FGF8 induces formation of an ectopic isthmic organizer and isthmocerebellar development via a repressive effect on Otx2 expression

Beads containing recombinant FGFS (FGF8-beads) were implanted in the prospe ctive caudal diencephalon or midbrain of chick embryos at stages 9-12, This induced the neuroepithelium rostral and caudal to the FGF8-bead to form tw o ectopic, mirror-image midbrains, Furthermore, cells in direct contact wit h the head formed an outgrowth that protruded laterally from the neural tub e. Tissue within such lateral outgrowths developed proximally into isthmic nuclei and distally into a cerebellum-like structure. These morphogenetic e ffects were apparently due to FGF8-mediated changes in gene expression in t he vicinity of the bead, including a repressive effect on Otx2 and an induc tive effect on Enl, Fgf8 and Wntl expression. The ectopic Fgf8 and Wntl1 ex pression domains formed nearly complete concentric rings around the FGF8-be ad, with the Wntl ring outermost. These observations suggest that FGFS indu ces the formation of a ring-like ectopic signaling center (organizer) in th e lateral wall of the brain, similar to the one that normally encircles the neural tube at the isthmic constriction, which is located at the boundary between the prospective midbrain and hindbrain, This ectopic isthmic organi zer apparently sends long-range patterning signals both rostrally and cauda lly, resulting in the development of the two ectopic midbrains. Interesting ly, our data suggest that these inductive signals spread readily in a cauda l direction, but are inhibited from spreading rostrally across diencephalic neuromere boundaries. These results provide insights into the mechanism by which FGFS induces an ectopic organizer and suggest that a negative feedba ck loop between Fgf8 and Otx2 plays a key role in patterning the midbrain a nd anterior hindbrain.