(A) Gene expression of AM, CRLR, and RAMPs in E13.5 WT and RAMP2^–/– embryos, assessed by real-time PCR of total RNA. No RAMP2 expression was detected in RAMP2^–/– mice, confirming RAMP2 was successfully destroyed. Conversely, RAMP3 expression did not differ between RAMP2^–/– and WT mice, showing that the absence of RAMP2 did not induce compensatory upregulation of RAMP3 during development. AM expression was upregulated more than 5-fold in RAMP2^–/– mice. n = 6 per group. **P < 0.01 vs. WT. (B–L) Development of blood vessels in E13.5 WT and RAMP2^–/– mice. Appearance of the yolk sac (B) and vitelline arteries (C and D). (E and F) CD31 immunostaining of sections of yolk sacs. Arrows indicate sections of vitelline arteries. (G and H) Whole-mount immunofluorescence staining of CD31 in yolk sacs. In C–H, vitelline arteries were well developed on the yolk sacs of WT mice but poorly developed on those of RAMP2^–/– mice. (I–L) TUNEL staining of sections of vitelline artery (I and J) and umbilical vessel (K and L) in E13.5 WT and RAMP2^–/– embryos. Apoptosis was visualized in green fluorescence. Arrows indicate vessel lumens. Some ECs in RAMP2^–/– mice were TUNEL positive. (M and N) Severe systemic edema observed in RAMP2^–/–. Front (M) and side (N) views of WT and RAMP2^–/– embryos at midgestation. Some RAMP2^–/– embryos showed severe systemic edema. (O–R) Pericardial effusion in RAMP2^–/– mice. (O and P) Magnified side view of embryos at midgestation revealing the appearance of the pericardial space in RAMP2^–/– embryos. (Q and R) Sagittal sections showing the pericardial space in embryos at midgestation. The pericardial space was larger in RAMP2^–/– than WT embryos and showed the accumulation of pericardial effusion. (S–U) Severe hemorrhagic changes in RAMP2^–/– mice. (S) Side view of WT and RAMP2^–/– embryos at midgestation. (T and U) Sections of the liver at the same stage. Some RAMP2^–/– embryos showed severe hemorrhagic changes that were apparent on their surface and within the liver. Scale bars: 20 μm (E and F); 50 μm (I–L, T, and U); 200 μm (Q and R).