A tumour does not solely consist of transformed cancer cells but represents a complex tissue that also includes a wide array of stromal cells. The importance of stromal components in tumour phenotype is highlighted by the fact that four out of the seven genes in the genetic profile used by the recently developed Oncotype DX Colon Cancer Assay to predict the risk of recurrence in colon cancer are stromal factors. Cancer progression occurs within the context of complex interactions between multiple cell types, which can be major drivers of the evolution of tumour cells. For example, the ability to recruit immune cells can engender a significant advantage to tumour cells because intratumoural inflammatory cells secrete cytokines and growth factors that promote tumour cell proliferation and survival as well as angiogenesis. Recruitment of immune suppressor cells also helps tumours evade immune surveillance. As a result, many cancers evolve to recruit inflammatory cells, and infiltration by these cells is often seen in tumour biopsies.
Other cellular components of the stroma, such as fibroblasts, can also secrete growth factors that support survival and proliferation of cancer cells. Direct interaction of integrins on cancer cells with the extracellular matrix and other cells also enhances tumorigenesis by activation of focal adhesion kinase (FAK) and several oncogenic signalling pathways, including Mek/Erk and PI-3K/Akt signalling.