The ultrastructure of the anastomosed vein of experimental arteriovenous fistulae in sheep.

The ultrastructural changes in the anastomosed veins of chronic experimental carotid-jugular arteriovenous fistulae in sheep have been contrasted with the changes in a) the veins of sheep subjected to phlebotomy from the contralateral side and b) the veins of sheep without arteriovenous fistulae. The anastomosed veins exhibited a profound phlebosclerosis which was in marked contrast with the mild degree observable in the contralateral veins of the experimental sheep and with the minimal alterations in control animals. There was progressive and extensive loss of elastic tissue, as well as thickening and proliferation of basement membranes, separation of basement membranes from both the endothelium and smooth muscle cells, and the accumulation of much amorphous material in the interstitium. Collagen was relatively sparse. Some endothelial gaps and denudation were observed and associated with the presence of fibrin and red cells in the intima. Muscle cells exhibited necrosis and degeneration with much cell debris and some extracellular lipid. The results indicate that hemodynamic stress is capable of inducing, in the vessel wall, profound cellular and connective tissue changes.