Early incorporated endothelial cells as origin of metastatic tumor vasculogenesis

Vascularization of solid tumors is thought to occur by sprouting or intussusceptive angiogenesis, co-option of existing vessels, and vasculogenic mimicry after the onset of tumor hypoxia, when the tumor radius exceeds the oxygen diffusion distance. In contrast, here we show that individual endothelial cells that are incorporated into pre-hypoxic tumors give rise to tumor blood vessels via vasculogenesis. Small metastatic lung tumor sections obtained after tail-vein injection of a syngeneic breast cancer cell line in the nude mice were labeled with antibodies against endothelial cell markers. Immunofluorescence showed the incorporation and mixed growth of CD31-, Tie-2-, and CD105-positive endothelial cells in tumors with radii less than oxygen diffusion distance and subsequent development of blood vessels from these early-incorporated endothelial cells. This observation lays the foundation of a novel vasculogenic paradigm of tumor vascularization, where incorporation of endothelial cells and their growth among tumor cells occur before the onset of core hypoxia in lung metastatic tumors.