Vascularization of the small intestine occurs through organ-specific mechanisms. (A) Schematic representation of the small intestine and its vasculature at E13.5. The small intestine develops as a simple tube, called the midgut loop, during ontogeny. (B-G) Whole-mount immunohistochemical analysis of midgut loops from wild-type embryos using pan-endothelial marker PECAM-1 (B-F) or PECAM-1 (green) and smooth muscle α-actin (red) (G). Images were collected by standard confocal microscopy (B-G) and used to build the 3-dimensional projection shown in B-F. (B) At E11.5, SMA runs along the primary capillary plexus surrounding the primitive gut. (C) Proximal part of mesentery and midgut loop boxed in B. Many short interconnecting vessels form between SMA and the neighboring capillary plexus. (D-G) Proximal parts of mesenteries and midgut loops at E12.0 (D), E12.5 (E, G), and E13.5 (F). The interconnecting vessels between SMA and the neighboring capillary plexus have elongated at E12.0, E12.5, and E13.5 (D-F). Each interconnecting vessel runs along the vein at E13.5 (F). (G) An optical slice is shown. Arteriogenesis has been initiated in the interconnecting vessels at E12.5. Bars: B, 500 μm; C-G, 100 μm.