Biological immortality (sometimes referred to as 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,
poison,
disease,
predation, lack of available resources, or changes to
environment.
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]
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.
The term "immortalization" was first applied to
cancer cells that expressed the telomere-lengthening
enzymetelomerase, 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.[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]Embryonic stem cells and
germ cells have also been described as immortal.[8][9]
According to the Animal Aging and Longevity Database, the list of animals with negligible aging (along with estimated longevity in the wild) includes:[12]
Greenland shark (Somniosus microcephalus) - 250 to 500 years
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.[13] 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 is not restored and will go on to age and die. In a similar manner
stem cells and
gametes can be regarded as "immortal".
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 contain post-
mitotic cells (cells that will never divide again) only in the extremities.[14] All hydra cells continually divide.[15] 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 subsequently able to maintain
telomere lengths.
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.[21] However, unlike vertebrates, lobsters express telomerase as adults through most tissue, which has been suggested to be related to their longevity.[22][23][24] Contrary to popular belief, lobsters are not immortal. Lobsters grow by
moulting, which requires considerable energy, and the larger the shell the more energy is required.[25] 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, causing them to die.[26] The
European lobster has an average life span of 31 years for males and 54 years for females.
Planarian flatworms
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".[27]
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".[28][29]
^Shay, J. W. & Wright, W. E. (2000). "Hayflick, his limit, and cellular ageing". Nature Reviews Molecular Cell Biology. 1 (1): 72–76.
doi:
10.1038/35036093.
PMID11413492.
S2CID6821048.
^Bulzomi, Pamela. "The Pro-apoptotic Effect of Quercetin in Cancer Cell Lines Requires ERβ-Dependant Signals." Cellular Physiology (2012): 1891-898. Web.
^Reitzer, Lawrence J.; Wice, Burton M.; Kennel, David (1978), "Evidence That Glutamine, Not Sugar, Is the Major Energy Source for Cultured HeLa Cells", The Journal of Biological Chemistry, 254 (April 25): 26X9–2676,
PMID429309
^Irfan Maqsood, M.; Matin, M. M.; Bahrami, A. R.; Ghasroldasht, M. M. (2013). "Immortality of cell lines: Challenges and advantages of establishment". Cell Biology International. 37 (10): 1038–45.
doi:
10.1002/cbin.10137.
PMID23723166.
S2CID14777249.
^Current Biology: Volume 23, Issue 19, 7 October 2013, Pages 1844–1852 "Fission Yeast Does Not Age under Favorable Conditions, but Does So after Stress." Miguel Coelho1, 4, Aygül Dereli1, Anett Haese1, Sebastian Kühn2, Liliana Malinovska1, Morgan E. DeSantis3, James Shorter3, Simon Alberti1, Thilo Gross2, 5, Iva M. Tolić-Nørrelykke1
^De Vito; et al. (2006). "Evidence of reverse development in Leptomedusae (Cnidaria, Hydrozoa): the case of Laodicea undulata (Forbes and Goodsir 1851)". Marine Biology. 149 (2): 339–346.
doi:
10.1007/s00227-005-0182-3.
S2CID84325535.
Dr. R. Michael Perry. Forever For All: Moral Philosophy, Cryonics, and the Scientific Prospects for Immortality, Universal Publishers, 2001.
ISBN1-58112-724-3
Martinez, D.E. (1998) "Mortality patterns suggest lack of senescence in hydra." Experimental Gerontology 1998 May;33(3):217–225.
Full text.
Rose, Michael; Rauser, Casandra L.; Mueller, Laurence D. (Spring 2011). Does Aging Stop?. Oxford University Press.