Wnt Signal Specifies the Intrathalamic Limit and Its Organizer Properties by Regulating Shh Induction in the Alar Plate

The structural complexity of the brain depends on precise molecular and cellular regulatory mechanisms orchestrated by regional morphogenetic organizers. The thalamic organizer is the zona limitans intrathalamica (ZLI), a transverse linear neuroepithelial domain in the alar plate of the diencephalon. Because of its production of Sonic hedgehog, ZLI acts as a morphogenetic signaling center. Shh is expressed early on in the prosencephalic basal plate and is then gradually activated dorsally within the ZLI. The anteroposterior positioning and the mechanism inducing Shh expression in ZLI cells are still partly unknown, being a subject of controversial interpretations. For instance, separate experimental results have suggested that juxtaposition of prechordal (rostral) and epichordal (caudal) neuroepithelium, anteroposterior encroachment of alar lunatic fringe (L-fng) expression, and/or basal Shh signaling is required for ZLI specification. Here we investigated a key role of Wnt signaling in the molecular regulation of ZLI positioning and Shh expression, using experimental embryology in ovo in the chick. Early Wnt expression in the ZLI regulates Gli3 and L-fng to generate a permissive territory in which Shh is progressively induced by planar signals of the basal plate.