Secreted Crb2b-sfEX affects photoreceptor patterning and maintenance. (A) The expression patterns of Crb genes are highlighted with different colors superimposed on a transmission electron microscopic image of a tangential section through the photoreceptor layer. The Crb2b-sfEX transgene in pt108b is mostly expressed in blue cones; endogenous crb2a is expressed in all photoreceptors and Müller cells; endogenous crb2b is expressed selectively in RGB cones. (B) In flat-mount retinal preparations at 4 mpf, ZO-1 immunolabeled apical profiles of photoreceptors in pt108b retina form a mosaic pattern, but the regular organization is degraded by 18 mpf. (C, C') JB4-Feulgen staining of retinal cross sections from wild-type (WT) (C) and pt108 (C') illustrates severe reduction of RGB cones (arrows) in pt108b retina at 27 mpf, although rods and UV cone nuclei (arrowheads) are maintained. Dashed lines, OLM. (D, D') In WT retina (D) at 27 mpf Zpr1, immunolabeled green and red cones have elongated nuclei (arrows), but in pt108 (D'), the nuclei of the few remaining Zpr1-labeled cones are less elongated (arrow). UV opsin immunoreactivity (asterisks) verified the UV cone identity of nuclei (arrowheads) that are abnormally positioned apical to the OLM. DAPI (4′,6-diamidino-2-phenylindole), nuclear stain. Dashed lines, OLM. (E, E') ZO1 immunostaining (arrows) revealed the OLM, which is positioned apical to the UV cone nuclei in WT (E, arrowheads) but basal to UV cone nuclei in pt108b (E', arrowheads). UV cone outer segments were visualized by UV opsin immunostaining (asterisks).