Fig. 6. Mouse-turtle interspecies grafting experiments reveal conserved guidance mechanisms in mammals and reptiles. (A-B') When DiI-labeled explants of PSB and MGE of stage 16 to 18 turtle embryos were grafted into E13.5 mouse forebrain slices, the released labeled cells (red) migrated within the host tissue. Cells from PSB explants grafted into the corticostriatal boundary (A) migrated along the corticostriatal boundary and accumulated ventrally (arrow), whereas MGE cells grafted into the basal telencephalon (B,B') migrated in the orthogonal direction, across the corticostriatal boundary. Turtle MGE cells dispersed and migrated as individuals to colonize the mouse cortex (B') and some developed branched neurites in the host tissue (B''). (C-F) GFP-expressing MGE explants from E12.5 mouse embryos grafted into stage 16 turtle slices (C) or into stage 17 telencephalic vesicles in ovo (D) were no longer visible after a few days. Individual GFP-positive mouse MGE cells migrated long distances within the telencephalon of turtle embryos, colonized the entire slice (C), or the pallium in the in ovo experiments (D). By contrast, cortical explants (E,F) still formed a compact mass of tissue several days after grafting and released very few cells. The few cells that were released did not migrate very far, either in grafted slice (E) or grafted hemisphere (F). Scale bars: 500 µm in A,B,C-F; 200 µm in B'; 40 µm in B''.