Hayflick limit

The Hayflick limit (or Hayflick phenomenon) is a phenomenon of cell division in tissue culture. It is the number of times a normal human cell population divides until cell division stops.^[1][2] We now know that the telomeres on each cell's chromosomes get slightly shorter with each new cell division, until they reach a critical length.^[3][4]

The idea of the Hayflick limit was suggested by American anatomist Leonard Hayflick in 1961,^[3] at the Wistar Institute in Philadelphia, Pennsylvania. Hayflick showed that a population of normal human fetal cells in a cell culture will divide between 40 to 60 times. The population then goes into senescence. This refutes the idea of Nobel laureate Alexis Carrel that normal cells are potentially immortal.^[5]

Each mitosis slightly shortens each of the telomeres on the chromosomes of the cells. Telomere shortening eventually makes cell division impossible, and this ageing of the cell population may cause the overall physical ageing of the human body.

The process of telomere shortening leads to apoptosis, the technical name given to programmed cell death. Although the Hayflick experiments were done on human cells in tissue culture, there is nothing special about human cells which would not be found in the cells of other living organisms. Apoptosis is thought to happen in all multicellular organisms.^[6] The 2002 Nobel Prize in Medicine was awarded to Sydney Brenner, H. Robert Horvitz and John E. Sulston for their work identifying genes that control apoptosis. The genes were found by studies in the nematode Caenorhabditis elegans. These same genes function in humans for apoptosis.

1. ↑ Sir Macfarlane Burnet, a Nobel laureate from Australia, coined the phrase "the Hayflick limit" for the first time in his book Intrinsic mutagenesis: a genetic approach to ageing, published in 1974.
2. ↑ Shay J.W. & Wright W.E. 2000. Hayflick, his limit, and cellular ageing. Nature Reviews Molecular Cell Biology. 1 (1): 72–76. Hayflick, his limit, and cellular ageing | Nature Reviews Molecular Cell Biology
3. ↑ ^{3.0 3.1} Hayflick L. & Moorhead P.S. (1961). "The serial cultivation of human diploid cell strains". Exp Cell Res. 25 (3): 585–621. doi:10.1016/0014-4827(61)90192-6. PMID 13905658.
4. ↑ Hayflick L. (1965). "The limited in vitro lifetime of human diploid cell strains". Exp. Cell Res. 37 (3): 614–636. doi:10.1016/0014-4827(65)90211-9. PMID 14315085.
5. ↑ Cancerous cells are the exception. Witkowsky, Jan 1985. The myth of cellular immortality. Science Direct. [1]
6. ↑ Green, Douglas R. (2011). Means to an end: apoptosis and other cell death mechanisms. Cold Spring Harbor Laboratory Press. ISBN 978-0-87969-888-1.