A specific projection of retinal displaced ganglion cells to the nucleus of the basal optic root in the chicken.

The displaced ganglion cells of Dogiel are a class of retinal ganglion cells whose perikarya are located along the inner margin of the inner nuclear layer. Found in all vertebrate classes, they are particularly conspicuous in avians. Recently, Karten, Fite &; Brecha (1977) found that these cells in the pigeon gave rise to a seemingly exclusive projection to the contralateral nucleus of the basal optic root, the major component of the avian accessory optic system. In the present work, the projections of displaced ganglion cells were investigated in hatchling and adult chickens. The cells were found to project to the nucleus of the basal optic root but not to the tectum. Labeled displaced ganglion cells following injections of horseradish peroxidase into the nucleus of the basal optic root were 15 × 20μm in size in both hatchlings and adults. Labeled cells tended to have a higher concentration in the peripheral than in the central retina. Cells were widely but irregularly spaced, with adjacent cells seldom closer than 100 μm. Up to 7700 displaced ganglion cells were labeled in the adult chicken.These results, together with those of Kartenet al. (1977), suggest that in birds, displaced ganglion cells may constitute a unique class of retinal ganglion cells that project exclusively to the nucleus of the basal optic root. In light of the projections of the nucleus of the basal optic root to the oculomotor complex and vestibulocerebellum, the displaced ganglion cells may be an initial link in a visual pathway involved in the control of oculomotor reflexes.