The Hayflick limit, or Hayflick phenomenon, is the number of times a normal human cell population will divide before cell division stops.
The concept of the Hayflick limit was advanced by American anatomist Leonard Hayflick in 1961. 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 French Nobel laureate Alexis Carrel that normal cells are immortal.
Each time a cell undergoes mitosis, the telomeres on the ends of each chromosome shorten slightly. Cell division will cease once telomeres shorten to a critical length. Hayflick interpreted his discovery to be aging at the cellular level. The aging of cell populations appears to correlate with the overall physical aging of an organism.
The Hayflick limit has been found to correlate with the length of the telomeric region at the end of chromosomes. During the process of DNA replication of a chromosome, small segments of DNA within each telomere are unable to be copied and are lost. This occurs due to the uneven nature of DNA replication, where leading and lagging strands are not replicated symmetrically. The telomeric region of DNA does not code for any protein; it is simply a repeated code on the end region of linear eukaryotic chromosomes. After many divisions, the telomeres reach a critical length and the cell becomes senescent. It is at this point that a cell has reached its Hayflick limit.
Hayflick was the first to report that only cancer cells are immortal. This could not have been demonstrated until he had demonstrated that only normal cells are mortal. Cellular senescence does not occur in most cancer cells due to expression of an enzyme called telomerase. This enzyme extends telomeres, preventing the telomeres of cancer cells from shortening and giving them infinite replicative potential. A proposed treatment for cancer is the usage of telomerase inhibitors that would prevent the restoration of the telomere, allowing the cell to die like other body cells.