We then explored the cellular basis of tissue mechanics. Lowering division frequency (via temperature) or blocking division increased viscosity dramatically (4-fold), demonstrating that cell division is an active epithelial fluidizer.

The tip of the cantilever consists of a fluorescent bead that contacts the epiblast. Because we know the force necessary to bend the cantilever, we can perform mechanical assays by driving the bead across the embryo and measure in real time the force applied to the tissue.

This work uncovers an unusual mechanical behavior. Embryonic tissues are classically described as materials with short-time elasticity and long-time viscosity. Here, we show that the elastic and viscous responses co-exist on long morphogenetic timescales within the epithelial embryo.

This model recapitulates the deformation of the entire epiblast (embryonic + extraembryonic tissues).

We showed that this stretching is elastic, as the deformation is reversible, which leads to tension buildup, specifically in the extra-embryonic territory, while it was previously believed that the whole epiblast was under tension

We then performed high-resolution imaging in the extra-embryonic territory and found that cells extensively increased their apical area by stretching

To investigate this process, we quantified the deformation of the embryo over time and showed that area expansion is mostly restricted to the extra-embryonic territory.

I then performed high-resolution imaging in the extra-embryonic territory and found that cells extensively increased their apical area

I quantified the deformation of the embryo over time and showed that area expansion is mostly restricted to an annular region in the extra-embryonic territory

With @mongeralab.bsky.social, we could show that a substantial increase in the extra-cellular space drives tissue elongation with little contribution from proliferation.
More details in my previous tweetorial:
x.com/a_michaut/st...

During my PhD in O. Pourquie's lab at
@harvardmed.bsky.social, I studied how the anteroposterior axis elongates in #chicken embryos 🐣

To do so, I confined in vitro tissue explants and showed they were capable of autonomous elongation, while they are fluid tissues.