Retinal projections in adult and newborn grey squirrels

Retinal projections in newborn squirrels were compared to those in adults by using horseradish peroxidase (HRP) as a highly sensitive anterograde tracer. In both newborn and adult squirrels, the HRP reaction product was found in the dorsal lateral geniculate nucleus, the superior colliculus, the pretectal nuclei, and the nuclei of the accessory optic tract. Thus, newborn squirrels have retinal input to most or all structures normally innervated in the adult. However, the pattern of terminations differed in the newborn from that in the adult, and this was especially apparent in the dorsal lateral geniculate nucleus and the superior colliculus. In the dorsal lateral geniculate nucleus, the regions of ipsilateral and contralateral retinal inputs were clearly less segregated than in adults, although the adult laminar pattern of retinal terminations was partially apparent, even though there was yet no cytoarchitectural evidence of the adult lamination pattern. In the superior colliculus, a marked difference was seen in the pattern of ipsilateral retinal terminations. In the adult, ipsilateral retinotectal input was restricted to a narrow, dense, patchy, mediolateral band in stratum opticum in the rostral colliculus. In the newborn, the ipsilateral retinotectal input was less dense, free of patches, spread in thickness to include much of the stratum opticum and the superficial grey, and spread in extent to include all but the caudal pole of the colliculus. These observations are consistent with the prevailing view that visual connections are initially widespread and become restricted during the course of development.