Precise targeting of retinal projections is required for the normal development of topographic maps in the mammalian primary visual system. During development, retinal axons project to and occupy topographically appropriate positions in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC). Phr1 retinal mutant mice, which display mislocalization of the ipsilateral retinogeniculate projection independent of activity and ephrin-A signaling, were found to have a more global disruption of topographic specificity of retinofugal inputs. The retinocollicular projection lacks local refinement of terminal zones and multiple ectopic termination zones originate from the dorsal–nasal (DN) retinal quadrant. Similarly, in the dLGN, the inputs originating from the contralateral DN retina are poorly refined in the Phr1 mutant. These results show that Phr1 is an essential regulator of retinal ganglion cell projection during both dLGN and SC topographic map development.

The topographic order of the retinocollicular projection in the rat was examined from birth until maturity. Small, localized deposits of rhodamine-filled latex microspheres were placed into the superior colliculus at different locations. To minimize labeling fibers of passage deposit sites were typically, although not exclusively, placed into the caudal-lateral pole of the colliculus. Examination of the area and density of labeled cells in the retinae of these animals led to the following conclusions: (1) At each age examined, the location of the majority of labeled cells was observed to be in appropriate topographic register with the deposit site in the superior colliculus. (2) Confirming the work of previous investigators, errors in topographic projection were observed. These were present in both the contralateral and ipsilateral retinae and decreased with increasing postnatal age. The mature pattern was present by P10. (3) Quantitatively, the number of retinal ganglion cells terminating nontopographically within the colliculus constituted a relatively minor proportion of the total number of labeled cells in both retinae. It is concluded that the majority of the retinal ganglion cells make topographically appropriate terminations within the superior colliculus during development.