Fig. 9. Differentiation of mesDA neurons requires Otx2. (A-P'') Adjacent sections through the intermediate mesencephalon of E11.5 and E12.5 wild type, En1^Cre/+; tOtx2^ov/ov and En1^Cre/+; Otx2^flox/flox embryos immunostained with Nkx6.1 and Nkx2.2 (A-B''), Mash1 and Sox2 (G-H''), Ngn2 and Sox2 (I-J''), Nurr1 and Ngn2 (K-L''), Nurr1 and Pitx3 (M-N''), Lmx1b and Otx2 (O,O''), Lmx1b and 5-HT (P,P'') or hybridized with Lmx1a (C-D'') or Msx1 (E-F'') probes. In En1^Cre/+; Otx2^flox/flox mutants, Lmx1b is dorsally expanded within the Nkx2.2⁺ domain that generates 5-HT⁺ neurons, and is abundant at E12.5 in the Sox2⁺ mesDA progenitors (O'',P''). The arrow indicates the ventral border of Nkx6.1⁺ domain in wild-type and En1^Cre/+; tOtx2^ov/ov embryos and the ventral border of the Nkx2.2⁺ domain in En1^Cre/+; Otx2^flox/flox mutants.