Fig. 4. Homeogenetic signaling induce the expression of RP markers. (A-D) Dorsal view of the midbrain region of HH19-21 embryos that received a small midbrain midline ablation at the 10-12ss as schematized on the left. Corresponding transverse cryostat sections counterstained with Nuclear Fast Red are illustrated in B',C'. The embryos were treated for the detection of Gdf7 (A-C) or Wnt1 (D) transcripts. The ablation resulted in complete regeneration (A, 1/6), or in the formation of partial (B, 4/6) or complete (C, 1/6) gaps in the midbrain Gdf7 expression domain. Scattered cells in the partial gaps expressed Gdf7 (B) or Wnt1 (D). (E,F) Lateral views (anterior is to the left) of Gdf7 midline expression in control (E) and ablated (F) embryos. Gdf7 expression is perturbed (*) on both sides of the ablation (delimited by the arrowheads). (G) Posterior view of a HH19 embryo illustrating the lack of Gdf7 expression (between arrowheads) on the midline of the mesencephalic vesicle after inversion of its anteroposterior axis as schematized in g. (H) Dorsal view of the midbrain of a 4-day-old chimera. An anteroposterior strip of quail midbrain neuroepithelium was transplanted at HH10 perpendicular to the host midline as schematized in h. H1-H3 illustrate the same chimera. The host roof plate (RP), labeled with chWnt1 (Fast Red), is seen in red using fluorescent optics (H1); the induced RP (purple arrowheads in H2 and H3), labeled with QWnt1 (NBT/BCIP), appears purple under bright-field optics (H2). After dissection of the dorsal midbrain, a faint QCPN labeling delineates the quail transplant (H3). Anterior is to the right.