Postnatal development of ipsilateral retino-geniculate projections in normal albino rats and the effects of removal of one eye at birth.

The postnatal development of ipsilateral retinofugal projections to the lateral geniculate body in normal albino rats, and in rats unilaterally enucleated at birth has been examined. At postnatal ages ranging from 1 day to 6 months, horseradish peroxidase was injected into one eye of normal rats and into the remaining eye of neonatally enucleated animals. After approximately 20 hours, the animals were perfused, the brains sectioned and reaction product visualised using tetramethylbenzidine. Ipsilateral retinal projections to the lateral geniculate body in normal animals were extensive on postnatal day 1 and became reduced over the next few days, the adult pattern being established between days 9 and 12. In the enucleated group, the terminal fields of the ipsilateral projections to the lateral geniculate body from the remaining eye remained larger and displayed a greater density of terminal labelling than in age-matched controls. In addition, the ipsilateral terminal field in the dorsal lateral geniculate nucleus occupied a more lateral position than in control animals. These findings support previous suggestions that the abnormally large ipsilateral retino-fugal projections observed in adult rats following removal of one eye, at or close to, birth, result from a failure of the ipsilateral projection to become restricted and that terminal or preterminal sprouting of retinal axons may also make a small contribution to the formation of the exuberant ipsilateral projection.