Zebrafish otp1 expression in the hindbrain. Anterior is left in all images except for C. All views are dorsal except for E, Gand Hthat are lateral. A, At 11 hpf stage, a narrow transverse stripe encompasses the neural tube in its anterior half. During somitogenesis, a second narrow stripe appears more anteriorly (data not shown). B, Double labelling with otp1 (blue) and krox20 (red) shows that otp1 expression in the hindbrain labels rhombomere 3 (r3) and 5 (r5) at 14 hpf stage. This developmental window coincides with the restriction of the two otp1 stripes toward the lateral margins of the neural tube, and with the early extension of the signal throughout the rhombencephalon. C, Transverse section of a 24 hpf stage zebrafish embryo through the hindbrain shows that these lateral clusters belong to the mantle layer of the ventral alar plate and are placed medially along the dorsoventral axis. D, Following (19 hpf), otp1 signals extend anteriorly, and then posteriorly, in the shape of repeated patterns. E, 24 hpf embryo; otp1 mRNA signal localizes in restricted groups of cells, in a pattern reminiscent of rhombomere segmentation. F, G, otp1 WISH combined with acetylated α-tubulin (TU) immunochemical staining confirms that otp1 transcripts occur in paired clusters within each rhombomere (30 hpf) (open arrowheads in Findicate the axons in the central part of every rhombomere). H, At 34 hpf, otp1 domains have elongated dorsally in parallel columns (vertical dashes in E, Fand H, and dashed line in G, indicate rhombomere boundaries and profile, respectively). ce, cerebellum; dap, dorsal alar plate; is, isthmus; n, notochord; ov, otic vesicle; ra, reticulospinal axons; vap, ventral alar plate.