The hippocampus plays very important roles in higher brain functions. It has been said that abnormal formation of layers of the hippocampal CA1 region might be involved in epilepsy and schizophrenia. CA1 neurons have been known to be generated in the ventricular zone (VZ) and migrate to their final locations taking a longer time than that in the neocortex, even though the distance of migration of the CA1 neurons is shorter than that of the neocortical neurons. However, the precise mode of migration in the developing hippocampus is not yet completely investigated. In the neocortex, neurons have been described to migrate using several different migration modes "Multipolar migration", "Locomotion", "Somal translocation", and "Terminal translocation". To track the migrating cells in the CA1 region, we transfected a green fluorescent protein (GFP)-expression plasmid into the VZ cells of mouse hippocampus by using in utero electroporation. In the late stage of development, most of the postmitotic migrating cells assumed a multipolar shape and accumulated just above the VZ. When we performed a sequential in utero electroporation, the late-born neurons showed a multipolar shape underneath the axon bundles deriving from the early-born neurons that had already settled in the pyramidal layer. We speculate that the migrating CA1 neurons might need to take a long time to acquire some mechanisms that enable them to pass through the overlying fiber bundles.
The multipolar cells then transformed into well-branched bipolar cells to migrate through the pyramidal layer. They made contacts with many radial fibers at a time by their leading processes. In the "locomotion" mode in the neocortex, cells have an unbranched (or slightly branched) leading process to migrate along the radial fibers in a saltatory manner. In contrast, time-lapse imaging of the CA1 neurons revealed that they changed their scaffolds from the original radial fibers to other radial fibers. In addition, the migration speed of CA1 neurons were more slowly than that of neocortical neurons. Because they proceed in a zigzag manner that is distinct from the known modes of neocortical migration, we refer to this migration as the "climbing." mode.