The timing of dopaminergic cell losses in the PG from bdnf ^−/−and gdnf^−/− mutant mice overlap. The number of dopaminergic (TH⁺) neurons in the PG (top panel) and NG (bottom panel) from wild-type (+/+, filled bars) and homozygous (−/−, open bars) mice on E14.5, E15.5, and E17.5 were counted after TH immunostaining. At each developmental age, TH⁺ cell numbers in wild-type and knock-out animals were compared separately within each genotype using Student'st test (α = 0.05). Significant differences between wild-type and homozygotes were first detected in the PG on E14.5 for bdnf^−/− mice and on E15.5 for gdnf ^−/−animals (top panel), whereas no differences in TH⁺ cell number were detected in the NG between wild-type and gdnf^−/− mutant mice at any of the developmental ages tested (bottom panel). Data are presented as means ± SEM. Sample sizes ranged from five to eight. *p < 0.05; **p < 0.01.