Hey2 functions in parallel with Hes5 and Hes1 in regulating outer hair cell and inner hair cell patterning, respectively. Cochlear whole mounts were prepared from wild-type (A-C), Hey2-/- (D-F), Hes5-/- (G-I), Hes5-/- ;Hey2-/- (J-L), Hes1-/- (M-O), and Hes1-/-;Hey2+/- (P-R) embryos at E18.5, and stained for a hair cell marker, Myosin6. Images from the apical (A, D, G, J, M, P), middle (B,E,H,K, N, Q), and basal (C,F,I,L, O, R) regions of the cochlea along the longitudinal axis were included. Arrowheads indicate the region that separates the inner from outer hair cells. Extra rows of outer hair cells (D,E,G-L, N, P, indicated by brackets) and inner hair cell doublet (E, H, M-R, indicated by asterisks) were seen in mutant samples in comparison to the wild-type control (A-C). Scale bar: 10 μm.