Live imaging and functional characterization of the avian hypoblast redefine the mechanisms of primitive streak induction.

In birds and mammals, the formation of the primitive streak, the hallmark of the primary axis and site of gastrulation, is thought to occur when the anterior displacement of the hypoblast (visceral endoderm in mice) lifts its inhibition on NODAL signaling in the posterior epiblast. Although the anterior movement of the murine visceral endoderm is well documented, the dynamics of the avian hypoblast remain poorly understood. Using live imaging and quantitative analyses, we find that the hypoblast is mechanically coupled to the epiblast and does not migrate away from its posterior end. Instead, the hypoblast mostly moves and deforms passively through forces that shape the primitive streak, transmitted from the epiblast. We further show that the posterior hypoblast does not inhibit the epiblast but instead expresses NODAL, which activates primitive streak formation and concomitantly patterns the hypoblast along the anteroposterior axis. Our results thus redefine the cellular and molecular mechanisms establishing the avian primary axis, demonstrating that the hypoblast motion and anteroposterior patterning are consequences rather than drivers of primitive streak induction, downstream of NODAL signaling.