Formation of large brains such as the mammalian cerebral cortex depends on continuous and efficient cell production by neural progenitor cells ( NPCs ) 1 , 2 , 3 ., NPCs are elongated , spanning a developing brain wall ( up to 300 μm thick in mice ) , and divide at the apical surface 4 , 5 , 6 , 7 , 8 ( Fig 1A ) ., They individually move the nuclei and somata in a cell cycle–dependent manner ( apical during G2 and basal during G1 ) , and these nuclear/somal movements are called interkinetic nuclear migration ( IKNM ) 9 , 10 , 11 , 12 ( Fig 1A–1C , S1 Movie ) ., Because IKNMs of different NPCs are not synchronized , IKNM-undergoing regions ( 100 μm thick , called the neuroepithelium NE or ventricular zone VZ ) display nuclei/somata at diverse apicobasal positions , a histological appearance referred to as pseudostratification ( Fig 1A ) ., The degree of neuroepithelial pseudostratification ( i . e . , the thickness of NE/VZ ) increases from mice to monkeys 13 to humans 14 , 15 ., Extensive pseudostratification is thought to contribute to increasing the frequency of cell divisions per unit apical surface 16 ., However , the accompanying dense countercurrents ( i . e . , apical and basal IKNMs ) run the risk of “traffic jams” 17 ., For example , experimentally induced failures in basal nucleokinesis lead to near-apical nuclear/somal overcrowding and abnormal brain histogenesis 18 ., Despite recent advances in understanding intracellular molecular mechanisms that move the nucleus apically or basally in each NPC 19 , 20 , 21 , 22 , 23 , how heterogeneous IKNMs are safely and efficiently organized ( or horizontally bundled ) into an ordered VZ remains unknown ., In the embryonic cerebral cortex , cell production ( i . e . , M-phase NPCs’ divisions ) accompanied by massive flows of the “material” ( somata of G2-phase NPCs ) and the “product” ( those of G1-phase NPCs ) occurs most extensively and continuously among a wide variety of developing tissues ., The high pseudostratification of the cerebral cortical VZs has led to it becoming a model system to study “production logistics” in organogenesis ., To understand a system-level device to make the intra-VZ nuclear/somal traffic as efficient and energy saving as possible , we sought to determine how active and passive movements are coordinated ( or collaborate with each other ) ., Previous quantitative imaging studies revealed that apical nucleokinesis during G2 phase ( Fig 1B ) was faster than basal nucleokinesis during G1 phase ( Fig 1C ) 18–21 , 23–26 and judged to be more directional , with a superlinear mean-squared displacement ( MSD ) pattern , than more fluctuating basal nucleokinesis showing a linear MSD pattern 18 , 21 , 25 , 27 ) ., When apical nucleokinesis was inhibited through blocking NPCs’ entrance into G2 phase , basal nucleokinesis was retarded ( Fig 1D ) 25 , 26 ., These results therefore suggested that apicalward nuclear flows in the VZ may induce passive basalward nucleokinesis of G1-phase cells ( Fig 1E ) ., However , the cell–cell interactions that would mechanically contribute to such a hypothetical passive mechanism remained unknown ., Especially , how G2-phase cells’ nuclear/somal movement towards/to the apical surface can induce or lead to such hypothetical passive basal nucleokinesis of early G1-phase cells has not been studied ., We here show that mechanical energy provided by an apically dividing ( M-phase ) NPC is transiently stored in the surrounding subapical space ( within 10 μm from the surface ) and elastically returned to its daughter cells , promoting passive basalward nucleokinesis ., The elasticity in the subapical space is established by physiological cell crowding , in which neighboring M-phase and non–M-phase cells collaborate mechanically , thereby enabling energy-efficient IKNMs and benefiting the systematic “production logistics” in the VZ ., We aimed at elucidating where and how the hypothetical passive nuclear/somal movements are induced in the mouse neocortical VZ ( Fig 1E ) ., One possibility is that nuclei/somata of early G1-phase cells ( i . e . , newly generated daughter cells ) in the subapical space ( within 10 μm from the apical surface ) of the VZ are propelled through direct soma–soma collision or pushing by G2- or M-phase cells ( Fig 2A ) ., To evaluate this “direct collision” model that we hypothesized in the subapical space , we first examined the density of somata by visualizing the plasma membranes of all cells ( Fig 2B ) ., We found that the subapical space had no extracellular gaps and that apical processes ( which appeared as fine meshes in horizontal sections ) were more abundant ( about 60% ) than somata ( about 40% ) , whereas the process/soma ratio decreased in deeper ( more basal ) VZ regions ., Time-lapse observations in horizontal sections of the subapical space ( 5 μm from the apical surface ) ( Fig 2C and 2D , S2 and S3 Movies ) revealed that direct contacts between somata of newly generated daughter cells and those of neighboring G2/M-phase cells , including slight soma–soma contacts , were observed in only 45% of all divisions ( n = 80 ) ., Importantly , the time that elapsed from the generation of daughter cells to their nuclear departure from the subapical space was similar between cases in which the nuclei/somata of daughter cells had direct contacts with G2- or M-phase somata ( Fig 2C ) and those without such soma–soma contacts ( i . e . , cases in which the nuclei/somata of daughter cells were neighbored by apical processes , Fig 2D ) ( Fig 2E ) ., These results do not support the direct soma–soma collision model in the subapical space ., Therefore , we sought to explore an alternative possibility ., Even without direct soma–soma collisions , daughter cells’ nuclei/somata could be propelled passively if the surrounding subapical space works like a pressurized air chamber , in which G2-phase cells’ somal influxes and M-phase cells’ voluminous divisions act as an indirect contributor to the displacement of daughter cells’ nuclei/somata ( Fig 3A ) ., By analogy , the aorta acts like a compressed air chamber ( “Windkessel” ) to elastically recoil and forward blood ejected by the heart 28 ( S1A Fig ) ., If the subapical space were similarly elastic , as predicted by recent simulation studies 18 , 29 , mechanical force provided centrifugally by a given mother M-phase cell while expanding to round up would result in storage of elastic energy in the surrounding process-rich subapical space ., The stored elastic energy could then be released and returned centripetally to the nuclei/somata of daughter cells generated by that mother cell , thereby externally inducing the daughter cells’ passive nuclear/somal displacement in the basal direction ( Fig 3A , S1B and S1C Fig , S4 Movie ) , just as the aorta forwards blood in a Windkessel-like manner ., Laser ablation on the apical surface resulted in centrifugal recoiling of the surface from the ablated vertex , indicating that it was tangentially contractile ( under tension ) ( Fig 3B and S5 Movie ) ., Quantitative monitoring further revealed that the apical meshwork was quite stable even at sites of cell division , with no local tilting or floppiness ( Fig 3C and 3D , S2A Fig , S6 and S7 Movies ) ., Inhibition of actomyosin to diminish the contractility of the apical surface changed the original apically concave morphology of cerebral walls to an apically convex shape ( Fig 3E ) ., This pharmacologically induced recoiling response ( i . e . , the expansion of the subapical space relative to its original volume ) suggests that the subapical space may have originally been laterally compressed , as previously suggested in the chicken midbrain tube 30 ., To further test the possibility of lateral compression in the subapical space , we performed a novel laser ablation test targeting the center of the soma ( Fig 3F ) ., We reasoned that if a cell’s soma under centripetal compression by the surrounding space is ablated to reduce its original physical stability , it would lead to shrinkage of the ablated soma and centripetal displacements of elements in the surrounding space ., Subapical laser ablation of M-phase cells’ somata in normal/untreated ( apically concave ) cerebral walls resulted in a quick centripetal shift of the surrounding processes ( Fig 3F and 3G , S8 and S9 Movies ) ., This soma-directed laser ablation quantitatively showed that the subapical space was under compression and ready to recoil ( Fig 3G and 3H ) ., While fiber-like apical processes are flexible and can be bent 18 , they are individually under tension along the apicobasal axis 31 ., We observed that apical processes contain microtubules bundles , which provide the structural basis for flexural rigidity 32–35 , most densely in the region near the apical surface ( S2E Fig ) , which was also F-actin rich ( S2F–S2H Fig ) ., Horizontal packing of such rubber string–like fibers by the narrowing of the apical surface seemed to contribute to subapical elasticity ., Furthermore , imaging in slice culture revealed a novel subapical-specific ( within 5 μm from the surface ) cellular structure; lamellipodia-like protrusions were dynamically extended laterally into the surrounding space from almost all apical processes of non–M-phase VZ cells ( Fig 3I , S2I Fig and S10 Movie ) ., They were inhibited by blebbistatin ( S2J Fig and S11 Movie ) ., The in vivo existence of this novel structure was confirmed by serial block-face scanning electron microscopy ( SBF-SEM ) ( Fig 3J and S12 Movie ) ., The close spatial juxtaposition of this volume-increasing microstructure with the narrowing/contractile apical surface may efficiently increase local subapical elasticity ., Together , these results suggest that the subapical space , where basal IKNM ( i . e . , newly generated daughter cells’ nucleokinesis ) begins , is indeed elastic ., To evaluate our new model , in which elastic energy stored in local subapical tissue is released to generate a force that lifts up daughter cells’ nuclei/somata , we performed both in silico and slice culture analyses , focusing on the origin and redistribution of mechanical forces during the transition from M phase to early G1 phase ., In our mechanical simulation , ( 1 ) the elastic subapical space was depicted in a simplified manner as two springlike strings fixed at the apical end , and ( 2 ) the time-dependent position changes of a single soma ( as of G2 , M , and early G1 phases ) were animated ( Fig 4A and 4B , S3A and S3B Fig , S13 Movie ) ., We first applied a constant apicalward force to the center of nucleus/soma in G2 phase and further , until the end of M phase ., Reaction forces from the elastic strings generated vertical ( basal ) components , i . e . , a lifting force ., Thus , the strings successfully mediated a basal bouncing-like movement of the circle ( daughter cell’s nucleus/soma ) away from the apical surface through sequential transfers of mechanical energy from each laterally expanding M-phase progenitor cell ( providing a pushing force ) to ( 1 ) the subapical space ( storing elastic energy ) and ( 2 ) back to the nuclei/somata of that progenitor cell’s daughters ., In silico basalward bouncing of the daughter cell’s soma/nucleus occurred following a timely apicalward force from the M-phase mother cell’s round soma , as if it were pushed by a finger ( S1B and S1C Fig , S4 Movie ) , at its basal pole ., Because previous studies have suggested that actomyosin localized basal to the nucleus/soma of G2/M-phase cells may contribute to apical nucleokinesis and rounding up 21 , 36–38 , we speculated that actomyosin provides the cell-biological basis for such transient apicalward pushing ( until the end of M phase ) ., To explore this possibility , we live monitored actin dynamics ., Interphase cells exhibited F-actin enrichment at the apical junction ., During the G2-to-M transition , F-actin accumulated near the basal pole of the soma ( Fig 4C and 4D , S14 and S15 Movies ) , followed immediately by thinning of the basal process and subsequently by rounding up of the soma ., F-actin accumulation at the basal side continued during rounding up of the soma and the growth of cleavage furrow in a basal-to-apical direction and then became undetectable after completion of cytokinesis ., Immunohistochemistry revealed that the basal F-actin–rich region accumulated phospho-myosin light chain ( Fig 4E ) ., To determine whether the basal myosin functions as the apicalward presser of the soma ( i . e . , the squeezer of the cytoplasm ) , we live monitored M-phase cells in cerebral walls treated with blebbistatin ( 20 μm ) ., Although this treatment caused medium-immersed ( unembedded ) cerebral walls to freely undergo a concave-to-convex change ( Fig 3E ) , embedding of cerebral wall slices in collagen gel allowed them to fully maintain their original apically concave shape even in the presence of blebbistatin , thereby keeping the subapical space laterally compressed ( at least to a considerable degree ) ., We observed abnormal basal bouncing of the blebbistatin-treated M-phase cells’ somata ( Fig 4F , S3C Fig , S16 Movie ) ., These data are consistent with our model , in which each M-phase cell’s soma is squeezed from the basal pole during its rounding-up step in an actomyosin-dependent manner and thus laterally pushes the surrounding subapical space ., Based on previous studies that well described the IKNM dynamics 18–21 , 23–26 , we extended our analysis on the behaviors of newborn ( ≤3-hr-old ) daughter cells to characterize the relationship between the shapes of their apical cytoplasmic portions and the positions of their nuclei ( Fig 5 ) ., Because we reasoned that the initial step would be more susceptible to the hypothetical elasticity-based pushing by the subapical space than later steps , it was also important to determine whether basalward nucleokinesis proceeded constantly or , instead , exhibited transitions from a fast mode to a slower mode ., Soon after their generation , most daughter cells remain connected to the apical surface 6 , 39 ., Sister daughter cells’ nuclei depart from the subapical space sequentially; one daughter cell inherits the basal process from its mother cell 5 , 6 and initiates basal nucleokinesis more quickly than its sibling ( non–process-inheriting ) cell 18 , 27 , 40 ., Despite this morphology-dependent intra-clonal difference in nuclear-departure time , we identified two key rules common to the early-nucleokinetic ( E-IKNM ) and late-nucleokinetic ( L-IKNM ) daughter cells ., First , the reduction of the horizontal sectional area of the apical cytoplasm ( at 3 . 5 μm , 5 . 0 μm , and 6 . 5 μm from the apical surface ) was earliest at 3 . 5 μm and later at more basal positions , suggesting an apical-to-basal cytoplasmic thinning , and always preceded the onset of basal nucleokinesis ( Fig 5C–5E ) ., Second , basal nucleokinesis , once started , was mostly biphasic , with an initial ( within 30-min ) quick displacement phase occurring in the subapical space and a subsequent slower phase occurring in more basal regions ( Fig 5F and 5G ) ., When myosin II was pharmacologically inhibited with 20 μm blebbistatin , both the initial quicker phase and the subsequent slower phases were decelerated ( in both E-IKNM and L-IKNM ) ( Fig 5H , S3D and S3E Fig ) ., The results described above support non–cell-autonomous basal nucleokinesis driven by a force externally applied from the subapical space , where actomyosin serves to make the entire apical surface contractile ( thereby contributing to the subapical elasticity ) and to squeeze/push each M-phase cell’s cytoplasm apically from its basal pole ( to begin the lateral storage of elastic energy ) ., However , they are also consistent with a cell-autonomous mechanism in which daughter cell–intrinsic actomyosin near the apical surface might transiently squeeze/push daughter cells’ nuclei basally 22 ., In silico increase and decrease of the spring constants of the two lines resulted in accelerated and retarded basal nucleokinesis , respectively ( Fig 4B ) ., Accordingly , to functionally assess the former ( passive ) mechanism , we performed a set of mechanical experiments to determine whether external forces , as expected to arise in the elastic ( Windkessel-like ) subapical space , are necessary and sufficient for the basal nucleokinesis ., Specifically , we locally laser ablated the subapical space that surrounded cells immediately ( <5 min ) after cytokinesis ( Fig 6A ) ., Successful ablation of the surrounding space , not the nuclei/somata whose movement should be evaluated , was reflected by tissue irregularity or shrinkage ( Fig 6B , S3F Fig , S18 and S19 Movies ) ., Although nuclei of daughter cells in control slices departed from the subapical space in 33 ± 6 min in E-IKNM cases or 58 ± 17 min in L-IKNM cases ( n = 5 pairs ) , nuclei of daughter cells whose neighboring subapical space was laser ablated ( decompressed ) did not move for up to 80 min ( n = 5 pairs ) ( Fig 6C ) ., We then asked whether compression to increase the subapical elasticity would accelerate daughter cells’ initial nucleokinesis ., To this end , we placed a cerebral wall slice embedded in collagen gel in a silicone-rubber chamber , to which an external force can be applied along a single axis to provide about 14% compression ( and about 8% stretching along the orthogonal axis ) of the subapical space ( Fig 6D–6H , S3G Fig , S20–S22 Movies ) ., From this artificially compressed subapical space , both E-IKNM daughter cells’ and L-IKNM daughter cells’ nuclei/somata departed significantly earlier than in control cases ( Fig 6H ) ., Next , we sought to determine whether a local external strain , e . g . , a strain provided from the subapical space to the apicalmost cytoplasm of a newly generated cell , can actually move that cell’s nucleus/soma against the expected basal mechanical resistance ( e . g . , intracellular viscosity and/or the extendibility of plasma membrane ) ., To mimic the in vivo situation in a more directly manipulable manner , we “imprinted” VZ tissues onto a culture dish so that the bipolar-shaped geometry of VZ cells could be maintained to a considerable degree 41 ( Fig 6I ) ., In imprinted cells successfully pushed by a microcapillary from one pole of the nucleus/soma , nuclear/somal displacement to the opposite side was observed either relatively quickly ( within 5 min in 3 out of 4 cases ) or slowly ( more than 15 min in 1 out of 4 cases ) , whereas control uncompressed cells did not show such displacements ( Fig 6J and 6K , S3H Fig , S1 Data ) ., These results , together with the aforementioned basalward bouncing of M-phase cells’ somata that occurred through overcoming potential apicalward resistance ( Fig 4F ) , imply that ( 1 ) external forces are necessary and sufficient for the initial passive movement of nucleus/soma in daughter cells , and ( 2 ) the retardation in the initial quick basal nucleokinesis step caused by blebbistatin ( Fig 5H and S3E Fig ) can be most reasonably explained by the absence or reduction of external forces in the Windkessel-like subapical space , although we do not exclude cell-intrinsic actomyosin’s partial contribution to this initial basalward step ., Based on these results , we sought to determine the contribution of the initial elasticity-based boosting step provided by the Windkessel-like subapical space to all IKNMs and the formation of highly pseudostratified NE/VZ ., For technical reasons , the aforementioned pharmacological and mechanical perturbations did not permit long-term assessments of neuroepithelia ., Hence , we aimed at mathematically removing or inhibiting only the initial 10-μm basalward nucleokinesis phase , to which this Windkessel-like mechanism seemed to critically contribute ., We developed a new simulation to systematically describe movements of all VZ cells’ nuclei , individually linked with cell cycle progression ( Fig 7A–7D , S4A Fig , S23 Movie ) , based on previously reported parameters for cell cycle progression and cell differentiation 3 , 42 as well as nuclear densities and migration velocities obtained from live-tracked nuclei 18 in the mid-embryonic mouse cerebral VZ ., The two major physical forces applied to induce nuclear/somal displacements along the apicobasal axis were ( 1 ) direct soma–soma ( nuclear–nuclear ) interactions ( i . e . , collisions and repulsions ) and ( 2 ) mechanical forces that act in a cell cycle phase–dependent manner , independently of such direct soma–soma repulsions ( S4A Fig ) ., The latter ( designated as “non-collision” forces ) included both intracellular forces dependent on microtubules or actomyosin and external forces such as those provided elastically from the cellular processes to the nuclei/somata ., For example , the in silico G2-phase cells’ rapid ( highly directional ) apical displacement 18–21 , 23–26 was reproduced by application of the “non-collision” force ., Horizontal assembly/bundling of the nuclear/somal movements of individually simulated clonally related cells ( exemplified in Fig 7A , S4A Fig , S23 Movie left , showing hairpin loop–like , distally bifurcated “production lines” ) successfully established a virtual ( NE/VZ-like ) pseudostratified tissue ( Fig 7B , S23 Movie right ) , which recapitulated the overall IKNM trajectories in vivo ( Fig 7C and 7D , S4B and S4C Fig ) ., Consistent with real observations ( Fig 2B–2E ) , soma–soma collisions/repulsions were much less frequent in the subapical ( within10 μm ) part than in more basal parts of the virtual VZ ( S4D Fig ) ., Interestingly , basal nucleokinesis in this simulation was functionally biphasic ( S4E–S4G Fig , S24 Movie ) , reminiscent of real observations ( Fig 5F and 5G ) ., The simulations showed that G1-phase nuclei/somata existing >10 μm away from the apical surface could be automatically displaced through direct nucleus–nucleus repulsions ( probably between G1-phase cells ) alone , as long as movements of other chronologically different nuclei ( i . e . , apical arrival of G2-phase cells’ nuclei/somata and the initial basal displacement of early G1-phase cells’ nuclei/somata in the subapical within 10 μm space ) were secured ( S4B–S4G Fig ) , supporting the previously proposed idea of passive IKNM 9 , 21 , 25 , 26 ., By contrast , the subapical ( within 10 μm ) nuclei/somata strongly required a basal acceleration by the “non-collision” mechanism , the absence of which resulted in severe disruption of overall IKNM and the VZ structure ( Fig 7E–7G , S25 Movie ) , suggesting that this initial basalward step may be critical or rate limiting for a high degree of overall pseudostratification ., As we experimentally demonstrated in Fig 6 , the initial 10-μm basalward step may strongly depend on an external , elasticity-dependent mechanism ., Therefore , it is very likely that the initial basal nucleokinesis step , which is more rapid and directional than the subsequent ( more basal ) nucleokinesis ( Fig 5 ) and requires external elasticity , is critical for the ordered brain histogenesis ., In diverse proliferative cell types , each non–M-phase cell will eventually enter M phase , and the M-phase cell will then generate two non–M-phase ( daughter ) cells ., Non–M-phase cells and M-phase cells are therefore in a producer–product relationship ., This chronological ( cell cycle ) partnership depends on intracellular chemical reactions , such as the activation and disappearance of cyclins , but a recent study on the Drosophila epithelium showed M-phase cells’ nonchemical ( physical ) contribution to tissue morphogenesis 44 ., Following the previously established concept of a mother-to-daughter morphological gift in the developing mouse cortical VZ , i . e . , asymmetric inheritance of each M-phase cell’s basal process/fiber by one daughter cell 6 , 18 , the present study revealed that each M-phase cell also gives mechanical energy to both daughter cells , with elastic assistance from the densely packed apical processes of neighboring non–M-phase cells ( via contractility of the apical surface ) ( Fig 8 ) and other M-phase cells that divide ( to generally increase the pressure of the subapical space ) ( S4H Fig ) ., These mother-to-daughter ( intra-clonal ) physical gifts assist daughter cells’ prompt nucleosomal movement away from the subapical space ., Thus , such established initial basal nucleokinesis enables non–M-phase cells to have thin and flexible apical processes throughout the subapical space , which is permissive for the voluminous division of new M-phase cells ., This permissiveness can be regarded as a mechanical gift from non–M-phase cells to M-phase cells ., These bidirectional physical collaborations ( S4H Fig ) may underlie efficient and safe intra-neuroepithelial nuclear/somal logistics and protect the subapical space from local overcrowding ., Existence of too many nuclei/somata in the subapical space prevents progenitor cells from freely dividing at the apical surface and induces them to abnormally leave the apical surface , leading to heterotopic divisions and disruption of histogenesis 18 ., Thus , give-and-take relationships ( i . e . , mutualism ) exhibited spatially and physically between M-phase cells and non–M-phase cells ( S4H Fig ) are essential for ordered brain development ., Previous studies showed that the basal nucleokinesis is mediated by active intracellular mechanisms dependent on kinesin/microtubules 23 and actomyosin 22 ., Our IKNM simulation ( virtual VZ ) ( Fig 7 ) showed that collective basalward IKNMs at basal VZ levels ( more than 10 μm from the apical surface ) can occur almost passively using soma–soma collisions between G1-phase cells’ nuclei/somata as a major driving force ., Although this result is consistent with a model of the passive basal IKNM suggested by Norden et al . 21 , 25 and Kosodo et al . 26 , the present study cannot address the relative importance of the collision-based passive mechanism compared to the cell-intrinsic ( kinesin- and/or actomyosin-dependent ) basal nucleokinesis mechanisms ., In the subapical space ( within 10 μm from the apical surface ) , we found a novel passive IKNM mechanism mediated by tissue elasticity and indirect energy transfer ., We cannot precisely determine the relative importance of this boosting mechanism compared to the kinesin- and/or actomyosin-dependent mechanisms in the subapical space ., Nevertheless , the existence of the Windkessel-like ( elasticity-based ) boosting mechanism is strongly suggested based on ( 1 ) the elastic property of the subapical space ( Fig 3 ) , ( 2 ) the more directional displacement of nuclei/somata during the initial 30 min than later periods ( with more superlinear MSD curves ) ( Fig 5 ) , and ( 3 ) the decompression and compression experiments that resulted in deceleration and acceleration of initial basal nucleokinesis , respectively ( Fig 6 ) ., We speculate that a Windkessel-like boosting mechanism may collaborate with cell-intrinsic basal nucleokinesis mechanisms ., For example , centripetal mechanical stimuli applied externally from the subapical space might also trigger intracellular molecular machinery for active nucleokinesis , a possibility to be studied in the future ., Similarly , cell cycle progression , which is associated with ( or upstream to ) intracellular molecular mechanisms for nucleokinesis , might also be modulated by external mechanical stimuli—another question to be addressed for better system-level understanding of intra-VZ collective behaviors of NPCs ., Atomic force microscopy ( AFM ) revealed that the elastic modulus on/near the apical surface was much greater ( 1 , 400 Pa ) 45 than that in more basal VZ regions ( about 100 Pa ) 46 ., The present study showed that an elasticity-based mechanism assists in an initial basalward step in IKNMs ., Actually , diverse biological systems for coordinating heterogeneous movements or flows similarly utilize elasticity as a means to minimize total energetic expenditure ., In mammalian running 47 , 48 or insect flying 49 , elastic energy stored at one stage in the stride or wingbeat is released at another ., Likewise , in blood circulation , the aorta flexibly receives blood ejected from the heart and elastically recoils to forward it ( Windkessel effect 28 ) ., Thus , such a commonly used strategy might participate in multiple aspects or phases in the VZ growth or the overall brain development , and perhaps in the neocortical evolution ., We previously showed that elasticity ( stiffness ) measured on/near the apical surface of the neocortical VZ by AFM was greater in ferrets than in mice 45 ., We also found in slice culture that NPCs’ initial basal nuclear/somal movement was quicker in ferrets than in mice 27 ., It is therefore possible that elasticity in the subapical space participates in the differential IKNM behaviors between mice and ferret ., Our mathematically IKNM-simulated virtual VZ showed that the initial basalward nucleokinesis step is important for the overall high-degree pseudostratification ( Fig 7 ) ., The degree of pseudostratification ( i . e . , the thickness of NE/VZ ) increases from mice to monkeys 13 and even further in humans 14 , 15 ., It would be meaningful to study the possible contribution of tissue-level or cell-level elasticity to the thickening of VZ during evolution ., Our AFM revealed that single dissociated NPCs are stiffer in mice than in ferrets 45 ., Adding such measurable parameters more into our virtual VZ method would be effective to improve our simulation and to expand its feasibility in species other than mice ., The animal experiments were conducted according to Japanese Act on Welfare and Management of Animals , Guidelines for Proper Conduct of Animal Experiments ( published by Science Council of Japan ) , and Fundamental Guidelines for Proper Conduct of Animal Experiment and Related Activities in Academic Research Institutions ( published by Ministry of Education , Culture , Sports , Science and Technology , Japan ) ., All protocols for animal experiments were approved by the Animal Care and Use Committee of Nagoya University ( No . 29006 ) ., R26-Lyn-Venus transgenic mice ( accession No . CDB0254K ) , R26-H2B-mCherry transgenic mice ( accession No . CDB0239K ) 18 , 50 , and R26-ZO1-EGFP transgenic mice ( accession No . CDB0260K ) 45 , 51 were provided by Toshihiko Fujimori ( NIBB , Japan ) ., Pregnant ICR mice were obtained from SLC ., Embryos at the mid-embryonic stage ( embryonic day E 13 ± 1 ) were used ., Image processing , nuclear tracking , and calculation of cross-sectional area and fluorescent intensity were performed using ImageJ ., Fluorescent intensity of Lifeact-EGFP at the level of the soma or the basal pole ( as depicted in Fig 4C and 4D ) was determined by measuring the maximal pixel intensity ( i . e . , brightest spots or bands ) on the horizontally sectioned cell cortex ., Reconstructed fluorescent 3D images were obtained using Volocity ( PerkinElmer ) ., “Departure” of each newborn daughter cell’s nucleus/soma from the subapical space under en face observation ( as discussed in Fig 6 ) was defined in the horizontal sectional plane 5 μm from the apical surface as the time when its diameter became smaller than 3 μm ( including complete disappearance ) ., Onset of basal displacement of the nucleus/soma of a newborn daughter cell under cross-sectional observation ( as discussed in Fig 5 , S3D and S3E Fig ) was defined as the time when the mass center of the nucleus/soma moved 1 . 5 μm basally ., MSD was obtained based on time-dependent changes of nuclear position along the apicobasal axis , as described previously 18 ., To determine whether the apical surface was stable , without exhibiting local tilting or floppiness of apices even where mitosis occurred , we analyzed a 3-min-interval movie obtained by en face observation of the apical surface of a cerebral wall prepared from a R26-ZO1-EGFP transgenic mouse 51 at 0 . 5-μm z intervals ., Cell borders at the apical surface level were extracted from the zero-crossing points for first-order derivatives obtained by applying 1D Savitzky-Go
Neural progenitor cells ( NPCs ) , which are apicobasally elongated and densely packed in the developing brain , systematically move their nuclei/somata in a cell cycle–dependent manner , called interkinetic nuclear migration ( IKNM ) : apical during G2 and basal during G1 ., Although intracellular molecular mechanisms of individual IKNM have been explored , how heterogeneous IKNMs are collectively coordinated is unknown ., Our quantitative cell-biological and in silico analyses revealed that tissue elasticity mechanically assists an initial step of basalward IKNM ., When the soma of an M-phase progenitor cell rounds up using actomyosin within the subapical space , a microzone within 10 μm from the surface , which is compressed and elastic because of the apical surface’s contractility , laterally pushes the densely neighboring processes of non–M-phase cells ., The pressed processes then recoil centripetally and basally to propel the nuclei/somata of the progenitor’s daughter cells ., Thus , indirect neighbor-assisted transfer of mechanical energy from mother to daughter helps efficient brain development .
The development of large brain structures , such as the mammalian cerebral cortex , depends on the continuous and efficient production of cells by neural progenitor cells ., Neural progenitor cells are elongated and span the developing brain wall ., The nuclei and bodies of these cells move cyclically between the apical and basal surfaces , and they divide every time they reach the apical surface ., While we understand how individual cells achieve this cycle , how the movements of several progenitor cells are coordinated with one another remains elusive ., By using a combination of live imaging and mechanical experiments , coupled with mathematical simulations , we show that cell crowding at the apical surface , where progenitor cells divide , creates a subapical microzone that is compressed and elastic ., We then show that when each mother cell rounds up , preparing for division , it pushes this elastic microzone laterally , thereby storing mechanical energy ., After cell division , this mechanical energy is transferred to the daughter cells , propelling them along the axis of movement in the direction of the basal surface , in an energy-saving manner ., Our mathematical simulations show that timely departure of newly generated daughter cells is critical for the overall tissue structure of the cerebral proliferative zone .
g1 phase, classical mechanics, engineering and technology, cell cycle and cell division, lasers, cell processes, g2 phase, mechanical energy, molecular motors, actin motors, stem cells, microscopy, optical equipment, motor proteins, research and analysis methods, contractile proteins, animal cells, proteins, scanning electron microscopy, physics, biochemistry, cytoskeletal proteins, cell biology, equipment, electron microscopy, myosins, biology and life sciences, cellular types, physical sciences