Fig. 3. Lrig3 acts in the non-fusing epithelium to prevent premature and ectopic fusion. (A-F) Dorsal views of paintfilled Lrig3^+/- (A-C) and Lrig3^-/- (D-F) inner ears collected at 6-hour intervals from E12 to E12.5. Anterior is towards the right. In mutants, early outgrowth is normal (D). However, fusion initiates too early (E) and occurs over a larger area (asterisk) than in control embryos (B,C), resulting in truncation by E12.5 (F). (G-I') Adjacent sections of Lrig3 heterozygotes through the lateral pouch at the level indicated (broken line, A) were processed for β-galactosidase histochemistry to reveal Lrig3-βgeo activity (G-I) or for in situ hybridization with a probe to Ntn1 (G'-I'). Lrig3 is transcribed throughout the lateral pouch prior to fusion (G). Levels become reduced (H) in the nascent fusion plate (arrowhead) just as Ntn1 expression initiates here (H'). Lrig3 expression is further diminished (I) as transcription of Ntn1 expands (I'). Scale bar: 50 µm.