Summary. Wnt3 and Wnt3a signaling maintains the MDO primordium by protecting the cells from apoptosis (A). The mature MDO secrets organizer signals, such as Shh, which drives thalamic development (B). In Wnt3/Wn3a deficient embryos, MDO cells go into apoptosis (D). Loss of the organizer leads to stalled development of the thalamic complex (D).