Some animals like salamanders or starfish can regrow lost limbs completely.
Why can’t humans or most mammals do that?
Is it something we lost in evolution, or were we never capable of it?
Just curious how regeneration works and why it’s limited in us.
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https://www.youtube.com/shorts/Vecx0CRwKKk
It’s a field of active research, so nobody knows and there is tons of speculation…
So “higher” vertebrates don’t form a specialised wound epidermis thought to enable limb regeneration. Why they do not, is very unclear, depending on what why you are asking, there is an easy why and a difficult why.
The principle is, that you get a layer of cells at the tip of the limbs that are somehow reverted to a developmental state and are able to recreate development. The easy answer to why not is, that regenerative vertebrates express to much of inhibitors (like nogging) and the cells don’t form the apical-ectodermal ridge.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8217717/
But what evolutionary drove them to its loss is debated. So often scaring is thought to inhibit regeneration and scaring has the benefit of quickly closing the wound and prevent infection, so people speculate it’s that. Others say that the cell states are to complex in these animals so its hard to get the cells to a point where they can reform a limb correctly (I don’t really buy it), other say its there is a more tight control on cancer, which inhibits strong growth in adults. Some say its the transition to endothermy and the required changes in metabolism (which might be related to the cancer thing).
There is of course plenty of work trying to engineer systems to switch from healing to regenerative capabilities, quite recently this paper made headlines: https://www.science.org/doi/10.1126/science.adp0176
where they activated retinoic acid to restore wound healing in mouse ears/pinna comparable to the one found in rabbit.
I haven’t heard something super convincing yet, but its not my field directly and I might be to uneducated to judge and would be happy to hear more ideas!
Imagine you are a cell. What can you know ? (This is the core of the problem)
Most cells cell only know which other cells is around them.
Am I a muscle cell in the arm? Are far along? Should my neighbours start thinking about fingers ? Etc.
Regeneration is about deciding what a new cell should be. Say your arm could regrow and make more arm cells; how does the regeneration process know when to stop and start making have cells ?
When you loose a limb, something critical is lost : _information__. Information about the placement and role of every cell in that limb, billions of them.
Now, how can a salamander regrow a limb ? We don’t really know, it’s an active field of research. It is studied for example in the field of natural intelligence, because it is a problem about computing information.
Some humans have done so. The field of regenerative medicine has a long way to go but a woman recently regrew a severed finger tip using this technology.
http://www.cnn.com/2010/HEALTH/09/09/pinky.regeneration.surgery/index.html
Multiple cases of regrown fingertips here:
https://www.npr.org/sections/health-shots/2013/06/10/190385484/chopped-how-amputated-fingertips-sometimes-grow-back