Figure 1. The metastatic cascade.

Metastatically competent primary tumours grow, invade the local host tissue and eventually shed tumour cells into the circulation. These cells travel to and colonize distant organs, and their subsequent growth at the secondary sites constitutes metastatic disease^37,73. Spontaneous metastasis assays involve establishing a primary tumour that is allowed to grow and spread in the host, whereas experimental metastasis assays circumvent the initial growth and invasion stages as a result of directly injecting tumour cells into the circulation. Other assays mimic aspects of metastatic growth by seeding tumour cells at the secondary site (for example, by intrasplenic injection of colorectal cancer cells, which directly targets the cells to the liver⁵⁵ where they grow as metastases). Figure is modified, with permission, from REF. 37 © (1982) Amerian Association for the Advancement of Science.