Fig. 6. Mutually repressive interactions establish segregated domains in the anterior neural plate. (A-C) Early expression of Six3 (A, red staining in C), Irx3 (B, blue staining in C) in HH6 chick embryos revealed by whole-mount in situ hybridization. The anterior side of the embryos is up. (D-F) Dissected brains from HH13 embryos in situ hybridized for Six3 (D), Irx3 (E), and Pax6 (F). Note that the anterior border of Irx3 expression (arrow in E) is located anterior to the posterior boundary of the Pax6-expressing domain (arrow in F). (G-J) Six3 (G,G'') or Otx2 (H) expression in presumptive HH10 embryos that had been electroporated with Irx3 at HH4. HH21 dissected brains that had been electroporated with Six3 (I,I') and ADSix3 (J,J') at HH8 stained for Irx3. Dorsal (G,G',H,H'), frontal (G''), and lateral views (I-J') of the specimens. Arrows indicate sites where Six3 or Irx3 expression was suppressed (G,G'',I',I''), and arrowheads mark ectopic expression of Irx3 in the ventral thalamus (J,J'). Asterisk in I indicates a crack in the specimen. The ZLI is represented by dashed lines in J. The expression of transgenes is monitored by GFP fluorescence (G',H',I''). Bars, 0.5 mm (A-H), 0.25 mm (I',I'',J'). hf, head fold; nc, notochord; ps, primitive streak.