Retinoid X receptor ? gene transcripts are expressed by a subset of early generated retinal cells and eventually restricted to photoreceptors

We have examined the distribution of the retinoid X receptor gamma (RXRgamma) in the developing chicken retina by using in situ hybridization and RNase protection assays. We detected RXRgamma transcripts as early as 4 days of embryonic development (d4) in central regions of the retina, spreading to more peripheral regions by d8. The first few RXRgamma-positive cells were scattered within the depth of the retinal neuroepithelium, but as they increased in number they became localized predominantly to the apical (outer, ventricular) layer. The identity of the RXRgamma-positive cells at these stages is unknown, due to the lack of cell type-specific markers. By d10, when photoreceptors and ganglion cells have been generated and begun to establish their definitive layers, RXRgamma-positive cells were virtually restricted to the photoreceptor layer, and maintained this distribution to posthatch stages. RNase protection assays were performed on whole retinae to verify the temporal pattern of in situ hybridization results and showed that between d5 and d16 there was a significant increase in the mRNA levels of the RXRgamma2 isoform. Between d16 and early posthatch stages the level of RXRgamma2 mRNA did not change significantly. Consistent with previous studies, mRNA levels of the RXRgamma1 isoform were substantially lower than mRNA levels of the RXRgamma2 isoform at all time points examined. These results demonstrate that RXRgamma mRNA is expressed in photoreceptors in the developing chicken retina and implicate RXRgamma as the earliest marker of photoreceptor differentiation documented to date.