Role of medial lamellar architecture in the pathogenesis of aortic aneurysms.

The human abdominal aorta is particularly susceptible to the formation of aneurysms with atrophic walls. This aortic segment normally has fewer medial lamellar units than would be expected for a mammalian aorta of comparable diameter as well as far fewer medial vasa vasorum than would be expected for an aortic wall of comparable thickness. To test the hypothesis that ischemia and/or loss of normal lamellar architecture are predisposing factors for aneurysm formation, we used the pig thoracic aorta, which is furnished with 75 medial layers and vasa supplying the outer two thirds. Vasal blood flow was surgically ablated, and crushing injury was used to reduce the number of intact lamellar units. Mural ischemia alone resulted in necrosis of cells in the medial zone furnished by vasa but did not lead to aneurysmal dilatation, and all the fibrous tissue layers persisted during the 2-month observation period. Mechanical injury resulted in aneurysms in both ischemic and nonischemic aortic segments, but only if fewer than 40 intact lamellae remained and the average tension per lamellar unit exceeded three times the normal value of 1316 +/- 202 dynes/cm (4543 +/- 1624 for ischemic and 4087 +/- 871 for nonischemic segments; p less than 0.01 for each). We conclude that a critical reduction in the number of intact lamellar units results in aneurysmal dilatation. Protracted medial ischemia due to intimal plaque formation in the avascular abdominal aorta may eventually reduce the number of intact lamellae and favor the development of aneurysms.