Fig. 7. A gradient of apoptosis in Math1^–/– organ of Corti parallels the normal base-to-apex gradient of hair cell differentiation. Sections were prepared from wild type [A-F, (WT)] and Math1^–/– [G-L (KO)] littermates at E15.5 (A-C,G-I) and E17.5 (D-F,J-L). Apical regions (A,D,G,J) of the developing organ of Corti were compared with basal regions (B,C,E,F,H,I,K,L) in order to observe the gradient of differentiation. All sections were stained with antibody to p27^Kip1 to reveal the sensory epithelium (green). In addition, sections were double-labeled with antibody to either Activated Caspase 3 (ActCasp3) (red; A,B,D,E,G,H,J,K) to reveal apoptotic cells, or MyosinVIIa to reveal the state of hair cell differentiation (red; C,F,I,L). Brackets indicate the extent of the p27^Kip1-containing region, arrowheads indicate inner hair cells and arrows indicate outer hair cells. Scale bar: 50 µm.