Shear stress gradient over endothelial cells in a curved microchannel system

Our purpose was to test a scale model of the microcirculation by measuring the shear forces to which endothelial cells were exposed, and comparing this to computer simulations. In vitro experiments were performed to measure the 2-dimensional projected velocity profile along endothelial cell lined microchannels (D-shaped, 10-30 mu m radius, n = 15), or in microchannels without endothelial cells (n = 18). Microchannels were perfused with fluorescently labeled microspheres (0.5 mu m dia., <1%) suspended in cell culture media. The velocity of individual microspheres was obtained off-line (videorecording), using an interactive software program; velocity was determined as the distance traveled in one video field (1/60 s). Mass balance was verified in the microchannels by comparing the microsphere velocities to the perfusion pump rate. In confluent endothelial cell lined microchannels, a velocity profile was obtained as microspheres passed an endothelial cell nucleus (identified by fluorescent dye), and again, for a paired region 100 mu m away without nuclei (cytoplasm region). The velocity profile was significantly shifted and sharpened by the endothelial cell nucleus, as anticipated. Over the nucleus, data are consistent with a normal sized nucleus extending into the lumen, further confirming that this scale model can be used to determine the wall shear stress to which endothelial cells are exposed. Using the experimental bulk phase fluid parameters as boundary conditions, we used computational fluid dynamics (CFD) to predict the expected wall shear stress gradient along an endothelial cell lined D-shaped tube. The wall shear stress gradient over the nucleus was 2-fold greater in the radial versus axial directions, and was sensitive to lateral versus midline positioned nuclei.