Figure 1

The metastatic process. The initial steps of metastasis require proliferation of the primary tumor and invasion through adjacent tissues and basement membranes. This process continues until the tumor invades blood vessels or lymphatic channels, when individual tumor cells detach from the primary tumor mass and are carried via the blood or lymph to a distant target organ. Subsequently, tumor cells arrest in small vessels within the distant organ, extravasate into the surrounding tissue and proliferate at the secondary site. All of these steps must be performed while tumor cells avoid and survive apoptotic signals and host immune responses.