Vascular morphogenesis in Snai1-cko allantois and allantois explants plated on collagen. (a, b) Histological sections of PECAM-1 immunostained E8.5 littermate control (a) and Snai1-cko (b) allantois. Vascular endothelial cells are present in both control and Snai1-cko allantois in vivo. (c, d) PECAM-1 immunostained allantois explants derived from E8.5 control (c) and Snai1-cko (d) allantoises, grown on collagen. Snai1-cko explants are less expanded than control littermate explants, and instead of the interconnected vascular network that can be seen in the littermate control explant, clusters of PECAM-1 positive cells that do not form a network are present in the Snai1-cko explant. Scale bar in (a, b): 20 μm; (c, d) 100 μm.