Cancer cells that have made in into the bloodstream are enjoying the ride of their lives (if you want to know more about how they got into the bloodstream, click here). They are being transported at incredible speed through the system of veins and arteries that reaches every remote corner of the human body – until they make the molecular-scale decision to stop. Interestingly, cancer cells stop and get out of blood vessels much in the same way that cells of the immune system do – which is yet another example of how cancer cells can hijack strategies usually adopted by other cells in the body.
This stopping strategy can be summarized in 3 simple words: “stop, drop and roll”. Cancer cells stop by forming microscopic molecular threads to attach themselves to the cells of the edge of the blood vessel. Since they are being hurled along at tremendous speed, these tethers tend to break, while the cells rolls along the side of the blood vessel wall. eventually, it manages to build up strong enough bonds to come to a complete stop – at which point the cancer cell in questions starts to sneak out of the blood vessel. This is a time-sensitive issue, as the other cells in the bloodstream are still flying by at incredible speed and could know into it at any moment. The way cancer cells exit the blood vessel is simply by squeezing themselves in between two of the cells that make up the cell wall itself, known as endothelial cells. They do so by breaking up the tight molecular staples that hold endothelial cells together – as well as by convincing the endothelial cells to contract by secreting a variety of signals. Once the cells of the blood vessel wall contract, the gap between them becomes larger, making it even easier for cancer cells to squeeze through.
Ultimately, cancer cells manage to sneak out for the blood vessel in a matter of hours. This process is known as extravasation – which literally means “getting out of the cell”. Once the cell is “out”, it can finally start forming a secondary tumor, or metastasis, in a different location of the body from where the tumor originally developed.
Interestingly, it is currently believed that some cancer cells are attracted to specific parts of the body as they can “smell” the chemical substances they secrete. This explains why certain types of cancer prefer to form metastases in certain organs: for example, brest cancer cells often end up in the bone and melanoma cells (a type of skin cancer) tend to make their way into the lung and brain.