CXCL12 is essential for filopodial extension and intussusception from endothelial cells of SMA. Whole-mount immunohistochemical analysis of midgut loops from wild-type (A, B) or CXCL12^–/– (C) embryos. Images were collected by standard confocal microscopy and used to build the 3-dimensional projection shown here. (A, B) Higher magnification of PECAM-1–stained E11.5 wild-type embryos demonstrates filopodial extension (A-B, arrowheads) and intussusception (B, arrow) from endothelial cells of SMA. The filopodial processes that arise from the endothelial cells of SMA connect to the primary capillary plexus surrounding the primitive gut. (C) Higher magnification of PECAM-1–stained E11.5 CXCL12^–/– embryos reveals that filopodial extension and intussusception from endothelial cells of SMA are undetectable in CXCL12^–/– embryos. Bars, 20 μm.