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Biological immortality refers to a stable or decreasing rate of mortality from cellular senescence as a function of chronological age. Various unicellular and multicellular species may 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 new 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.

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 apoptosisi.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 Tantigen,[8] commonly introduced through simian virus 40 (SV-40).[9]

Tardigrades, also known as "water bears", are highly resilient microscopic animals. They are capable of surviving extremes such as heat, radiation, drought, and the vacuum of space by going into suspended animation, where their metabolism slows to near zero and they simply wait out the harsh conditions until the environment is more favorable.

Bacteria are said to be biologically immortal, but only at the level of the colony. The two daughter bacteria resulting from cell division of a parent bacterium can be regarded as unique individuals or as members of a biologically "immortal" colony. The two daughter cells can be regarded as "rejuvenated" copies of the parent cell because damaged macromolecules have been split between the two cells and diluted. In the same way stem cells and gametes can be regarded as "immortal".

Hydras are a genus of simple, freshwater animals possessing radial symmetry and no post-mitotic cells. All hydra cells continually divide. It has been suggested that hydras do not undergo senescence, and, as such, are biologically immortal.[10] However, this does not explain how hydras are consequently able to maintain telomere lengths.

Turritopsis dohrnii, or Turritopsis nutricula, is a small (5 millimeters (0.20in)) 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.

Excerpt from:
Biological immortality - Wikipedia, the free encyclopedia