CXCL12 is essential for the formation of interconnecting vessels between SMA and the primary capillary plexus surrounding the primitive gut but not the venous network. Whole-mount immunohistochemical analysis of midgut loops from wild-type or CXCL12^–/– embryos using pan-endothelial marker PECAM-1. Images were collected by standard confocal microscopy and used to build the 3-dimensional projection shown here. (A) The interconnecting vessels between SMA and the primary capillary plexus surrounding the primitive gut are almost absent in E12.5 CXCL12^–/– embryos. (B) A collateral (arrow) arises from SMA around the apex of the midgut loop in the mutants. (C, D) There is no significant difference in SMV (arrowheads) and the venous network arising from SMV between E12.5 wild-type (C) and CXCL12^–/– (D) embryos. Bars, 100 μm.