Spinning nuclei in the brain of the zebrafish embryo

I wish to report here a striking phenomenon that arises in the developing brain of the zebrafish embryo. High-resolution differential interference contrast (DIC) time-lapse video microscopy in the live embryo reveals that throughout the second day of embryogenesis, the nuclei of some of the still-undifferentiated brain cells undergo a sustained spinning behaviour, usually around a constant axis specific to each nucleus. Figure 1 shows several nuclei in the brain that were spinning around an axis perpendicular to the focal plane (see Supplementary material). The mean spin rate is about 10 rotations per hour, but some nuclei were spinning at up to one rotation per minute over a 30-minute period, with a peak velocity of two rotations per minute (Figure 1e). Spinning can occur around any axis (but see below) and can be continuous or saltatory. Most nuclei spin in the same direction for the whole duration of the video recording (up to 2 hours); cases of reversal, or change in spinning axis, are shown in Figure 1b,d. In cell fields in which several nuclei were spinning simultaneously in the same vicinity, no obvious correlation could be discerned between their individual axes of rotation. The spinning behaviour also seems to be unlinked to mitosis, which can be readily observed in the brain. In the hindbrain, spinning nuclei were first observed at about 24 hours post-fertilization (hpf, in standard developmental time [1]), and from then up to the latest stage examined (42 hpf). In the midbrain, spinning nuclei were first observed by 30 hpf, both in the optic tectum and in the tegmentum (Figure 1f), although in the latter, spin rates as Magazine R627