Fig. 1. Rhombomere 4 differentiates earliest in the zebrafish hindbrain. (A-D) Single optical sections from a confocal time-lapse recording showing early rhombomere boundary formation. Images show dorsal views with anterior towards the left. Rhombomere boundaries are marked with arrowheads, and the r4/5 boundary arrowheads are labeled with a dot. ot, otic vesicle. (E-H) In situ hybridization of isl1 in blue and, in red, krox-20 in r3 and r5 and pax2a at the midbrain-hindbrain boundary (mhb). Arrowheads in F label early isl1-expressing nVII neurons in r4. isl1 also labels peripheral cranial ganglia, such as the trigeminal ganglion (tg in E). Anterior is towards the left. Somite stages are indicated (lower right-hand corners). Scale bars: in A, 50 µm in A-D; in E, 50 µm in E-H.

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