Rat Cells Human cells, extracted from the body and grown in a culture dish, don't divide forever. Until now, it had been a riddle to many researchers as to why cultured rodent cells did stop dividing, despite the fact that they have an enzyme called telomerase, which keeps their telomeres at a constant length. But the reason, as it turns out, is probably that these cells were kept under the wrong conditions. Unlike cancer cells, though, which also grow forever but are unstoppable, these cells still stop dividing if, for example, they get damaged by x-rays.
Humans are larger and live longer than rats.
It is more likely that one of their many cells, over a life time, will acquire enough mutations to turn into a cancer cell. One way to prevent this malignant transition is to limit the number of cell divisions. In a rat, this safety measure might not be necessary. Turning Back the Strands of Time , Scientific American , February 1998.