Cellular migration patterns in the developing mouse cerebral cortex.

The migration patterns of embryonic mouse cortical cells were investigated using a replication-incompetent retrovirus vector (BAG). The lateral ventricles of embryonic day 12 mouse embryos were infected with BAG and brains were harvested 2, 3, 4 and 6 days after infection. The location and morphology of all infected cortical cells were recorded from serial sections of entire brains, which were then reconstructed in three dimensions. Examination of the distribution of labelled cells revealed that there were migration patterns characteristic of each medial-lateral domain of the cortex. In the medial and dorsal areas, migration was often radial, although tangential spread increased with survival time, in large part due to ramification of cells in the intermediate zone. In the dorsolateral and lateral areas of the cortex, radial migration was generally not observed. Rather, variable extents of tangential migration occurred, and often resulted in wide separation of cells in the cortical plate. Almost all of the cellular dispersion occurred in the intermediate zone, although a modest degree of dispersion also occurred within the cortical plate itself. Most dispersion occurred in the mediolateral plane, with relatively little dispersion along the anteroposterior axis. Though characteristic migration patterns could be defined, wide variability in the extents of radial migration and tangential separation of cells was seen. The patterns of migration paralleled the distribution of radial glial fibers in all areas, and are most likely a reflection of the role of this network in supporting the migration of cortical neurons. The extent and variability of cellular dispersion supports a lineage-independent mechanism of cortical column ontogenesis.