In vivo subcellular imaging of INM and mitotic cleavages of THN progenitors. Lateral view of THN progenitors in the cerebellum of an ∼24-hpf Tg(atoh1a:Gal4TA4)^hzm2 transgenic embryo injected with Medusa vector M1. Centrosomes are shown in yellow, cell nuclei in blue, and cellular membranes in red. (A) Centrosomes (arrows) were found to line the fourth ventricle at the apical side of the four THN progenitors undergoing INM between the midbrain–hindbrain boundary (MHB) and the fourth ventricle. Throughout INM, the centrosomes did not change their positions. Green asterisk demarcates a nucleus moving from apical to basal (A and B), while the corresponding centrosome (green arrow) stays at the apical side. The orange asterisk demarcates a nucleus that moves from basal to apical (A–C) to undergo a mitotic cleavage at the apical side (C). The corresponding centrosome (orange arrow) stays at the apical side, replicating to build the two spindle poles of the spindle apparatus (C, orange arrows). Thus, THN progenitors along the URL show characteristic INM behavior. Images are taken from Video 5. Note: some yellow-only labeling may suggest insufficient coexpression of transgenes from Medusa vectors. However, dependent on the z-level position of organelles and different intensities in expression levels, proper co-labeling can only be observed in cells of interest for which z-stacks were recorded. MHB, midbrain–hindbrain boundary; URL, upper rhombic lip.