The altered physiological responses of the retina in ASIC3 knockout mice led us to investigate the retinal structural integrity in these animals. Development of the retina appeared normal in ASIC3
−/− mice at 3 months compared with wild-type animals
(Figs. 5A 5B) , and no apparent change was observed in the thickness of the inner and outer retinal layers. Electron micrographs
(Figs. 5C 5D 5E 5F)showed no apparent morphologic changes of inner segments (IS;
Figs. 5C 5D ) or outer segments (OS;
Figs. 5E 5F ) of rod photoreceptors. However, the situation was dramatically changed at 12 months
(Figs. 6A 6B) . ASIC3
−/− retina displayed an apparent disorganization of the outer nuclear layer (ONL) and of the OS and IS. The thickness of rod OS was increased in ASIC3
−/− mice, and some vacuolated areas appeared in OS and IS as well as holes in the outer plexiform layer (OPL). The number of rows in the ONL was reduced to eight
(Fig. 6B) . At 16 months, the OS was more vacuolated and the IS exhibited many vacuolated areas, suggesting massive cell death
(Fig. 6C) . The ONL, inner nuclear layer (INL), and OPL were thinner. To determine the cause of retinal disorganization, we examined the ultrastructure of the retina in ASIC3
+/+ and ASIC3
−/− mice. No significant difference between 12-month-old and 3-month-old retinas was observed in wild-type animals because each OS was flat, well organized, and closely aligned with the other (compare
Figs. 6D and 5E ). At 12 months, the retinas of ASIC3
−/− mice showed gross disorganization of OS and IS
(Figs. 6E 6H)in rod photoreceptors, with many gap area degradation products around the OS and around the mitochondria in the IS. At 16 months, the OS and IS from ASIC3
−/− mice appeared consistently affected by an enhancement of the gap areas, suggesting the degeneration of IS and the concomitant degradation of OS
(Figs. 6F 6I) . The INL was also affected and showed some pyknotic nuclei with condensed chromatin
(Fig. 6C) . The ganglion cell layer exhibited some displaced amacrine cells, ganglion cells, or both, in a degenerating state (compared
Figs. 6K and 6Lwith 6J). These observations are in good agreement with the functional data showing altered retinal physiological responses in knockout mice at 12 and 16 months, and they demonstrate an important role for ASIC3 in the long-term structural integrity of the retina.