Fig. 3. RA signalling is required for the specification of intermediate cells. (A) Expression of Raldh3 in the head ectoderm adjacent to the rostral forebrain in a stage 14 chick embryo (lateral view, rostral towards the right). The dorsal-to-ventral line indicates the level of the transversal section shown below. Boxed area indicates intermediate telencephalic region of the embryo. (B) Expression of transcription factors in stage 14 intermediate explants grown alone or with BMS189453 (an antagonist of retinoid signalling via receptors RAR/) (n=25). Stage 14 intermediate explants (n=25) grown alone generated MEIS2⁺ cells (22±6%) and PAX6⁺ cells (23±4%), but no NKX2.1⁺ or EMX1⁺ cells were detected. In the presence of BMS453, the generation of MEIS2⁺ cells was blocked and the generation of PAX6⁺ cells was markedly reduced (7±3%). A small number of NKX2.1⁺ cells (5±3%) were generated, but no EMX1⁺ cells were detected. Scale bar: 30 µm.