Regulation of axon number in primate optic nerve by prenatal binocular competition.

We report here that in mature rhesus monkeys in which one eye was removed during the first half of gestation, the optic nerve of the remaining eye is larger and contains significantly more retinal axons than in age-matched control animals. Such supernumerary fibres in monocularly enucleated monkeys probably result from an arrest in the normal process of elimination of excess embryonic optic axons. Although the function of retained supernumerary optic axons is unknown, this finding demonstrates that (1) competition between the two eyes for synaptic territory occurs prenatally, before visual experience and (2) an early lesion in one brain area can adjust or enhance the size and perhaps the performance of other synaptically related structures.