The immortal embryo of german ponds
In the quiet freshwater ponds and streams of Germany lives a creature challenging definitions of aging and death. The Hydra vulgaris, a freshwater polyp measuring only about 10 millimeters in length, has a form of biological immortality. These simple invertebrates, relatives of jellyfish and corals, show no signs of deterioration with age. Studies have demonstrated that under the right lab conditions, their mortality rate remains constant, regardless of how old they are. Some researchers estimate that 5% of a hydra population could be alive after 1,400 years.
This continuous life is possible because a hydra's body is composed almost entirely of stem cells. It has three distinct stem cell lineages that constantly renew all its tissues; an adult hydra replaces all of its approximately 100,000 cells every 20 days. This process is regulated by the FoxO gene, a transcription factor also found in humans and linked to long life. In hydra, FoxO is highly expressed in all three stem cell lineages, driving their continuous self-renewal and preventing the decline of aging in most other multicellular organisms. Researchers in Kiel, Germany, have shown that suppressing FoxO in hydra leads to a reduction in stem cells and changes to the immune system that mirror those in elderly humans.
Regeneration from cellular soup
The hydra's regenerative capabilities were first documented in detail by Swiss naturalist Abraham Trembley in 1744. Trembley discovered that when he cut a polyp in half, each piece would regenerate into a complete, new organism. He even created a seven-headed hydra by repeatedly splitting the head partway down. Today, it is known that a hydra can regrow its entire body, including its simple nervous system, from a tissue fragment in about 72 hours.
The most extreme demonstration of this ability is regeneration from dissociated cells. If an adult polyp is mechanically broken apart into a suspension of individual cells, those cells can reaggregate and reorganize themselves. Within the first few hours, the epidermal and gastrodermal cells sort themselves into the original two-layer structure. Over the next three to five days, a complete new polyp, with a mouth, foot, and tentacles, forms from the cellular pellet. This process of self-organization, where the body plan is re-established from a state with no positional information, makes the hydra a model for how cells coordinate to build and repair complex tissues.