Inductive signal and tissue responsiveness defining the tectum and the cerebellum

The mes/metencephalic boundary (isthmus) has an organizing activity for mes encephalon and metencephalon. The candidate signaling molecule is Fgf8 whos e mRNA is localized in the region where the cerebellum differentiates. Resp onding to this signal, the cerebellum differentiates in the metencephalon a nd the tectum differentiates in the mesencephalon. Based on the assumption that strong Fgf8 signal induces the cerebellum and that the Fgf8b signal is stronger than that of Fgf8a, we carried out experiments to misexpress Fgf8 b and Fgf8a in chick embryos, Fgf8a did not affect the expression pattern o f Otx2, Gbx2 or Irx2. En2 expression was upregulated in the mesencephalon a nd in the diencephalon by Fgf8a, Consequently, Fgf8a misexpression resulted in the transformation of the presumptive diencephalon to the fate of the m esencephalon, In contrast, Fgf8b repressed Otx2 expression, but upregulated Gbx2 and Irx2 expression in the mesencephalon, As a result, Fgf8b complete ly changed the fate of the mesencephalic alar plate to cerebellum. Quantita tive analysis showed that Fgf8b signal is 100 times stronger than Fgf8a sig nal. Cotransfection of Fgf8b with Otx2 indicates that Otx2 is a key molecul e in mesencephalic generation. We have shown by RT-PCR that both Fgf8a and Fgf8b are expressed, Fgf8b expression prevailing in the isthmic region, The results all support our working hypothesis that the strong Fgf8 signal ind uces the neural tissue around the isthmus to differentiate into the cerebel lum.