Fig. 3. Ptc1^loop2 affects cell-fate specification in a radial manner. (A) `Haloes' of PHOX2A-negative/Ptc1<sup>loop2</sup>-negative (uncolored) cells surround Ptc1<sup>loop2</sup>+/PHOX2A-negative (blue) cells. (B) Cross-section through whole-mount shown in A, demonstrating that the `haloes' are columns of PHOX2A-negative/Ptc1^loop2-negative cells radially associated with more-pially located Ptc1^loop2+ cells. (C) Ptc1^loop2-electroporated embryos at E4 display midbrain columns in cross-section. (C, inset) Magnified view of a single column of cells (indicated by arrowhead). Individual cells are marked by asterisks. (D) Cross-section through an E4 embryo electroporated with low concentrations of EGFP, displaying bipolar-radial glia-like midbrain progenitors. Note that, when multiple cells are present in a single column, they are cytoplasmically continuous (arrowhead). (E) Close-up of boxed area in D, highlighting the radial glial-like morphology of the midbrain progenitors, including the presence of end-feet at the ventricular surface. (F) Demonstration of dye-coupling through gap junctions among three ventral midbrain cells following the injection of Alexa-Fluor 488 into the central cell (^*). H&H stage 10 explant presented in whole-mount view (rostral is to the top and ventricular surface faces the viewer; orientation is the same as in Fig. 1A). Each cell is approximately 7.5 µm across and the cells are spaced approximately 5 µm apart. The central cell is ventricular with respect to the other two cells. H&H, embryonic stages according to Hamburger and Hamilton (Hamburger and Hamilton, 1951); rFP, rostral floor plate.