The displaced ganglion cell in the avian retina: developmental and comparative considerations.

Abstract

Quantitative and qualitative aspects of the retinal displaced ganglion cells (DGC's) were studied in the chick embryo, hatchling and adult, using retrograde labeling techniques. Retrograde axonal transport following discrete injections of horseradish peroxidase into the dorsolateral optic tectum was demonstrated from day 10.5 of incubation. The labeled retinae revealed numerous DGC's at all stages studied, with an average of 1183 cells labeled in 10.5 day embryos and as many as 9500 labeled in hatchlings. These population estimates imply that at least this many DGC's were present at these ages (many cells could have gone unlabeled by our procedures). More localized injections in adults, revealed an average of 2238 cells. In every instance in which orthotopic ganglion cells were labeled, numerous DGC's were similarly labeled. The DGC's were distributed throughout the retinae in embryos and hatchlings, but often tended to be most numerous in the peripapillary regions. In adults, this tendency was even more marked. These results demonstrate that the DGC, has a direct tectal projection in the chicken, in contrast to the almost exclusive, ectomamillary (EM) projection which has been demonstrated in the pigeon. It is speculated that this disparity may be due to species differences, which could be manifested by a dual tectal-EM projection in the chicken. The differential distribution of the DGC in these species (peripheral in the pigeon, peripapillary or panretinal in the chicken) supports this speculation.