Asymmetric production of surface-dividing and non-surface-dividing cortical progenitor cells

Mature neocortical layers all derive from the cortical plate (CP), a transient zone in the dorsal telencephalon into which young neurons are continuously delivered. To understand cytogenetic and histogenetic events that trigger the emergence of the CP, we have used a slice culture technique. Most divisions at the ventricular surface generated paired cycling daughters (P/P divisions) and the majority of the P/P divisions were asymmetric in daughter cell behavior; they frequently sent one daughter cell to a non-surface (NS) position, the subventricular zone (SVZ), within a single cell-cycle length while keeping the other mitotic daughter for division at the surface. The NS-dividing cells were mostly Hu⁺ and their daughters were also Hu⁺, suggesting their commitment to the neuronal lineage and supply of early neurons at a position much closer to their destiny than from the ventricular surface. The release of a cycling daughter cell to SVZ was achieved by collapse of the ventricular process of the cell, followed by its NS division. Neurogenin2 (Ngn2) was immunohistochemically detected in a certain cycling population during G1 phase and was further restricted during G2-M phases to the SVZ-directed population. Its retroviral introduction converted surface divisions to NS divisions. The asymmetric P/P division may therefore contribute to efficient neuron/progenitor segregation required for CP initiation through cell cycle-dependent and lineage-restricted expression of Ngn2.