Fig. 2. Bipolar differentiation of the midbrain roof plate. (A-A'') BMP signaling is required for RP differentiation later than stage HH10. Lateral (A), posterior (A') and dorsal (A'') views of the same embryo fixed 2 days after insertion of a noggin-soaked bead in the midbrain and labeled for Gdf7 transcripts. The midbrain RP is mostly missing (A, arrowhead) except for a faint labeling at its caudal (A', arrowhead) and rostral (A'', arrowhead) ends. (B,C) Local and distant effects of FGF8 beads. Lateral views of embryos labeled for Wnt1 2 days after implantation of FGF8 beads. In most cases, the induced RP extends from the dorsal midline to the bead (B, arrowhead). In cases where the FGF8 bead induces locally a small RP segment (C, arrow), a small deflection of the endogenous RP also points in the direction of the bead (C, arrowhead).