The Hayflick limit is the number of times a normal human cell population will divide
until cell division stops. Empirical evidence shows that the telomeres associated with each
cell's DNA will get slightly shorter with each new cell division until they shorten
to a critical length. The concept of the Hayflick limit was advanced
by American anatomist Leonard Hayflick in 1961, at the Wistar Institute in Philadelphia,
Pennsylvania. Hayflick demonstrated that a population of normal human fetal cells in
a cell culture will divide between 40 and 60 times. The population will then enter a
senescence phase, which refutes the contention by Nobel laureate Alexis Carrel that normal
cells are immortal. Each mitosis slightly shortens each of the telomeres on the DNA
of the cells. Telomere shortening in humans eventually makes cell division impossible,
and this aging of the cell population appears to correlate with the overall physical aging
of the human body. This mechanism also appears to prevent genomic instability. Telomere shortening
may also prevent the development of cancer in human aged cells by limiting the number
of cell divisions. However, shortening of telomeres impairs immune function and thus
might also increase susceptibility to cancer.
History Belief of cell immortality
Prior to Hayflick's discovery, it was believed that vertebrate cells had an unlimited potential
to replicate. Alexis Carrel, a Nobel prize-winning surgeon, had stated "that all cells explanted
in culture are immortal, and that the lack of continuous cell replication was due to
ignorance on how best to cultivate the cells". He supported this hypothesis by having cultivated
fibroblasts from chicken hearts, and to have kept the culture growing for 34 years. This
indicated that cells of vertebrates could continue to divide indefinitely in a culture.
However, other scientists have been unable to repeat Carrel's result.
Carrel's result is suspected to be due to an error in experimental procedure. To provide
required nutrients, embryonic stem cells of chickens may have been readded to the culture
daily. This would have easily allowed the cultivation of new, fresh cells in the culture,
so there was not an infinite reproduction of the original cells. If this is true, it
has been speculated that Carrel knew about the error, but he never admitted it.
Also, it has been theorized that the cells Carrel used were young enough to contain pluripotent
stem cells, which, if supplied with a supporting telomerase-activation nutrient, would have
been capable of staving off replicative senescence, or even possibly reversing it. Cultures not
containing telomerase-active pluripotent stem cells would have been populated with telomerase-inactive
cells, which would have been subject to the 50–60 mitosis cycles until apoptosis occurs
as described in Leonard Hayflick's findings. Experiment and discovery
Dr. Leonard Hayflick first became suspicious of Carrel's theory while working in a lab
at the Wistar Institute. Hayflick was preparing normal human cells to be exposed to extracts
of cancer cells when he noticed the normal cells had stopped proliferating. At first
he thought that he had made a technical error in preparing the experiment, but later he
began to think that the cell division processes had a counting mechanism. Working with Paul
Moorhead, a cytogeneticist, he designed an experiment to test Carrel's theory of cell
division. The experiment proceeded as follows. Hayflick
and Moorhead mixed equal numbers of normal human male fibroblasts that had divided many
times with female fibroblasts that had divided only a few times. Unmixed cell populations
were kept as controls. When the male control culture stopped dividing, the mixed culture
was examined and only female cells were found. This showed that the old male cells remembered
they were old, even when surrounded by young cells, and that technical errors or contaminating
viruses were unlikely explanations as to why only the male cell component had died. The
cells had stopped dividing and had become senescent based purely upon how many times
the cell had divided. These results disproved the immortality theory
of Carrel and established the Hayflick limit as a credible biological theory that, unlike
Carrel's experiment, has been repeated by other scientists.
Cell phases Hayflick describes three phases in the life
of a cell. At the start of his experiment he named the primary culture "phase one".
Phase two is defined as the period when cells are proliferating – Hayflick called it the
time of "luxuriant growth". After months of doubling the cells eventually reach phase
three, a phenomenon of senescence – cell growth diminishes and then cell division stops
altogether. Telomere length
The Hayflick limit has been found to correlate with the length of the telomere region at
the end of a strand of DNA. During the process of DNA replication, small segments of DNA
at each end of the DNA strand are unable to be copied and are lost after each time DNA
is duplicated. The telomere region of DNA does not code for any protein; it is simply
a repeated code on the end region of DNA that is lost. After many divisions, the telomeres
become depleted and the cell begins apoptosis. This is a mechanism that prevents replication
error that would cause mutations in DNA. Once the telomeres are depleted, due to the cell
dividing many times, it will no longer divide. This is when the cell has reached its Hayflick
limit. This process does not take place in cancer
cells due to an enzyme called telomerase. This enzyme maintains telomere length, which
results in the telomeres of cancer cells never shortening. This gives these cells 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. On the other hand, telomerase activators might repair or extend the telomeres
of healthy cells, thus extending their Hayflick limit. Telomerase activation might also lengthen
the telomeres of immune system cells enough to prevent cancerous cells from developing
from cells with very short telomeres. Carnosine can increase the Hayflick limit
in human fibroblasts, and also appears to reduce the rate of telomere shortening.
In popular culture The concept of Hayflick limit was used in
the SF movie "Anacondas 2 - The search for Blood orchid".
See also Apoptosis
Biological immortality HeLa cells
Induced stem cells Leonard Hayflick
Telomerase Notes
References
Literature Watts, Geoff. "Leonard Hayflick and the limits
of ageing". The Lancet 377: 2075. doi:10.1016/S0140-6736(11)60908-2. Harley, Calvin B.; Futcher, A. Bruce; Greider,
Carol W.. "Telomeres shorten during ageing of human fibroblasts". Nature 345: 458–60.
doi:10.1038/345458a0. PMID 2342578. Gavrilov LA, Gavrilova NS. The Biology of
Life Span: A Quantitative Approach. New York: Harwood Academic Publisher. ISBN 3-7186-4983-7.
Gavrilov LA, Gavrilova NS. "How many cell divisions in 'old' cells?". Int. J. Geriatric
Psychiatry 8: 528–528. Wang, Richard C.; Smogorzewska, Agata; De
Lange, Titia. "Homologous Recombination Generates T-Loop-Sized Deletions at Human Telomeres".
Cell 119: 355–68. doi:10.1016/j.cell.2004.10.011. PMID 15507207.
Watson, J. M.; Shippen, D. E.. "Telomere Rapid Deletion Regulates Telomere Length in Arabidopsis
thaliana". Molecular and Cellular Biology 27: 1706–15. doi:10.1128/MCB.02059-06. PMC 1820464.
PMID 17189431. External links
Cell immortality and cancer Historical review of studies on cell division
limit