08-P001 Interkinetic nuclear migration in the zebrafish retina: Actomyosin contraction is the prime mover

As the neurogenerative epithelium of the vertebrate central nervous system expands during embryonic development, nuclei of neural progenitors undergo a process called interkinetic nuclear migration (IKNM). The nuclear movements of IKNM are generally believed to involve gradual migrations from apical to basal and back during the G1 and G2 phases of the cell cycle, respectively. Yet the kinetics of IKNM and the mechanisms that drive it have never been systematically studied. We use a high-resolution time-lapse confocal microscopy approach to record nuclear movements in zebrafish retinal neuroepithelial cells and show that, rather than being gradual and directed, nuclear movements are mainly stochastic except for brief apical nuclear translocations preceding mitosis. We also demonstrate that IKNM is driven largely by actomyosin contractions rather than transport along microtubules, as these movements still occur when the microtubule cytoskeleton is destabilized or demolished yet they are severely inhibited when Myosin II activity is blocked. Based on our findings we propose that nuclear movements during IKNM are reminiscent of movements of people at a crowded party held in a room with a bar at one end. People get thirsty and go to the bar to get a drink. Then they move or are pushed away to make room for others at the bar. Between drinks the partygoers jostle around the room, but when one of them gets thirsty again, he returns to the bar in a fast and directed manner.