What is the unknown cellular and mechanical mechanism that enables the passive IKNM in the proliferative zone of the embryonic cerebral wall?

(A) Schematic representation of the embryonic mouse cerebral wall subjected to the present study. SEM picture of a cerebral wall isolated from an E13 mouse is combined with a cross-sectional scheme (right) depicting NPCs spanning apicobasally across the wall and neurons accumulating in the outer zone. While the outer territory composed of differentiated neurons is really stratified, the inner progenitor territory, called the ventricular zone, is “pseudostratified,” with each nucleus migrating in a cell cycle–dependent manner within an elongated progenitor cell. As exemplified in a clone sparsely labeled (green, plasma membrane; magenta, nucleus) in slice culture, nuclear/somal movement is apical during G2 phase (progenitor, until 160 min) and basal during G1 phase (daughter cells, until 440 min), whereas mitosis occurs at the apical surface (160 min) (see also S1 Movie). These movements are collectively referred to as interkinetic nuclear migration (IKNM). Scale, 20 μm. (B and C) Trajectories of nuclei/somata that migrated apically (B) or basally (C). Modified from Okamoto et al. [18], which is consistent with other previous observations [18–21, 23–26]. (D) Previous studies by Leung et al. [25] and Kosodo et al. [26] showed that pharmacological inhibition of the apical nucleokinesis during G2 resulted in retarded basal nucleokinesis during G1. (E) Current understanding that apicalward nuclear flows in the VZ may induce passive basalward nucleokinesis of G1-phase cells. By which cellular mechanisms this hypothetical passive nucleokinesis occurs is unknown. E, embryonic day; IKNM, interkinetic nuclear migration; NPC, neural progenitor cell; SEM, scanning electron microscopy; VZ, ventricular zone.