There are countless potential ways to use stem cells to improve health. These are still the very early stages of stem cell medicine, when looking at the long term. Researchers have barely scratched the surface of what could be accomplish once patient-matched stem cells of any arbitrary type can be reliably and rapidly generated to order. To give just a few examples that are already possible: deliver vast numbers of immune cells into the body to attack cancer, a specific pathogen, or simply to boost immune function in the elderly; deliver neural stem cells to improve plasticity in the brain; repairing worn joints and heart tissue with stem cells that alter signaling in tissues to instruct native cells to get back to work. But there is much, much more than this presently under investigation in the laboratory.
A great deal of present work on stem cells focuses on muscle and the various types of stem cell that support it. These stem cells are better understood than others, and the techniques for working with them are more robust and reliable. In addition, muscle tissue is easier to work with in animal models in comparison to internal organ tissue. All of this translates to a lower cost of research in money and time, and greater ease in raising funds and producing results. Details matter. On the topic of muscles and stem cells I noticed this report of an interesting application of stem cell infusions today:
Mesenchymal stem cells (MSCs) occur naturally in the body and may differentiate into several different cell types. They form part of the stroma, the connective tissue that supports organs and other tissues. MSCs also excrete growth factors and, according to the new study, stimulate muscle precursor cells, called satellite cells, to expand inside the tissue and contribute to repair following injury. Once present and activated, satellite cells actually fuse to the damaged muscle fibers and form new fibers to reconstruct the muscle and enhance strength.
By injecting MSCs into mouse leg muscles prior to several bouts of eccentric exercise (similar to the lengthening contractions performed during resistance training in humans that result in mild muscle damage), researchers were able to increase the rate of repair and enhance the growth and strength of those muscles in the exercising mice. "We have an interest in understanding how muscle responds to exercise, and which cellular components contribute to the increase in repair and growth with exercise. But the primary goal of our lab really is to have some understanding of how we can rejuvenate the aged muscle to prevent the physical disability that occurs with age, and to increase quality of life in general as well. Satellite cells are a primary target for the rejuvenation of aged muscle, since activation becomes increasingly impaired and recovery from injury is delayed over the lifespan. MSC transplantation may provide a viable solution to reawaken the aged satellite cell."
Satellite cells themselves will likely never be used therapeutically to enhance repair or strength in young or aged muscle "because they cause an immune response and rejection within the tissue." But MSCs are "immunoprivileged," meaning that they can be transplanted from one individual to another without sparking an immune response.
I think that last prediction about the use of satellite cells will be quickly proven wrong. The trajectory of research is clearly toward the ability to generate large numbers of any type of cell as needed from a sample of a patient's skin or similar. So why not satellite cells? The only good reason that immediately springs to mind is that it may turn out to be more efficient or effective to use patient-matched mesenchymal stem cells instead. Either way, ultimately the transfer of stem cells itself will most likely vanish for the majority of therapeutic applications, to be replaced with direct programming of existing in situ cell populations. Stem cell medicine is a bridge technology in this sense, though one likely to last decades.
Here is another thought for the day: how long before athletes are engaging in the use of stem cell treatments to build muscle? I would not be entirely surprised to find haphazard attempts at this taking place in today's medical tourism industry, but I suspect we are still some years away from more reliably effective treatments if enhancement of youthful muscle is the end goal.