Hayflick limit
Encyclopedia
The Hayflick limit is the number of times a normal cell population will divide before it stops, presumably because the telomeres reach a critical length.
The Hayflick limit was discovered by Leonard Hayflick
in 1961, at the Wistar Institute
, Philadelphia, when Hayflick demonstrated that a population of normal human
fetal
cells in a cell culture
divide between 40 and 60 times. It then enters a senescence phase (refuting the contention by Alexis Carrel
that normal cells are immortal
). Each mitosis
shortens the telomeres on the DNA of the cell. Telomere shortening in humans eventually makes cell division impossible, and it correlates with aging. This mechanism appears to prevent genomic instability and the development of cancer
.
cells had an unlimited potential to replicate. Alexis Carrel
, a Nobel prize
-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 claiming to have cultivated fibroblasts from chick
hearts, and to have kept the culture growing for 34 years. This indicated that cells of vertebrate
s could continue to divide indefinitely in a culture. However, other scientists were unable to reproduce Carrel's result.
In fact, Carrel's result was due to an error in his experimental procedure: chick embryonic stem cells were added to the culture daily. This allowed for the cultivation of new fresh cells in the culture, and not simply the infinite reproduction of the original cells present in the culture. It has been speculated that Carrel knew about the error, but he never admitted it.
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 Morehead, he designed an experiment that showed the truth about normal cell division.
The experiment proceeded as follows. Hayflick and Morehead mixed equal numbers of normal human male fibroblasts that had divided many times (cells at the 40th population doubling) with female fibroblasts that had divided only a few times (cells at the 10th population doubling). 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 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 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 accredited biological theory which, unlike the experiment of Carrel, has been reproduced by other scientists.
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 (telomeres) 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, the cell will no longer divide and the Hayflick limit has been reached.
This process errs in cancer cell
s. Cancer cells turn on an enzyme
called telomerase
which is able to restore telomere length. Thus the telomere of cancer
cells is never shortened, giving these cells infinite replicative potential. A proposed treatment for cancer is a telomerase inhibitor 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 perhaps extend the telomeres, thus extending the Hayflick limit of healthy cells. This might strengthen 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.
The Hayflick limit was discovered by Leonard Hayflick
Leonard Hayflick
Leonard Hayflick , Ph.D., is Professor of Anatomy at the University of California, San Francisco, School of Medicine, and was Professor of Medical Microbiology at Stanford University School of Medicine. He is a past president of the Gerontological Society of America and was a founding member of the...
in 1961, at the Wistar Institute
Wistar Institute
The Wistar Institute is a biomedical center, with a focus on cancer research and vaccine development. It is located in the University City section of Philadelphia, Pa...
, Philadelphia, when Hayflick demonstrated that a population of normal human
Human
Humans are the only living species in the Homo genus...
fetal
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
cells in a cell culture
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
divide between 40 and 60 times. It then enters a senescence phase (refuting the contention by Alexis Carrel
Alexis Carrel
Alexis Carrel was a French surgeon and biologist who was awarded the Nobel Prize in Physiology or Medicine in 1912 for pioneering vascular suturing techniques. He invented the first perfusion pump with Charles A. Lindbergh opening the way to organ transplantation...
that normal cells are immortal
Biological immortality
Biological immortality refers to a stable rate of mortality as a function of chronological age. Some individual cells and entire organisms in some species achieve this state either throughout their existence or after living long enough. This requires that death occur from injury or disease rather...
). Each mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
shortens the telomeres on the DNA of the cell. Telomere shortening in humans eventually makes cell division impossible, and it correlates with aging. This mechanism appears to prevent genomic instability and the development of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
.
Belief of cell immortality
Prior to Hayflick's discovery, it was believed that vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
cells had an unlimited potential to replicate. Alexis Carrel
Alexis Carrel
Alexis Carrel was a French surgeon and biologist who was awarded the Nobel Prize in Physiology or Medicine in 1912 for pioneering vascular suturing techniques. He invented the first perfusion pump with Charles A. Lindbergh opening the way to organ transplantation...
, a Nobel prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
-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 claiming to have cultivated fibroblasts from chick
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
hearts, and to have kept the culture growing for 34 years. This indicated that cells of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s could continue to divide indefinitely in a culture. However, other scientists were unable to reproduce Carrel's result.
In fact, Carrel's result was due to an error in his experimental procedure: chick embryonic stem cells were added to the culture daily. This allowed for the cultivation of new fresh cells in the culture, and not simply the infinite reproduction of the original cells present in the culture. It has been speculated that Carrel knew about the error, but he never admitted it.
Experiment and discovery
Dr. Leonard HayflickLeonard Hayflick
Leonard Hayflick , Ph.D., is Professor of Anatomy at the University of California, San Francisco, School of Medicine, and was Professor of Medical Microbiology at Stanford University School of Medicine. He is a past president of the Gerontological Society of America and was a founding member of the...
first became suspicious of Carrel’s theory while working in a lab at the Wistar Institute
Wistar Institute
The Wistar Institute is a biomedical center, with a focus on cancer research and vaccine development. It is located in the University City section of Philadelphia, Pa...
. Hayflick was preparing normal human cells to be exposed to extracts of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
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
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
processes had a counting mechanism. Working with Paul Morehead, he designed an experiment that showed the truth about normal cell division.
The experiment proceeded as follows. Hayflick and Morehead mixed equal numbers of normal human male fibroblasts that had divided many times (cells at the 40th population doubling) with female fibroblasts that had divided only a few times (cells at the 10th population doubling). 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 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 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 accredited biological theory which, unlike the experiment of Carrel, has been reproduced 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 stops altogether.Telomere length
This limit has been found to correlate with the length of the telomereTelomere
A telomere is a region of repetitive DNA sequences at the end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos "end" and merοs "part"...
region at the end of a strand of DNA. During the process of DNA replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
, small segments of DNA at each end of the DNA strand (telomeres) 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
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
. 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, the cell will no longer divide and the Hayflick limit has been reached.
This process errs in cancer cell
Cancer cell
Cancer cells are cells that grow and divide at an unregulated, quickened pace. Although cancer cells can be quite common in a person they are only malignant when the other cells fail to recognize and/or destroy them. In the past a common belief was that cancer cells failed to be recognized and...
s. Cancer cells turn on an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
called telomerase
Telomerase
Telomerase is an enzyme that adds DNA sequence repeats to the 3' end of DNA strands in the telomere regions, which are found at the ends of eukaryotic chromosomes. This region of repeated nucleotide called telomeres contains non-coding DNA material and prevents constant loss of important DNA from...
which is able to restore telomere length. Thus the telomere of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
cells is never shortened, giving these cells infinite replicative potential. A proposed treatment for cancer is a telomerase inhibitor 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 perhaps extend the telomeres, thus extending the Hayflick limit of healthy cells. This might strengthen the telomeres of immune system cells enough to prevent cancerous cells from developing from cells with very short telomeres.
Carnosine
Carnosine
Carnosine is a dipeptide of the amino acids beta-alanine and histidine. It is highly concentrated in muscle and brain tissues....
can increase the Hayflick limit in human fibroblasts, and also appears to reduce the rate of telomere shortening.
Literature
- Harley C, Futcher A & Greider C (1990) Telomeres shorten during ageing of human fibroblasts, Nature, 345, 458–460.
- Leonid A. Gavrilov & Natalia S. Gavrilova (1991) The Biology of Life Span: A Quantitative Approach. New York: Harwood Academic Publisher, ISBN 3-7186-4983-7 (see section 5.6 there)
- Gavrilov, L.A., Gavrilova, N.S. (1993). How many cell divisions in 'old' cells? Int. J. Geriatric Psychiatry, 8(6): 528-528.
- Wang R, Smogorzewska A & Lange T (2004) Homologous Recombination Generates T-Loop-Sized Deletions at Human Telomeres, Cell, 119, 355–368.
- Watson J & Shippen D (2007) Telomere Rapid Deletion Regulates Telomere Length in Arabidopsis thaliana, Molecular and Cellular Biology, 27(5), 1706-1715.