We should all be rightly concerned about the future of brain rejuvenation - whatever the technologies used, repairing (or even identifying and understanding) age-related damage to the brain looks to be a lot harder than anything else we have on our plate. It's also a great deal more crucial. After all, the worst case scenario for, say, a liver or heart is that we hook it out and replace it with a new one grown from our own tissue. That won't be science fiction a decade from now, and nor will more subtle methods of dealing with age-damaged organs using stem cell based regenerative medicine. We may even solve the problem of growing new organs from our aged cells that are youthful in aspect within the next decade or two - undamaged mitochondria, long telomeres, repaired DNA. But we are our brains, and the brain is a good deal more complex than the rest of the body - so the hardest repair job is also the one that has to be done in situ with no wholesale replacement of parts.
Randall Parker discussed these issues in connection with a recent study of changing gene expression in the brain with age. It's well worth reading:
I see the brain as by far the toughest challenge for the development of rejuvenation therapies. For many parts of the body the simplest approach to rejuvenation will be parts replacement. Once tissue engineering and stem cell research advance far enough we'll be able to replace bad parts just as mechanics do with old cars. Got old failing kidneys dodgy lungs ruined by emphysema? Grow new ones. Is your liver shot? If you don't want to get a whole new liver then send in stem cells that programmed to gradually replace the existing cells with new ones. Got liver scar tissue that doesn't want to go away? Send in cells programmed to eat it up to make room for new liver cells made from stem cells. But the brain's three dimensional network of neural connections defines who you are. Put a new brain in place of your own and that body will no longer be you for most practical purposes.
To rejuvenate the brain each cell in the brain must be repaired. But the scope of such a repair job is enormous. While estimates on the number of neurons in a human brain vary the range goes from 10 billion to 100 billion or 100 billion to 200 billion with the number of neuroglial support cells ranging from 5 to 10 times the number of neurons or perhaps 50 to 100 time sthe number of neurons. So we might have a half trillion or even a trillion cells in our brains, all aging and accumulating DNA mutations, intracellular lysosomal junk, and other damage. To develop methods repair all those cells right in the brain is an enormous scientific and engineering challenge.
While stem cell therapy gets a great deal of press (and deservedly so) and while stem cell therapy does have a crucial role to play in brain rejuvenation stem cells can not do most of the brain repair job. Much of brain rejuvenation probably requires highly advanced gene therapy delivery methods and basically DNA programs to send into cells to carry out repair tasks. Future advances in nanotechnology will eventually produce nanobots that can carry out many brain repair tasks. But to repair DNA we need gene therapy to send in corrective sequences to replace mutated sequences and deleted sequences.
If you treasure your ability to think and your mental identity then support a rapid increase in the rate of development of gene therapies and other therapies aimed at brain rejuvenation.
It's a big task - recall that Paul Allen is putting a hundred million dollars into simply understanding the genetics of the brain. That's a start, but it is only a start. There is much more to come.