Development of vascular networks during the morphogenesis of intestinal villi in the fetal mouse.

The formation of villi was investigated in the proximal small intestine of fetal mice between 12 and 19 days of gestation in association with the development of vascular networks. Three-dimensional vascular networks were observed with confocal laser scanning microscopy after perfusing the fetuses with RITC-labelled gelatin. Some specimens were simultaneously stained for laminin by immunofluorescence to examine the epithelio-mesenchymal boundary. At 12 and 13 days of gestation, the epithelium of small intestine formed cylindrical tubes, and a loose plexus of large vessels formed in the mesenchyme. At 14 days of gestation, a dense plexus of smaller vessels was formed inside of the loose plexus and some capillary branches from this dense plexus developed beneath the epithelium. The epithelium began to make ingrowths in places where no capillaries were situated beneath the epithelium. The capillaries formed a loop in a concavity made between the ingrowths. As development proceeded, this concavity deepened and formed a villus. The top of the capillary loop always places at the top of the mesenchyme in the villus and the capillary loop then developed into a capillary network within the villus. These results suggest that the initial formation of intestinal villi in the fetal mouse occurs by an ingrowth of the epithelium and the developing vascular nets appear to play some role in villi formation. Throughout the villus formation, the top of the capillary loop in a villus would be anchored to the epithelial basement membrane or to the adjoining matrixes.