Thanks a lot Diana !!

Our results thus show that primitive streak formation is induced by NODAL activation in the posterior hypoblast. The movement of the hypoblast (which is a counter-rotating movement) is a consequence of primitive streak formation – and not a cause of it, as previously thought.

Finally, we investigated whether the anterior hypoblast could inhibit primitive streak formation, as previously thought. By grafting the anterior hypoblast onto the forming primitive streak, we could show that it does not. On the contrary, the grafted hypoblast starts to express NODAL.

In this movie, you can for example see that the full ablation of the hypoblast completely inhibits primitive streak formation, while a partial ablation that leaves a piece of posterior hypoblast on each side leads to the formation of two embryonic axes (forming conjoined twins).

Using functional analyses and tissue-recombination experiments, we confirmed that the posterior hypoblast is indeed the inducer of primitive streak formation through NODAL signaling activation.

Using HCR-RNA-FISH, we observed that the expression of NODAL, a master activator of primitive streak formation, initiates in the posterior hypoblast and then spreads to the entire posterior blastoderm.

The hypoblast thus remains in constant contact with the epiblast, which is incompatible with the idea that its movement controls primitive streak formation by locally lifting NODAL inhibition. This contradiction led us to reinvestigate the molecular mechanisms underlying primitive streak formation.

By intercalating a porous filter between the epiblast and the hypoblast, we further showed that the hypoblast requires physical contact with the epiblast to achieve counter-rotating flows. Thus, forces generated in the epiblast propagate by mechanical coupling to the hypoblast, setting it in motion.

By generating a two-color chimera, we observed that the counter-rotating flows of the epiblast and hypoblast are highly synchronized. As a result, the hypoblast and epiblast move very little in relation to each other.