The Composition of the Inner Nuclear Layer of the Cat Retina

The cellular composition of the inner nuclear layer (INL) is largely conserved among mammals. Studies of rabbit, monkey, and mouse retinas have shown that bipolar, amacrine, Müller, and horizontal cells make up constant fractions of the INL (42, 35, 20, and 3%, respectively); these proportions remain relatively constant at all retinal eccentricities. The purpose of our study was to test whether the organization of cat retina is similar to that of other mammalian retinas. Fixed retinas were embedded in plastic, serially sectioned at a thickness of 1 microm, stained, and imaged at high power in the light microscope. Bipolar, amacrine, Müller, and horizontal cells were classified and counted according to established morphological criteria. Additional sets of sections were processed for protein kinase C and calretinin immunoreactivity to determine the relative fraction of rod bipolar and AII amacrine cells. Our results show that the organization of INL in the cat retina contains species-specific alterations in the composition of the INL tied to the large fraction of rod photoreceptors. Compared with other mammalian retinas, cat retinas show an expansion of the rod pathway with rod bipolar cells accounting for about 70% of all bipolar cells and AII cells accounting for nearly a quarter of all amacrine cells. Our results suggest that evolutionary pressures in cats over time have refined their retinal organization to suit its ecological niche.