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Regenerating hydra

Virtually immortal!

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The regenerative capacity of some sea creatures can be considered “superhuman”. Particular genes are crucial. They are the key to stem cells constantly renewing themselves.

Dr. Klaus Duffner | Germany

When the Greek mythological hero Heracles set out to hunt down the quasi-serpentine Hydra, because she was always preying on herds of cattle, he had a problem. Whenever he lopped off one of Hydra’s nine heads, she grew two more.

The Genevan scholar Abraham Trembley must have had this legend in mind when he set about describing the native fresh water polyp in the 18th century. His experiments and observations on the barely one centimeter long, tentacled animal were seminal. He cut up the polyps, but they still clung to life. What is more, the severed body parts were fully able to regenerate, thus giving rise to several completely new polyps from one animal. Trembley could not resist naming this strange organism “hydra.”


Tremendous ability to regenerate

Experiments have shown that one hydra cut into tiny sections can bring forth up to 100 complete polyps, if there are at least 300 to 500 cells in the individual parts. So-called interstitial cells (I-cells) are the key to this amazing regenerative capacity. These undifferentiated stem cells continue to be able to divide throughout their lives and are continuously forming new nerve cells, gland cells, muscle cells, germ cells and – typical for the cnidaria phylum – cnidoblasts.


No aging, but death?

Are hydra immortal? To probe this question, researchers at the Rostock Max-Planck institute offered the “little critters” a completely carefree life for almost ten years: constantly uniform water temperature, regular food and no predators. The findings of their recently published study? Although individuals do succumb to a natural death, the mortality rate is identical at any age. No matter whether an individual is one or ten years old, the mortality risk remains the same – unlike humans whose mortality rate increases with age. In other words, a hydra does not age, and every cell is continuously renewed.


Genes for immortality

Why are their cells so durable? In March 2010 international scientists announced that they had succeeded in completely unraveling the genome of fresh water polyps. The genetic material of primitive polyps is unexpectedly voluminous and, at approximately 20,000 genes, is just as complex as that of vertebrates. The core gene for longevity is the so-called “FoxO.” It is not only found in fresh water polyps but in all animals, and it controls stem cell formation. If the FoxO gene is experimentally “switched off” in hydra, stem cell activity slows down drastically.

We humans also lose more stem cells as we age, and the remaining cells become less active. This is why aging tissues are so difficult to regenerate. Conversely, post-centenarians have been found to have particularly high levels of active FoxO, which gives rise to the name “Methuselah gene.” Aging processes are no issue for hydra. A polyp can replace its full complement of body cells in only five days. For this reason the researchers are certain that a hydra’s FoxO genes are the key to understanding (an infinitely) long life.

Dr. Klaus Duffner

Dr. Klaus Duffner | Germany

Scientific Journalist
Medizin & Wissen Freiburg


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