We do grow limbs routinely, in utero. Typically 4 limbs per person. I would assume that we would be looking to activate the genes responsible for that same process. You can look up timelapse videos of embryos on youtube to answer your question.

It seems more likely for there to be a single set of genes responsible for growing a limb than 2 sets in the Axolotl. Especially since the new limbs seem identical to the old ones, rather than following a distinct backup blueprint.

From what I’ve read, Salamanders of all kind retain their stem-cell embryonic growth capabilities in their early life (after birth). But once they become adults, they become more mortal like the rest of us. But Axolotls for some reason never lose their ability.

An axolotl is a salamander that remains in its larval form throughout its life, while most salamanders go through a metamorphosis during sexual maturity, so it’s not surprising it retains embryonic growth capabilities.

There’s actually a theory that hominid ancestors at some point split off from other great apes by also not going through the typical great ape sexual maturity. For examples humans look a lot more like juvenile chimps that we do sexually mature chimps.

My theory would be that neoteny goes hand-in-hand with the traits of domestication (social friendliness, high tolerance of strangers, etc), and that humans are effectively self-domesticated.

Some of this is alluded to in https://en.wikipedia.org/wiki/Neoteny_in_humans which is a fascinating read, as is this very thread. Thanks for the food for thought!

I believe there's also a hypothesis that our ancestry back to the Devonian or Cambrian was based on neoteny/paedomorphism where a particular creature living on the sea floor, when in the infant stage, has a limb for propulsion which it loses when it finds a nice spot to settle. Our very distant ancestors didn't lose those limbs and evolved into the distant precursors to placaderms, bony fish, and, well, terrestrial fauna.

Isn’t the whole body growing from the same point at the same pace in a lot of instances when someone is in the womb (except when something like thalidomide is introduced)? It would seem a big challenge would be to have a part of a human body, at any number of stages after birth, maybe especially after full development, regrow from nothing, and excluding, at the very least, the areas of the body in proximity to it.

I’m obviously not an expert.

Why we can't do it: https://www.popsci.com/newts-toads-regrow-limbs-humans-evolu...

Seams reasonable that it would probably form in the same way that it forms in the embryo. But i guess it would depend on how we manage to convince the body to do this.

Introducing pluripotent cells seems like a recipe for disaster.

We should really just grow clones in labs and harvest them for parts.

Remove the brains at week 16 through genetic and surgical means. Keep the rest of the body artificially alive. Expensive upfront, but massively scaleable.

MHC, ABO, etc. complexes engineered to be transplant compatible.

We could replace organs and blood as we age. In fifty years, full head transplants could tackle every disease except brain and blood cancers and neurodegeneration. Every other disease solved.

It's so simple and obvious, but nobody can get over the egocentric morality qualms and superstitious ick factor.

Our bodies are plants. It's our minds that are special. We should be able to transplant every other part.

Brain is necessary for growth , repair , and several immune responses. Do you want a zombie arm?

Gets you a factor of two if you're lucky. Imagine the mental fidelity of presidents 46 and 47 when they're ready for a third full body transplant.

It worked for Dick Cheney

I think Dick Cheney at 70% neurodegeneration would be smarter than 43 at his best!

Idk, Doubleya wasn’t “smart” but he was “smart”, ya know? I wouldn’t want him making complicated calculations, and to his credit he knew that you get people better at counting, or with more fingers, to do that work.

But for getting us into a ridiculously drawn out war, and another war, and…oh and paving the way for drone strikes, for helping lead the country into a recession, for being - at the time - really good at embarrassing the office, he still pretty much gets to just paint or whatever and fuck off.

A bit mad to think about how much I miss him compared to gestures at everything. “Mission Accomplished”, vomiting on (iirc) the Japanese PM, being oddly good at dodging shoes to the head, this is like seeing a toddler destroy a car and going “cuuuuuute”.

Cheney, on the other hand, shot a guy in the head “on accident” and by my count has no redeeming qualities unless…nope, just checked and he’s still alive.

What if we transplant just part of the brain?

I think the big factor there is that you'd have to wait over a decade for the transplant to be the correct size. I'm also not sure that we have the technology to keep a brainless body alive for such a long period - the brain is involved in a large number of processes that we don't yet have a way to replicate. And then you'd need a complex surgery to perform the transplant.

Pluripotent cells work fine in many animals with no apparent problems and avoid all of the issues with the clone approach. If pluripotent cells turn out to cause problems, then we could always engineer a kill switch to make sure they die off after the limb is regrown.

ya, but no be riding the glitch train I have a atrong sense that the brain is very much involved in growing the rest of the nervous system......which the "parts" need to function these hypothetical procedures will be hugely complex surgeries that will take a long, realy long time, and there will be complications, so the risk to benifit ratio will only be good in a few scenarios....for which you need a wildly complex and expensive facility to prepare for many years in advance. gets worse, as the timing of procedures may require "parts" that are critical for the....."chasis" to self maintain, then requiring other back up "chasis" gets even worse, as the learning curve is going to be exceptionaly steep, and long, almost certainly meaning that who ever trys first, wont live long enough to benifit. all for what? a new chasis for a worn out brain?

Yes. There is a big heuristic lookup network involved.

Ever notice as we age, get hurt, that table, network, updates?

For some injuries, part of the imperfect process, is a relearn, recalibrate step involved. Once that has played out. The whole thing works as a unit.

I can sometimes remember what it used to be like. Being able to move jn some way, or some rate.

And when I try and do it today, there is a very deep inhibition. My brain / body knows it no longer happens that way,

And these things may be why physical therapy helps. No pain. No gain. We have someone else pushing us. Manipulating our bodies in ways that challenge those built in metrics.

So, it hurts, but it is good hurt. Triggers that network to reconsider, update, whatever.

> "It's our minds that are special. We should be able to transplant every other part."

The gut has the enteric nervous system with half a billion nerve cells and a hundred million neurons. Where's the clear divide between 'brain' and 'everything else'?

> "full head transplants could tackle every disease except"

except being quadraplegic and in an Iron Lung because reconnecting the spinal column is indistinguishable from magic at this point. What about the risks involved in major surgery and rehab? The hospital staffing and effort and costs involved in doing multiple organ transplant surgeries per lifetime for each of hundreds of millions of people? Saying "it's so simple" doesn't make it simple.

It turns out that you can induce pluripotency with all four Yamanaka factors, but there's a significant risk of the pluripotent cells turning into cancer.

But by using only three of the four factors, you can still induce pluripotency, but with much lower cancer risk.

I think that cloning individual organs is much more promising than cloning entire bodies and then harvesting. Much less work, you don't have an entire body you need to either upkeep or dispose of.

Maybe like so?

https://youtu.be/p1BxsqDKH3Y

I’ve cut off my fingertip accidentally (only about 3mm) and it grew back as it healed. This feature only works as far as the white crescent of the fingernail. Past that point and it won’t grow back.

Interesting. Did nerves grow back too?

Well, it's not numb, but I don't know how to test if that is the intact nerves behind the cut.

I found this diagram:

https://jbiol.biomedcentral.com/articles/10.1186/jbiol105/fi...

It's fairly reminiscent of how limb growth looks during embryogenesis - and I think considering that is how other vertebrate (such as axolotl) regeneration looks, that it's probably what human limb regrowth would be like too. So your second description sounds more like it, but with continuing development of 'prior' parts while the 'later' parts (fingers etc) are still developing.

Michael Levin's group have experimented with a 'bioreactor' that can be attached to the stump of an amputated limb in frogs (that don't ordinarily regenerate limbs) and after a short period of treatment (days), the reactor is removed and the limb then regenerates on its own over the next few months. They are looking into reproducing this in rats now, with the clearly-stated aim of similar human regeneration being the ultimate goal.

I wholeheartedly recommend watching some of Levin's lectures on YouTube, there are a great many of them and I think they're both illuminating and fascinating!