Fig. 1. Background on midbrain roof plate properties: schematic representation of previous experiments that disclosed unexpected properties of the RP near the isthmic organizer (IsO). (A-C) For simplicity, although the midbrain RP is not yet differentiated from the neural folds at stage HH10, the experimental manipulation performed at stage HH10 and its outcome on RP patterning 1 or 2 days later are both represented on the same outline of a stage HH10 chick neural tube. (A) Cells from a restricted isthmic domain (red circle isthmic node, Isnode) extend to populate the midline on both sides of the MH junction, marked by an arrow (Millet et al., 1996; Louvi et al., 2003; Alexandre and Wassef, 2003). (B) Interfering with local patterning at the Isnode prevents RP formation on the adjacent midline (Alexandre and Wassef, 2003). (C) A lateral source of FGF8 created by insertion of a FGF8-soaked bead or of an IsO transplant induces RP duplication/bifurcation (Bally-Cuif and Wassef, 1994; Crossley et al., 1996). (D) The RP fails to develop on the dorsal midline of midbrain vesicles whose AP axis has been inverted by a 180° rotation (Marin and Puelles, 1994; Alexandre and Wassef, 2003).