Real time analysis of pontine neurons during initial stages of nucleogenesis

During development, neurons migrate from their origin to their final destinations where they form neuronal architectures such as layers and nuclei. While the mechanisms for the formation of laminated structures have been studied extensively, little is known about nucleogenesis. Previously, we analyzed nucleogenesis in neurons from four types of mossy-fiber projecting precerebellar nuclei neurons by gene transfer, and obtained evidence suggesting that the change from tangential to radial migration occurs at the region that will develop into the nucleus (Kawauchi, D., Taniguchi, H., Watanabe, H., Saito, T., Murakami, F., 2006. Direct visualization of nucleogenesis by precerebellar neurons: involvement of ventricle-directed, radial fibre-associated migration. Development 133, 1113-1123). Here we analyzed the dynamics of these neurons using mice embryos. We electroporated genes of fluorescent proteins to the lower rhombic lip at embryonic day (E) 12.5 and carried out time-lapse analyses at E14.5, when pontine nuclei begin to be formed. We found that many labeled neurons showed transition from tangential to radial migration in the region that will develop into the nucleus. This transition occurred in two ways. One was initiated by a leading process extending radially while the second was caused by a newly developed radial process from the cell soma. Curiously, we observed that many neurons stopped tangentially migration, paused, and then began radial migration. These findings indicate that a signal to stop and cause the change in tangential to radial migration is critical for nucleogenesis by pontine neurons.