Fig. 2. Defective vasculogenesis in the yolk sacs of Tie2cre;Tgfbr2^loxp/loxp;R26R embryos. (A,B) Embryos with yolk sacs were isolated at E10.5 (A) and E11.5 (B). The Tie2cre;Tgfbr2^loxp/loxp;R26R embryos were arrested at around the 20-25 somite stage. No blood vessels were visible in their yolk sacs. (C) Embryos from A stained with X-gal to detect endothelial cells. In contrast to the well-formed blood vessel nets in the control embryo yolk sac, formation of blood vessels in the mutant was initiated only in the region close to the allantois (arrow). (D,E) Mutant embryo (E) that escaped global vascular compromise and survived to E12.5 without growth retardation or obvious yolk sac vascular abnormalities. cko, Tie2cre;Tgfbr2^loxp/loxp;R26R embryos; ctrl, Tie2cre;Tgfbr2^loxp/+;R26R embryos.