Cell colonies under pressure: How growth can prevent motion

The ability to actively migrate is a fundamental property of living matter such as cells. Scientists at the MPI-DS have investigated how this motion interacts with the growth of the entire colony, which can be observed in a wide variety of cellular aggregates. Such growth happens when cells in tissues, bacterial colonies, cell cultures in the laboratory or in tumors divide continuously and take up more and more space.

The researchers recreated this scenario in a minimal computer model of a growing three-dimensional cell colony and also gave the cells a certain amount of force to actively move, an ability known as motility. In their simulations, they found that more frequent cell divisions and thus faster growth can restrict the motion of cells, resulting in less mixing of the colony. In this case, hardly any migration of individual cells is visible, even if they have the potential to move.

"Surprisingly, we found that there is a relatively sharp threshold of motility up to which the growth of the colony almost completely inhibits the migration of cells," says Torben Sunkel, first author of the study. Cells only begin to move through the tissue, once a certain ratio of motility to growth rate is exceeded. In biology, it is well-known that cells can switch their motility on or off in response to biochemical signals or due to other mechanisms. "But in our model, the transition arises all by itself purely from mechanical interactions -- a prime example of collective behavior that arises from the interaction of many individual parts," emphasizes Philip Bittihn, senior author of the study and group leader in the Department of Living Matter Physics at MPI-DS. As the researchers discovered, the reason for this behavior is two-fold: On the one hand, cell division and growth cause a lack of space inside the colony, which prevents motion directly through mechanical contacts. On the other hand, rapid spatial expansion of the colony means that cells have to travel further, which makes any existing motion less effective.

How cell colonies organize and structure themselves is important in many areas. The study may therefore also provide new starting points for experimental and medical research -- for example for bacterial colonies, wound healing processes, tissue engineering or in cancer research.