Fig. 7. Apoptosis and proliferation in Tbx20^–/– hearts. (A-D) TUNEL assay on transverse sections of E8.5 (A,B) and E9.5 (C,D) embryos does not detect a difference in apoptosis between hearts of Tbx20 mutant embryos (B,D) and wild-type littermates (A,C). However, note the increase in apoptosis in extracardiac tissues such as the neural tube (nt) in the Tbx20 mutant. White boxes highlight the heart regions. (E,F) Analysis of cell proliferation in transverse sections of the (ventricular) heart region of wild-type (E) and Tbx20 mutant (F) embryos at E8.75, by BrdU incorporation assay. A black box indicates the mutant heart in F. (G) Quantification of cell proliferation by the ratio of BrdU-positive cells to total cell number, the BrdU-labeling index, in the analyzed heart area. The labeling index is significantly reduced in the mutant heart (0.0298±0.0033) when compared with the wild type (0.1325±0.0089). avc, atrioventricular canal; lv, left ventricle; rv, right ventricle.