ANEURYSM INDUCED BY PERIARTERIAL APPLICATION OF ELASTASE HEALS SPONTANEOUSLY

Background. The pathogenesis of aortic aneurysms remains largely unkno wn, despite aneurysmal rupture being an increasingly common catastroph e. Methods. This study was designed to elucidate the mechanism of arte rial dilatation histologically and electron microscopically, utilising a new animal model. Elastase, 3.0 mg/ml, was applied to the abdominal aortae of 18 New Zealand white rabbits from the adventitia side for 3 hours. The rabbits were sacrificed at 0, 3, 14, 28, 42 and 90 days af ter the procedure (n=4, 3, 2, 2, 2, 3). Results. Two rabbits were foun d to have developed aortic rapture. On day 0, elastase application ind uced fusiform aneurysms up to 1.62+/-0.14 times the pre-elastase appli cation aortic diameter. Dilated walls revealed medial elastolysis, deg eneration of smooth muscle cells (SMCs) and damaged endothelial cells. By day 3, the smooth muscle cells had changed to the synthetic type. Aneurysms did not progress, and after 42 days, showed gradual shrinkag e. By day 90, aortic diameters had nearly normalised. Conclusions. Aor tic walls also returned to the pre-elastase application thickness and some mature medial elastic lamellae showed regeneration. Medial smooth muscle cells reverted to the contractile type. Aortic dilatation indu ced by peri-aortic application of elastase heals spontaneously, accomp anied by regeneration of smooth muscle cells. Irreversible degeneratio n of medial smooth muscle cells appears to be more critical to aneurys m formation than degeneration of elastic lamellae.