Fibronectin stimulates formation of a vascular plexus in the Snai1-cko allantois explants. (a, b) PECAM-1 immunostained allantois explants derived from E8.5 littermate control (a) and Snai1-cko (b) allantoises grown on fibronectin. Although morphologically abnormal, formation of a vascular network can be observed in the Snai1-cko explant. Snai1-cko explants also exhibited PECAM-1 positive clusters of cells that did not adhere to the plate but grew up into the culture media. (c, d) Confocal imaging of DAPI (blue) and PECAM-1 (green) immunofluorescent allantois explants from E8.5 control (c) and Snai1-cko (d) allantoises. The Snai1-cko explant exhibits an intense aggregation of endothelial cells in one region of the explant, and the absence of connections between endothelial cells in other regions. Scale bar in (a, b): 100 μm; (c, d) 50 μm.