Microvascular circulation of the descending colon in horses

The microvascular circulation of the descending colon was studied in 5 adult horses, using microangiography and light microscopy combined with gross studies and scanning electron microscopy of vascular replicas. After heparinization, horses were euthanatized, and 3 segments of the descending colon and its mesentery containing 1 vascular arcade were removed from each horse. The fecal bars were gently massaged from the lumen, and the blood was flushed free of the circulation with isotonic NaCl. In 5 segments, the vascular system was injected with a modified radiopaque medium and evaluated radiographically. Specimens examined radiographically also were prepared for histologic examination, using standard methods. Ten segments were injected with 1 of 2 types of plastics and studied grossly or by scanning electron microscopy. Arcuate arteries gave rise to a descending colonic rete that surrounded the vein and supplied numerous descending colonic lymph nodes. The rete also supplied the mesocolon and the descending colonic tissue. Short filamentous vessels arising from the rete directly penetrated the mesenteric tenia to supply an intermuscular plexus between the longitudinal and circular muscle layers of the muscularis externa. Larger vessels arising from either side of the rete divided into the long- and short-terminal arteries that supplied an extensive submucosal plexus, which was continuous around the circumference. The submucosal plexus supplied the mucosa, the tunica muscularis, and the serosa. Vessels running centrifugally from the submucosal plexus formed an intermuscular plexus between the longitudinal and circular muscle layers of the muscularis externa. The intermuscular plexus at the mesenteric angle also was supplied by vessels branching from the short-terminal arteries as they penetrated the muscularis externa. At the antimesenteric tenia, the submucosal plexus gave rise to larger vessels that formed a subserosal loop. From this loop, 5 vessels penetrated the longitudinal muscle layer to contribute to the intermuscular plexus. Vessels within the longitudinal and circular muscles of the muscularis externa ran parallel to the muscle fibers and, consequently, perpendicular to each other. Arteries supplying the mucosa penetrated the muscularis mucosa and branched into a capillary network at the base of the descending colonic glands. These capillary networks anastomosed with the networks around adjacent glands at the luminal surface, forming a honeycomb-like pattern. Drainage was facilitated by more sparsely distributed venules that united with venules from adjacent areas and descended to the submucosal plexus. These veins were characterized by regular, helical, smooth muscle constrictions.