Active and Passive Intravasation
Tumors can use both active and passive methods to enter vasculature. Some studies suggest that cancer cells actively move towards blood or lymphatic vessels in response to nutrient or chemokine gradients, while others provide evidence for the hypothesis that metastasis in the early stages is more accidental.
In active intravasation, cancerous cells actively migrate toward and then into nearby blood vessels. The first step in this process is specific adhesion to venular endothelial cells, followed by adherence to proteins of the subendothelial basement membrane, such as laminin and types IV and V collagen. The final step is the adhesion of the metastatic tumor cell to connective tissue elements such as fibronectin, type I collagen, and hyaluronan, which is required for the movement of the tumor cell into the subendothelial stroma and subsequent growth at the secondary site of colonization.
Passive intravasation refers to a process in which tumors metastasize through passive shedding. Evidence for this is seen when the number of tumor cells released into the blood stream increases when the primary tumor experiences trauma. Additionally, cells growing in restricted spaces have been shown to push against each other causing blood and lymphatic vessels to collapse, potentially forcing cells into the vessels.
Read more about this topic: Intravasation
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