Morphological and mechanical examinations suggest a Windkessel-like elastic property of the subapical space and raise a possibility of elasticity-based nucleokinesis via a mother-to-daughter energy transfer.

(A) A model of elasticity-based passive movement of daughter cells’ nuclei/somata (see also S1B and S1C Fig, S4 Movie). (B) En face observation of centrifugal recoiling of laser-ablated apical surface (ablation occurred at the vertex indicated by a yellow circle) EGFP-ZO1–labeled, see also S5 Movie), showing the tangential pretension/contractility of the surface. (C) Time-lapse imaging showing that the apical surface (ZO1-EGFP, white) was stable even at sites of cell division (a clone sporadically labeled with H2B-RFP, magenta) (upper panels, en face projected view; lower panels, orthogonally projected view; see also S6 Movie). (D) Graph showing that “tilting” of apices was minimal (the inner products for the normal vectors to the apices, x axis, was close to 1.0) (see also S2A Fig and S7 Movie). (E) Concave-to-convex conversion of apical surface geometry by myosin II inhibition. (F) Centripetal recoiling of the subapical space immediately after laser ablation of an M-phase cell (cyan, the center of the soma was targeted) in a cerebral wall stained with FM4-64 (displacements of the surrounding apical processes [yellow] are summarized in the right panel; see also S8 Movie). (G, H) Quantitative assessment of the change in horizontal sectional area of laser-ablated somata. M-phase cells in cerebral walls sporadically LynN-mCherry–labeled (via in utero electroporation) were ablated at the center of their somata. Graph (H) shows that shrinkage of M-phase somata, as exemplified in F, were consistently reproduced (see also S9 Movie). (I) Three-dimensionally reconstructed live image of an apical process of a slice-cultured VZ cell that dynamically extended lamellipodia-like protrusions only in the subapical space (see also S10 Movie). (J) Serial block-face SEM analysis of the in vivo subapical surface, showing three apical processes (highlighted in a section [left], 3D reconstructed [right]) with lamellipodia-like structures filling the apicalmost (about 5 μm) zone (see also S2J Fig and S12 Movie). Scale, 2 μm in B and J; 5 μm in I; 10 μm in C, F, and G; and 100 μm in E. Underlying data can be found in S1 Data. SEM, scanning electron microscopy; VZ, ventricular zone.