Analysis: the regeneration of limbs, organs and even heads is no big deal for many animals
By David Duffy, UCD
It's time to say a big thank you to your amazing regenerating liver. Too often, we take advantage of the punishment our livers can take with excess alcohol and late night parties, but this organ's surprising ability to regenerate helps to protect us from our own misbehaviour (well, up to a point: think hangover and long-term liver damage).
The largest internal organ, your liver has far better regenerating capabilities than the other organs in your body. It plays a key role in removing toxins and bacteria from your blood (alcohol is of course a toxin, hence the term "intoxicated"). However, despite their amazing healing potential and ability to handle the gallons of alcohol we throw at them, livers are not immune to the abuse we inflict upon them and any doctor will surely warn of the damage excess drinking will ultimately cause, including irreparable liver damage.
If a flatworm is cut into numerous pieces with as few as 300 cells per piece, every piece will regenerate into a full individual
The regular damage inflicted on the liver by such ingested toxins means that the organ has evolved remarkable regenerating capacity. Regeneration is the replacement of all or part of a lost or damaged organ by the formation of new tissue. Imagine what it would be like if your arm was amputated above the elbow and was able to just fully regrow. Essentially, the liver already possesses such a superpower. If 75% of your liver was removed, the remaining 25% would regrow the liver back to its original mass. Intriguingly though, the human liver has not necessarily gained regeneration abilities over evolutionary time, but rather the rest of our body has lost them.
But how does this stack up to the regenerative abilities of other organs and other animals? It turns out that even the human liver's amazing regenerative potential fails to impress when compared with much of the rest of the animal kingdom. We humans have only feeble regenerative abilities when compared with other species for whom regeneration of limbs, organs and even heads is no big deal.
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How the axoloti can regenerate its brain, heart and limbs
The unassuming axolotl (a type of Mexican salamander, lizard-like in appearance) is un-phased by a crushed spine or the loss of a limb. This master regenerator will simply regrow and replace lost or damaged parts, even rapidly growing a new fully-functional limb, complete with bones, muscle, nerves and skin.
Regeneration gets even more fascinating as we get more evolutionarily distant from humans. Flatworms can regenerate entire bodies, including eyes and brains. If one gets cut in half, both halves will regenerate the missing parts (head or tail), resulting in two complete and genetically identical animals, which are essentially clones of the original. Even more surprisingly, if a flatworm is cut into numerous pieces with as few as 300 cells per piece, every piece will regenerate into a full individual, forming a veritable clone army!
Not to be outdone by the mighty flatworm, sponges and cnidarians (the group of animals including jellyfish and corals), can even withstand grating. If you grate these types of animals into tiny pieces, they have the uncanny ability to re-aggregate and reform into correctly patterned animals, with everything from head to guts in the correct place. It would appear Terminator 2's T1000 isn’t so far-fetched after all. Another example is sea turtles, who can regenerate their skin, and some fish can regenerate fins.
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From RTÉ 1's Nine News, report on how warm sea turtles washed up on Irish shores in 2016
Now for the truly mind-warping part. It turns out that humans aren’t such poor regenerators as was originally assumed. While adult humans are pretty inept at regeneration - for instance, healing a wounded limb but not re-growing it - this is not the case throughout our entire lifetimes. In fact, each one of us is a pretty good regenerator as an embryo and our regenerative potential is gradually lost as we grow older and as adults our regenerative capacity is limited. Human embryos can regrow hearts, spinal cords and digits (fingers and toes). We do possess the genes that govern regeneration, just like other master regenerator animals, but in adult humans they just do not get activated after wounding.
Regenerative medicine is a branch of medical research which is pursuing the goal of reawakening these dormant genes to reactivate our regenerative capabilities. While re-growing limbs may sound like science fiction, regenerative medicine is estimated to be a $67 billion market this year. From recent work re-growing organs and entire limbs in labs, to the discovery of scarless wound healing in African spiny mice, the potential in unlocking humans’ dormant regenerative abilities is vast. Here’s to a regenerative future as we look to our animal cousins for inspiration.
Dr David Duffy is a researcher at Systems Biology Ireland at UCD
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
