Fig. 9. Midbrain dopaminergic neurons are ectopically expressed in the rostral hindbrain of Sp8^-/-embryos. Sagittal sections of E11.5 (A-D), E12.5 (E-H) and E17.5 (I-P) control and mutant embryos showing the expression of tyrosine hydroxylase (TH, green) and 5-hydroxytryptamine (5-HT, red) revealed by double immunohistochemistry. The expression is also shown at two different magnifications (low: A,C,E,G,I,K,M,O; high: B,D,F,H,J,L,N,P). At E11.5, although in the control embryo (A,B) 5-HT-expressing cells are found just posterior to the MHB (A), there are no such cells in the mutant (C, at higher magnification in D). At this stage of development, dopaminergic neurons (DA) were detected at ectopic positions in the rostral hindbrain of mutant embryos (C, arrowheads). In contrast to E11.5, at E12.5 in the mutant embryo, serotonergic neurons are detectable (H, arrows). However, in the most rostral domain occupied by ectopic dopaminergic neurons, 5-HT-positive cells are not present (arrowhead in H). At E17.5 serotonergic neurons, although slightly reduced in close vicinity to DA (arrows in K and L), appear similar to controls, while the population of DA is increased. There are also some ectopic DA neurons located posteriorly, where in the control only serotonergic neurons can be detected (arrowheads in L and P). These findings indicate that beside the expansion of midbrain DA in the rostral hindbrain, it seems that the onset of serotonergic neuron differentiation is at least delayed in mutant embryos.