanism by which collective tissue dynamics guide individual cell behaviour.