Endothelial-specific TAK1 deficiency causes vessel regression and cell death at E10.5. (A) Confocal Z-stack images of whole-mount PECAM-1 and collagen IV staining of control (Tak1^flox/flox) and TAK1^ecko (Tie2-Cre Tak1^flox/flox) yolk sacs at E10.5. Arrows indicate regions positive for collagen IV and negative for PECAM-1. Photos are representative of 3 independent litters (n = 3). Scale bar indicates 40 μm. The number of vessel regions that were positive for collagen IV but negative for PECAM-1 were defined as regressing vessels and were counted in 30 randomly chosen areas (total in 2 mm²) from 3 different yolk sacs (graph). Data are shown as means ± SD (n = 3). ***P < .001. (B) TUNEL (green) and PECAM-1 (red) staining of TAK1 control (Tak1^flox/flox) and TAK1^ecko (Tie2-Cre Tak1^flox/flox) yolk sacs at E10.5. Arrows indicate TUNEL positive cells. Scale bar indicates 40 μm. The percentages of TUNEL⁺ cells in the total cell population of TAK1 control and TAK1^ecko yolk sacs are shown. Data are shown as means ± SD (Tak1^flox/flox, n = 5; Tie2-Cre Tak1^flox/flox, n = 4). ***P < .001.