When Do Telomeres Matter?

Cultured human and rodent cells divide a set number of times before they cease dividing, a process called replicative senescence. But how do they know when to stop dividing? It appears that human cells have a way to count the number of divisions they have undergone: They monitor the progressive shortening of the ends of their chromosomes with each division. However, rodent cells do not appear to have any intrinsic counting method. In their Perspective, Shay and Wright now reveal that rodent cells, when grown under the correct culture conditions, can divide continuously ( Mathon et al., Tang et al.). They propose that cells from long-lived humans and short-lived rodents have adopted different methods to respond to the insults that they accumulate through their lifetimes.