The organization of cerebellar afferent projections to the paramedian lobule in neonatal cats.

This study sought to determine whether cerebellar afferent pathways, that are topographically organized in adult cats, are similarly ordered during the postnatal development and maturation of the cortex, or whether the projections are first distributed randomly in the cortex before becoming organized. Injections of wheat germ agglutinin-horseradish peroxidase were made into dorsal (dPML) or ventral (vPML) divisions of the paramedian lobule (PML) in neonatal (0- to 21-days-old) and adult cats and the ensuing distributions of retrogradely labeled neurons in the lateral reticular nuclei, the inferior olive and the pontine nuclei were compared. Magnocellular and parvicellular neurons in the dorsomedial and dorsolateral parts of the ipsilateral lateral reticular nuclei project respectively to dPML and vPML in all neonatal and adult cats. Olivocerebellar projections were entirely crossed, in most cases, with neurons projecting to the dPML more rostral and medial in the dorsal and medial accessory nuclei and in the principal olive than neurons which project to the vPML. A parasagittal zonal organization of olivocerebellar projections was present in newborn cats. Neurons were labeled in the ipsilateral inferior olive following dPML injections in 1- to 4-day-old kittens, but not in older kittens or in adult cats. Pontocerebellar projections were bilateral with a contralateral predominance. In adult and neonatal cats, labeled neurons were clustered together and formed rostral-caudal oriented columns dorsomedial and ventromedial to the pyramidal tract after injections in the contralateral dPML and vPML and bilaterally in the dorsolateral pons after dPML injections. These results show that lateral reticulo-, olivo- and pontocerebellar projections to the PML which are topographically organized in adult cats are organized similarly in newborn cats. Studies in prenatal cats are required in order to determine whether these cerebellar afferents are ever randomly distributed in the cerebellar anlage or whether these projections are ordered as they grow into the cerebellum.