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Normal somatic cells when grown in culture will become growth inhibited when they encounter another cell.  The cells in our bodies are governed by growth control mechanisms and cellular senescence (aging). Cell aging puts a limit on the number of times a cell can divide: the more a cell has divided, the less likely it will be to divide again. Growth mechanisms are in place to stop or continue cell growth depending on the conditions. Contact inhibition is a growth mechanism. In most cases when two cells collide they attempt to move in a different direction to avoid future collisions.
Contact Inhibition of Growth
Contact inhibition is a growth mechanism which functions to keep cells growing into a layer one cell thick (a monolayer). If a cell has plenty of free space, it replicates rapidly and moves freely. This process keeps happening until the cells have divided so many times there is no longer any room in the layer for them to replicate. At this point, normal cells will stop replicating.
Cancerous cells typically lose this property and thus grow in an uncontrolled manner even when in contact with neighbouring cells. They aren't motivated to change direction upon contact, so they pile up and grow over each other. Cells of naked mole rats, a species in which cancer has never been observed, show hypersensitivity to contact inhibition.
Contact Inhibition of Locomotion
In most cases, when two cells collide they attempt to move in a different direction to avoid future collisions. As replication increases the amount of cells, the number of directions those cells can move without touching another is decreased. As the two cells come into contact, their locomotive process is paralyzed. Cells will also attempt to move away from another cell because they stick better to the area around them, a structure called the substratum, than on other cells. When the two cells colliding are different types of cells, one or both may respond to the collision.
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