TEMPORAL EXPRESSION OF ROD AND CONE OPSINS IN EMBRYONIC GOLDFISH RETINA PREDICTS THE SPATIAL-ORGANIZATION OF THE CONE MOSAIC

Purpose. Cone photoreceptors in teleost fish retina are organized into a precise, crystalline mosaic in which the four spectral subtypes hav e a consistent position relative to each other. The objective of the c urrent study was to describe the spatial and temporal progression of p hotoreceptor differentiation in the embryonic goldfish retina to under stand how the retinal cone mosaic might be produced. Methods. To ident ify developing photoreceptors when they first begin to express a speci fic opsin, the authors used in situ hybridization with cRNA probes gen erated from cDNA for rod opsin and red, green, blue, and ultraviolet c one opsins from goldfish (Carassius auratus). Results. In the retina, rod opsin was expressed first, and it was restricted to a small patch of regularly spaced, precocious rods located near the ventronasal edge of the retina, close to the choroid fissure. The patch enlarged by re cruitment of additional rods in a circular path, moving from ventral t o nasal to dorsal to temporal retina. Expression of cone opsins began approximately 10 hours after rod opsin was first expressed, and differ entiation of cone photoreceptors followed the spatial pattern laid dow n by the early rods. The temporal order of onset of cone opsin express ion was red, then green, then blue, then ultraviolet. When rod and red cone opsin probes were combined, the number of labeled cells was addi tive, suggesting that these two opsins are expressed in separate popul ations of photoreceptors. Conclusions. The onset of opsin expression i n goldfish retina follows a highly ordered spatiotemporal pattern. Ear ly differentiation and regular spacing of the precocious rods was unex pected and suggested that they may play a role in cone mosaic patterni ng. The order of subsequent cone opsin expression was related to the r elative positions of cone subtypes in the mosaic, suggesting the possi bility that inductive interactions among developing photoreceptors may be responsible for patterning the cone mosaic array.