Vascular endothelial cells appear to be aligned with the flow in the immediate vicinity of the arterial wall and have a shape which is more ellipsoidal in regions of high shear and more polygonal in regions of low shear stress. In order to study quantitatively the nature of this response, bovine aortic endothelial cells grown on Thermanox plastic coverslips were exposed to shear stress levels of 10, 30, and 85 dynes/cm2 for periods up to 24 hr using a parallel plate flow chamber. A computer-based analysis system was used to quantify the degree of cell elongation with respect to the change in cell angle of orientation and with time. The results show that (i) endothelial cells orient with the flow direction under the influence of shear stress, (ii) the time required for cell alignment with flow direction is somewhat longer than that required for cell elongation, (iii) there is a strong correlation between the degree of alignment and endothelial cell shape, and (iv) endothelial cells become more elongated when exposed to higher shear stresses.

Experimental studies have shown that endothelial cells which have been exposed to shear stress maintain a flattened and elongated shape after detachment. Their mechanical properties, which are studied using the micropipette experiments, are influenced by the level as well as the duration of the shear stress. In the present paper, we analyze these mechanical properties with the aid of two mathematical models suggested by the micropipette technique and by the geometry peculiar to these cells in their detached post-exposure state. The two models differ in their treatment of the contact zone between the cell and the micropipette. The main results are expressions for an effective Young's modulus for the cells, which are used in conjunction with the micropipette data to determine an effective Young's modulus for bovine endothelial cells, and to discuss the dependence of this modulus upon exposure to shear stress.

A quantitative study of the en face size and shape of endothelial cells from aortic intercostal ostia has been carried out in rabbits. Photomicrographs were taken from vascular casts of the rabbit aorta and the endothelial cell outlines were analyzed quantitatively using a digitizer and digital computer. The morphology of the endothelial cells was described using 8 calculated parameters (area, perimeter, length, width, angle of orientation, width: length ratio, axis-intersection ratio and shape index). Marked changes in cell morphology were found in the regions proximal and distal to ostia as well as around flow dividers. Cells on the aorta are aligned with the flow direction, and the endothelial cells within the ostia have an angle of orientation of approximately 45 deg to the axis of the vessel. The results obtained to date suggest that endothelial cell morphology and orientation around a branch vessel may be a natural marker or indicator of the detailed features of blood flow.

The mechanical properties of cultured bovine aortic endothelial cells exposed to a fluid-Imposed shear stress were studied using the micropipette technique. The cells, which were attached to a Thermanox plastic substrate, were exposed to a specific steady shear stress of either 10,30, or 85 dynes/cm 2 and for a duration ranging from 0.5 to 24 hours. Morphological changes In shape and orientation were observed, and following each experiment, the mechanical properties were measured using the micropipette aspiration technique applied to cells detached from the substrate. Fluorescent microscopy was carried out to observe cytoskeletal F-actln filaments stained with rhodamlne phalloldln. During exposure to shear, the en face shape of the endothelial cells on the substrate became more elongated and their long axis became oriented to the direction of flow. There was also an alteration In the F-actln filaments. These changes were dependent on both the level of shear stress and the duration of exposure. After detachment, the cells exposed to shear maintained their deformed shape. This Is In contrast to cells In a static, no-flow environment which became spherical in shape upon detachment. Cells exposed to shear stress demonstrated a mechanical stiffness significantly greater than that of control cells, which was dependent on both the level of shear stress and the duration of exposure. Furthermore, It appears that the influence of shear stress on endothelial cell mechanical stiffness may be related to alterations in cytoskeletal structure. (Arteriosclerosis 7:276-286, May/June 1987) A lthough there is considerable indirect evidence that hemodynamic forces are a factor in the process of atherogenesis, the detailed mechanisms are still poorly understood and the precise role is uncertain.1 ' 2 The accumulation of lipids within the intima is believed to be an important factor in the early stages of atherosclerosis, and the process of transendothelial macromolecule transport and the influence of hemodynamic-related events, in particular shear stress, received early attention.34 More recently, hemodynamic forces have been found to affect the shape and orientation of endothelial cells studied both in vivo and in vttro.5 " 10 Furthermore, the cytoskeleton (in particular stress fibers), which is an important structure in the support of the membrane and in the maintenance of cell shape, also appears to differ in regions of differing shear stress.11 " 13It thus appears that when endothelial cells are exposed to a fluid-imposed shear stress the process of adaptation or response involves not only cell orientation and elongation, but also a change in the supporting, internal structure. Furthermore, this could be reflected in the mechanical properties of the endothelial cells, and any change in these properties could be important to the deformation a cell Received August 11, 1986; revision accepted January 27, 1987. undergoes as part of the adaptation process. For this reason, it was believed that a measurement of the mechanical prop...

The pattern of endothelial geometry at various locations along stenosed dog aortas was examined. This was done to test the hypotheses that the shape of an endothelial cell is related to the local wall shear stress associated with the flowing blood and that alterations in hemodynamics, produced by vascular geometrical changes, influence endothelial cell geometry. Aortic stenosis with a reduction of 71% of the cross-sectional area was produced. The animals were sacrificed 12 weeks later, and the endothelial cell geometry and orientation were studied using the vascular casting technique and a computerized analysis to determine cell area and shape index. The regions of the stenosis examined were those known to experience different hemodynamic conditions. The value of the shape index was found to fall rapidly in the convergent region of the stenosis and to increase suddenly in the divergent region, eventually returning to the prestenotic value at a more distal site. Using a model of a stenosis made from a vascular cast, laser Doppler anemometry was applied to measure velocity profiles and to estimate the local wall shear stress in a stenosed aorta. It is shown that the shape index distribution along these stenosed vessels may be correlated with the level of wall shear stress, with more elongated cells occurring in regions of higher shear stress. (Arteriosclerosis 6:220-229, March/April 1986) T he evidence for the involvement of fluid dynamics in the atherosclerotic process centers primarily on the pattern of the disease.1 " 3 It is often regions of arterial branching and sharp curvature that have the greatest predilection for the development of atherosclerosis. These are also regions where the flow will assume unusual characteristics or at least deviate from what otherwise might be considered a well-behaved arterial flow. The indictment provided by this indirect evidence, particularly as it relates to the bifurcations and geometrical contortions of the arterial vasculature, presently motivates much of the interest in arterial fluid dynamics.It has been suggested that vascular geometry may affect the atherogenic process through its influence on the hemodynamic environment to which the intima is exposed. 4 ' 5 Under such a hypothesis, whatever affects vascular geometry would alter the local detailed flow characteristics and correspondingly influence endothelial morphology and function. Our interest in hemodynamic Received November 5,1984; revision accepted November 12, 1985. forces, in particular arterial wall shear stress, stems from the belief that it is through this hemodynamically imposed frictional force that a fluid mechanic effect on the endothelium becomes manifest. In this context, the topic of interest is the influence of hemodynamic forces on endothelial morphology, cell function, cell turnover rate, and transendothelial transport, and it is the first of these, endothelial morphology, more specifically, en face cell shape, which is the subject of this investigation.There is a body of accumulating data th...