Active cell divisions generate fourfold orientationally ordered phase in living tissue

Morphogenesis, the process through which genes generate form, establishes
tissue scale order as a template for constructing the complex shapes of
the body plan. The extensive growth required to build these ordered
substrates is fuelled by cell proliferation, which, naively, should
destroy order. Understanding how active morphogenetic mechanisms couple
cellular and mechanical processes to generate order — rather than
annihilate it — remains an outstanding question in animal development. We
show that cell divisions are the primary drivers of tissue flow leading to
a fourfold orientationally ordered phase. Waves of anisotropic cell
proliferation propagate across the embryo with precise patterning. Defects
introduced into the nascent lattice by cell divisions are moved out of the
tissue bulk towards the boundary by subsequent divisions. Specific cell
proliferation rates and orientations enable cell divisions to organize
rather than fluidize the tissue. We observe this using live imaging and
tissue cartography to analyse the dynamics of fourfold tissue ordering in
the trunk segmental ectoderm of the crustacean Parhyale hawaiensis
beginning 72 hours after egg laying. The result is a robust, active
mechanism for generating global orientational order in a non-equilibrium
system that sets the stage for the subsequent development of shape and
form.