SPATIOTEMPORAL COORDINATION OF ROD AND CONE PHOTORECEPTOR DIFFERENTIATION IN GOLDFISH RETINA

In this study, we have compared spatial and temporal aspects of develo pment of new rods and cones in the adult goldfish by using a combinati on of bromodeoxyuridine immunocytochemistry and opsin in situ hybridiz ation to determine the intervals between terminal mitosis (cell ''birt h'') and expression of opsin mRNA for each photoreceptor cell type. Th e goldfish opsins include rod opsin and four different cone opsins: re d, green, blue, and ultraviolet. In a cohort of photoreceptors born at the same time, rods expressed opsin mRNA within 3 days of cell birth, while expression of cone opsin mRNA required at least 7 days. This te mporal discrepancy in differentiation, coupled with a discordance in t he site of cell genesis of rods and cones, allowed opsin expression to commence in both cell types in approximately the same retinal locatio n. Commitment to the generic cone phenotype occurred within approximat ely 6 days throughout the cone cohort, as indicated by expression of i nterphotoreceptor retinoid-binding protein (IRBP) mRNA, but expression of a specific spectral phenotype was delayed until rods differentiate d nearby. Onset of expression of cone opsin mRNA followed a phenotype- specific sequence: red, then green, then blue, and finally ultraviolet ; in situ hybridization with two opsin probes confirmed that individua l photoreceptors expressed only one type of opsin as they differentiat ed. This stepwise process of cone differentiation is consistent with t he hypothesis that cell-cell interactions among developing photorecept ors may coordinate selection of specific photoreceptor phenotypes. (C) 1997 Wiley-Liss, Inc.