The infrarenal aorta is especially prone to the development of aneurysm, suggesting an intrinsic structural and molecular difference in different parts of the aorta. Previous study from this laboratory has implicated a potential role of integrin alpha5beta1 in maintaining aortic integrity. The aim of this study was to investigate the expression of integrin alpha5beta1 and the relationship to collagen and elastin content between 2 different aortic segments. In this study, the variation of smooth muscle cells and the localization of integrin alpha5beta1 in the suprarenal and infrarenal aorta tissues removed from organ donors were studied immunohistochemically. The biochemical analysis for matrix proteins and integrin alpha5beta1 protein was done by Western Blot on the corresponding tissues. All interested protein content was normalized to the smooth muscle alpha-actin protein. No significant difference of smooth muscle cells density between the 2 segments of aortas was observed. Integrin alpha5beta1 was localized in the outer layer of all aortic media. The authors found that the ratio of collagen/elastin in infrarenal aortas was significantly increased 2-fold because elastin content in infrarenal aortas decreased 49% as compared with suprarenal aortas. Integrin alpha5beta1 content relative to its specific ligand-collagen-did not differ between these 2 different aortic segments. These results suggest that the infrarenal aorta differed biochemically from the suprarenal aorta. A decrease in infrarenal elastin without a corresponding decrease in collagen and integrin alpha5beta1 may affect the compliance and integrity of the distal aorta. These intrinsic anatomic differences may be important in the susceptibility of the infrarenal aortas to aneurysm formation.