Tumor progression has long been recognized to involve a crosstalk between different cell types within the tumor mass and its supporting tissue. Established tumors are complex masses composed of two interconnected compartments: parenchyma and stroma. The tumor parenchyma comprises the malignant cells themselves, whereas the stroma is a very heterogeneous microenvironment including a variety of non-malignant cells and the connective tissue. The connective tissue contains the extracellular matrix and adhesion molecules that provide the structural support of the tumor microenvironment, while the diverse stromal cell types consist of fibroblasts, immune cells, as well as cells that participate in the construction of the tumor-associated vasculature, specifically endothelial cells, pericytes and smooth muscle cells. These stromal cells are recruited within the mass and are vital for tumor functional activity, maintenance and growth.1
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