Fig. 5. Lrig3 and Ntn1 participate in cross-repressive interactions that define the fusing and non-fusing domains of the lateral pouch. (A,E) In situ hybridization of Ntn1 on transverse sections through E12 Lrig3^+/- (A) and Lrig3^-/- (E) embryos. In Lrig3 mutants, Ntn1 expression is expanded to fill the lateral pouch (outlined). (B,F) β-galactosidase histochemistry of E12 Lrig3^+/- (B) and Lrig3^-/- (F) littermates. As previously demonstrated (Fig. 3), Lrig3-βgeo levels are reduced in fusion plate cells (arrowhead, B) compared with the surrounding epithelium. However, in Lrig3 mutants (F) reporter activity is present at high levels throughout the pouch. (C,G) Lrig3 and Ntn1 mutant mice were generated with two different gene trap vectors, so only Lrig3^LST016 mice carry a placental alkaline phosphatase (PLAP) reporter. Hence, PLAP histochemistry reveals Lrig3 transcription in Ntn1^+/+;Lrig3^+/- (B) and Ntn1^-/-; Lrig3^+/- embryos (G) at E12.5. Like β-geo, PLAP staining of Lrig3 heterozygotes (C) is absent from the fusion plate at E12.5 (arrowhead). By contrast, Lrig3 transcription is sustained in the fusion plate of age-matched Ntn1 homozygotes (G). Note that these embryos are 12 hours older than those in A,B,E,F. (D,H) β-Galactosidase histochemistry of E12 Ntn1^+/- (D) and Ntn1^-/- (H) littermates. Ntn1-βgeo is active in the fusion plate (arrowhead) in heterozygotes (D), consistent with in situ hybridization results (see Fig. 3). However, no activity is detected in the lateral pouch of Ntn1 homozygotes (H). Ntn1-βgeo expression is unchanged in the dorsal pouch (asterisks). Scale bar: 50 µm.