Time course of the regression of intimal hyperplasia in experimental vein grafts

A previous study in which vein grafts were removed from the arterial circulation and reimplanted into the venous circulation of the same animal demonstrated regression of vein graft intimal hyperplasia and medial thickening within 14 days. The present study was designed to characterize the kinetics of the morphological and ultrastructural changes over this 14-day period. Twenty-one male New Zealand White rabbits received a reversed vein interposition bypass graft of the right common carotid artery. Fourteen days after the procedure, 21 vein grafts were isolated, removed, and reimplanted into the contralateral external jugular venous system as veno-venous interposition bypass grafts (reversal grafts). The grafts were harvested at 60 minutes, 1 day, 3 days, 5 days, 7 days, and 14 days after reversal. Before insertion into the venous circulation, the vein graft had a confluent endothelial cell surface with multiple layers of smooth muscle cells representing intimal hyperplasia. After 1 hour, the reversal graft retained an intact endothelial cell layer with no evidence of tissue edema or cellular disruption. By 24 hours, there were a few blood cells on the endothelial cell surface. There was no inflammatory infiltrate seen in the subendothelium, and the smooth muscle cells were unaltered. At 3 days, the endothelial cell lining remained intact with no polymorphonucleocytes in the subendothelium or within the graft wall. Underlying smooth muscle cells at this time were noted to contain cytoplasmic vacuoles. At 5 days, there were no inflammatory cells seen on the surface or within the vein graft wall, but many of the underlying smooth muscle cells within the intimal hyperplasia were noted to be fragmented and to have clumping of chromatin. After 7 days, the endothelial cells remained intact and there was widespread evidence of apoptosis beneath the subendothelium with highly fragmented smooth muscle cells, some of which were histologically in the process of breaking up. At 14 days, the grafts retained uniform endothelial cell surfaces. Most of the smooth muscle cells that composed the intimal hyperplasia seen before implantation as a reversal graft were gone. Areas of newly laid down collagen could be observed. There were no acute inflammatory cells but for some mast cells seen in the graft wall. This study demonstrates that in this model, regression of intimal hyperplasia was associated with apoptosis of the smooth muscle cells and the deposition of collagen. There was no evidence that this process is mediated by an acute inflammatory response. Regression therefore appears to be due to induction of smooth muscle cell apoptosis by either a reduction in pressure or flow or a combination of both factors. The findings will enable a systematic cellular and molecular analysis of the biology of regression, which may afford clues to better understand the biology of the developing intimal hyperplasia.