Modification of neuron numbers in the visual system of the rat.

After unilateral lesions of the posterior cerebral cortex at birth, the ipsilateral dorsal lateral geniculate nucleus disappears within 96 hours. Several of the remaining visual centers may be distorted, atrophic or hypertrophic. Examples of the last are the ipsilateral nucleus of the optic tract (NOT) and superficial layers of the superior colliculus (SC), which usually show a volume increase. In order to determine changes in neuron numbers in these two regions, we exposed rats to 3H-thymidine on specific embryonic days. Some of these rats received poterior cortical lesions on the day of birth and others served as controls. Counts from adult rats exposed to 3H-thymidine on embryonic days 13 and 14 reveal a significant increase in the number of labeled neurons in both the NOT and SC on the side of the lesion. The retinal projections in rats with posterior lesions at birth were determined by the application of silver degeneration methods after eye removal. Atypical optic projections are found to the lateral posterior nucleus and the NOT on the side of the lesion. We conclude that the redistribution of optic collaterals after early destruction of the dorsal lateral geniculate nucleus serves to increase neuron numbers in the NOT and SC. Therefore, the present findings are comparable to the results in nonmammals after transplantation of additional peripheral organs.