Fig. 9. Role of Lmx1b and Wnt1 in isthmus organizing activity. At 10-somite stage, Lmx1b and Wnt1 (blue) are expressed in the mesencephalon and metencephalon (A). Their expression overlaps with Fgf8 (red) expression around the isthmic region. Lmx1b represses Fgf8 cell-autonomously; however, it induced Fgf8 expression non cell-autonomously in adjacent cells. In cell-autonomous repression of Fgf8 by Lmx1b, Grg4 may intervene. In non cell-autonomous induction of Fgf8 by Lmx1b, Wnt1 may be involved. Fgf8 could induce Lmx1b. Gbx2 and Fgf8 induces each other's expression, and Gbx2 repressed Lmx1b expression. Otx2 and Gbx2 repress each other's expression. As a result of this complicated gene expression cascade, Fgf8 expression may be set and kept in the isthmic region just rostral to the Lmx1b and Wnt1 expression ring by E2.5 (B).

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