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 thi s to computer simulations. In vitro experiments were performed to measure t he 2-dimensional projected velocity profile along endothelial cell lined mi crochannels (D-shaped, 10-30 mu m radius, n = 15), or in microchannels with out endothelial cells (n = 18). Microchannels were perfused with fluorescen tly labeled microspheres (0.5 mu m dia., <1%) suspended in cell culture med ia. The velocity of individual microspheres was obtained off-line (videorec ording), using an interactive software program; velocity was determined as the distance traveled in one video field (1/60 s). Mass balance was verifie d in the microchannels by comparing the microsphere velocities to the perfu sion pump rate. In confluent endothelial cell lined microchannels, a veloci ty profile was obtained as microspheres passed an endothelial cell nucleus (identified by fluorescent dye), and again, for a paired region 100 mu m aw ay without nuclei (cytoplasm region). The velocity profile was significantl y shifted and sharpened by the endothelial cell nucleus, as anticipated. Ov er the nucleus, data are consistent with a normal sized nucleus extending i nto the lumen, further confirming that this scale model can be used to dete rmine the wall shear stress to which endothelial cells are exposed. Using t he experimental bulk phase fluid parameters as boundary conditions, we used computational fluid dynamics (CFD) to predict the expected wall shear stre ss gradient along an endothelial cell lined D-shaped tube. The wall shear s tress gradient over the nucleus was 2-fold greater in the radial versus axi al directions, and was sensitive to lateral versus midline positioned nucle i.