Fig. 4. Yolk sac angiogenesis is defective in conditional ephrinB2 knockout embryos. (A-F) Restriction of ephrinB2 mRNA expression to endothelial cells of the E9 yolk sac is revealed by in situ hybridization with ephrinB2 (A-C) and Flk1 (an endothelial-specific marker, D-F) RNA probes on sections of ephrinB2^lacZ/+ control (A,D) ephrinB2^lacZ/lacZ (B,E) and conditional ephrinB2 knockout (C,F) yolk sacs at E9. EphrinB2 mRNA in controls is expressed in Flk1-positive endothelial cells (A versus D), and is lost in both conventional (B) and conditional (C) ephrinB2 mutants. (G-I) Identical defects in yolk sac vessel remodeling are seen in ephrinB2^lacZ/lacZ (H) and conditional ephrinB2 (I) knockout embryos compared with ephrinB2^lacZ/+ controls (G) are revealed by whole-mount anti-PECAM1 staining of E9.5 yolk sacs. The large (arrow) and small (arrowheads) branches in control yolk sacs (G) are instead a plexus of equally sized capillaries (arrowheads) in both mutants (H,I).

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