The nasotemporal division of retinal ganglion cells with crossed and uncrossed projections in the fetal rhesus monkey.

The development of the partial decussation pattern in the primate retina was studied in fetal rhesus monkeys of known gestational ages. Retinal ganglion cells with either crossed or uncrossed projections were identified by labeling with HRP following unilateral injections of this tracer into the optic tract. At all fetal ages, very few cells (less than 0.5% of the total ganglion cell population) were found to project to the inappropriate hemisphere. The nasotemporal overlap zone, defined as the retinal region along the vertical meridian containing cells with either crossed or uncrossed projections, also appeared equivalent to that described for the adult animal. A temporal offset in the decussation pattern of large ganglion cells, similar to that of the mature retina, could be recognized as early as 50 d before birth. These results indicate that an adultlike retinal decussation pattern is evident in the fetal primate at a stage when projections from the 2 eyes are completely intermingled within retinorecipient nuclei, and prior to the onset of retinal ganglion cell loss. Moreover, the primate visual system exhibits a degree of precision in the specification of the nasotemporal division unrivaled among the mammalian species studied to date. The developmental specificity evident in the decussation pattern of the fetal rhesus monkey appears to reflect the specialized organization of this primate's retina for binocular focal vision.