Study of novel threadlike structures on the intestinal fascia of dogs.

The primo-vascular system was visualized in the mesentery surrounding the small intestine of a dog using Trypan blue. This structure, which was first observed in a rat, formed a network as primo-vessel branches were joined to primo-nodes. Other characteristic features of the primo-vascular system, such as bundles of tubes with fibrous extracellular matrix in a primo-vessel and a broken-line alignment of rod-shaped nuclei along the primo-vessel, were observed. Blood vessels, lymph vessels, and primo-vessels were present in the same mesentery, and they could clearly be distinguished by histological differences.