USH1 and USH2 proteins in developing photoreceptors in Xenopus laevis. (A–I) Scanning electron microscopy (left panels) and confocal microscopy analysis of USH protein immunolabeling at various stages of Xenopus development. (A–C) At stage 37–38, a few photoreceptors display developing calyceal processes (CP) that protrude from the distal region of the inner segment (IS) (A). Occasionally, a more developed outer segment with a basket of calyceal processes is observed (A, asterisk). Immunostaining for protocadherin-15 (B) and myosin VIIa (C) is detected in the distal region of the inner segment, at about the level at which the connecting cilium (CC) develops. (D–F) At stage 39–40, well-delimited calyceal processes surround the growing outer segments (D). At this stage, multiple myosin VIIa–immunoreactive spots are observed above the connecting cilium and at the tip of the inner segment, which is visualized by F-actin labeling (E). The USH2 protein Vlgr1 is detected in the periciliary ridge complex (PRC) region (F). (G and H) At stage 41–42, well developed calyceal processes are observed in almost all the photoreceptors (G). The protocadherin-15 staining coincides with the presence of the calyceal processes, which are visualized by F-actin immunostaining (red), at both stage 40–41 (H) and stage 45 (I). Bars, 2 µm.