The Immortal Animal

It has been said: "We are born; life is a bitch; then we die." How accurate is this? Well, no one can argue with the "being born" part. Of course, some creatures are hatched, some are created through budding or sprouting, some through other odd means -- but in the end, a new creature exists, is "born." As for the middle part of the quotation, there is some debate. Usually, a person with a large bank account will hold a different view than someone with no account at all. Opinions also vary according to present circumstances -- for instance, someone who generally disagrees with the statement will suddenly change sides when the air conditioner breaks down and eggs are frying on the sidewalk. As for the final part, it is generally agreed that death is inevitable for all living things -- the "great equalizer" -- although some people use this same phrase to describe gun ownership.

But do all things die? Viruses might not, but we are not yet sure if they are "alive" in the first place, or if they are merely strings of amino acids that fit into the wrong places at the wrong times. Also, it is too difficult to keep just one in a jar and observe it for a thousand years. Some bacteria could claim to go on forever, but when one reproduces by splitting into two, the two new creatures are not quite the same as the one original "parent." So bacteria are disqualified.
In an earlier blog entry ("Scientists Find World's Oldest Animal, Murder It"), I reviewed some of the longest living creatures on Earth, and although a 410-year-old clam won the title for animals, a 4,770-year-old pine tree has stood motionless the longest. However, even in these cases, the creatures are expected to die eventually.

So, it seems that all creatures die. Well, except one. There is one multi-celled organism, and apparently the only one on Earth -- animal or plant, fish or fowl, mammal or reptile -- that is biologically immortal.

First I need to explain "biologically immortal." Such a creature can be killed by predators, accidents, or disease. However, in the absence of those fates, a biologically immortal creature should never die of "old age." First I need to explain "die of old age." Do you ever wonder why people age and then die, even if they kept a good diet, exercised regularly, and did not play with guns? After all, the body is a well-oiled machine that replaces old cells with new ones, right? Why would such a process eventually break down?

Well, first I need to explain the Hayflick Limit, which describes the process of replicative senescence. Ok, I just threw in that last phrase, "replicative senescence," to scare you, or to provide you with a pickup line in a bar. Let's just stick with the Hayflick Limit. The point is that our body heals itself through cell division, replacing damaged cells with shiny new ones. However, it turns out that there is a limit to how many times a cell can divide, and that number is about 50. After that, the cell remains as it is, and eventually becomes damaged. As enough cells grow older and more damaged, the body's functions begin to break down and fail, and death is always the result. In some species, the rate of dividing cells is slower than others, thus longer lifespans for different animals.

So, because of the Hayflick Limit, cells stop dividing and the body will die of "old age". A creature that is biologically immortal can never die of old age. However, there is only one creature in the world who has beaten the system. Do the cells of this creature divide more than 50 times, or do they not need to divide, or do they divide so slowly that the number 50 is never reached? None of the above. Apparently, this creature has found a way to restart the 50 count from the beginning.

Turritopsis nutricula

Turritopsis nutricula is a jellyfish whose bell-shaped medusa is about 4.5 mm across, and the body is about the same length. It originated in the Caribbean, but it now is distributed around the world. Living forever apparently has its benefits for traveling to exotic places. So, how does it get around that nasty Hayflick thing? I am going to have to use another fancy phrase: cellular transdifferentiation.

You know how stem cells can become any other type of cell? No, you don't? Oh, well, "stem cells" -- specific cells often present in the fetal form of creatures -- can become any other type of cell (skin cell, muscle cell, brain cell, etc.), based on genetic input and the overall need of the emerging organism. Once a cell becomes a "differentiated" cell (a specific type), it cannot reverse backwards. In other words, a skin cell is a skin cell, and cannot transform into a brain cell, and vice versa. Otherwise, a stomach cell could become a skin cell, and beauty would no longer be skin deep. That is not the scientific explanation of this phenomenon.

Well, our immortal jellyfish somehow has the ability to perform cellular transdifferentiation -- transforming a non-stem cell into a different type of cell. This is a very rare occurrence in nature. In one example, if you remove the lens of an eye from a chicken, its iris cells can transform into lens cells, repairing the eye. This still does not explain, however, why you are removing a chicken's eye when all you are interested in is whether the wings should be breaded with regular or spicy herbs. In the case of the jellyfish, it transforms its entire body from a reproductive adult into its polyp form, which is the stage where the jellyfish begins its growth into a jellyfish. In human terms, it is like a middle-aged guy who has a wild night out on the town (my euphemism for "reproducing the species"), and then comes home and transforms himself into a stem-cell-rich fetus, to be reborn that morning. The cells believe they are new, not middle-aged. The downside is that the new baby will not remember which pickup lines worked the night before, except for the phrase "replicative senescence."

In essence, the jellyfish is changing its cells from old worn-out cells nearing the 50 count to brand new cells, restarting the count from zero. This process can go on indefinitely, so the jellyfish is biologically immortal.