Migratory pathways and neuritic differentiation of inferior olivary neurons in the rat embryo. Axonal tracing study using the in vitro slab technique.

The use of the HRP retrograde tracing method, applied in vitro to embryonic (E15-E20) cerebellum-brainstem slabs, has allowed the identification of single young postmitotic olivary neurons. Labeled neurons move within two migratory streams: one superficial, under the pia (the marginal stream), and the other, of earlier onset, deeper in the medullary parenchyma (the submarginal stream). All neurons in the latter converge to the inferior olive ipsilaterally to their proliferation site; whereas, most neurons within the marginal stream cross the midline and bypass the olivary domain. Only a few HRP-labeled neurons leave the marginal stream towards the olivary territory, on their proliferation side. Hence, contrary to previous reports, the submarginal stream provides almost all the olivary neurons (95% at least), while the contribution of the marginal stream is very small (5% at the most). Axonogenesis is the earliest event in neuritic differentiation. By E15, 48 h after proliferation, the axons at the front of the migrating neurons have already crossed the interolivary commissure, and reached at least the site of HRP application, while the cell bodies have not yet penetrated their terminal domain. An ipsilateral component of this axonal tract was never detected. Hence, the olivocerebellar projection is formed very early, and is entirely crossed from its onset. Dendritogenesis was also analyzed during intra-uterine life; olivary neurons evolve from a fusiform shape (typical of migrating neurons) to a stellate form, with long and straight dendrites (once arrived at their ultimate location). Thus the acquisition of their mature spherical 'ball of wool' shape is a postnatal event, most probably concomitant with the major synaptogenetic phase.