Fig. 8. Homeogenetic induction of Shh expression in the floor plate. (A) Schematic of the mouse node depicting the segregation of gene expression patterns in the dorsal (blue) and ventral (red) layers (DL and VL, respectively). Whereas Shh and Sfpe2 reporter activity are restricted to the ventral layer, the Shh target genes Ptc and Foxa2 are expressed in both the DL and VL. (B) Schematic of the notochord and floor plate showing the transcriptional cascade resulting in Shh expression. In the notochord, Foxa2 activates Shh, which then signals to the overlying neural plate. For simplicity, only transcriptional components of the Shh pathway resulting in its expression in the floor plate is shown. In response to Shh signaling, Gli2 activates Foxa2 expression in the neural plate. Foxa2, in cooperation with a homeodomain (HD) protein and at least one other transcriptional activator (X) bind Shh regulatory elements (Sbe1, gray oval and Sfpe2, yellow oval) to stimulate Shh transcription in the floor plate.