Schematic configuration and flow pattern in VSF channel. (A) Schematic diagram of the flow channel and test section. ECs were seeded on the collagen gel in the presence of SMCs. The dimensions of the test section are described in “Materials and methods.” The thickness (t) of the gel was measured to be approximately 0.03 mm. (B, top) Phase-contrast photomicrograph (top view) of experimental flow patterns in the VSF channel. (B, bottom) Schematic drawing of the side view of the streamlines in the VSF deduced from the top-view photomicrograph. Flow is from left to right and made visible with marker particles, as described in “Materials and methods.” Flow separation occurs in the region distal to the step, forming 4 specific flow areas: (a) the stagnant flow area, (b) the area of the recirculation eddy, (c) the reattachment flow area, and (d) the area where the flow has developed again to laminar flow, as denoted in the bottom part of the figure. The span of the areas (represented as length) and the mean shear stress (MSS) in each of the areas were measured as indicated. The movie for online microscopic observation of the flow patterns is provided in Video S1.