Shear force and its effect on cell structure and function.

Cell culture studies of the influence of a laminar flow and the associated wall shear stress on bovine aortic endothelial cells are reviewed. These experiments, taken together with those of others, demonstrate that in response to a steady state flow the cells elongate in shape, orient their major axis with the direction of flow, and reorganize their F-actin structure, with a concomitant increase in cell stiffness. There also is an influence of flow on cell function, including the ability to replicate, to secrete vasoactive substances, and on such processes as endocytosis. Such effects of flow are dependent on the level of shear stress, the duration of exposure, the surface to which the cells are adherent, and the media employed. Furthermore, the use of a 1 Hz sinusoidal, non-reversing pulsatile flow was found to, in general, enhance the effects observed for a steady state flow. Although these studies were motivated by an interest in investigating the role of flow in the initiation and progression of atherosclerosis, the results obtained provide evidence that relatively small physical forces can have an important influence. This suggests it is possible that the unloading represented by placing cells in the microgravity environment of space may be an observable influence. More importantly, it also indicates that cell biology experiments on orbit need to be conducted under conditions where the influence of other forces, e.g., those due to a flowing cell culture system, are negligible.