  During embryogenesis tissue layers continuously rearrange and fold into
specific shapes. Developmental biology identified patterns of gene expression
and cytoskeletal regulation underlying local tissue dynamics, but how actions
of multiple domains of distinct cell types coordinate to remodel tissues at the
organ scale remains unclear. We use in toto light-sheet microscopy, automated
image analysis, and physical modeling to quantitatively investigate the link
between kinetics of global tissue transformations and force generation patterns
during Drosophila gastrulation. We find embryo-scale shape changes are
represented by a temporal sequence of three simple flow field configurations.
Each phase is accompanied by a characteristic spatial myosin distribution,
quantified in terms of a coarse-grained 'myosin tensor' that captures both
concentration and anisotropy. Our model assumes tissue flow is driven by stress
proportional to the myosin tensor, and is effectively visco-elastic with two
parameters that control 'irrotational' and 'divergence-less' components of the
flow. With just three global parameters, this model achieves up to 90%
agreement between predicted and measured flow. The analysis uncovers importance
of a) spatial modulation of myosin distribution and b) long-range spreading of
its effect due to mechanical interaction of cells. In particular, we find
germband extension phase is associated with the onset of effective areal
incompressibility of the epithelium, which makes the relation of flow and
myosin forcing strongly non-local. Our analysis also revealed a new function
for basal myosin in generating a dorsally directed flow and, combined with
mutant analysis, identified an unconventional control mechanism through twist
dependent reduction of basal myosin levels on the ventral side. We conclude
that understanding morphogenetic flow requires a fundamentally global
perspective.

Tissues and Organs;Cell Behavior;Quantitative Methods