Fig. 9. Generation of V2a and V2b INs from common progenitors in the p2 domain. Multipotent neuroepithelial (radial) progenitors (A), which do not express Foxn4, generate a population of V2a/V2b (p2) progenitors (B). All V2a/V2b progenitors express Foxn4, which induces the expression of Dll4, Gata2 and Mash1. These common progenitors also start to express Lhx3 at their final division (C). Notch1 is expressed in all p2 progenitors (Lindsell et al., 1996), so Notch1/Dll4 reciprocal cell-cell interactions are initiated (opposing arrows in C). This situation resolves into two populations of progenitors, one with activated Notch1 (Notch1^*) and the other with Dll4 (D). Notch1^* blocks the V2a fate and, in cooperation with Foxn4 and Mash1, specifies V2b IN fate (E). The complementary set of p2 progenitors (Dll4-positive) that fails to activate Notch1 adopts the V2a fate instead, possibly under the control of Lhx3 (Tanabe et al., 1998) (E). In this way, V2a and V2b INs are generated in a salt-and-pepper fashion during the same time window from a homogeneous population of p2 progenitors.