Thank you Mathieu! 😁

This is a question for #LISH25 :)

And in case you were wondering if Hydras look like their mythological counterparts, the regenerated aggregates can indeed have multiple heads! And their number is scaling with the size of the aggregate.

You can find more movies here, and do not miss the supplementary figures: tinyurl.com/3wz4mavb. Thanks to the people who contributed to this work, Giulia Serafini, Heino Andreas, Chris Zechner, Carl Modes, @paveltomancak.bsky.social and @mpi-cbg.de. A big thank to the Hydra community for the help 💚

Embryos hatch in a configuration similar to our stretch experiments: they break the shell on one side and stretch outside this hole. Actin is indeed aligned with the direction of the stretch. The mechanism we uncovered could also provide robustness to the embryo patterning. (9/9)

We think this is a great example of how mechanics impacts and potentially drives self-organization and the emergence of order in collectives of cells, and how analogies from physics can help understand it. Of course, ‘what about embryos?’ asked Pavel. (8/9)

Altogether, this suggests that the ordering of actin in aggregates happens through cell-cell interaction and is favored by anisotropic stretch but delayed by supracellular (smectic-like) fibers. Wnt head organizers would only play a role later. (7/9)

What about stretch? We constrained the aggregates in agarose except for a hole. The tissue protrudes inside the hole, and the resulting stretch has a fast effect on actin alignment in the whole aggregate. Importantly, Wnt head organizers were not yet present in the protrusion. (6/9)

We mechanically perturbed the aggregate by changing topology ('skewer') and geometry ('sausage'). Topological changes alone do not significantly bias the actin orientation. Geometry has an indirect effect on actin orientation but is not associated with curvature anisotropy. (5/9)

Measurement of the nematic (orientation) order parameter in space and time showed growing ordered domains and line defects. This suggests that local cell-cell interactions drive the transition from disorder to order. (4/9)

First, we quantified the emergence of order over days using analogies with liquid crystals. Rotational symmetry is broken. Surprisingly, translation symmetry is also partially broken. This smectic behavior is associated with the formation of supracellular actin fibers. (3/9)

Hydra is not only immortal but also can self-organize a whole organism from a clump of thousands of cells. In these aggregates, actin polarity is completely lost in cells and is re-established de novo. We studied how the transition from disorder to order happens. (2/9)

Publication @pnas.org www.pnas.org/doi/10.1073/...