They don't die. They just sit there. What causes them to stop dividing is one of the major questions in aging research. Part of the question may have been answered earlier this year by investigators at the Geron Corporation of Menlo Park, CA, and the University of Texas Southwestern Medical Center at Dallas. Using cultures of cells from the human retina and skin, researchers found that replicative senescence could be greatly delayed if they inserted a gene coding for an enzyme called telomerase into these cells.
All human chromosomes have telomere structures at their ends.
Each time the cell divides, the telomeres get a bit shorter. When they become too short, the cells apparently are prevented from continuing to divide. Telomerase reverses this process, lengthening the telomeres and delaying replicative senescence. Effros has measured telomeres of human T-cells and observed a similar effect. They actually reach a certain 'magic' telomere length beyond which they will not divide any further.