Nobody likes the idea of getting old. Despite our many efforts to escape or delay the aging process, it seems to be an inevitable part of life. IT SEEMS. But why? Why do living things gradually fall apart as they age?
There's a word for that: senescence. It is the state of gradual deterioration of our body, the reason why we do not stay young for long. At the cellular level, cells stop dividing and eventually die. It can also apply to an entire organism (a living being can no longer respond adequately to external stressors) or to specific organs or tissues (such as leaves dying and falling from trees in autumn). Yes, there are ways we can slow (or speed up) the rate at which senescence occurs, and it happens one way or another. However, some species can escape the aging process completely and essentially live forever.
The “immortal” jellyfish, turritopsis dohrnii
To date, there is only one species that has been defined as "biologically immortal": the turritopsis dohrnii jellyfish or turritopsis nutricula jellyfish. This small transparent animal is found in oceans around the world and practically does not die. The jellyfish that never dies can “go back in time” to an earlier stage in its life cycle.
The jellyfish's new life begins with a fertilized egg that grows into a larval stage called a planula. After a quick swim, the planula attaches itself to a surface (like a rock, or the ocean floor, or the hull of a boat), where it develops into a small organism. A sort of “octopus”: a tube-shaped structure with a mouth at one end and a sort of “foot” at the other end. It remains stuck in place for some time, becoming a small colony.
Eventually, depending on the jellyfish species, one of these “polyps” will form a growth called a “bud,” or produce segments that can break away from the rest of the colony. This process leads to the next stages of the jellyfish life cycle: the ephyra (a small jellyfish) and the actual “adult” jellyfish, capable of sexual reproduction.
For most other jellyfish, this stage is the end of the line. But turritopsis dohrnii (and perhaps other jellyfish species as well) has a trick. When the “eternal jellyfish” faces some sort of environmental stress, such as starvation or injury, it can revert to a small patch of tissue, which then transforms into a sexually immature polyp phase of life.
The immortal jellyfish is a bit like a butterfly turning into a caterpillar or a frog turning back into a tadpole.
Of course, turritopsis dohrnii can always be killed. Jellyfish are biologically immortal, but not invulnerable to trauma. Except, in fact, the jellyfish animal does not die of old age. In theory it can live forever. But is it the only immortal animal? We see.
Can the mythical hydra animal live forever?
The hydra looks a bit like the “polyp” stage of a jellyfish (which makes sense, given that jellyfish and hydra are grouped in the same phylum, Cnidaria): a tubular body with a tentacle-shaped ring mouth one end and an adhesive foot at the other. If jellyfish are immortal, the animal hydra holds its own quite well in terms of animal longevity. Hydras are very simple organisms that spend their days mostly in one place in freshwater ponds or rivers and using their stinging tentacles to grab any prey that comes within range.
Instead of gradually deteriorating over time, a Hydra's stem cells have the ability to “live forever,” to infinitely self-renew. This appears to be due to a particular set of genes called FoxO genes, which are found in living things (from worms to humans) and play a role in regulating cell lifespan.
In the case of Hydra stem cells, there appears to be an overabundance of FoxO gene expression. When the researchers prevented the FoxO genes from working, they found that the Hydra cells began to show signs of aging, and the Hydras were no longer young for long. We don't yet know exactly how it works, but we know that these genes clearly play an important role in maintaining Hydra's infinite youth.
Even immortal lobsters? Almost
Lobsters also do not experience senescence. Unlike Hydra's reliance on certain genes, however, their longevity is due to their ability to endlessly repair their DNA.
Normally, during the process of copying DNA and cell division, the protective caps on the chromosomes, called telomeres, become increasingly short and, when they are too short, a cell goes into senescence and can no longer continue to divide. In other words, they cannot live forever.
Lobsters don't have this problem thanks to an endless supply of an enzyme called telomerase, which works to maintain telomere regeneration. They produce a lot of this enzyme in all of their cells during adult life, allowing them to keep their DNA and functions young for a long time (indefinitely).
Telomerase is not unique to lobsters. It is present in most other animals, including humans, but after passing the embryonic life stage, telomerase levels in most other cells drop and are not enough to constantly rebuild telomeres.
Unfortunately for lobsters, though, there's a problem: they literally outgrow their shells. Lobsters get bigger and bigger, but their shells cannot change size, which means a lifetime of shells that are too small and a continuous rebuilding of a shell that is too tight, which tends to break sooner. This requires a fair amount of energy. Eventually, the amount of energy needed to move one shell and grow another is simply too much, and the lobster succumbs. A significant difference compared to a jellyfish animal.
Forever young, young forever (or long).
There are many other animal (and non-animal!) species that show prospects for an existence without old age. The risk of dying for naked mole rats appears not to increase with age. The world's oldest animal, a sea-dwelling (very stress-resistant) Quahog mollusk called Ming, died (by chance) only after a full 500 years when researchers dragged it out of the ocean to find out how old it was. Incredibly old bristlecone pines appear to have the same fluidity as younger trees. One particular colony of quaking aspens is thought to be around 80.000 years old… and there are many other species of exceptional longevity that seem to defy time. Animals that start with o and animals that start with e seem to be the only ones that have to die. Joke. Although “E” is also the initial of “Human Beings”. Already. Jellyfish, animals and lobsters are doing well: but can they help us too?
Do they hold the key to eternal youth for humans too? Can man become another immortal animal?
We know that aging in humans is due to a multitude of factors, many of which we still do not fully understand. Perhaps these examples from other species can shed light on these processes and also allow humans to live forever, or stay young for a long time.