(A, B) Phalloidin (A) and p75 immunostaining (B) of E18.5 whole cochlea showing hair cells (HCs) (phalloidin) and pillar cells (p75) in the cochlear duct of Fgf9−/+;Fgf20lacZ/+, Fgf9−/−, Fgf20lacZ/lacZ, Fgf9−/−;Fgf20lacZ/+, Fgf9−/+;Fgf20lacZ/lacZ and, Fgf9−/−;Fgf20lacZ/lacZ embryos. (C) Phalloidin (green) and p75 immunostaining (red) showing the orientation of HCs, pillar cells, and gaps in the sensory epithelium. (D) Prox1 (green) and Sox2 (red) co-immunostaining showing supporting cells (SCs) (yellow, Prox1 and Sox2) and undifferentiated sensory progenitors (red, Sox2). (E–G) Measurement of HC number (E), SC number (F), and length of cochleae (G) of E18.5 embryos. Scale bars, A, 500 μm; C, 100 μm. For statistical analysis, all samples were compared with Fgf9−/+;Fgf20lacZ/+ double heterozygous controls. *p < 0.001. Sample numbers (n) are indicated in data bars.