How Do Tumors Recruit Blood Vessels? Blood vessels are indispensible for tumor growth and metastasis. Hence, tumors exploit multiple avenues to recruit blood vessels. Angiogenesis—the sprouting of new blood vessels from the existing vasculature—is the most widely investigated mode of new vessel formation in tumors. There are five other mechanisms of new vessel recruitment (top panels; adapted from Carmeliet and Jain, 2011). However, their relevance in cancer is still being debated, and their molecular mechanisms are not well understood. Vasculogenesis involves vessel formation by endothelial progenitor cells (EPCs), which are recruited from the bone marrow and/or are resident in vascular walls. Intussusception is the splitting of pre-existing vessels to give rise to daughter vessels. Vessel co-option occurs when cancer cells grow around and co-opt the existing vasculature. Vascular mimicry is a process in which cancer cells get incorporated into the blood vessel wall. Tumor stem cell to EC differentiation occurs when cancer stem-like cells differentiate into endothelial cells (ECs). For historical reasons and, now, for convenience, the term “angiogenesis” is used to describe all of these methods of blood vessel recruitment by tumors.