Fig. 2. Telencephalic progenitor cells gradually acquire their intermediate character. (A) A stage 12 and a stage 14 chick embryo (side view, rostral towards the right). Black squares indicate intermediate neuroectoderm explant regions of the embryo. Explants were analysed for expression of transcription factors after 40-48 hours in culture. (B) Stage 12 intermediate explants (n=15) generated MEIS2⁺ (14±7%), PAX6⁺ (22±7%), NKX2.1⁺ (7±6%) and EMX1⁺ (8±6%) cells in different domains. Scale bar: 30 µm. (C-F) Expression of transcription factors in stage 14 intermediate explants grown alone, with purified SHH protein, with mFrz8CRD (an antagonist of WNT signalling) or with conditioned medium containing WNT3A (n=15 for each). (C) Stage 14 intermediate explants (n=25) grown alone generated MEIS2⁺ cells (22±6%), PAX6⁺ cells (23±4%) but no NKX2.1⁺ or EMX1⁺ cells were detected. (D) In the presence of SHH, the generation of MEIS2⁺ cells was blocked, and NKX2.1⁺ (65±5%) but no PAX6⁺ or EMX1⁺ cells were detected. (E) In the presence of WNT3A, the generation of MEIS2⁺ cells was blocked, and PAX6⁺ cells (61±3%) and EMX1⁺ (25±5%) but no NKX2.1⁺ or cells were detected. (F) In the presence of mFrz8CRD, MEIS2⁺ cells (20±5%) were still generated, while the generation of PAX6⁺ cells was blocked. No NKX2.1⁺ or EMX1⁺ cells were detected. Scale bar: 30 µm.