Birth-date-dependent segregation of the mouse cerebral cortical neurons in reaggregation cultures

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Abstract
Cerebral cortical neurons form a six-layered structure in which their position depends on their birth date. This developmental process requires the presence of Reelin, which is secreted by Cajal-Retzius cells in the cortical marginal zone (MZ). However, it is still unclear whether the migration from the ventricular zone (VZ) to beneath the MZ is essential for the neurons to segregate into layers. Previous transplantation studies of ferret cerebral cortical neurons suggested that their ultimate laminar fate is, at least to some extent, determined in the VZ but it is unknown how 'laminar fate' eventually positions cells in a specific layer. To explore the segregation properties of mouse cortical cells that have not yet arrived beneath the MZ, embryonic day (E)16 VZ and intermediate zone (IMZ) cells were dissociated and allowed to reaggregate for 1-4 days in vitro. The results suggested that the migrating neurons in the IMZ at E16 preferentially located near the centre of the aggregates, more than did the proliferative cells from the VZ. The birth-date labelling followed by the dissociation-reaggregation culture suggested that the segregation properties of the E16 IMZ was characteristic of the E14-born cells, which were migrating in the IMZ at E16, but they were not general properties of migrating IMZ cells. This birth-date-dependent segregation mechanism was also observed in the Reelin signalling-deficient yotari cells. These findings suggest that cortical neurons acquire a birth-date-dependent segregation mechanism before their somas reach the MZ.