Alteration of the retinotectal projection map by the graft of mesencephalic floor plate or Sonic hedgehog

The floor plate plays crucial roles in the specification and differentiatio n of neurons along the dorsal-ventral (DV) axis of the neural tube. The tra nsplantation of the mesecephalic floor plate (mfp) into the dorsal mesencep halon in chick embryos alters the fate of the mesencephalon adjacent to the transplant from the tectum to the tegmentum, a ventral tissue of the mesen cephalon. In this study, to test whether the mfp is involved in the specifi cation of the DV polarity of the tectum and affects the projection patterns of retinal fibers to the tectum along the DV axis, we transplanted quail m fp into the dorsal mesencephalon of chick embryos, and analyzed projection patterns of dorsal and ventral retinal fibers to the tectum. In the embryos with the mfp graft, dorsal retinal fibers grew into the dorsal part of the tectum which is the original target for ventral but not dorsal retinal fib ers and formed tight focuses there. In contrast, ventral retinal fibers did not terminate at any part of the tectum, Transplantation of Sonic hedgehog (Shh)-secreting quail fibroblasts into the dorsal mesencephalon also induc ed the ectopic tegmentum and altered the retinotectal projection along the DV axis, as the mfp graft did. These results suggest that some factors from the mesencephalic floor plate or the tegmentum, or Shh itself, play a cruc ial role in the establishment of the DV polarity of the tectum and the reti notectal projection map along the DV axis.