The Hayflick limit was discovered by Leonard Hayflick in 1965. Hayflick observed that cells dividing in cell culture divided about 50 times before dying. As cells approach this limit, they show more signs of old age.
Except for HeLa cells the limit to the number of times a cell divides has been noted in all human cell types that have been fully differentiated, as well as in other organisms. It varies from cell type to cell type, and more significantly from organism type to organism type. The human limit is around 52. The limit has been linked to the shortening of telomeres, a region of DNA at the end of chromosomes?.
The limit is believed to be one of the causes of aging. It is believed that if the shortening of telomeres can be slowed or prevented, life expectancy can be extended. Much research is happening in this area.
Stem cells, by definition, have not yet been fully differentiated, and therefore many of these cells may continue to regenerate new cells for the entire lifespan of the organism, without limit, thus constituting a notable exception to the Hayflick limit in humans and other organisms. While the manifestations of the constant regenerative effects of stem cells is most easily seen in tissues which must constantly produce replacements for existing cells, such as skin and blood cells, stem cells of one form or another are found in every tissue of the human body, even if only as dormant stem cells known as "spore-like cells" ().
Cancer cells constitute the other main exception to the limits on cell division. It is believed that the Hayflick limit exists principally to help prevent cancer. If a cell becomes cancerous and the Hayflick limit is approaching, the cell will only be able to divide a certain number of times. Once it reaches this limiting number of divisions, the formed tumour will no longer be able to reproduce and the cells will die off. Cancers become problems after having found ways around the Hayflick limit. Cells that have found a way around the limit are referred to as "immortal". Such immortal cells may still die, but the group of immortalized cells produced from cell division of an immortal cell has no limit as to how many times cell division might take place among the cells that constitute such a group of immortalized cells.