| Abstract: | In most mammals with frontalized eyes, retinal ganglion cells in the nasal or temporal retina send their axons to the contralateral or ipsilateral half, respectively, of the brain. Previous studies in the cat, however, have indicated a retinal region of “nasotemporal overlap” from which arise the retinal projections to both the contralateral and ipsilateral halves of the brain. The present study thus examined in the cat whether any retinal ganglion cells give rise to bifurcating axons that innervate both halves of the brain. By employing fluorescent retrograde double labeling, we investigated whether or not single retinal ganglion cells project bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. After Fast Blue was injected into the lateral geniculate nucleus on one side and Diamidino Yellow was injected contralaterally into the lateral geniculate nucleus, 100–200 ganglion cells in each retina were double labeled with both tracers. These double-labeled cells were distributed primarily in the temporal retina, including the region around the vertical meridian and, additionally, in the nasal retina. About 60–80% of the double-labeled cells had large cell bodies (more than 25 μm in diameter), and the others had medium-sized ones (15–25 μm in diameter). The pattern of distribution of double-labeled cells, which was observed after the combined injection into both superior colliculi, was similar to that seen after the combined injection into both lateral geniculate nuclei; more than 9% of double-labeled cells, however, were large. The results indicate that a certain population of ganglion cells in the cat retina send their axons bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. The bilaterally projecting ganglion cells are mostly large, corresponding probably to α cells (the morphological counterparts of Y cells). In comparison with the patterns of bilateral projections of single retinal ganglion cells in the rat and monkey, the pattern of the bilateral retinofugal projections in the cat could represent an intermediate between those in the rat and monkey. © 1994 Wiley-Liss, Inc. |
