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]

According to the Animal Ageing and Longevity Database, the list of organisms with negligible aging (along with estimated longevity in the wild) includes:[10]

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 a 'tun' - essentially a shrivelled blob - a type of suspended animation (Cryptobiosis) where their metabolism slows to near zero and they simply wait out the harsh conditions until the environment is more favorable.

However they are not "immortal".[11] It has been estimated that a tardigrade would have a life span of less than a year if it never entered the cryptobiotic state. However, one that alternates active with cryptobiotic periods might survive for as long as sixty years - though more typically up to 10 years. The maximum known was a museum specimen of moss that had been kept dry for one hundred and twenty years which yielded a number of tardigrades. When the animals were moistened, a few of them revived, but all died within a few minutes.

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.[12] 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".

Excerpt from:
Biological immortality - Wikipedia, the free encyclopedia