Restitution of aortic wall after sustained necrotizing transmural ligation injury. Role of blood cells and artery cells.

Partial ligation of the rabbit abdominal aorta with fine silk suture for 48 hours produced a circular band of transmural necrosis. On release of the ligature, blood cells from the lumen and from adventitial vasa vasorum, as well as cells derived by mitosis from the adjacent surviving endothelium and media, participated in the restitution of a continuous endothelial lining and an intact media containing well-differentiated smooth muscle cells within normal medial lamellar units. Initial deposition of a layer of blood platelets on the fibrillar material coating the denuded lumenal surface was followed by ingress from the lumen of polymorphonuclear granulocytes and mononuclear cells. These changes preceded the appearance of mitoses in surviving endothelial and medial smooth muscle cells at the margin of injury. By 24 hours, poorly differentiated cells had accumulated in the central portion of the intima and inner media. Similar cells formed a more extensive, nearly complete lumenal layer which was eventually continuous with and indistinguishable from the adjacent uninjured endothelium. By 7 days, smooth muscle cells repopulated the media, and a collection of less differentiated cells persisted between the restored endothelium and media. By 28 days, the only deviation from normal arterial structure was the persistence at the point of ligature of intimal thickening, consisting of smooth muscle cells and collagen and elastin fibers. Though still present at 6 weeks, this zone became increasingly compact and layered. There was no evidence that fibrin thrombus formation was a consistent feature of the initial reaction or that it played a role in the healing process or in the formation of the intimal lesion. Despite complete circumferential necrosis at the site of ligature, there was no evidence of medial rupture or intramural hemorrhage.