This is the second simulation, with contractility and stiffness increasing over time (the latter derived from what we learnt using Brillouin microscopy):

Finally, we developed a physical toy model of mesoderm folding to test if the dynamic, localised increase in stiffness facilitates folding the mesoderm? The answer is YES!. See the simulations ⤵️: first movie: ✅contractility ❌stiffness; second movie: ✅contractility ✅ stiffness.

How do microtubules make mesoderm cells -transiently- stiffer? Using live imaging & 3D-SIM, we found microtubules reorganise within the cell's sub-apical compartment, -where we measure the stiffening- by becoming increasingly aligned with each other during mesoderm folding:

We wanted to understand what makes the mesoderm transiently stiff during its folding & invagination. We did not see a correlation between actomyosin and the increase in stiffness. But surprise!: We depolymerised microtubules using Colcemid (right panel) and mesoderm cells remained in a softer state😱

On the contrary, the ectodermal cells located on the dorsal side of the embryo become softer! In sum, the embryo undergoes two diverging mechanical transitions: mesoderm: stiffening followed by softening; ectoderm: softening. See ⤵️ the softening of dorsal cells when they flatten and stretch:

Gastrulation starts when the mesoderm (bottom in ⬆️), folds and invaginates, and next, undergoes EMT. Here, we measured fast and biphasic mechanical dynamics: during mesoderm invagination, cells stiffen (Brillouin shift map -bottom panel-, transient yellow); during EMT, mesoderm cells soften.

Using Brillouin microscopy, we measured changes in stiffness within the whole-cell of an intact organism.
We focused on the most important event in our life: gastrulation, which in Drosophila is really fast (~30'):