Grain Boundaries Form Dynamically in the Zebrafish Cone Mosaic*

In zebrafish retinae, cone photoreceptors self-organize by subtype into a crystalline lattice called the cone mosaic. This system is a striking example of ordered cell packing in a vertebrate epithelium. This lattice lies on the surface of the retinal hemisphere. The lattice grows in such a way that topological defects, called Y-junctions, must be inserted to maintain constant cell spacing. Y-junctions form grain boundaries, running in the polar direction. These grain boundaries are similar to those observed in physical crystals at a free energy minimum on a curved surface.
The existence of these grain boundaries raises the question of how they form. The defects could form randomly in the azimuthal direction, and then coalesce into grain boundaries. Alternatively, the defects could be created in a spatially correlated way such that defect motion is unnecessary. Tracking individual UV cones via a photoconvertible fluorescent reporter in transgenic zebrafish, we rule out grain boundary formation by defect motion. This means that while adding photoreceptors, zebrafish retinae dynamically cluster defects, leaving large domains with few defects. By phase-field crystal simulations, we demonstrate dynamic formation of grain boundaries in comparable geometries.