Transient retinofugal pathways in the developing chick.

The central retinal projections have been examined in normal chick embryos early in development to determine if a transient ipsilateral projection is present. Anterograde transport of HRP identified using tetramethyl benzidine as a chromogen and degeneration techniques were used on embryos of successive ages between 6 and 16 days of incubation to show the central distribution of the retinal axons from one eye. Besides the anticipated contralateral projection, a small projection was identified to primary visual nuclei on the side of the brain ipsilateral to the injected or lesioned eye in embryos between days 6 and 12 of incubation. A projection from the injected eye into the contralateral optic nerve was also identified in a number of embryos. By embryonic day 15 the retinal projection to the ipsilateral side of the brain and into the contralateral optic nerve had disappeared. This loss of the anomalous projections coincides with a period of substantial cell death in the retinal ganglion cell layer. It appears, therefore, that in the chick, like the rat, the ipsilateral retinofugal projection resulting from an embryonic enucleation may in part be due to retention of a normal ipsilateral projection.