Blood vessels of the allantois arise via vasculogenesis from TAL1⁺/Flk1⁺ cells (angioblasts).

In panel A, isolated TAL1⁺ cells populate the allantoic mesoderm at 7.5 dpc. In panel B, PECAM⁺ vascular networks are evident in the 8.3-dpc allantois. In panel C, explanted prevascular (7.5 dpc) allantoides cultured for 24 hours establish a PECAM⁺ vasculature. Panel D is a high magnification image of the vessels depicted in panel C. Magnification bars: panels A, B, and D = 50 μm; panel C = 100 μm.