Extensive recrossing of retinotectal axons after neonatal unilateral superior collicular lesions in hamster.

Anterograde transport methods were used to examine the ipsilateral retinocollicular projections of adult hamsters subjected to ablation of one superior colliculus (SC) on the day of birth. In many of these animals ipsilateral retino-SC axons were not limited to the medial part of the remaining tectum as previously reported but encompassed the entire rostrocaudal and mediolateral extents of this SC. A common pattern was the existence of a dense patch of label contiguous with the midline, a more lateral region of diffuse labeling, a second dense lateral patch and diffuse labeling which extended to the lateral boundary of the tectum. The dense patches of label were usually aligned into rostrocaudally oriented bands. Additional experiments in which one optic tract was sectioned a week prior to the eye injection showed that the reorganization was primarily due to axons which recrossed the midline at the level of the midbrain. However, the uncrossed retinotectal projection was also abnormally dense in the animals subjected to neonatal SC lesions. Dense patches of label in the caudal half of the tectum were only observed when recrossing fibers were labeled. In a final set of experiments, both the ipsilateral and contralateral projections were labeled with different tracers in individual hamsters. These experiments showed a clear tendency for fibers from the two eyes to segregate in the remaining SC. This segregation was, however, incomplete, even in regions where labeling from each eye was quite dense.