What are the signals that make cells stop dividing?
A lack of positive external signals causes cells to stop dividing.
Contact inhibition
Cells are also able to sense their surroundings and respond to changes. For instance, if a cell senses that it is surrounded on all sides by other cells, it will stop dividing. In this way, cells will grow when needed but stop when their goal has been met. To revisit our wound example, the cells fill in the gap left by the wound but then they stop dividing when the gap has been sealed. Cancer cells do not exhibit contact inhibition. They grow even when they are surrounded by other cells causing a mass to form. The behavior of normal (top animation) and cancer cells (bottom animation) with regard to contact inhibition is depicted below.
The round containers in which the cells are depicted in the animations are called petri dishes. In the laboratory, cells are often grown in these, covered with a nutrient-rich liquid.
Cellular Senescence
Most cells also seem to have a pre-programmed limit to the number of times that they can divide. Interestingly, the limit seems to be based, in part, on the cell's ability to maintain the integrity of its DNA. An enzyme, telomerase, is responsible for upkeep of the ends of the chromosomes. In adults, most of our cells don't utilize telomerase so they eventually die. In cancer cells, telomerase is often active and allows the cells to continue to divide indefinitely. For more information on telomerase, see the Cancer Genes section
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