Fig. 2. Delta-Notch signaling maintains olig2 expression and inhibits ngn1 expression. (A-C) Transverse sections, dorsal up, of 24 hpf embryos. Whereas a discrete group of ventral spinal cord cells expressed olig2 (A), cells that expressed ngn1 were scattered throughout the spinal cord (B). In situ double RNA hybridization revealed that a subset of olig2⁺ cells (C, red) expressed ngn1 (blue, arrows). (D-G) Dorsal views, anterior left, of 12 hpf embryos. (D,E) Embryos hybridized with probes for olig2 and tlxa. Brackets indicate olig2 expression. tlxa expression marks prospective Rohon-Beard (RB) sensory neurons. In wild-type embryos, olig2 expression was maintained throughout the length of the trunk neural keel (D, arrow). In dla^-;dld^- mutant embryos, identified by the excess Rohon-Beard phenotype, anterior neural keel cells did not maintain olig2 expression (E). (F,G) Embryos hybridized with probe to reveal ngn1 expression. Brackets indicate region of neural keel that expressed olig2. Excess cells in dla^-;dld^- mutant embryos expressed ngn1 at high level (G) compared to wild type (F). Scale bar: 20 µm (A-C); 50 µm (D-G).

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