Theoretically, the only organ that cannot be replaced is the brain. In practice, lifespan is equivalent to brainspan. Following an earlier discussion, it is open to debate whether aging is caused by factors that do not have their origin in the brain. Perhaps our brain just ages because the other organs in the body can no longer support it (but see below). If we could change the body at regular intervals to keep it always young, it might happen that our brain would never age. Technically, we might soon be able to transplant entire bodies. Robert White has already experimented body transplants in animals: separating the head from the body and then insert the head in a new body. (Notice the fact that I call it “body transplants” and not “brain” or “head transplants” because size does not matter here; the brain is us, and can never be changed, but the body can, and therefore it is the body that is transplanted.) Of course, body transplants, even if they are made to work, are a difficult and expensive technique.

It is also possible, though speculative, that future developments in cybernetics and therapeutic cloning will make it possible to replace all other organs. But even if we could replace the most vital organs with either new or artificial organs, it appears to be a difficult, dangerous, and unpredictable approach. Also, many theories center aging on post-mitotic tissues such as neurons, so the idea that we could avoid brain aging by replacing or rejuvenating the rest of the body is likely incorrect. Perhaps it is worth to try in animals as we might learn something out of it. For now, we must focus on trying to discover a way to stop aging in all the body, having, of course, the brain as top priority.

Short-term memory loss, personality and cognitive changes with age, dementia, general decline of the nervous system and senses, and many other changes are likely to occur with aging. Until recently, it was thought that neuronal loss, due to the accumulation of damage–such as oxidative damage–was the main cause of brain aging. Nowadays, it appears that neurons can remain relatively healthy through life, with the exception of pathologies. In fact, until recently, neurons were not thought to be able to divide. Now it appears that neurons can replicate in adult monkeys, in an area of the brain called hippocampus, which is used for long-term memory. Similar results have been reported in humans, and Fernando Nottebohm reported brain rejuvenation in birds. Instead of seeing brain aging as a mere consequence of the death of neurons, it appears that, even without neuronal death, biochemical and structural changes compromise neuron function. With age, what changes is the wiring, the complex network of connections between cells. It has even been suggested that brain aging is an extension of brain development, in line with a linkage between development and aging. While the jury is still out, the debate of whether aging is a result of damage accumulation or of programmed events also extends to brain aging.

(article source: senescence.info)