Fig. 1. Expression of TGFß2/Smad3 in the developing chick spinal cord. (A) At stage HH13, ActR-II is expressed throughout the DV axis. (B) TGFß2 is highly expressed in the notochord and ventral mesoderm at HH13, and weakly in the floor plate. (C) At stage HH18, TGFß2 is highly expressed in the notochord and the floor plate. (D) Expression of Smad3 in the opened neural plate of a stage 6 chick embryo. (E) Smad3 is expressed at rostral levels of the neural tube by stage 13, along the DV axis, excluding dorsal and ventral midline cells. (F,G) Smad3 expression at stage 18 predominates in two stripes encompassing dorsal and intermediate domains of the VZ. (F) A third ventral stripe is detected at brachial levels. (H) Summary of the pattern of Smad3 expression along the DV axis of the stage 18 chick neural tube. Panels on the right indicate progenitor domains. (I,J) From stage HH18-19, Smad3 is absent from Olig2-expressing pMN progenitors (I) and from postmitotic Isl1+ MNs (J). (K) Smad3 is co-expressed with Nkx2.2 within p3 progenitor domain. (L) Smad3 is absent from Foxa2-expressing floor plate cells. (M) Double-labelling Smad3 with anti-Lhx1/5 assigns Smad3-expressing domains to V1-dI6, dI4 and dI1-2. (N) Double-labelling Smad3 with anti-Pax2 assigns Smad3 expressing domains to V1-dI6 and dI4. (O) At stage 25, Smad3 expression is high at the lateral edge of the VZ at most levels of the DV axis except in the dorsal domain. Virtually all Smad3+ cells at the edge of the VZ co-express Pax2.