In situ hybridization of S1PR1 mRNA expression in E10.5 embryos. (A) Smad2fl/fl;Smad3+/−;Tie2-Cre embryo. (B) EC-Smad2/3 KO embryo. DA indicates dorsal aorta; H, heart; and ISV, intrasomitic vessel. Arrowheads indicate the representative ISVs expressing S1PR1 mRNA.