Fig. 3. Expression of RP competence factors in rotated dorsal midbrain transplants. Dorsal views of four HH22-23 quail-chick chimeras (A-A'',B,B',C,C',D-D'') that received a 90° rotated dorsal midbrain transplant 3 days earlier at stage HH10, as schematized on the left (black, quail donor). The embryos were treated for the detection of Wnt1, Lmx1b or Gdf7 transcripts, as indicated together with the color of detection (red or purple) at the top of each picture. Species-specific probes were used in A-A'',C-D'' to discriminate between chick and quail Wnt1 transcripts. After using the second probe, the quail transplant was identified by QCPN immunocytochemistry (A'',B',D''). RP-like structures developed in the graft in contact with one or both ends of the host RP in about 25% of the transplants (16/63, A-A''). In more than 70% of cases, Wnt1 remained widely expressed in the transplants (B-D'). Lmx1b was expressed ectopically in part of the Wnt1 expression domain (4/4, B'). Except in the smallest transplants, Wnt1 expression was regulated along the DV axis, leading to its reorganization in a wide band that flanked the host midline (C',D'). In these cases, the transplant straddles the midline (D', see also F, a transverse section through the transplant, double arrowhead) without differentiating a RP (compare with E, a section through the host RP, arrowhead).