  A major hallmark of neural development is the oscillatory movement of nuclei
between the apical and basal surfaces of the neuroepithelium during the process
of interkinetic nuclear migration (IKNM). Here, we employ long-term, rapid
lightsheet and two-photon imaging of zebrafish retinas $in~vivo$ during early
development to uncover the physical processes that govern the behavior of
nuclei during IKNM. These images allow the capture of reliable tracks of
nuclear movements and division during early retinogenesis for many tightly
packed nuclei. These tracks are then used to create and test a theory of
retinal IKNM as a diffusive process across a nuclear concentration gradient
generated by the addition of new nuclei at the apical surface. The analysis
reveals the role of nuclear packing at the apical surface on the migration
dynamics of nuclei, provides a robust quantitative explanation for the
distribution of nuclei across the retina, and may have implications for
stochastic fate choice in this system.
