Characteristic expression of CXCR4 in developing mesenteries. (A-I) Whole-mount in situ hybridization analysis of CXCR4. Whole-mount view of the midgut loops from wild-type embryos at E11.5 (A), E12.5 (B), and E13.5 (C). CXCR4 expression is observed predominantly in SMA (A, B) and newly formed branches arising from SMA (B, C) but not in the primary capillary plexus surrounding the primitive gut (A-C). Arrows indicate SMA (A-C). (D) Analysis of the transverse sections of E11.5 midgut loop. CXCR4 is detected in endothelial cells of SMA (arrow) but not in those of SMV (arrowhead). (E-H) Analysis of the transverse sections of the midgut loop from wild-type and CXCL12^–/– embryos at E10.5 (E, F) and E11.5 (G, H). (E, F) CXCR4 expression is observed in SMA from wild-type and CXCL12^–/– embryos at E10.5. (G, H) Note the severe reduction of CXCR4 expression in CXCL12^–/– embryos at E11.5. (I) Whole-mount view of the lung from wild-type embryos at E12.5. (J, K) Whole-mount immunohistochemical analysis of the lungs from wild-type (J) and CXCL12^–/– (K) embryos at E12.5 using pan-endoth elial marker PECAM-1. CXCR4 expression is not observed in blood vessels of the lung (I, J) and vascular defects are not observed in the mutant lung (K).