Time-lapse analysis of THN progenitor behavior. (A) Lateral view of the MHB region of a Tg(atoh1a:Gal4TA4)^hzm2 x Tg(shhb:Gal4TA4,5xUAS:mRFP)^hzm4 embryo at 24 hpf. URL-derived THN progenitors are labeled by mRFP expression. The boxed area is enlarged in B–F. (B) mRFP-expressing THN progenitors are connected to the apical surface by thin processes (arrow). (C) THN progenitors divide at the apical side (arrows indicate dividing cell). (D) During radial migration these cells maintain apical processes (arrows) that are retracted (E) once the nuclei reach the MHB. (F) Around the same time, axon-like processes become visible (arrow). These time-lapse data show that transgene expression mediated by Gal4 in Tg(atoh1a:Gal4TA4)^hzm2 embryos reveals cell behavior consistent with that previously observed for URL-derived THN neurons (Köster and Fraser, 2001a; Volkmann et al., 2010). Images were taken from Video 4. MHB, midbrain–hindbrain boundary; URL, upper rhombic lip.