Nuclear movements during the majority of the cell cycle, in G1 and S phases, have been less thoroughly examined. In the mammalian cortex, they are thought to involve microtubules, as well as various microtubule motors and actomyosin ( Xie et al., 2007 Tsai et al., 2007), while in the zebrafish retina, it appears to be the actomyosin complex alone that moves the nuclei to the apical surface during G2 ( Norden et al., 2009 Leung et al., 2011). The molecular mechanisms that drive the rapid nuclear movement in G2 have been investigated in a number of tissues ( Norden, 2017). In the second gap phase (G2), nuclei migrate rapidly toward the apical surface where they divide again ( Del Bene, 2011 Sauer, 1935 Baye and Link, 2007 Leung et al., 2011 Kosodo et al., 2011 Norden et al., 2009).
During the first gap phase (G1) of the cell cycle, nuclei migrate away from this surface to reach more basal positions by synthesis phase (S), when DNA is replicated. Under normal conditions, nuclei of proliferating cells undergo mitosis (M) exclusively at the apical surface. More than 80 years ago, these movements, termed interkinetic nuclear migration (IKNM), were shown to occur in synchrony with their cell cycle ( Sauer, 1935). Within these cells, striking nuclear movements take place during the proliferative phase of neural development. The vertebrate nervous system arises from a pseudostratified epithelium within which elongated proliferating cells contact both the apical and basal surfaces. Considerations of nuclear motion constrained inside the enveloping cell membrane show that concentration-dependent stochastic forces inside cells, compatible in magnitude to those found in cytoskeletal transport, can explain the observed magnitude of the diffusion constant. The time-varying concentration profiles show clear evidence of crowding as nuclei reach close-packing and are quantitatively described by a nonlinear diffusion model. We employ long-term, rapid light-sheet and two-photon imaging of early zebrafish retinogenesis to track entire populations of nuclei within the tissue. Yet, this hypothesis has not yet been tested, and the forces involved not quantified.
Tracking of nuclear subpopulations has shown evidence of diffusion - mean squared displacements growing linearly in time - and suggested crowding from cell division at the apical surface drives basalward motion. An important question in early neural development is the origin of stochastic nuclear movement between apical and basal surfaces of neuroepithelia during interkinetic nuclear migration.
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