Fig. 8. Organization of the MHB region in the presence (wt) and absence (ace) of functional Fgf8 signaling. In the presence of the functional organizer signal Fgf8 (red block) the MHB region is correctly patterned, and the mid- and hindbrain regions are separated by the isthmic constriction (dotted line). The mesencephalic tectum (Mes) is polarized, as revealed by the graded expression of various markers (gradient green). In the ace mutant, where functional Fgf8 is absent, the isthmic indentation fails to form, the tectum expands caudally at the expense of the metencephalon (Met), and it loses its polarized character, showing gene expression profiles characteristic of the anterior tectum in wild-type embryos (gray block).