Fig. 9. Model for IZ formation in zebrafish. One side of the neural plate is represented (gray bar, the axial midline; red, MIZ and gli1 expression domain; black, LIZ). For simplicity, Gsk3β/PKA are shown together, although they might act in parallel. The E(Spl) and Gsk3β/PKA pathways inhibit neurogenesis throughout the IZ in a dose-dependent and cooperative manner. Gli1, expressed in the MIZ, acts antagonistically and promotes neurogenesis. Gli1 activity might be partially downregulated by Gsk3β/PKA. Responding to the sum of these activities, MIZ cells are more prone to commit towards neurogenesis than are LIZ cells. Gli1Act, activated Gli1; Gli1P, phosphorylated Gli1.