Figure 2. Light- and electron-microscopic retinal morphology in normal and canine cone-rod dystrophy 3 (crd3) affected dogs. In the retina of a 27-weeks-old non-affected dog (A), the outer nuclear layer (ONL) comprises approximately 10 rows of rod nuclei and a single distal row of cone nuclei. The inner and outer segments of the photoreceptors (IS, OS) are of consistent proportions, tightly aligned, and parallel, and the distal OS tips are in close proximity to the apical membrane of the retinal pigment epithelium (RPE). In retinas of 4.7- and 13.4-weeks-old crd3-affected dogs (B, C), rod and cone IS and OS lack the tightly packed highly parallel organization of a normal photoreceptor layer, and the distal OS tips appear to be more distant from the RPE apical membrane than in normal dogs. In the retina of an 18-weeks-old crd3-affected dog (D), IS and OS are disarrayed and disorganized, and a distinct gap is observed between the RPE and the OS (arrows). The retinas of 26-weeks- and 5 years-old crd3-affected dogs (E, F) exhibit continued photoreceptor degeneration as evidenced by loss of cone and rod IS, OS, and nuclei. Electron micrograph of the retina of a 27-weeks-old nonaffected dog (G) shows that the microvilli from the RPE apical membrane extend to invest the photoreceptor OS. Electron micrographs of the retina of a 13.4-weeks-old crd3-affected dog (H, I) show that the RPE apical microvilli form a tangled flattened mat that does not extend to invest the photoreceptor OS (arrows).