Limb generation may be a long way down the road, but genetic studies give a glimmer of hope.
Although humans cannot regrow lost limbs, there is a range of species that can regenerate lost appendages.
These animals include echinoderms, such as starfish and sea cucumbers; amphibians, including the axolotl and newt; and certain fish species.
Although these animals are considered to be far-removed from humanity, because we all evolved from a joint ancestor, we still share large quantities of genetic information.
And, humanity’s ability to regenerate has not been completely lost. Although it is now limited to regrowing fingertips and healing wounds, similar genetic mechanisms are thought to be at work.
The genetics of regrowth
Recent research, published in PLOS One, used sophisticated genetic and computational techniques to investigate tissue regeneration in three species.
A team from MDI Biological Laboratory in Maine studied the regrowth of axolotl’s forelimbs, zebra fish’s caudal fins, and a ray-finned fish called the bichir’s pectoral fins. The team of scientists, led by Benjamin L. King, Ph.D., and Voot P. Yin, Ph.D., searched out the genetic signature of the mechanisms responsible.
After tracking down the genes and processes responsible for tissue regeneration, the researchers were surprised by the results.
“We didn’t expect the patterns of genetic expression to be vastly different in the three species, but it was amazing to see that they were consistently the same.”
Benjamin L. King, Ph.D.
The axolotl, zebra fish, and bichir are not near relatives – their last shared ancestor lived on earth around 420 million years ago. That they all share a similar mechanism is a fascinating discovery.
Of particular interest to the team was a mass of cells called a blastema. The blastema contains cells that have not differentiated into different types and acts as a reservoir for regenerating tissues; it is a vital first step in the regeneration process. Within these blastema, King and Yin found a set of genes in each of the three species, all of which are controlled by genetic regulators called microRNAs.
MicroRNAs are a relatively recent discovery; they are short sections of RNA that do not code for specific proteins and are responsible for fine-tuning and regulating the expression of genes.
Genes and limb regrowth
After examining the genes at work during fin and limb regrowth, a microRNA section known as miR-21 was found to be most highly expressed in all three animals.
MiR-21 has been well conserved throughout evolution and can be found in a number of species, including humans.
Understanding the genetic pathways at work in limb regeneration is the first step on a long path toward manipulating them within humans.
Axolotls can regrow entire limbs with ease.
Although the idea of regrowing limbs sounds firmly within the realms of fantasy, King believes that, given enough time and money, it could become a reality, he says:
“The fact that we’ve identified a genetic signature for limb regeneration in three different species with three different types of appendages suggests that nature has created a common genetic instruction manual governing regeneration that may be shared by all forms of animal life, including humans.”
Although altering and working with genes in humans to produce limb regrowth will be decades away, there are other treatments that might be reachable in the near future.
For instance, wound healing, which requires the replacement of damaged tissues, uses similar genetic mechanisms.
If these mechanisms could be influenced by medical interventions, wound healing could potentially be sped up, decreasing infection risks and reducing pain.
Also, when a limb is amputated, nerves can be damaged. If these nerves could be repaired and regenerated, there is a potential for prosthesis to interface with the nerves, making them more sensitive.
When asked how soon this new technology might be realized, King says: “It depends on the pace of discovery, which is heavily dependent on funding.” King also remarks that “unfortunately, we are in a period of greatly diminished funding for scientific research.”
Although the research is fascinating, and its potential is nothing short of miraculous, only time will tell whether it will become a reality in the long-term.