Germ band retraction involves a dramatic rearrangement of the tissues on the surface of the Drosophila embryo. As germ band retraction commences, one tissue, the germ band, wraps around another, the amnioserosa. Through retraction the two tissues move cohesively as the highly elongated cells of the amnioserosa contract and the germ band moves so it is only on one side of the embryo. To understand the mechanical drivers of this process, we designed a series of laser ablations that suggest a mechanical role for the amnioserosa. First, we find that during mid retraction, segments in the curve of the germ band are under anisotropic tension. The largest tensions are in the direction in which the amnioserosa contracts. Second, ablating one lateral flank of the amnioserosa reduces the observed force anisotropy and leads to retraction failures. The other intact flank of amnioserosa is insufficient to drive retraction, but can support some germ band cell elongation and is thus not a full phenocopy of ush mutants. Another ablation-induced failure in retraction can phenocopy mys mutants, and does so by targeting amnioserosa cells in the same region where the mutant fails to adhere to the germ band. We conclude that the amnioserosa must play a key, but assistive, mechanical role that aids uncurling of the germ band.

Holographic laser microsurgery is used to isolate single amnioserosa cells in vivo during early dorsal closure. During this stage of Drosophila embryogenesis, amnioserosa cells undergo oscillations in apical surface area. The postisolation behavior of individual cells depends on their preisolation phase in these contraction/expansion cycles: cells that were contracting tend to collapse quickly after isolation; cells that were expanding do not immediately collapse, but instead pause or even continue to expand for ∼40 s. In either case, the postisolation apical collapse can be prevented by prior anesthetization of the embryos with CO2. These results suggest that although the amnioserosa is under tension, its cells are subjected to only small elastic strains. Furthermore, their postisolation apical collapse is not a passive elastic relaxation, and both the contraction and expansion phases of their oscillations are driven by intracellular forces. All of the above require significant changes to existing computational models.

Background Heat shocks applied at the onset of gastrulation in early Drosophila embryos frequently lead to phenocopies of U-shaped mutants – having characteristic failures in the late morphogenetic processes of germband retraction and dorsal closure. The pathway from non-specific heat stress to phenocopied abnormalities is unknown. Results Drosophila embryos subjected to 30-min, 38-°C heat shocks at gastrulation appear to recover and restart morphogenesis. Post-heat-shock development appears normal, albeit slower, until a large fraction of embryos develop amnioserosa holes (diameters > 100 μm). These holes are positively correlated with terminal U-shaped phenocopies. They initiate between amnioserosa cells and open over tens of minutes by evading normal wound healing responses. They are not caused by tissue-wide increases in mechanical stress or decreases in cell-cell adhesion, but instead appear to initiate from isolated apoptosis of amnioserosa cells. Conclusions The pathway from heat shock to U-shaped phenocopies involves the opening of one or more large holes in the amnioserosa that compromise its structural integrity and lead to failures in morphogenetic processes that rely on amnioserosa-generated tensile forces. The proposed mechanism by which heat shock leads to hole initiation and expansion is heterochonicity – i.e., disruption of morphogenetic coordination between embryonic and extra-embryonic cell types.