Fig. 6. A model for effects of Patched (Ptc) on neural patterning. (A; left half) Shh emanating from the notochord (N) induces formation of the floor plate (FP), and subsequent shh expression in the FP generates a ventral-dorsal activity gradient of Shh (as indicated by the density of the blue dots). (B; left half) The activity gradient of Shh promotes the specification of a series of ventral cell types: p0, p1, p2, pMN and p3, which are progenitor domains from which distinct V0 neurons, V1 neurons, V2 neurons, motoneurons and V3 neurons are generated respectively. Production of a mutated form of the Shh receptor Ptc (Ptc1^loop2; A; right half; light green), which does not bind Shh but antagonizes its signaling, causes cell-autonomous abnormal dorsal spread of Shh and (B; right half) ventral-todorsal switches in neural progenitor identity. Modified with permission from Briscoe et al. (Briscoe et al., 2001).