A tumor doesn't stay put. Individual cells or whole groups break off and set off on a journey through the body. Some crawl solo, like scouts; others stick together, like a flock. Scientists built a computer model to understand how these cells pick a direction.
In the model, cells respond to two signals. The first is an attraction to stiffer tissues: the cell senses where the tissue is denser and crawls toward it. The second is an active force pulling the cell away from the tumor, as if someone were tugging on a string. It turns out these two signals don't always work in harmony. When a cell stretches out, one signal pulls it one way, and the other pulls it the opposite way. If you switch both on at once, the model shows cells getting stuck and unable to move.
So the scientists split the process into two steps. First, the cell responds to stiffness; then, it responds to the breakaway command. This trick is called a fractional step. The model began to show more realistic scenarios: cells indeed crawl singly or in groups, just like in a real tumor.
This is only the first stage. In the future, scientists will add cell proliferation and biochemical mechanisms that determine whether a cell is a loner or a team player. Then it will become clearer how cancer conquers new territories.
Cancer cells sense the stiffness of surrounding tissue — a process called durotaxis. Tumors are often stiffer than healthy tissue, and cells crawl toward them, as if following a slope.