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.