534 R.M.Nerem research where the importance of physical factors as an effect on cellular behavior is recognized,and in which engineers have been involved.In the early days this in- volvement was due to the realization that the physical forces imposed by flowing blood might be an important influence on the biology of the arterial wall.In vivo studies demonstrated differences in endothelial cell shape and F-actin localization,the permeability of the endothelium,and the recruitment of monocytes into the arterial wall in regions of differing hemodynamic environment (35).Such studies also sug- gested a possible influence of hemodynamic environment on the endothelial cell turn- over rate (9),the process of cell replication whereby a confluent endothelial monolayer is maintained,as aging cells die.Unfortunately,it is very difficult to quan- tify in vivo the detailed characteristics associated with the flow of blood;thus these in vivo results,in terms of any demonstration of a hemodynamic effect,had to be viewed as qualitative in nature. For this reason,more than a decade ago a number of laboratories began to use cell culture in the study of flow effects on vascular biology.The first effort in studying cultured vascular endothelial cell monolayers in the presence of flow was that of Dewey et al.(17)who used a cone-plate viscometer to investigate shear stress effects. Other laboratories,including my own (29,30),have joined in the use of cell culture to study the effects of a cell's biomechanical environment.This has included studies on the influence of a fluid-imposed shear stress,on changes due to the direct effect of pressure,and on alterations associated with the cyclic stretching of cells grown on a compliant membrane(35).Recently a workshop on mechanical stress effects on vas- cular cells was held in Atlanta,and the abstract proceedings of this meeting are rep- resentative of the current state of knowledge(3).What is clear from this meeting is that,in response to its biomechanical environment,the vascular endothelial cell re- sponds by altering its cytoskeletal structure.With this alteration there is a change in cell shape and an alignment of the cell's major axis within the stress field,as illus- trated in Fig.2,and there are a variety of associated changes in cell function. As part of this there is an influence of mechanical stress on the cell's ability to rep- licate.As noted earlier,in vivo observations suggest an influence of a hemodynamic environment on endothelial cell turnover rates,and it is believed that higher rates oc- cur in regions characterized hemodynamically as low-shear stress in comparison to those characterized as high-shear stress(9).Whether or not it is in fact a shear-stress effect cannot be determined from in vivo experiments.Furthermore,it is well known that the turnover rate of an endothelial monolayer in culture is far higher than that observed in vivo in endothelium.Thus,in our own laboratory we have focused on studying the effects of laminar flow and the associated fluid-imposed shear stress on the process of cell replication in cultured endothelial monolayers.What we have found is that in the presence of shear stress,the overall proliferation rate of sub-con- fluent endothelial cells is altered(30).For endothelial cells cultured on a polyester sub- strate,e.g.,Thermanox or Mylar,and for low-shear stresses,e.g.,5 dynes/cm2 or less,any effect is absent as was noted in the earlier study of Dewey et al.(17).How- ever,for shear stresses in the range from 30 to 90 dynes/cm2,cell proliferation is sig- nificantly slowed,with the growth at 90 dynes/cm2 being almost totally arrested,at least for 24 hours. These earlier results were based on cell density data,and to confirm this 3H-thy- midine incorporation and autoradiography measurements were performed.These were conducted post-shear,i.e.,after removal of the coverslip from the flow cham-534 R.M. Nerem research where the importance of physical factors as an effect on cellular behavior is recognized, and in which engineers have been involved. In the early days this in￾volvement was due to the realization that the physical forces imposed by flowing blood might be an important influence on the biology of the arterial wall. In vivo studies demonstrated differences in endothelial cell shape and F-actin localization, the permeability of the endothelium, and the recruitment of monocytes into the arterial wall in regions of differing hemodynamic environment (35). Such studies also sug￾gested a possible influence of hemodynamic environment on the endothelial cell turn￾over rate (9), the process of cell replication whereby a confluent endothelial monolayer is maintained, as aging cells die. Unfortunately, it is very difficult to quan￾tify in vivo the detailed characteristics associated with the flow of blood; thus these in vivo results, in terms of any demonstration of a hemodynamic effect, had to be viewed as qualitative in nature. For this reason, more than a decade ago a number of laboratories began to use cell culture in the study of flow effects on vascular biology. The first effort in studying cultured vascular endothelial cell monolayers in the presence of flow was that of Dewey et aL (17) who used a cone-plate viscometer to investigate shear stress effects. Other laboratories, including my own (29,30), have joined in the use of cell culture to study the effects of a cell's biomechanical environment. This has included studies on the influence of a fluid-imposed shear stress, on changes due to the direct effect of pressure, and on alterations associated with the cyclic stretching of cells grown on a compliant membrane (35). Recently a workshop on mechanical stress effects on vas￾cular cells was held in Atlanta, and the abstract proceedings of this meeting are rep￾resentative of the current state of knowledge (3). What is clear from this meeting is that, in response to its biomechanical environment, the vascular endothelial cell re￾sponds by altering its cytoskeletal structure. With this alteration there is a change in cell shape and an alignment of the cell's major axis within the stress field, as illus￾trated in Fig. 2, and there are a variety of associated changes in cell function. As part of this there is an influence of mechanical stress on the cell's ability to rep￾licate. As noted earlier, in vivo observations suggest an influence of a hemodynamic environment on endothelial cell turnover rates, and it is believed that higher rates oc￾cur in regions characterized hemodynamically as low-shear stress in comparison to those characterized as high-shear stress (9). Whether or not it is in fact a shear-stress effect cannot be determined from in vivo experiments. Furthermore, it is well known that the turnover rate of an endothelial monolayer in culture is far higher than that observed in vivo in endothelium. Thus, in our own laboratory we have focused on studying the effects of laminar flow and the associated fluid-imposed shear stress on the process of cell replication in cultured endothelial monolayers. What we have found is that in the presence of shear stress, the overall proliferation rate of sub-con￾fluent endothelial cells is altered (30). For endothelial cells cultured on a polyester sub￾strate, e.g., Thermanox or Mylar, and for low-shear stresses, e.g., 5 dynes/cm 2 or less, any effect is absent as was noted in the earlier study of Dewey et al. (17). How￾ever, for shear stresses in the range from 30 to 90 dynes/cm 2, cell proliferation is sig￾nificantly slowed, with the growth at 90 dynes/cm 2 being almost totally arrested, at least for 24 hours. These earlier results were based on cell density data, and to confirm this 3H-thy￾midine incorporation and autoradiography measurements were performed. These were conducted post-shear, i.e., after removal of the coverslip from the flow cham-