<P> The Hayflick limit or Hayflick phenomenon is the number of times a normal human cell population will divide before cell division stops . </P> <P> The concept of the Hayflick limit was advanced by American anatomist Leonard Hayflick in 1961, at the Wistar Institute in Philadelphia, Pennsylvania, US . Hayflick demonstrated that a normal human fetal cell population will divide between 40 and 60 times in cell culture before entering a senescence phase . This finding refuted the contention by Nobel laureate Alexis Carrel that normal cells are immortal . </P>

The hayflick limit is a natural limit to the ____
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