Evidence of intracerebellar collateralization of nucleocortical cell processes in a prosimian primate (Galago): a fluorescence retrograde study.

The distribution of single and/or double-labeled neurons in the cerebellar nuclei was investigated in a prosimian primate (Galago) by means of fast blue (FB) and nuclear yellow (NY) as retrograde tracers. Injections were made into spatially separate regions of cerebellar cortex on one side (zones C1-C3 in lobules IV and V and in the paramedian lobule) and into the same zones of lobules IV and V on both sides. Following unilateral injections single- and double-labeled somata were seen in the anterior (NIA) and posterior (NIP) interposed cerebellar nuclei on the same side. Single-labeled cells were, in general, more concentrated in NIA from the lobule IV-V injection and in NIP from the PML placement; double-labeled somata were about evenly distributed between NIA and NIP. Cell labeling was sparse in the contralateral NIP. Large and small somata were filled with FB, NY, or FB + NY ipsilateral to the injection sites while the majority of labeled neurons on the contralateral side had small oval- or fusiform-shaped somata. Subsequent to bilateral injections cells labeled with each tracer were concentrated in the NIA, moderate in number in the NIP, both on the ipsilateral side, and sparse in the contralateral NIP. The double labeling of cells in the present study indicates that these neurons project to spatially separated, yet possibly functionally related, areas of ipsilateral cortex. Since larger and small somata were double labeled it is possible that those cells with cerebellofugal processes to extracerebellar targets may simultaneously relay information to divergent cortical sites. In contrast, the few labeled cells seen in the contralateral NIP have small oval and fusiform somata. These neurons may be specifically involved in cerebelloolivary and olivocerebellar feedback loops.