In order to elucidate the mechanism for appearance of no-flow areas (areas where tumor blood flow temporarily cease), we directly observed the process of tumor-induced neovascularization and measured pressure change in a feeding vessel (starting vessel) which supplies blood to the tumor vascular network. Total length of tumor vascular network from one starting vessel increased exponentially as the tumor increased in size exponentially. The pressure of the starting vessel increased from approximately 40 cmH2O to 120 cmH2O with enlargement of the tumor size. As soon as the pressure of the starting vessel reached a plateau, however, there was a rapid increase in low-flow or no-flow areas in places within the tumor. We considered that no-flow areas were produced by the imbalance between the pressure elevation of a starting vessel and the enlargement of the vascular network from that vessel.
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