Fig. 2. Boundaries of hindbrain RA activity along the anteroposterior axis. All embryos are homozygous for the RARE-lacZ RA-indicator transgene; stages are indicated on each panel. Anterior is oriented towards the right in A-D (lateral views) and towards the top in E-O (dorsal views). (A-C) Embryos were double-stained for RARE-lacZ expression (ß-galactosidase activity), which is observed posteriorly, followed by Cyp26a1 expression (in situ hybridization), which is observed anteriorly with its posterior border at r2. RARE-lacZ expression is first seen at E7.5 (B) and the r2/r3 division marked in C indicates the anterior extent of RARE-lacZ, which reaches r3 at E7.6. (D,E) Raldh2 expression detected posteriorly in the paraxial mesoderm at E7.6. (F-K) Dorsal view of embryos double-stained for expression of RARE-lacZ and Cyp26a1 from E7.4-E7.9. RARE-lacZ expression has reached r3 by E7.6 (H), but begins to clear from r3 by E7.9 (K). (L-N) Double-stained embryos showing RARE-lacZ expression (white arrows indicate its anterior extent at the r4/r5 boundary) and expression of either Krox20 in r3 at E8.0 (L), Krox20 in both r3 and r5 at E8.25 (M) and Hoxb1 in r4 at E8.5 (N).