Fig. 9. Schematic showing the migration of vagal NC-derived cells (green dots and shading) and quail NCC (red dots and shading) from the neural tube towards and along the gut. (A) In normal embryos NCC migrate from somites 1-7, `fill' the precursor pool, and thus have sufficient population pressure to colonise the entire length of the gut. (B) In 3-6 ablated embryos, the numbers of NCC in the precursor pool are reduced, such that there is insufficient population pressure to advance their migration into and along the gut. (C) Addition of the quail neural tube from somite 3 to 3-6 ablated embryos provides the critical number of cells to the precursor pool to allow complete colonisation of the gut. (D,E) 3-6 ablated + Q sacral (D) and 3-6 ablated + Q trunk (E) show that the sacral and trunk NCC can contribute sufficient cells to the precursor pool to enable full colonisation of the gut. The sacral NCC maintain their intrinsic identity and only contribute to the ENS in the proximal gut. Trunk NCC do not contribute any cells to the ENS, thus their role is restricted to providing critical numbers to the precursor pool to allow the remaining host vagal NCC to colonise the gut. (F) Grafting of somite 3 neural tube into the position of the third somite in 1-7 ablated embryos results in complete colonisation of the gut since these cells, which are more proliferative than their rostral neighbours, are placed directly into the optimal pathway to the gut.