Fig. 2. Vascular defects in a Notch1-null mouse embryo. (A) Extraembryonic yolk sac from a wild-type mouse embryo exhibits remodeling of the yolk sac vasculature to generate vessels of different sizes. (B) Yolk sac from a Notch1^-/-mutant mouse embryo. The yolk sac has arrested at the primitive vascular plexus stage, and has not undergone any angiogenic vascular remodeling. Both yolk sacs have been immunostained with an antibody to a protein expressed on vascular endothelial cells. Reproduced with permission from Krebs et al. (Krebs et al., 2000). Copyright (2000) Cold Spring Harbor Laboratory Press.