Molecular pathways: Not a simple tube-the many functions of blood vessels

Although the ability of blood vessels to carry fluid and cells through neoplastic tissue is clearly important, other functions of vascular elements that drive tumor growth and progression are increasingly being recognized. Vessels can provide physical support and help regulate the stromal microenvironment within tumors, form niches for tumor-associated stem cells, serve as avenues for local tumor spread, and promote relative immune privilege. Understanding the molecular drivers of these phenotypes will be critical if we are to therapeutically target their protumorigenic effects. The potential for neoplastic cells to transdifferentiate into vascular and perivascular elements also needs to be better understood, as it has the potential to complicate such therapies. In this review, we provide a brief overview of these less conventional vascular functions in tumors.