Isthmus organizer and regionalization of the mesencephalon and metencephalon.

The brain vesicles that are formed at an early stage of neural development are the fundamentals of the brain plan. Heterotopic transplantation revealed that the diencephalon could change its fate when juxtaposed to the isthmus (mes-metencephalic boundary), which indicated that the isthmus functions as an organizer for the mesencephalon and metencephalon. Fgf8 is identified as an isthmus organizing signal. Misexpression of Fgf8a and Fgf8b indicated that a strong Fgf8 signal organizes cerebellar development. The transcription factors define the fate of the region. Overlapping expression of Otx2, En1 and Pax2 may define the mesencephalic region and additional expression of Pax3/7 may instruct the mesencephalic region to differentiate into the tectum. The di-mesencephalic boundary is determined by repressive interaction between Pax6 and En1/Pax2 and the mes-metencephalic boundary is defined by repressive interaction between Otx2 and Gbx2. Fgf8 is induced at the border of the Otx2 and Gbx2 expression domain, overlapping with Gbx2 expression.