Purplehermann's Shortform

In general the human body is only capable of healing injuries that are the kind of thing that, if they were smaller, would still leave the victim alive, in the Stone Age.  If an injury is of a type that would be immediately fatal in the Stone Age, there’s no evolutionary pressure to make it survivable.  For example, we can regrow peripheral nerves, because losing a peripheral nerve means a numb patch and a weak limb, but you could live with this for a few months even if you’re a caveman.  On the other hand, we can’t regrow spinal cord, because a transected spinal cord  is fatal within a day or two even given the finest Stone Age nursing care (it didn’t become survivable until about 1946.). On the third hand, we can heal brain from strokes, even though brain is more complex than spinal cord, because a small stroke is perfectly survivable as long as you have someone to feed you until you get better.  We can survive huge surgical incisions, even though those would be fatal in the Stone Age, because small penetrating wounds were survivable, and the healing mechanisms can just do the same thing all along the incision.  This is why we sew wounds up: to convince the healing mechanisms that it’s only a small cut.

Unfortunately this argument suggests regrowing limbs is impossible.  An amputation is bad but survivable, and after it heals, you can still get around.  But many years of spending a lot of bodily energy on regrowing a limb that is pretty useless for most of that time doesn’t seem worthwhile.

Some particular problems I see:

In humans, there’s no mechanism for a growing limb to connect correctly to an adult injury site.  For example, there’s already a bunch of scar tissue there, which has to be cleared away progressively as the limb grows.  Evolution has not seen fit to provide us with this complex biochemistry, unlike the case of salamanders.

Children have a high level of circulating growth hormone, which tells the arm cells how fast to grow.  If you tried to provide this to an adult, their other bones would also grow, causing deformity (acromegaly).

It’s odd that we can’t grow new teeth when the old ones fall out.  More than once, I mean.  Drilling for cavities makes sense because the enamel (outer tooth layer) is essentially dead, and doesn’t regrow.  But we should be able to grow a whole new tooth from the root when we get a cavity.

The regrowing limb thing is a nonstarter due to the issue of time if I understand correctly. Salamanders that can regrow limbs take roughly the same amount of time to regrow them as the limb takes to grow in the first place. So it would be 1-2 decades before the limb was of adult size. Secondly it's not as simple as just smearing on some stem cells to an arm stump. Limbs form because of specific signalling molecules in specific gradients. I don't think these are present in an adult body once the limb is made. So you'd need a socket which produces those which you'd have to build in the lab, attach to blood supply to feed the limb, etc.

There seem to be clinical trials underway for regrowing teeth in Japan: https://www.popularmechanics.com/science/health/a60952102/tooth-regrowth-human-trials-japan/

Research companies work best when there's plenty of infrastructure that can supply stuff they need to do the research. Including, to mention one recent case, electricity [EA · GW]. It also helps to be in an area where there is stable government that can protect the research site from civil or military unrest, and too much (or too unpredictable) corruption. You also want it to be a place where your researchers are happy to live while they do their research, and where you can relatively easily recruit other skilled workers.

China does meet these requirements, but it is not exactly lacking in bureaucracy so I'm not sure why it made the list. If you're doing research involving human trials of some sort, you also want to be able to communicate well with the participants so extensive knowledge of the language and culture will be very useful.

All that said, plenty of organizations do carry out research all over the world, not just in rich countries with a lot of bureaucracy.

Speaking as a low-level employee in the medtech industry, I can report that a lot of companies are satisfied doing human research in Eastern Europe, where costs are low and regulations lax, but infrastructure is still available. I have heard that there is a bit of "rotation" among these countries over time, since costs and regulations tend to increase wherever human research becomes popular.