Development of the spinocerebellar projection in the prenatal mouse.

An abundance of information is available concerning the spinocerebellar projection in adult mammals. However, only a few studies have attempted a developmental analysis of this important projection system in early postnatal and/or prenatal animals. The present study provides an analysis of the development of the projection from the spinal cord to the cerebellum in fetal mice using anterograde tracing techniques in an in vitro preparation. After applications of biocytin to the caudal cervical spinal cord, anterogradely labelled fibers were present in the brainstem of embryonic day 12 (E12/13) mice, however, there was no indication of label in the cerebellum. At E13/14, labelled fibers were evident in the rostrolateral portions of the cerebellum/isthmus region. By E15/16, labelled spinocerebellar fibers had progressed farther into the cerebellum and were seen crossing the midline in a very superficial position. At older ages, the number of crossing fibers increased, and they became more ventrally positioned within the cerebellum. At E17/18 and E18/19, labelled spinocerebellar fibers were observed to branch and invade deeper portions of the cerebellum including the cerebellar nuclei. However, at E18/19, there was no indication of the parasagittal organization characteristic of this projection in the adult animal. The results of this study indicate that spinocerebellar fibers are present within the cerebellum significantly earlier than the development and differentiation of their primary targets, the granule cells. Furthermore, these data suggest that spinocerebellar fibers may form associations with cerebellar nuclear cells during fetal development.