The origins of vacularization in tumors.

Vascularization is crucial for tumor growth and metastasis. Angiogenesis and vasculogenesis are widely accepted processes of tumor vascularization, particularly for endothelium-dependent vessels. In both these processes, the tumor vascular endothelial cells are derived from the host cells, including cells in normal tissues around the tumor or endothelial progenitor cells. In addition, the mosaic vessels occur as a transitional pattern between endothelium-dependent vessels and vasculogenic mimicry (VM), wherein both host endothelium and tumor cells participate in tumor vascularization. VM provides a special passage not involving endothelial cells and is conspicuously different from angiogenesis and vasculogenesis. The biological features of the tumor cells that form VM remain unknown. Tumor stem cells may participate in VM. In this review, we discuss the patterns involved in the origin of vascularization in tumors.