Fig. 1

Neural plate olig2:EGFP⁺ precursors generate PMNs and KA' interneurons at the same time. (A) Dorsal view of the posterior neural plate at 11.5 hpf (five-somite stage) and (B-F) lateral view of 6- to 12-somite regions of spinal cord at 25 hpf in Tg(olig2:egfp) zebrafish embryos. (A) Heterogeneous primary neuronal populations within the EGFP⁺ domain. Asterisks and arrowheads mark EGFP⁺ Hu⁺ Isl⁺ PMNs and EGFP⁺ Hu⁺ Isl^- interneurons, respectively. Isl protein is nuclear, whereas Hu is cytoplasmic. (B-F) Embryos were incubated with BrdU at successive timepoints (as shown to the left of each panel) and labeled with anti-GABA (red) and anti-BrdU (blue) antibodies. Arrows and arrowheads mark BrdU⁺ and BrdU^- KA' interneurons, respectively. (B) Four KA' interneurons were formed from EGFP⁺ precursors that underwent S phase at 10 hfp (arrows). Arrowhead marks BrdU^- KA' interneuron, indicating that the postmitotic cell was formed before or after 10 hpf. (C) Two BrdU⁺ KA' interneurons (arrows) and two BrdU^- KA' interneurons (arrowheads) were detected in the embryos that were incubated with BrdU at 11 hpf. (D,E,F) Embryos incubated with BrdU at 12, 14 and 18 hpf. No BrdU⁺ KA' interneurons were evident. (G) Average of all KA' cells (squares) versus BrdU⁺ KA' cells (triangles) (n=4 animals each). S phase for the last KA' interneurons produced occurs between 10 and 12 hpf. Error bars represent s.e.m. Scale bar: 25 µm.