The Hayflick limit was discovered by Leonard Hayflick in 1965, at the Wistar Institute (Philadelphia), when Hayflick demonstrated that normal human cells in a cell culture divide about 52 times in 20% oxygen (i.e., practically normal air) or 70 times in 3% oxygen (which is the same as human internal conditions). It then enters a senescence phase (refuting the contention by Alexis Carrel that normal cells are immortal). Each mitosis shortens the telomere appendix on the DNA of the cell, thus ticking back an \"inner clock\" for each subsequent copy of the cell. Some organisms\' cells do not encounter the Hayflick limit due to telomere lengthening; for example, the cells of some long-lived sea-birds such as Leach\'s Petrel are technically immortal
It will thus be seen that the stoppage of further division of the cells is not because of damage in all cases but the basic makeup of the cells.