It may seem like mythology when talking about hydra and immortality however amazingly it seems both hydra and the turritopsis nutricula are biologically immortal with some key differences.
Hydra is a genus that encompasses small fresh water animals, which diverged from metazoans very early on in the history of evolution. As many people may know, there is a myth about a gigantic creature with three/more heads and whenever one was cut off two more would sprout in its place. Today’s hydra are not quite so big (30mm with only 4-25 tentacles) and feed on creatures such as daphnia and cyclops - the tiny freshwater invertebrates not fairies and one-eyed giants. They can move in an inchworm fashion and reproduce asexually - the young bud off from the parent. However these facts are not nearly as interesting as the fact that it can undergo morphalaxis - pretty much you can cut it in half and it will form two working independent hydras. The new Hydra are smaller and contain relatively the same amount of cells as half the original organism but reorganized. Some biologists in the 19th century even reported regrouping of the cells after the organism after it had been forced through a fine mesh! So this organism can be cut up - but it will still die of old age right? Well a paper by D.E. Martinez suggests that in fact hydras don't undergo senescence a process where we age and cells develop so that an organism is in its prime state for reproduction and then it deteriorates there after. His paper describes a trend whereby the reproduction rates of individual organisms were not related to their age but rather the environment. This suggests that senescence does not occur. Along with this he studied mortality rates. During his four-year study mortality rates were very low compared with the age of first reproduction. There is a graph on his paper showing the relationship between these two factors for various animals. The hydra is a clear outlier of this graph and with the minimum life span of a hydra being at least twenty times as long as the expected life span (1.2 - 2.6 months). Nobody knows how long the life span of the hydra actually is. Despite its amazing feats in survival the hydra is not a very complex organism - it lacks specific tissues and organs and as a hermaphrodite its method of reproduction leaves a lot to be desired.
The turritopsis nutricula outdoes hydra in that it gets younger after optimizing itself for sexual reproduction. Normally jellyfish go from the being polyps (these are sexually immature) to being medusae (sexually mature morphs of the jellyfish) and then they die - this is the usual process of senescence. However this particular jellyfish can revert back to being polyps after sexually maturing through the process of transdifferentiation which affects the exumbrellar epidermis and canal system. In this case transdifferentiation is the process by which non-stem cells transform into a different type of cell, this is very rare in the animal kingdom and is usually involved in regeneration of certain tissues. However it seems all T. nutricula undergo this process as a part of its life cycle. Basically the T. nutricula retracts its tentacles and flips inside out, after which it develops typical polyp components such as stolons. The 100% occurrence rate means that essentially this organism will not die of old age. Also the method reproduction is distinctly different to hydra in that it has eggs which need to be fertilized and deposited in the seabed.
If we were able to genetically modify human cells to transdifferentiate in a similar manner - perhaps we could achieve immortality. This is a topic that has been discussed for many centuries but now we have proof that it does exist to a certain extent.
For more information please visit the papers I have listed as references. They contain a lot more detail in experimental procedure and use a wider range of more specific biological terms, which have not been included in this blog post.
References
http://www.britannica.com/EBchecked/topic/278116/Hydra
http://www.ucihs.uci.edu/biochem/steele/PDFs/Hydra_senescence_paper.pdf
http://en.wikipedia.org/wiki/Turritopsis_nutricula
http://www.biolbull.org/cgi/reprint/190/3/302
http://8e.devbio.com/preview_article.php?ch=2&id=6
http://www.timesonline.co.uk/tol/news/uk/science/article5594539.ece
http://en.wikipedia.org/wiki/Hydra_(genus)
Lamont Lee
