Schematics of the process of metastasis. (a) Formation of metastatic tumor cell lines at primary sites like breast, lung, and skin (melanoma) seen as the red nodes. Metastasis from these primary sites then spreads to the brain via the circulatory system (red arrows) and also to adjacent sites like the liver, bone, lung, and lymph nodes (black arrows). The inset shows the primary site of melanoma cells proliferating and migrating towards the vasculature, subsequently disseminating to secondary organ sites. (b) The metastatic tumor cells detach from the primary site and penetrate the adjacent parenchyma to reach the blood vessels. On reaching the vessels, the cells invade and enter the circulation (intravasation) and then disseminate within the vascular system (left half of figure). These cells eventually adhere to secondary sites “soil” to then extravasate out of the blood vessels and for colonies of metastatic cells (right half of figure).
