Scanning electron microscopy analysis of the inner–outer segment interface in mouse and macaque photoreceptors. (A) Mouse retina (perfused animals). The connecting cilium (CC), which connects the inner segment (IS) and outer segment (OS) of photoreceptors, and the periciliary membrane complex (PMC) region are visible. (B–G) Macaque retina (perfused animals). Finger-like structures, the calyceal processes (CP), protrude from the apical region of the inner segment, and surround the base of the outer segment. The absence (A, in mouse) or presence (B, in macaque) of the calyceal processes is highlighted in the schematic diagrams illustrating the structural features of a mouse rod (A) and macaque cone (B) photoreceptors. (C and D) Macaque photoreceptors with outer segments broken at the base display numerous calyceal processes, crowning the apical region of the remaining inner segment. (E) Measurement of calyceal processes in rod and cone photoreceptors (values are presented as mean ± SEM). The cone cells have larger numbers of thicker and more regularly spaced calyceal processes than the rod cells. (F and G) Flat-mount (F) and longitudinal cross-section (G) views of phalloidin-stained macaque retinas (perfused animals) show the calyceal processes containing actin filament bundles, with their roots extending downward into the inner segment (arrowheads). (H) Longitudinal cross-section of a phalloidin-stained human retina, illustrating the presence of the F-actin–labeled calyceal processes and their roots (arrowheads) in the apical region of the inner segment of a cone photoreceptor cell. Bars: (A–D) 1 µm; (F–H) 2 µm.