Here is an interesting question for today's discussion: do present stem cell therapies produce results that we might in any way classify as rejuvenation? The therapies presently available in numerous clinics around the world vary in the type and quality of cells used, and whether they are derived from the patient's tissues. Simple approaches extract cells from fat tissue or cord blood or similar sources and use limited purification to enrich the proportion of stem cells and progenitor cells of various types. More sophisticated approaches standardize to proven, narrow methodologies for a single cell type at high purity levels, such as the widely used mesenchymal stem cells. Further variants involve the addition of scaffolds, nutrient gels, or adjuvant treatments to try to keep the transplanted cells alive and doing their thing for longer. The evidence gathered to date strongly suggests that near all of the stem cell transplant therapies deployed over the past fifteen years produce benefits through signaling: the transplanted cells don't as a rule hang around for long, but they alter the behavior of native cells, such as stem cells and immune cells.
This is a far cry from the class of stem cell treatment envisaged for the future of rejuvenation treatments after the SENS model. That would include the delivery of populations of engineered stem cells, derived from the patient, but with their age-related molecular damage removed. The intention would be for these cells to take up residence, possibly at the same time as the existing damaged stem cell populations are cleared out. It may involve repair of the stem cell niche as well, a more complex and daunting prospect given the complexities of the niches that are well understood. Many more are not, and even if simply delivering new stem cells for the long term, something that is presently beyond the state of the art, there are scores of such cell types. Each will require their own special handling, if the challenges seen so far in regenerative medicine continue to hold true. The reason that only a few cell types are widely used in today's stem cell treatments is that each cell type requires a very different recipe and methodology. It has taken a great deal of time and effort to arrive at the recipes presently in circulation. This SENS approach to stem cell replacement is clearly rejuvenation, however. The old is cast out and the youthful ushered back in.
The closest thing to a SENS-style stem cell therapy that has taken place are the immune system reboot treatments for type 1 diabetes and multiple sclerosis. Chemotherapy is used to kill off the old immune cells and stem cell therapy delivers a new set of cells, albeit not a set of cells with any age-related damage removed. Chemotherapy is an unpleasant thing to go through, but it actually worked. This is quite an old approach in comparison to much of what is discussed here; if you go digging you'll see that it was even attempted with mixed results for rheumatoid arthritis long enough ago to predate the advent of the modern standard treatment of biologics for immune suppression. The need for aggressive chemotherapy seems to have discouraged adoption of this approach, at least until targeted cell killing is a safer, less unpleasant undertaking.
Is this rejuvenation, however? Neither type 1 diabetes nor multiple sclerosis are age-related diseases; they are not a part of aging, though like all non-age-related disease they interact with aspects of aging in ways that are never good for the patient. I think we can all agree that fixing a broken leg isn't rejuvenation, and for the same reasons neither is fixing an immune system that breaks because of inherited genetic mutation, inflammatory injury, or simple bad luck. How about the rest of the modern panoply of stem cell therapies, the much more common and straightforward transplantation to enhance regeneration? These treatments produce benefits by providing a temporary period of (a) increased healing, sometimes regeneration that would never normally happen, such as in heart tissue, in other cases a matter of restoring more youthful levels of healing, (b) reductions in chronic inflammation via interaction with immune cells, and (c) other, less well cataloged changes such as lowered oxidative stress in tissues.
Perhaps the most common form of present day stem cell therapy are those intended to partially address joint wear in the old, such as early osteoarthritis, and the less dramatic but still potentially debilitating cases of middle-aged muscle and tendon damage. There is a high expectation of attaining modest benefits in terms of reduced pain and increased capacity for use, the risks are minimal, and the costs are reasonable - as little as a few thousand dollars with some footwork, even in the broken US medical system. You probably know someone who has investigated the treatment, and if not today then you will a few years from now. If an age-worn joint is marginally improved, but via the actions of your old cells, shoring up old tissues with more age-damaged cells, is that rejuvenation? You are better off, but the joint is no younger in any biochemical measure of aging. I can see the way to argue either side of that question with little difficulty, but on the whole I'm inclined to say that if there is room for debate then the results are not rejuvenation - and thus we're not close enough to the point in the road ahead where we can relax.