Cell Division: Interkinetic Nuclear… Mechanics

Generation of Neural Organoids and Their Application in Disease Modeling and Regenerative Medicine

The complexity and precision of the human nervous system have posed significant challenges for researchers seeking suitable models to elucidate refractory neural disorders. Traditional approaches, including monolayer cell cultures and animal models, often fail to replicate the intricacies of human neural tissue. The advent of organoid technology derived from stem cells has addressed many of these limitations, providing highly representative platforms for studying the structure and function of the human embryonic brain and spinal cord. Researchers have induced neural organoids with regional characteristics by mimicking morphogen gradients in neural development. Recent advancements have demonstrated the utility of neural organoids in disease modeling, offering insights into the pathophysiology of various neural disorders, as well as in the field of neural regeneration. Developmental defects in neural organoids due to the lack of microglia or vascular systems are addressed. In addition to induction methods, microfluidics is used to simulate the dynamic physiological environment; bio-manufacturing technologies are employed to regulate physical signaling and shape the structure of complex organs. These technologies further expand the construction strategies and application scope of neural organoids. With the emergence of new material paradigms and advances in AI, new possibilities in the realm of neural organoids are witnessed.

Cell cycle dynamics control fluidity of the developing mouse neuroepithelium

As developing tissues grow in size and undergo morphogenetic changes, their material properties may be altered. Such changes result from tension dynamics at cell contacts or cellular jamming. Yet, in many cases, the cellular mechanisms controlling the physical state of growing tissues are unclear. We found that at early developmental stages, the epithelium in the developing mouse spinal cord maintains both high junctional tension and high fluidity. This is achieved via a mechanism in which interkinetic nuclear movements generate cell area dynamics that drive extensive cell rearrangements. Over time, the cell proliferation rate declines, effectively solidifying the tissue. Thus, unlike well-studied jamming transitions, the solidification uncovered here resembles a glass transition that depends on the dynamical stresses generated by proliferation and differentiation. Our finding that the fluidity of developing epithelia is linked to interkinetic nuclear movements and the dynamics of growth is likely to be relevant to multiple developing tissues.

Mechanical constraints to cell-cycle progression in a pseudostratified epithelium

As organs and tissues approach their normal size during development or regeneration, growth slows down, and cell proliferation progressively comes to a halt. Among the various processes suggested to contribute to growth termination,1-10 mechanical feedback, perhaps via adherens junctions, has been suggested to play a role.11-14 However, since adherens junctions are only present in a narrow plane of the subapical region, other structures are likely needed to sense mechanical stresses along the apical-basal (A-B) axis, especially in a thick pseudostratified epithelium. This could be achieved by nuclei, which have been implicated in mechanotransduction in tissue culture.15 In addition, mechanical constraints imposed by nuclear crowding and spatial confinement could affect interkinetic nuclear migration (IKNM),16 which allows G2 nuclei to reach the apical surface, where they normally undergo mitosis.17-25 To explore how mechanical constraints affect IKNM, we devised an individual-based model that treats nuclei as deformable objects constrained by the cell cortex and the presence of other nuclei. The model predicts changes in the proportion of cell-cycle phases during growth, which we validate with the cell-cycle phase reporter FUCCI (Fluorescent Ubiquitination-based Cell Cycle Indicator).26 However, this model does not preclude indefinite growth, leading us to postulate that nuclei must migrate basally to access a putative basal signal required for S phase entry. With this refinement, our updated model accounts for the observed progressive slowing down of growth and explains how pseudostratified epithelia reach a stereotypical thickness upon completion of growth.