Fig. 8. Model of the initial dorsoventral patterning of the telencephalon. Proposed signalling events in neural cells during the initial specification of telencephalic cells of ventral, intermediate and dorsal character. At gastrula stages, most or all prospective telencephalic cells become specified as ventral NKX2.1⁺ cells in response to node-derived SHH signals (Gunhaga et al., 2000). WNT signals derived from adjacent dorsal ectoderm induce early dorsal PAX6⁺ cells (Gunhaga et al., 2003). RA promotes the generation of intermediate MEIS2⁺ and MEIS2⁺/PAX6⁺ cells. FGF8 derived from the anterior neural ridge (Crossley et al., 2001) maintains ventral character by opposing RA signalling in ventral cells, and FGF signals derived from the dorsal midline region induce definitive dorsal/precortical PAX6⁺/EMX1⁺ cells (Gunhaga et al., 2003). The specification of the most dorsal midline cells of the telencephalon appears to also require BMP signalling (Hebert et al., 2002), but is not indicated in the model.