Fig. 1. GDNF expression is restricted to the distal gut after colonization of the entire gut by ENS progenitors. (A) GDNF expression in gut at E15.5. The gut isolated from GDNF^lacZ/+ mouse embryos was stained with X-gal histochemistry. Intense X-gal staining was observed in the cecum and colon. (B) Wholemount preparations of the small intestine (upper panels) and colon (lower panels) of E15.5 GFR1^GFP/+ embryos (obtained by crossing GFR1^flox/+ mice to ß-actin Cre mice: see Materials and methods). GFR1-expression (as revealed by GFP) was confirmed in almost all PGP9.5⁺ neurons in the colon, whereas only a small neuronal population expressed GFR1 in the small intestine. (C) Quantitative analysis of GFR1-expressing neuronal population. Percentage of GFP⁺-PGP9.5⁺ neurons / total PGP9.5⁺ neurons is presented. Scale bar: 10 µm in B. Co, colon; SI, small intestine.