Backgroud Thoracic aortic aneurysm (TAA) disease is a serious condition with both high mortality and morbidity rates associated with surgical therapy. Further understanding of the formation and progression of TAAs may provide novel, less invasive therapeutic strategies in patients with this devastating disease. Aneurysm formation is a complicated, dynamic process involving both cellular and extracellular processes. The mechanisms of TAA formation are poorly understood, mainly due to the lack of a useful and reproducible model. In 2003, Ikonomidis in South Carolina developed a murine model of TAA.The mouse model was less satisfactory because the small size of the murine aorta. The rat model has the advantage of larger aorta and the ability to do more research. Proper animal models are useful to understand the aetiology and evolution and they can be used to evaluate the efficacy of new treatments.Objective The goal of this study was to test the hypothesis that abluminal calcium chloride application could creat TAAs in the SD rats.MethOds Eight-week-old male Sprague-Dawley(SD) rats(n=60) were divided into two groups equally. All rats were anesthetized. An angiocath was then advanced through the mouth into the trachea under direct vision and connected to a rodent ventilator set at a respiratory rate of 180 breaths per minute. And their thoracic aortas were exposed via left thoracotomy. In the experiment group,a sponge soaked in 0.5 mol/L CaCl2 was placed on the distal half of the exposed descending thoracic aorta under direct vision. As control group, 0.5 mol/L CaCl2 was instead of 0.9% NaCl. After 15 min, the sponge was removed. Then the chest was closed. At 4,8,12 weeks, all animals undergoing fixation and aortic harvest were reanesthetized and the thorax was then opened beneath the xiphoid process. Potassium chloride was injected into the left ventricle under direct vision to arrest the heart. Then it was perfusion program and the entire animal was immersed in formalin. Seven days after fixation, the animal was removed from formalin, and the aorta was carefully harvested from its root to the renal arteries. The aorta was then immersed in an individually labeled jar containing a mixture of 80% methanol and 20% dimethyl sulfoxide (Dent’s solution).Diameter measurements were made using Axioplan 2 imaging system. Hematoxylin and eosin staining and Van Gieson and Verhoeff staining showed the elastic architecture and collagen structure. Immunohistochemical staining was performed to evaluate the presence of lymphocytes and macrophages. Results 12 weeks of CaCl2 treatment resulted in visually obvious dilation of the exposed distal descending thoracic aortic segment.The distal descending thoracic aortic diameter at 4,8,12 weeks was (0.82±0.13), (1.12±0.43), (1.65±0.58)μm respectively. The control group at 4,8,12 weeks was (0.81±0.04) , (0.94±0.12), (1.09±0.03)μm respectively. The diameter of the ascending aorta (used as an internal control) were not significantly different between groups. Hematoxylin and eosin stained slides showed disruption of the normal elastic lamellar architecture of the CaCl2-exposed aortic wall compared to the contralateral wall or untreated control. Van Gieson-stained sections showed decreased collagen content in the exposed region of aortic wall. Immunohistochemical study indicated significantly more CD3+cells and CD68+ cells in the aortas of CaCl2 treated groups than in control aortas. CD3+ cells were located in the media and surrounding the vasa vasorum in the adventitia. Conclusion We conclude that abluminal application of CaCl2 to the thoracic aorta reliably produces dilation, wall-thinning, and disruption of mural architecture, the hallmark signs of aneurysm formation. These results suggest that the rat is a useful and technically feasible model for the study of TAAs. To our knowledge, these findings describe for the first time the generation of a reproducible model of isolated TAA formation in SD rat. Further understanding of the formation and progression of TAAs may provide novel, less invasive therapeutic strategies in patients with this devastating disease. Aneurysm formation The mechanisms of TAA formation are poorly understood, mainly due to the lack of a useful and reproducible model. In 2003, Ikonomidis in South Carolina developed a murine model of TAA. mouse model was less satisfactory because the small size of the murine aorta. The rat model has the advantage of larger aorta and the ability to do more research. Proper animal models are useful to understand the aetiology and evolution and they can be used to evaluate the efficacy of new treatments .Objective The goal of this study was to test the hypothesis that abluminal calcium chloride application could create TAAs in the SD rats. Methodes Eight-week-old male Sprague-Dawley (SD) rats (n = 60) were divided into two groups equally. All rats were anesthetized. An angiocath was then advanced through the mouth into the trachea under direct vision and connected to a rodent ventilator set at a respiratory rate of 180 breaths per minute. And their thoracic aortas were exposed via left thoracotomy. In the experiment group, a sponge soaked in 0.5 mol / L CaCl2 was placed on the distal As control group, 0.5 mol / L CaCl2 was instead of 0.9% NaCl. After 15 min, the sponge was removed. Then the chest was closed. At 4,8,12 weeks, all animals undergoing fixation and aortic harvest were reanesthetized and the thorax was then opened beneath the xiphoid process. Then it was perfusion program and the entire animal was immersed in formalSeven days after fixation, the animal was removed from formalin, and the aorta was carefully harvested from its root to the renal arteries. The aorta was then immersed in an individually labeled jar containing a mixture of 80% methanol and 20% dimethyl sulfoxide Dent’s solution. Diameter measurements were made using Axioplan 2 imaging system. Hematoxylin and eosin staining and Van Gieson and Verhoeff staining showed the elastic architecture and collagen structure. Immunohistochemical staining was performed to evaluate the presence of lymphocytes and macrophages. Results 12 weeks of CaCl2 treatment resulted in visually obvious dilation of the exposed distal descending thoracic aortic segment. Distal descending thoracic aortic diameter at 4,8,12 weeks was (0.82 ± 0.13), (1.12 ± 0.43), (1.65 ± 0.58) μm respectively. The control group at 4,8,12 weeks was (0.81 ± 0.04), (0.94 ± 0.12), (1.09 ± 0.03) μm respectively. The diameter of the ascending aorta (used as an internal control) were no t significantly different between groups. Hematoxylin and eosin stained slides showed disruption of the normal elastic lamellar architecture of the CaCl2-exposed aortic wall compared to the contralateral wall or untreated control. Van Gieson-stained sections showed decreased collagen content in the exposed region of aortic wall. Immunohistochemical analysis indicated significantly more CD3 + cells and CD68 + cells in the aortas of CaCl2 treated groups than in control aortas. CD3 + cells were located in the media and surrounding the vasa vasorum in the adventitia. Conclusion We conclude that abluminal application of CaCl2 to the results suggest that the rat is a useful and technically feasible model for the study of TAAs. To our knowledge, these findings describe for the first time the generation of a reproducible model of isolated TAA formation in SD rat.