Comparison of topographical patterns of ganglion and photoreceptor cell differentiation in the retina of the zebrafish, Danio rerio.

Earlier studies suggested retinal differentiation in the zebrafish commences ventrally rather than centrally as is the case in other vertebrates. Here we describe the topographical spread of cell differentiation for ganglion cells, double cones and rods in the zebrafish retina between 36 and 72 hours postfertilization (hpf), by using immunohistochemical markers in retinal wholemounts. Staining for all three cell types commenced within the ventral retina on the nasal side of the optic nerve and choroid fissure, at 38 hpf for ganglion cells and 50 hpf for double cones and rods. Within 3 to 4 hours, the staining of ganglion cells and double cones spread in a continuous wave-like fashion into the nasal region of the ventral retina. After this time, the staining patterns for ganglion cells and double cones progressed dorsally into the central and temporal retina. Finally, stained somata of ganglion cells were observed within the temporal-ventral region by approximately 48 hpf, more than 8 hours later than the first ganglion cells within the nasal retina. The topographical spread of double cone staining was slightly less orderly. After staining had extended into the nasal retina between 50 and 54 hpf, a small group of stained double cones often appeared at the temporal edge of the choroid fissure by 56 hpf, simultaneously with initial staining observed dorsal and temporal to the optic nerve. The topographical spread of rod staining in the ventral retina was more symmetrical. After rod staining appeared near the nasal edge of the choroid fissure at 50 hpf, rods accumulated within a localized patch nasal to the fissure. Approximately 5 hours after initial rod staining, scattered rod staining appeared on the temporal side of the choroid fissure (approximately 55-57 hpf). Rods increased rapidly within the ventral retina, and a dense symmetrical patch extended out from the choroid fissure into the nasal and temporal regions of the ventral retina by 70 hpf. A scattered pattern of rod staining also occurred within the dorsal retina at this time.