Mesenchymal Stem Cell Transition to Tumor-Associated Stromal Cells Contributes to Cancer Progression

The tumor microenvironment is a heterogeneous population of cells consisting of the tumor bulk plus supporting cells. It is becoming increasingly evident that these supporting cells are recruited by the cancer cells from nearby endogenous host stroma, and promote events such as tumor angiogenesis, proliferation, invasion, and metastasis, as well as mediate mechanisms of therapeutic resistance. In this chapter, we review current knowledge pertaining to the origins of recruited host stroma, the recruitment process by which tumor cells co-opt them, stromal contributions to tumor progression, and tumor-associated fibroblasts and their biomarkers. Additionally, we postulate on the hierarchical clustering of recruited stromal cells within a tumor.