The fruit fly wing epithelium transitions from disordered to crystalline cell packing during development. Cells rearrange from irregular polygonal shapes into a hexagonal lattice — the closest packing of approximately equal circles. Previous work attributed this ordering to tissue-level mechanical forces: shear flows push cells into alignment, like rolling cylinders between plates until they organize.
The paper identifies a different control variable: cell size polydispersity. Above a critical threshold of size variation, cells remain disordered regardless of mechanical input. Below it, crystalline packing emerges. The phase transition is controlled by statistics, not forces.
Tissue shear does contribute — but not as the ordering mechanism. Shear accelerates the transition by aligning already-ordered local domains into global crystal structure. It is a catalyst, not a cause. Without sufficient size uniformity, no amount of shear produces crystallization. With sufficient uniformity, crystallization occurs even with reduced shear, just more slowly.
The mechanism parallels colloidal crystallization exactly. A suspension of uniform colloidal particles crystallizes; a polydisperse suspension jams into a glass. The fly wing is a colloidal system where the colloids are living cells, and development reduces polydispersity by regulating cell growth until the system crosses the crystallization threshold.
The structural point: the transition from disorder to order is not driven by the organizing force (shear) but by the removal of the obstacle (size variation). The ordering was always energetically favorable — the cells just needed to become similar enough for it to happen. The force doesn't create the order; the uniformity permits it.