In vivo expression of lacZ reporter in endoglin gene-trapped embryos. (A-D,I) endoglin was specifically expressed in endothelial cells. Note that the expression was not detected in the chorion of the placenta (D) at E9.5. (E-H) endoglin expression was also detected in the extraembryonic ectoderm, chorion, and yolk sac mesoderm. al indicates allantois; ch, chorion; da, dorsal aorta; e, endocardium; exe, extraembryonic ectoderm; ys, yolk sac. Scale bars, 50 μm. Original magnifications: × 40 (A), × 25 (C), × 400 (E), and × 100 (F,G,I).