Figure 1.

The metastatic process. The complex process of metastasis includes induction of EMT, detachment, invasion of the tumor microenvironment and shedding of CTCs into the blood stream (intravasation). Apparently, the majority of CTCs relate to differentiated cancer cells that do not bear tumorigenicity and are rather prone to undergo anoikis. However, CTCs that bear cancer stem cell features, i.e., circulating cancer stem cells, are able to survive in the circulating blood, escape from immune surveillance, home to secondary sites, extravasate, undergo MET, and thus may eventually form distant metastatic lesions. EMT, epithelial-to-mesenchymal transition; CTCs, circulating tumor cells; MET, mesenchymal-epithelial transition.