Wouldn’t it be great if humans could regenerate their limbs? If it were possible for humans to “regrow” their limbs, then amputees would have no more worries. In a short time, they would not appear to be amputees any longer. They would have their old lives back. Already, there are some animals that have this ability. Salamanders, for instance are known for their ability to regrow limbs. If we could somehow harness their ability to regenerate, the world of prosthetic limbs could be a thing of the past.
In order to begin to figure out how to make humans regenerate limbs, we have to first understand how organisms like salamanders do it. When a salamander loses a limb, a “tumorlike” group of cells called a blastema forms where the limb was removed (Humphries). In only three weeks, this group of cells can form a fully developed limb (ScienceDaily). It was originally thought that the blastema was composed of pluripotent cells (cells that can differentiate into any type), like stem cells. However, in 2009, a group of scientists discovered that this is not the case. Actually, the cells retain their original identities, muscle cells regenerate the muscles, bone cells regenerate the bones, etc. This greatly simplified the matter for scientists as it revealed that the presence of pluripotent cells was not necessary for limb regrowth. In fact, the regeneration of a limb is very similar to the healing of cuts or broken bones (Humphries).
However, this brought up the question: If salamander limb regeneration follows a similar mechanism to human wound repair, why can’t humans regenerate their limbs? Actually humans do have this ability in our earlier stages of development. Fetuses actually have the ability to regrow limbs, and young children can regrow fingertips. But why is this ability lost with adulthood? I found an interesting segment from show on the Science Channel that goes into more detail on the subject.
In case you don’t feel like watching it, basically, it has to do with the extracellular matrix. The extracellular matrix is what carries information to the cells to effectively tell them what they should be doing: growing, moving somewhere else, differentiating, etc. In salamanders, the nature of the blastema allows the extracellular matrix to keep contact with the cells on the outermost of the wound. In humans, we don’t have this blastema, so the extracellular matrix is cut off from the outermost part of the wound, preventing the message of regeneration from getting to the outermost cells. This problem was solved by a scientist who decided to try isolating the extracellular matrix from pig bladders and applying it to the site of his brother’s severed finger. Lo and behold, the fingertip was able to regenerate in a few weeks, fingernail and all, a feat that was previously unheard of in older men. This scientist is particularly optimistic regarding the possibility of extending this to include the regeneration of an entire hand.
|Finger regeneration before and after from the above video|
Now, here comes the evolutionary biologist’s question: why would a salamander maintain contact with between its wounds and extracellular matrix while we don’t? What evolutionary process led to that difference? Well, the only explanation I could specifically find was that because salamanders are amphibians, their cells need to retain flexibility for metamorphosing (“Why can’t we…?”). However, I wasn’t satisfied with this explanation because while this explanation addresses why juvenile salamanders retain fetus-like flexibility, it does not fully explain why adult salamanders do not lose this ability just as adult humans do. So, I was thinking about it and I think I came up with a possible explanation. Because amphibians have evolved to be both aquatic and terrestrial animals, their skin has evolved in a different way than human skin. Amphibians must make sure to have their skin constantly moist while human skin is dry most of the time. Perhaps it is this necessity that fostered the development of a way to keep the cells always surrounded by the extracellular matrix. This is of course just my own speculation, so if you have another explanation that better explains it, please let me know!
Fortunately, we have made great strides in the study of regeneration technologies in the past few years. Hopefully, it won’t be long before we’ll only see prosthetics in the history books.
Humphries, Courtney. “A Limb Regeneration Mystery Solved” 2009. http://www.technologyreview.com/biomedicine/22955/page1/
Science Daily. “Salamanders, Regenerative Wonders” 2009. http://www.sciencedaily.com/releases/2009/07/090701131314.htm
“Why can’t we regenerate limbs like other species?” 1999. http://www.straightdope.com/columns/read/1660/why-cant-we-regenerate-limbs-like-other-species