MIGRATORY PATHWAYS AND SELECTIVE AGGREGATION OF THE LATERAL RETICULAR NEURONS IN THE RAT EMBRYO - A HORSERADISH-PEROXIDASE INVITRO STUDY, WITH SPECIAL REFERENCE TO MIGRATION PATTERNS OF THE PRECEREBELLAR NUCLEI

The migration and ultimate domain invasion of postmitotic lateral reticular nucleus (LRN) neurons were followed in embryonic day 15–20 (E15‐E20)rat embryos, by using a horseradish peroxidase (HRP) in vitro axonal tracing method. All of the LRN axons elongate and neuronal somata migrate via the subpial or marginal migratory stream (mms), circumnavigating the ventrolateral aspect of the medulla at the glial endfeet level. They reach the ventral midline at E16, bypass it, and begin to settle in their final territory at E17. At E18 the LRN anlage is fully formed, and at E19‐E20 its cells have finished their migration and are rapidly differentiating. Comparison of these sequential steps with their counterparts in the development of the inferior olive (ION) and external cuneatus (ECN) brings to light the essential role of the neuroepithelial cells of the interolivary commissure (the “floor plate”). This zone is likely to act as (1) a chemoattractant for the growth cones of the LRN, ION, and ECN, and (2) a decision‐making center, which instructs the somata of these neurons to cross the midline or not, ultimately governing the crossed or uncrossed pattern of their projection to their common target, the cerebellum. Finally, the ontogeny of the LRN and ECN provides a most surprising example, even unique in the central nervous system, of long‐distance, neurophilic migration that conveys neuronal cell bodies contralaterally to the side on which they proliferate. Copyright © 1990 Wiley‐Liss, Inc.