CXCR4 expressed in endothelial cells is required for vascularization of the gastrointestinal tracts. Whole-mount PECAM-1 staining of E12.5 midgut loops (A, B) and E18.5 stomachs (C, D). (A, B) Images were collected by standard confocal microscopy and used to build the 3-dimensional projection shown here. The interconnecting vessels between SMA and the primary capillary plexus surrounding the primitive gut seen in CXCR4^flox/+/Tie2-Cre embryos (A) are absent in CXCR4^flox/–/Tie2-Cre embryos (B). (C, D) Whole-mount view of the stomach. The large vessels distributed to the surface of the stomach seen in CXCR4^flox/+/Tie2-Cre embryos (C) are absent in CXCR4^flox/–/Tie2-Cre embryos (D).