The Emergence of Y-junctions in the Zebrafish Cone Mosaic

Abstract

Cone photoreceptor cells in the retina of teleost fish are arranged in a remarkably ordered, crystalline pattern. Just like an atomic crystal, this “cellular crystal” exhibits topological defects; they are known as Y-junctions in the fish literature and correspond to edge dislocations in the terminology of materials science. Although the presence of Y-junctions in the cone mosaic has previously been noted, they have never been studied systematically. We have characterized Y-junctions within the zebrafish cone mosaic using two methods. The first is a flat mount retina prepared from transgenic zebrafish in which the UV-opsin promoter drives expression of GFP in UV cones. With image processing software, we characterized the distribution of Y-junctions across the retina. We found that Y-junctions coalesce into grain boundaries and are distributed throughout the retina. Using this information, we developed a second live-imaging method that allows us to study the dynamics of Y-junctions. We generated a transgenic construct that expresses a photoconvertible protein, mEOS, in the nucleus of UV cones. Multiphoton microscopy allows us to visualize Y-junctions in live juvenile fish and photoconvert the cones around an identified Y-junction. We are able to track the Y-junction defect in the cone mosaic pattern over time to determine whether the defect is immobile after it forms or whether the pattern is reorganized as the retina grows.