Neurogenetical segregation of the vestibulospinal neurons in the rat

The time of origin of the vestibulospinal projection neurons was determined by a double-labeling method using 5-bromodeoxyuridine (BrdU), the thymidine analogue, and Fluoro-Gold (FG), a retrograde fluorescent tracer. Rat fetuses were exposed to BrdU in utero to label the vestibular neurons on one of the embryonic (E) days between E12 and E15. Upon reaching adulthood, the rats were given unilateral injections of FG into the cervical cord to identify the spinal projection neurons. Brainstem sections were immunohistochemically processed for BrdU and then examined for neurons that were both BrdU-positive and FG-positive in the vestibular nuclei. In the lateral vestibular nucleus (LVe), most of the vestibulospinal neurons were generated on E12. In the inferior vestibular nucleus (IVe), the vestibulospinal neurons were produced almost equally on both E12 and E13. In the medial vestibular nucleus (MVe), the vestibulospinal neurons were generated consistently on days between E12 and E14 with a mild peak on E13. The present results thus demonstrate that genesis of the vestibulospinal neurons occurs sequentially in the following order: firstly in the LVe, secondly in the IVe, and finally in the MVe. The different sequential generation of vestibulospinal neurons among the LVe, MVe and IVe may reflect the fact that the vestibulospinal projections are differentially organized depending on the nature of each subnucleus.