Fig. 2. Expression of Gdnf and Ret during mouse gut organogenesis. Wild-type (A-C), Ret.k— (D) embryos and gut preparations (E-I), were stained by whole-mount in situ hybridisation histochemistry using riboprobes specific for Gdnf (A,B,D,E,F,H) or Ret (C,G,I). In E9.0-9.5 embryos, Gdnf mRNA is detected in the splachnic mesenchyme of the stomach (indicated by arrow in all panels of the top row and by s in all panels of the bottom row) and in the branchial arches (black arrowheads in A,B). At this stage, RET-expressing ENS progenitors (white arrowhead in C) are starting to invade the GDNF-expressing region of the foregut (C; outline of the stomach is indicated by line and an arrow). (E) In the gastrointestinal tract of E9.5 embryos, expression of Gdnf is highest in the stomach region. However, in the gut of E10.5 embryos (F), the main site of Gdnf expression has shifted to the caecum (indicated by c). At this stage, the front of migrating NC cells was positioned rostrally to the high Gdnf-expressing caecum. At later stages, the domain of Gdnf expression in the hindgut extends posteriorly along with the wave of migrating NC cells (white arrows in H,I). Note the similar pattern of expression of Gdnf in RET-deficient mouse embryos (D). In addition to the gastrointestinal tract, Gdnf is also expressed in the pharyngeal pouches of the branchial arches (black arrowheads in A,B) and in a small region ventrally to the dorsal aortas (white arrowhead in A,B), the site where the progenitors of the superior cervical ganglia will first coalesce.