Fig. 2. HH blockade prevents differentiation and promotes proliferation in the ventral midbrain. (A,B) Increased cyclin B2 (A) and cyclin D1 (B) expression following unilateral misexpression of Ptc1^loop2. (C) BrdU labeling shown in a cross-section through an EGFP-electroporated embryo, in which it is confined to proliferating cells of the ventricular layer. (D) Massive increase in BrdU labeling (blue) following Ptc1^loop2 electroporation. Note that the increased thickness of the ventricular layer is associated with a reduction of the mantle layer, where differentiated neurons normally reside (compare double-headed arrows in C and D, which were photographed at the same magnification). (E) Cross-section through the ventral midbrain of a Ptc1^loop2-electroporated embryo, showing a reduction in class III ß-tubulin expression (brown, asterisk) following HH blockade. (E, inset) Whole-mount view of the cross-section in E. (F,G) SHH (brown) overexpression results in reduced cyclin D1 (blue) expression. The same embryo is presented in F (before) and G (after) the detection of SHH. (H) Embryos bilaterally electroporated with either SHH (light embryos, upper) or Ptc1^loop2 (dark embryos, lower) at H&H stage 9. Note the reduced size of SHH-electroporated embryos compared with Ptc1^loop2-electroporated embryos. Embryos are shown in sagittal view, with rostral to the left. III, third ventricle; bi, bilateral electroporation; Di, diencephalon; EP, electroporated; HB, hindbrain; rFP, rostral floor plate.