For a good while, I lived in Houston, TX, which is one of the places where experiements of this nature were being performed. It's a fascinating science to consider, especially that involving heart repair, which was as far as I'd seen being considered. For those that aren't aware of what is being considered, the experiments being run involved injecting stem cells into the inbound arteries of the existing heart and allowing them to differentiate into muscle tissue, thereby replacing damaged tissue or helping to strengthen weaker areas of the heart to improve throughput in a muscle that can't (by nature of it being rather required for ongoing needs) be isolated and allowed to recover the way that other msucle groups can.
At the time, the limitations involved a fairly short timescale where the stem cells needed to be introduced into the damaged (insulted) muscle mass before the muscle was isolated by the body into scar tissue. Outside of that limited window, the heart could be improved by helping increase the muscle strength (I rather envisioned it a bit like homeopathic steroids) but the outcome would have been less than if the original damage could have been repaired in time, i.e. the scar tissue is still ineffective and present resulting in a "weak link".