Fig. 8. Model depicting the Nolz1^−/− mutant phenotype.

a. A subset of DA axons crosses the ventral midline of the hypothalamus, while remaining axon bundles fail to innervate the striatum in Nolz1^−/− mutant embryos. b Striatopallidal projection neurons are born at the expense of striatonigral neurons in Nolz1^−/− mutant striatum. The striatonigral–pallidal lineage switch causes (c) reduced striatal axonal outgrowth and (d) altered composition of growth factors secreted from the striatum in the mutant embryos generating a repulsive environment for DA axons. The loss of attraction (d) causes DA axon guidance phenotype as described in (a), while reduced axonal outgrowth alone has no impact on DA axons.