Stiffness of primordial germ cells is required for their extravasation in avian embryos.

Unlike mammals, primordial germ cells (PGCs) in avian early embryos exploit blood circulation to translocate to the somatic gonadal primordium, but how circulating PGCs undergo extravasation remains elusive. We demonstrate with single-cell level live-imaging analyses that the PGCs are arrested at a specific site in the capillary plexus, which is predominantly governed by occlusion at a narrow path in the vasculature. The occlusion is enabled by a heightened stiffness of the PGCs mediated by actin polymerization. Following the occlusion, PGCs reset their stiffness to soften in order to squeeze through the endothelial lining as they transmigrate. Our discovery also provides a model for the understanding of metastasizing cancer extravasation occurring mainly by occlusion.