Biological immortality (sometimes referred to bio-indefinite mortality) is a state in which the rate of mortality from senescence is stable or decreasing, thus decoupling it from chronological
age. Various unicellular and multicellular species, including some
vertebrates, achieve this state either throughout their existence or
after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury or disease.
This definition of immortality has been challenged in the Handbook of the Biology of Aging,[1] because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high.[2] As a hypothetical example, there is only a 50% chance of a human surviving another year at age 110 or greater.
The term is also used by biologists to describe cells that are not subject to the Hayflick limit on how many times they can divide.
This definition of immortality has been challenged in the Handbook of the Biology of Aging,[1] because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high.[2] As a hypothetical example, there is only a 50% chance of a human surviving another year at age 110 or greater.
The term is also used by biologists to describe cells that are not subject to the Hayflick limit on how many times they can divide.
Cell lines
Biologists chose the word "immortal" to designate cells that are not subject to the Hayflick limit, the point at which cells can no longer divide due to DNA damage or shortened telomeres. Prior to Leonard Hayflick's theory, Alexis Carrel hypothesized that all normal somatic cells were immortal.[3]The term "immortalization" was first applied to cancer cells that expressed the telomere-lengthening enzyme telomerase, and thereby avoided apoptosis—i.e. cell death caused by intracellular mechanisms. Among the most commonly used cell lines are HeLa and Jurkat, both of which are immortalized cancer cell lines. HeLa cells originated from a sample of cervical cancer taken from Henrietta Lacks in 1951.[4] These cells have been and still are widely used in biological research such as creation of the polio vaccine,[5] sex hormone steroid research,[6] and cell metabolism.[7] Normal stem cells and germ cells can also be said to be immortal (when humans refer to the cell line).[citation needed]
Immortal cell lines of cancer cells can be created by induction of oncogenes or loss of tumor suppressor genes. One way to induce immortality is through viral-mediated induction of the large T‑antigen,[8] commonly introduced through simian virus 40 (SV-40).[9]
Organisms
According to the Animal Aging and Longevity Database, the list of organisms with negligible aging (along with estimated longevity in the wild) includes:[10]- Rougheye rockfish (Sebastes aleutianus) – 205 years
- Olm (Proteus anguinus) – 102 years
- Painted turtle (Chrysemys picta) – 61 years
- Blanding's turtle (Emydoidea blandingii) – 77 years
- Eastern box turtle (Terrapene carolina) – 138 years
- Red sea urchin (Strongylocentrotus franciscanus) – 200 years
- Ocean quahog clam (Arctica islandica) – 507 years
- Great Basin bristlecone pine (Pinus longaeva) – 4,713 years
- Greenland Shark – 400 years[11]
Bacteria and some yeast
Many unicellular organisms age: as time passes, they divide more slowly and ultimately die. Asymmetrically dividing bacteria and yeast also age. However, symmetrically dividing bacteria and yeast can be biologically immortal under ideal growing conditions.[15] In these conditions, when a cell splits symmetrically to produce two daughter cells, the process of cell division can restore the cell to a youthful state. However, if the parent asymmetrically buds off a daughter only the daughter is reset to the youthful state—the parent isn't restored and will go on to age and die. In a similar manner stem cells and gametes can be regarded as "immortal".Hydra
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Hydra
Hydras are a genus of the Cnidaria phylum. All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually. Hydras are simple, freshwater animals possessing radial symmetry and no post-mitotic cells. All hydra cells continually divide.[citation needed] It has been suggested that hydras do not undergo senescence, and, as such, are biologically immortal. In a four-year study, 3 cohorts of hydra did not show an increase in mortality with age. It is possible that these animals live much longer, considering that they reach maturity in 5 to 10 days.[16] However, this does not explain how hydras are consequently able to maintain telomere lengths.
Jellyfish
Turritopsis dohrnii, or Turritopsis nutricula, is a small (5 millimeters (0.20 in)) species of jellyfish that uses transdifferentiation to replenish cells after sexual reproduction. This cycle can repeat indefinitely, potentially rendering it biologically immortal. This organism originated in the Caribbean sea, but has now spread around the world. Similar cases include hydrozoan Laodicea undulata[17] and scyphozoan Aurelia sp.1.[18]Lobsters
Research suggests that lobsters may not slow down, weaken, or lose fertility with age, and that older lobsters may be more fertile than younger lobsters. This does not however make them immortal in the traditional sense, as they are significantly more likely to die at a shell moult the older they get (as detailed below).Their longevity may be due to telomerase, an enzyme that repairs long repetitive sections of DNA sequences at the ends of chromosomes, referred to as telomeres. Telomerase is expressed by most vertebrates during embryonic stages but is generally absent from adult stages of life.[19] However, unlike vertebrates, lobsters express telomerase as adults through most tissue, which has been suggested to be related to their longevity.[20][21][22] Contrary to popular belief, lobsters are not immortal. Lobsters grow by moulting which requires a lot of energy, and the larger the shell the more energy is required.[23] Eventually, the lobster will die from exhaustion during a moult. Older lobsters are also known to stop moulting, which means that the shell will eventually become damaged, infected, or fall apart and they die.[24] The European lobster has an average life span of 31 years for males and 54 years for females.
Planarian flatworms
Polycelis felina, a freshwater planarian
Planarian flatworms have both sexually and asexually reproducing types. Studies on genus Schmidtea mediterranea suggest these planarians appear to regenerate (i.e. heal) indefinitely, and asexual individuals have an "apparently limitless [telomere] regenerative capacity fueled by a population of highly proliferative adult stem cells". "Both asexual and sexual animals display age-related decline in telomere length; however, asexual animals are able to maintain telomere lengths somatically (i.e. during reproduction by fission or when regeneration is induced by amputation), whereas sexual animals restore telomeres by extension during sexual reproduction or during embryogenesis like other sexual species. Homeostatic telomerase activity observed in both asexual and sexual animals is not sufficient to maintain telomere length, whereas the increased activity in regenerating asexuals is sufficient to renew telomere length... "[25]
Lifespan: For sexually reproducing planaria: "the lifespan of individual planarian can be as long as 3 years, likely due to the ability of neoblasts to constantly replace aging cells". Whereas for asexually reproducing planaria: "individual animals in clonal lines of some planarian species replicating by fission have been maintained for over 15 years". They do not live forever.[26]
Attempts to engineer biological immortality in humans
Although the premise that biological aging can be halted or reversed by foreseeable technology remains controversial,[27] research into developing possible therapeutic interventions is underway.[28] Among the principal drivers of international collaboration in such research is the SENS Research Foundation, a non-profit organization that advocates a number of what it claims are plausible research pathways that might lead to engineered negligible senescence in humans.[29][30]In 2015, Elizabeth Parrish, CEO of BioViva, treated herself using gene therapy, with the goal of not just halting, but reversing aging.[31] She has since reported feeling more energetic, but long-term study of the treatment is ongoing.[citation needed]
For several decades,[32] researchers have also pursued various forms of suspended animation as a means by which to indefinitely extend mammalian lifespan. Some scientists have voiced support[33] for the feasibility of the cryopreservation of humans, known as cryonics. Cryonics is predicated on the concept that some people considered clinically dead by today's medicolegal standards are not actually dead according to information-theoretic death and can, in principle, be resuscitated given sufficient technological advances.[34] The goal of current cryonics procedures is tissue vitrification, a technique first used to reversibly cryopreserve a viable whole organ in 2005.[35][36]
Similar proposals involving suspended animation include chemical brain preservation. The non-profit Brain Preservation Foundation offers a cash prize valued at over $100,000 for demonstrations of techniques that would allow for high-fidelity, long-term storage of a mammalian brain.[37]
In 2016, scientists at the Buck Institute for Research on Aging and the Mayo Clinic employed genetic and pharmacological approaches to ablate pro-aging senescent cells, extending healthy lifespan of mice by over 25%. The startup Unity Biotechnology is further developing this strategy in human clinical trials.[38]
In early 2017, Harvard scientists headed by biologist David Sinclair announced they have tested a compound called NAD+ on mice and have successfully reversed the cellular aging process and can protect the DNA from future damage.[39] "The old mouse and young mouse cells are indistinguishable", David was quoted. Human trials are to begin shortly in what the team expect is 6 months at Brigham and Women's Hospital, in Boston.[citation needed]
