Fig. 1. TBX5 is required for cardiac proliferation. Whole-mount antibody staining with tropomyosin (Tmy) of stage 37 (A) control morpholino (CMO) or (B) TBX5 morpholino (T5MO) embryos. Transverse heart sections through (C,E) stage 33 and (D,F) stage 37 embryos stained with Tmy, to mark cardiac tissue, and DAPI, to mark cell nuclei; (C,D) CMO-derived tissue; (E,F) T5MO-derived tissue. (G-L) Examples of proliferating cardiac cells in transverse heart sections from (G-I) CMO and (J-L) T5MO embryos sectioned through the cardiac region at stages 29, 33 and 37, as indicated. Proliferating cardiomyocytes are identified as those positive for Tmy (myofibrilis shown as green) and anti-phosphohistone H3 (pH3; localized to the nucleus and shown in red). (M-R) Examples of cardiac cells undergoing apoptosis in transverse heart sections from (M-O) CMO and (P-R) T5MO embryos at stages 29, 33 and 37, as indicated. Apoptotic cardiomyocytes are identified as those positive for Tmy (myofibrilis shown as green) and anti-cleaved caspase 3 (CC3; localized to the nucleus and shown in red). Quantification of results from (S) total cardiomyocyte cell numbers, (T) mitotic index, and (U) programmed cell death. In all cases, bars represent the average of at least three embryos: CMO, red bar and T5MO, blue bar. Error bars denote the standard deviation, and ^* denotes a statistically significant difference (at P<0.05) between CMO and T5MO embryos at a given stage. Results are derived from a single set of experiments, all experiments being repeated at least once with an independent batch of embryos. Scale bars: 50 µm. a, atrium; i, inflow tract; o, outflow tract; v, ventricle.