Fig. 1. Domains of expression of transcription factors define ventral (V), intermediate (I) and dorsal (D) subdivisions of the developing telencephalon. (A) A schematic drawing of a HH stage 22 (E3.5) chick embryo. The dorsal (D) to ventral (V) line indicates the level of the transverse sections shown in the corresponding panel. (B) In the telencephalon of a HH stage 22 chick embryo, MEIS2 was expressed at high levels in the intermediate region. PAX6 was expressed in the dorsal region and in the most dorsal domain of the intermediate region of the telencephalon. NKX2.1 was expressed exclusively in the ventral region and EMX1 exclusively in the dorsal region of the telencephalon (top panel). Below, enlargements depict overlapping (MEIS2/PAX6, PAX6/EMX1) and largely non-overlapping (MEIS2/NKX2.1, MEIS2/EMX1) regions of the telencephalon. Yellow staining represents double-labelled cells. (C) The expression domains of transcription factors in the developing telencephalon.