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Basal lamina (BL) is a crucial mechanical and functional component of blood vessels, constituting a sensor of extracellular microenvironment for endothelial cells and pericytes. Recently, an abnormality in the process of matrix microfibrillar component remodeling has been advocated as a mechanism involved in the development of aortic dilation. We focused our attention on BL composition and organization and studied some of the main components of the Extracellular Matrix such as Tenascin, Laminins, Fibronectin, type I, III and IV Collagens. We used surgical fragments from 27 patients, submitted to operation because of aortic root aneurysm and 5 normal aortic wall specimens from heart donors without any evidence for aneurysmal or atherosclerotic diseases of the aorta. Two samples of aortic wall were harvested from each patient, proximal to the sinotubular junction at the aortic convexity and concavity. Each specimen was processed both for immunohistochemical examination and molecular biology study. We compared the convexity of each aortic sample with the concavity of the same vessel, and both of them with the control samples. The synthesis of mRNA and the levels of each protein were assessed, respectively, by RT-PCR and Western Blot analysis. Immunohistochemistry elucidated the organization of BL, whose composition was revealed by molecular biology. All pathological samples showed a wall thinner than normal ones. Basal lamina of the aortic wall evidentiated important changes in the tridimensional arrangement of its major components which lost their regular arrangement in pathological specimens. Collagen I, Laminin alpha2 chain and Fibronectin amounts decreased in pathological samples, while type IV Collagen and Tenascin synthesis increased. Consistently with the common macroscopic observation that ascending aorta dilations tend to expand asymmetrically, with prevalent involvement of the vessel convexity and relative sparing of the concavity, Collagen type IV is more evident in the concavity and Tenascin in the convexity.