Fig. 4. Arteriovenous malformations in Notch pathway mutant mice. (A) In a wild-type embryo, India ink is injected into the heart to visualize blood flow as it exits anteriorly through the aortic arch arteries (aa) and enters the descending dorsal aorta (da). (B) In an embryo with an endothelial cell-specific deletion of the Notch1 gene (Tek-Cre/+; Notch1^flox/-), arteriovenous malformations permit the injected ink to leak directly into the venous system. Original images provided by Luke Krebs in my laboratory; reproduced with permission from Weinmaster and Kopan (Weinmaster and Kopan, 2006).