Progenitor cell nuclei in the rapidly expanding epithelium of the embryonic vertebrate central nervous system undergo a process called interkinetic nuclear migration (IKNM). Movements of IKNM are generally believed to involve smooth migration of nuclei from apical to basal and back during the G1 and G2 phases of the cell cycle, respectively. Yet, this has not been formally demonstrated, nor have the molecular mechanisms that drive IKNM been identified. Using time-lapse confocal microscopy to observe nuclear movements in zebrafish retinal neuroepithelial cells, we show that, except for brief apical nuclear translocations preceding mitosis, IKNM is stochastic rather than smooth and directed. We also show that IKNM is driven largely by actomyosin-dependent forces as it still occurs when the microtubule cytoskeleton is compromised but is blocked when MyosinII activity is inhibited.
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