Experimentally induced enlargement of the uncrossed retinotectal pathway in rats.

The present experiments investigate the effects of neonatal lesions upon projection patterns of the uncrossed retinotectal pathway in albino rats. The results indicate that enlargement of the terminal field from one eye can be induced either by a contralateral optic tract lesion or by removal of the opposite eye at birth. The extent of the enlargement is more prominent in the latter case. If an optic tract lesion is accompanied by eye enucleation on the side ipsilateral to the tract lesion, the uncrossed retinotectal projection from the remaining eye will undergo further enlargement. However, optic fiber counts show that such an enlargement of the terminal field is not due to a significant increase in the number of uncrossed optic axons which contribute to the enlarged projection, but rather to an increased terminal arbor of individual axons (as shown by results from fiber counts). While a severe ganglion cell loss was observed in the retina contralateral to a tract lesion, a substantial population of cells persists in the ganglion cell layer and the number of cells appear much higher than the number of uncrossed optic axons arising from the same eye. The implications of these findings are discussed in relation to results reported in previous studies.