Ipsilaterally located olivocerebellar projection neurons of the chick.

The use of horseradish peroxidase (HRP) and 1,1'-Dioctadecyl-3,3,3',3'- tetramethyl-indocarbocyanine perchlorate (DiI) as retrograde tracers, applied in vitro within the olivocerebellar tract of both embryos (9 to 21 days old) and postnatal (3-60 days old) chickens, has allowed the observation of a small population of neurons located ipsilaterally to the placement of the tracer. These neurons, whose morphology indicated that they belong to the inferior olive rather than to the reticular formation or the raphe nuclei, followed the same developmental steps as normally placed inferior olivary neurons. Furthermore, pedunculotomy experiments made on 3-day-old chickens demonstrated that ipsilateral neurons sent their axons through the cerebellar peduncle. In contrast to the completely crossed arrangement of the olivocerebellar projection, the present results show the existence, as in the rat, of a few ipsilateral inferior olivary neurons whose significance is unclear.