Morphometric analysis of 35 biopsy specimens from patients with stable (n=10) and unstable (n=25) atherosclerotic lesions was carried out. The structure of the plaques and their connective tissue caps was studied by various methods of histological sections staining. A new morphometric approach to quantitative evaluation of atherosclerotic lesions instability is suggested. It consists in calculation of the morphological "rigidity" coeffi cient, due to which the plaque is characterized more accurately. The proportion of areas of the "rigid" (connective tissue and calcium salt deposition areas) to "soft" (atheronecrotic nuclei, microvessels, clots and hemorrhages) structures of the plaque is evaluated. Plaque instability (liability of a to rupture) is associated with changes in the extracellular matrix components in the cap: accumulation of collagen and reduction of elastic fi ber content reducing vessel elasticity and making its locally more rigid. . Address for correspondence: ShishkinaValya@mail.ru. V. S. ShishkinaAcute clinical manifestations of cardiovascular diseases are associated with the development of unstable atherosclerotic lesions liable to ruptures in the main arterial walls. The mechanisms of atherosclerotic plaque instability development are different and not amply studied. The risk of plaque rupture is explained primarily by its structure: thickness of the fi brous cap and content of extracellular matrix proteins (collagen, elastin, and glycosaminoglycans), size and consistence of atheromatous nuclei, and infl ammatory phenomena [4,7]. Rupture of an unstable plaque can lead to acute circulatory disorders (acute coronary syndrome and brain stroke) and death [6]. High positive correlation between atherosclerosis and increase of pulse wave velocity (PWV) has been demonstrated [13]. The PWV is determined by morphological characteristics of the vessel and other parameters [1-3]; it characterizes the elastic stress of the vascular wall and increases with increasing artery rigidity [5]. The content and proportion of collagen (responsible for the vessel strength) and elastic fi bers (responsible for elasticity and stretching/compression of the vessel) largely determine biomechanics of the vascular wall [11,13]. Arterial wall with increased content of collagen fi bers and reduced content of elastic fi bers becomes more rigid [5]. Pathological changes in the composition of vascular matrix components are fraught with higher risk of cardiovascular complications of hypertensive nature [8,9].Here we use morphological criteria for evaluation of vascular wall rigidity based on the morphometric analysis of carotid artery sections from patients with atherosclerosis. We hypothesized that vascular rigidity can serve as an indicator of plaque instability.The relationship between vascular wall rigidity and plaque instability was investigated. To this end, detailed morphological analysis of biopsy specimens of carotid arteries and extracellular matrix components was carried out and the plaque rigidity coeffi cient was

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