contact inhibition

Imagine people spreading out to fill an empty room: they keep moving until everyone has elbow room, and then they stop, because there is nowhere left that is not already taken. Normal cells growing on a surface behave much the same way. Contact inhibition is the rule that a healthy cell stops moving and stops dividing once it is surrounded on all sides by other cells — it senses that the neighborhood is full and settles down.

There are really two related effects. Contact inhibition of movement means a crawling cell that bumps into a neighbor stops crawling in that direction, so cells do not pile over one another. Contact inhibition of proliferation (also called density-dependent inhibition) means that as cells become crowded and touch on all sides, signals passed through their adhesion contacts — including cadherins — tell each cell to stop dividing. The result, in a dish, is that normal cells grow until they form a single complete layer covering the surface and then halt, rather than heaping into mounds.

Contact inhibition matters because it is one of the brakes that keep a tissue the right size and a single layer thick — important in wound healing, where cells crawl in to cover a gap and then stop once the gap is filled. Its failure is a hallmark of cancer: cancer cells often ignore contact inhibition, continuing to divide and crawl even when crowded, so in a dish they pile up into disordered heaps instead of a tidy single layer. That loss of the 'neighbors are full, stop' signal is part of how tumors grow without limit.