As solid tumors develop, a variety of physical stresses arise including growth induced compressive force, matrix stiffening due to desmoplasia, and increased interstitial fluid pressure and altered flow patterns due to leaky vasculature and poor lymphatic drainage [1]. These microenvironmental stresses likely contribute to the abnormal cell behavior that drives tumor progression, and have become an increasingly significant area of cancer research. Of particular importance, is the role of flow shear stress on tumor-endothelial signaling, vascular function, and angiogenesis. Compared to normal vasculature, blood vessels in tumors are poorly functional due to dysregulated expression of angiogenic growth factors, such as vascular endothelial growth factor (VEGF) or the angiopoietins. Also, because of the abnormal vessel structure, blood velocities can be an order of magnitude lower than that of normal microvessels. Recently published work utilizing intravital microscopy to measure blood velocities in mouse mammary fat pad tumors, demonstrated for the first time that shear rate gradients in tumors may help guide branching and growth of new vessels [2]. However, much still remains unknown about how shear stress regulates endothelial organization, permeability, or expression of growth factors within the context of the tumor microenvironment.

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