Intramural blood vessels of the large intestine of the horse (Equus przewalskii f. caballus)]

The vascular system of the large intestine of 12 horses was examined by means of vascular corrosion casts, histology and transmission electron microscopy providing the following results. The Aa. et Vv. breves et longae leave the mesenteric vessels, respectively the subserously on the teniae lying cecal vessels to reach the tela subserosa at the mesenteric margin. The short vessels enter the deeper layers of the wall instantly, whereas the Aa. et Vv. longae move towards the submucosa by penetrating the muscular layers after a variable subserous course. The tela submucosa contains an arterial and a venous vascular plexus. In broader areas of the submucosa a three-dimensional vascular network can be found. This consists of a deep and a superficial vascular plexus, which are closely interconnected. The deep plexus is applied to the inner circular muscles, whereas the superficial plexus is adjacent to the muscularis mucosae. The (deep) arterial plexus receives its afflux from the Aa. breves et longae and supplies parts of the circular muscle layer with recurrent muscle branches. The vascularisation of the mucosa also originates from the submucosal (superficial) plexus. In the basal tunica mucosa, the ascending arteries form a transversal network from which arterioles branch into periglandular capillaries around each Lieberkühn crypt. Close to the lumen, a polygonal subepithelial capillary system is formed. The capillaries turn into postcapillary venules immediately below the epithelium of the mucosal surface. Veins move vertically through the submucosa to enter the submucosal plexus after few inflowing side branches. Branches of the subserous-submucosal connections form an intermuscular plexus between the circular and longitudinal muscular layer. This plexus supplies the capillaries of the tunica muscularis. The subepithelial capillaries are predominantly lined with a fenestrated endothelium, whereas the capillaries of the pericryptal mucosa mainly show a continuous endothelial lining. The latter contain multiple vesicles, which may fuse in order to form transcytoplasmic channels. Sphincter-like muscle bundles at the transition points from capillaries to venules may provide hemodynamic regulatory structures in the submucosa of the horse. Veins with circumferential cushions of smooth muscle fibres, so-called 'throttle veins', are also found.