The vasculature's primary role is to supply nutrients and gases to and remove wastes from various tissues throughout the body. These substances exchange at the smallest blood vessels known as capillaries, which are made of a monolayer of endothelial cells that separate blood flow from other vascular tissues. The formation of new capillaries from preexisting blood vessels is known as angiogenesis, which is a multi-step process where endothelial cells migrate, proliferate and sprout to form new tubules. It is well known that there are many chemical factors (e.g., vascular endothelial growth factor and basic fibroblast growth factor) regulating angiogenesis; however, the role of mechanical factors, such as those associated with blood flow (i.e., hemodynamic stress) in angiogenesis remains to be explored.

Previously we developed a three-dimensional in vitro system that mimics the process of sprouting angiogenesis under the influence of circumferential stress in vivo. This system subjects endothelial cells to well-defined uniaxial cyclic stretch, and therefore enables the investigation of endothelial tubulogenesis under stretch. In the present study, we investigated the effect of strain magnitudes on endothelial tubulogenesis. Bovine aortic endothelial cells were cultured to confluence on the top of a three-dimensional collagen gel that was attached to a deformable silicone substrate. The endothelial cell monolayer was then stimulated for 1 day with basic fibroblast growth factor to promote tubulogenesis. Next, for two days the stimulated cells were kept as either static controls or stretched by cyclically elongating the silicone substrate at 10 or 20% strain, at 5 cycles per minute. At the end of experiments, the cells were fixed and the microscopic images of cord-like structures inside the gel were recorded using phase contrast video microscopy and analyzed using commercial imaging software. We found at 20% strain the number of endothelial cords was lower than that of static controls; however, this inhibitive effect was not observed at 10% strain. Cyclic stretch also induced the alignment of endothelial cells cords perpendicular to the principal axis of stretch; this inductive effect was greater at 20% than 10% strain. However, the length and depth of the invasive structures were not significantly altered by the cyclic stretch.