Early development of the rat precerebellar system: migratory routes, selective aggregation and neuritic differentiation of the inferior olive and lateral reticular nucleus neurons. An overview.

The migration, cytoarchitectonic segregation and neuritogenesis of the inferior olive (ION) and lateral reticular (LRN) neurons are described in the rat. Generated in the same primary precerebellar neuroepithelium, at embryonic days 12-13 (E12-E13) for the ION and E12-E14 for the LRN, the postmitotic cells take either the intraparenchymal (smms, for ION neurons) or the subpial migratory streams (mms, for LRN neurons and other populations, as those of the external cuneate nucleus, ECN). The ION neurons settle in their ultimate domain from E16 to E18, ipsilaterally to their proliferation side. The LRN (and ECN) neurons cross the midline at the "floor plate" (FP) level, and settle contralaterally to their birthplace between E17 and E19. In both cases, the acquisition of a mature dendritic tree is a late event when compared to the precocious axonogenesis. The FP structure may play a major role in i) attracting the axons of the precerebellar neurons, and ii) instructing these neurons whether to cross the midline or not. Thus, ultimately the FP may govern the pattern (crossed or uncrossed) of the projections of the ION and LRN to their common cerebellar target.