Bifurcating projections from the retinal ganglion cells to the primary visual targets (SC and LGN) in the cat.

Bifurcating projections of retinal ganglion cells to the primary visual targets were studied in the cat using a retrograde fluorescent double-labeling technique with Fluoro-Gold (FG) and Evans Blue (EB) as the tracers. Following injections of FG and EB into the left and right lateral geniculate nuclei (LGN), or into the left and right superior colliculi (SC), or LGN and SC on the same side, 4.56% of the single-labeled cells with one tracer were simultaneously labeled by the other tracer injected contralaterally in bilateral LGN-injection group; 12.18% of the single-labeled cells were further labeled by the other tracer injected contralaterally in bilateral SC injection-group; and 8.95% of the single-labeled cells in the ipsilateral retina, 10.94% of the single-labeled cells in the contralateral retina by the tracer injected into the LGN were labeled by the other tracer injected into the SC on the same side. All three retinal ganglion cell classes by size demonstrated double-labeled cell bodies. In the bilateral LGN- and bilateral SC-injection groups, the double-labeled cells were mainly of the small type (65% and 82.8%, respectively), while in the group of the injections into the LGN and SC on the same side, double-labeled cells were predominantly of the large type (55.45%). These results indicate that single ganglion cells of the retina send bifurcated projections to the bilateral LGN, bilateral SC, or unilateral LGN and SC, via axonal collaterals. These bifurcated axons can be regarded as an important way by which the information of the single ganglion cells can be conducted to the same primary visual targets on both sides, and two different optic centers on the same side of the brain.