Fig. 8. Model for the generation of ventral and midline cell types in the telencephalon. SHH, which is expressed ventral to the developing telencephalon, is required to generate not only ventral cell types, but also rostral and dorsal midline ones. SHH is likely to be required for these processes only indirectly by antagonizing GLI3 and thus relieving repression of Fgf gene expression (see Discussion). FGFs are then necessary to generate ventral and at least some rostral midline cell types. Previous evidence indicates that GLI3 is required for expression of WNTs and BMPs and for formation of the dorsal midline, and that FGF8 regulates Bmp4 expression in a dose-dependent manner. Hence, SHH is likely to promote ventral and midline development indirectly by regulating GLI3 and FGF signalling.