As always, good, thoughtful commentary from Randall Parker on some recent advances in stem cell research:
My guess is that in the future both the use of starter cells to grow replacement organs in a human body and the transfer of fully grown xenotransplants will become commonly used treatments for organ failures. These treatments might even be used in a complementary fashion. Get an emergency full sized xenotransplant organ with immunosuppressive drugs to meet an immediate emergency need and then once the patient is stabilized and healthy transfer in some more immunologically compatible cells to grow yet another organ that will serve as the permanent replacement.
In the longer run it seems reasonable to expect the development of techniques that allow the growth of full sized immunologically compatible organs. Alternatively, full sized organs will be grown up and then immune system modification therapies will be developed that will be able to adjust an immune system to teach it that a transplant organ should be treated as native tissue.
I'm not so confident here; my suspicious is that the level of biomedical competence required to overcome the various hurdles associated with xenotransplantation will also allow organs to be grown from scratch from a patient's own tissue or remove obstacles associated with immune rejections between humans. From my admittedly distant position of viewing, it looks like scientists have about the same level of work remaining in any of these fields.
This is an important report. But I repeat my caution above: If the presence or absence of some compound(s) in the blood is reducing the repair ability of a variety of tissue types (and it seems likely other tissue types will also be found to be affected by young versus old blood) then there is a decent chance that this reduction in repair ability was selected for to achieve some benefit, most likely a reduction in cancer risk.
Suppose that changes in levels (either increases in suppressor molecules or decreases in cell growth stimulating molecules) of one or more compounds in the blood as we age happens in order to reduce the risk of cancer. Well, this is problematic for hopes to derive maximal benefits from replacing aged stem cell reservoirs with youthful stem cells. The old stem cells could be replaced with younger cells. There'd be immediate gains from lowered risk of cancer and relative improvements in the vigor and health of adult stem cells. So that is still worth doing. Yet the young replacement stem cells would still be restrained by levels of compounds in the aged blood. Here's the problem: If some but not all stem cell reservoirs have their stem cells replaced with younger stem cells it might not be safe to change the blood to make it more like young blood. It might be necessary to rejuvenate all stem cell reservoirs before the blood can safely be made more like young blood.
Here is an analogy: Imagine you have a car. It is old and it has 4 bad axles that will fall off if the car is driven too fast as well as a steering column that will fall apart at high speeds. Suppose you know how to replace the 4 axles but not the steering column. Well, if you replace only the 4 axles you still can't safely drive your car at high speeds. But with humans this problem is even tougher because there are many stem cell reservoirs located near every muscle and organ that would need to be rejuvenated before they could all have their level of stimulation by the blood safely raised to youthful levels.
Once really effective anti-cancer treatments (even treatments that kill all precancerous cells) are developed then most (all?) safety worries from making blood young again would go away. Any cancers that popped up in response to having youthful and growth-stimulating blood could quickly be slain or they could be slain even before the blood was rejuvenated. So great cancer-slaying treatments would make rejuvenation treatments easier to implement.