In vivo subcellular imaging of centrosome dynamics in THN progenitors. Lateral view of the cerebellar anlage of an ∼36-hpf Tg(atoh1a:Gal4TA4)^hzm2 transgenic embryo injected with Medusa vector M1. Centrosomes (green arrow indicates the first centrosome, red arrow second, turquoise third, and yellow fourth; white arrows are shown when centrosomes are indistinguishable) are labeled in yellow, cell nuclei in blue, and cellular membranes in red. (A) Centrosomes (arrow) of the four THN progenitors were found to line the fourth ventricle. (B) Nuclei translocate basally toward the MHB, leaving behind a long trailing process containing the centrosome at its most apical part. This subcellular coordination argues that the final MHB-directed cell movement to initiate migration represents an extended final step of INM. (C–I) When nuclei reach the MHB, trailing processes containing the centrosomes at the most apical position are retracted, representing the initiation of THN neuron migration. Images are taken from Video 6. MHB, midbrain–hindbrain boundary; URL, upper rhombic lip.