Vascular disorganization in Flk1 p/e–HIFdn transgenic mice. Whole-mount immunohistochemistry for PECAM-1. (A) Wild-type embryo at E10.5 with well-developed vessel system. (B) The cephalic vasculature in transgenic littermates remained a primitive plexus. (C) Higher magnification of a wild-type heart. V indicates ventricle; A, atrium. (D-E) Transgenic hearts were not properly looped but remained linear tubes. The endocardium was patchy and irregularly shaped (arrows). Magnification: 32 × (A,B), 50 × (C,D,E).