Development of a rod photoreceptor mosaic revealed in transgenic zebrafish

The number and distribution of neurons within the vertebrate retina are tightly regulated. This is particularly apparent in the highly ordered, crystalline-like arrangement of the cone photoreceptors in the teleost. In this report, using a transgenic line of zebrafish, a novel and developmentally regulated mosaic pattern of the rod photoreceptors is described. The spatial and temporal expression of EGFP, under the control of the Xenopus rhodopsin gene promoter, was nearly identical to the endogenous rhodopsin. EGFP was first detected in the ventral nasal retinal in an area of precocious neurogenesis referred to as the "ventral patch". Subsequent expression of EGFP was observed in isolated cells sporadically distributed across the dorsal and central retina. However, confocal microscopy and spatial analysis of larval eyes or retinal explants from adults revealed a precise arrangement to the rod photoreceptors. The rod terminals were arranged in regularly spaced rows with clearly identifiable telodendria linking neighboring cells. The rod inner segments projected through the cone mosaic in a predictable pattern. In the adult, the rod mosaic originated near the retinal margin where clusters of rods differentiated around the immature short single cone. In the embryo, the sporadic differentiation of the rods led to the gradual formation of the mosaic pattern. With the growing interest in neuronal stem cells, revisiting this model of neurogenesis provides an avenue to uncover mechanisms underlying the precise integration of new neuronal elements into a preexisting neural network.